Patent Publication Number: US-2012033673-A1

Title: Systems and methods for a para-vitualized driver in a multi-core virtual packet engine device

Description:
RELATED APPLICATION 
     This present application claims priority to U.S. Provisional Patent Application Ser. No. 61/371,309, entitled “SYSTEMS AND METHODS FOR A PARA-VIRTUALIZED DRIVER IN A MULTI-CORE VIRTUAL PACKET ENGINE DEVICE”, filed on Aug. 6, 2010, incorporated herein by reference in its entirety for all purposes. 
    
    
     FIELD OF THE DISCLOSURE 
     The present application generally relates to data communication networks in a multi-core device. In particular, the present application relates to systems and methods for providing a para-virtualized driver in a multi-core device operating a plurality of virtualized packet engines. 
     BACKGROUND 
     In a multi-core system, any one of the cores may be performing the same functionality or different functionality. The multi-core system may deploy a Receive Side Scaler, such as Microsoft&#39;s receive side scaling technology to distribute packets received from a network interface card to any core for processing. The Receive Side Scaler may be agnostic to the functionality being performed on any of the cores. As the Receive Side Scaler receives network packets from a network interface card, it forwards the network packet to a core based on a predetermined function. The network packet may be part of a transaction or series of multiple network packets in some context. Some of these network packets may go to different cores because of the distribution function of the Receive Side Scaler. Furthermore, a virtualized environment may be operating on each of the cores. These virtualized environments may share physical network interface cards. This may cause challenges in the processing and coordination of network packets across cores and virtualized environments based on a context or data flow. 
     BRIEF SUMMARY OF THE DISCLOSURE 
     Multi-core systems can balance network traffic across one or more cores in the multi-core system. These multi-core systems can comprise any number of cores, or processors. In some embodiments, multi-core systems distribute network traffic according to flow distribution models such as functional parallelism, where each core in a multi-core system is assigned to a different function, or data parallelism where each core in a multi-core system is assigned to a different device or module. 
     In some instances, distribution of network traffic across one or more cores requires obtaining network traffic from a network interface card and storing network traffic on a selected core. Many network interface cards contain a transmit and receive queue within which recently received data packets can be stored prior to forwarding them through an internal system or transmitting them out over a network. Often times a core is not associated with a particular network interface card and therefore does not have access to transmit and receive queues typically included on network interface cards. Rather, each core in the multi-core system shares, in some embodiments, a common network interface card. 
     In some cases, the cores may be operating a virtualized environment in which a packet engine operates on a hypervisor. Each of the cores may operate a guest operating system, referred to as domains, via the hypervisor. The guest operating systems may have different privileges. One core may have privileges to access and interface directly with the hardware and drivers of the device, such as the physical NIC cards. This core may have direct access to the common network interfaces of the hardware shared by the cores and the virtualized environments. Other cores may not have such privileges and get access to the network traffic of the NICs via interfaces of the hypervisor and communications with the core with the privileges to access the NIC. 
     In some aspects, the present solution is directed to design and implementation of para-virtualized network drivers for handling network communications between a plurality of virtualized packet processing engines operating on corresponding cores in a multi-core system. A first core operating a first domain of a virtualized environment receives a packet from the NIC of a multi-core device. The first domain of the first core may establish a notification channel with a second domain of a second core. The first domain transmits these packets to a second domain via an RX ring provided via a shared memory interface of the hypervisor. The first domain sends a notification via the notification channel to the second domain. The second domain during network I/O processing may check the difference between read and write descriptors. Based on this difference, the second domain may read the packets from the RX ring. The second domains covers these packets from the RX ring into a form processed by the packet processing engine. 
     The packet processing engine of the second domain may have a network packet to transmit via the physical NIC. The second domain may convert the network packets in the form for transmission to memory buffers. The second domain stores these memory buffers to the shared TX ring. The second domain also sends a notification to the first domain indicating the storage of packets to the shared TX ring. The first domain, during network I/O operations, reads from the TX shared ring based on the difference between read and write descriptors to the shared ring. Based on the difference, the first domain reads the memory buffers from the shared TX ring, converts the packets into a form for the packet processing engine to transmit via the physical NIC. These packets are then transmitted via the physical NIC via the first domain. 
     In one aspect, the present solution is directed to a method for communicating packets between network interface hardware of a multi-core device and a plurality of virtualized packet processors executed by one or more cores of the multi-core device. A first virtualization domain executing on a multi-core device may receive a packet via a network interface hardware of the multi-core device. The first virtualization domain may comprise a privileged domain having direct access to the network interface hardware. The system may communicate the packet to a queue for a virtualized packet processor. The virtualized packet processor may be from a plurality of virtualized packet processors and executing within a second virtualization domain on a core of the multi-core device. The second virtualization domain may be from a plurality of virtualization domains not having direct access to the network interface hardware. The virtualized packet processor may determine that the queue includes a difference between a number of packets read from the queue and a number of packets written to the queue. The virtualized packet processor may process the packet from the queue responsive to the determination. 
     In some embodiments, the system may execute the virtualized packet processor in user memory space of the multi-core device. The system may establish the queue in kernel memory space of the multi-core device accessible by the first virtualization domain and the second virtualization domain. The system may assign each of the plurality of virtualized packet processors with a corresponding queue in kernel memory space of the multi-core device. The first virtualization domain may send the packet into a shared queue residing in kernel space of the multi-core device, prior to communicating the packet to the queue of the virtualized packet processor. The virtualized packet processor may generate a system call or hypervisor call to transfer the packet from the queue to the second virtualization domain responsive to identifying a difference between the number of packets read from the queue and the number of packets written to the queue. 
     In certain embodiments, the system converts the packet from a first format in the queue to a second format for processing by the virtualized packet processor. The system may transfer a plurality of packets from the queue to a receive queue of the virtualized packet processor residing in user memory space. The system may communicate the packet from the queue in kernel memory space to a receive queue of the second virtualization domain in user memory space of the multi-core device. In some embodiments, the system may execute a first portion of a driver in the first virtualization domain for receiving the packet from the network interface hardware, and executing a second portion of the driver in the second virtualization domain for receiving the packet from the queue. 
     In one aspect, the present solution is directed to a method for communicating packets between network interface hardware of a multi-core device and a plurality of virtualized packet processors executed by one or more cores of the multi-core device. A virtualized packet processor executing within a first virtualization domain on a core of a multi-core device may buffer a packet in a transmit queue of the first virtualization domain. The first virtualization domain may be from a plurality of virtualization domains not having direct access to a network interface hardware of the multi-core device. The virtualized packet processor may determine that the transmit queue has one or more packets to transmit. The system may transmit, responsive to the determination, the one or more packets from the transmit queue to a shared queue residing on kernel memory space of the multi-core device accessible by a second virtualization domain. The second virtualization domain may comprise a privileged domain having direct access to the network interface hardware. The second virtualization domain may access the one or more packets from the shared queue in response to a notification from the first virtualization domain, for transmission via the network interface hardware to a network. 
     In some embodiments, the shared queue may reside in kernel space of a hypervisor providing the first and second virtualization domains. The system may establish the shared queue in kernel space of the multi-core device accessible by the first virtualization domain. The virtualized packet processor may generate a system call to initiate communication of the one or more packets on the transmit queue to the shared queue in kernel memory space responsive to the buffering of the one or more packets on the transmit queue. 
     In certain embodiments, the virtualized packet processor may generate a hypervisor call in kernel memory space to initiate communication of the one or more packets on the transmit queue to the shared queue in kernel memory space. The system may convert the packet from a first format associated with the virtualized packet processor to a second format prior to buffering the packet in the transmit queue. The system may execute a first portion of a driver in the first virtualization domain buffer the packet in the transmit queue, and executing a second portion of the driver in the second virtualization domain to access the packet from the shared queue. 
     The details of various embodiments of the methods and systems described herein are set forth in the accompanying drawings and the description below. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The foregoing and other objects, aspects, features, and advantages of the methods and systems described herein will become more apparent and better understood by referring to the following description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1A  is a block diagram of an embodiment of a network environment for a client to access a server via an appliance; 
         FIG. 1B  is a block diagram of an embodiment of an environment for delivering a computing environment from a server to a client via an appliance; 
         FIG. 1C  is a block diagram of another embodiment of an environment for delivering a computing environment from a server to a client via an appliance; 
         FIG. 1D  is a block diagram of another embodiment of an environment for delivering a computing environment from a server to a client via an appliance; 
         FIGS. 1E-1H  are block diagrams of embodiments of a computing device; 
         FIG. 2A  is a block diagram of an embodiment of an appliance for processing communications between a client and a server; 
         FIG. 2B  is a block diagram of another embodiment of an appliance for optimizing, accelerating, load-balancing and routing communications between a client and a server; 
         FIG. 3  is a block diagram of an embodiment of a client for communicating with a server via the appliance; 
         FIG. 4A  is a block diagram of an embodiment of a virtualization environment; 
         FIG. 4B  is a block diagram of another embodiment of a virtualization environment; 
         FIG. 4C  is a block diagram of an embodiment of a virtualized appliance; 
         FIG. 5A  are block diagrams of embodiments of approaches to implementing parallelism in a multi-core system; 
         FIG. 5B  is a block diagram of an embodiment of a system utilizing a multi-core system; 
         FIG. 5C  is a block diagram of another embodiment of an aspect of a multi-core system; 
         FIG. 6  is a block diagram of an embodiment of a multi-core system; 
         FIGS. 7A-7D  are flow diagrams of embodiments of a method for distributing data packets across logical queues in a multi-core system; 
         FIG. 8  is a flow diagram of an embodiment of a method for distributing data packets across a multi-core system based on a hash; 
         FIG. 9  is a flow diagram of an embodiment of a method for distributing data packets across a multi-core system via core-to-core messaging; 
         FIG. 10  is a flow diagram of an embodiment of a method for transmitting data packets stored in logical transmit queues on cores in a multi-core system; and 
         FIG. 11  is a flow diagram of an embodiment of a method for distributing data packets to logical receive queues on cores in a multi-core system; 
         FIG. 12A  is a block diagram of an embodiment of a para-virtualized driver in an embodiment of a system; and 
         FIG. 12B  is a block diagram of an embodiment of a para-virtualized driver in an embodiment of a multi-core system; 
         FIG. 12C  is a flow diagram of an embodiment of a method for communicating packets between network interface hardware of a multi-core device and a virtualized packet processor; and 
         FIG. 12D  is a flow diagram of another embodiment of a method for communicating packets between network interface hardware of a multi-core device and a virtualized packet processor. 
     
    
    
     The features and advantages of the methods and systems described herein will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. 
     DETAILED DESCRIPTION OF THE DISCLOSURE 
     For purposes of reading the description of the various embodiments below, the following descriptions of the sections of the specification and their respective contents may be helpful:
         Section A describes a network environment and computing environment which may be useful for practicing embodiments described herein;   Section B describes embodiments of systems and methods for delivering a computing environment to a remote user;   Section C describes embodiments of systems and methods for accelerating communications between a client and a server;   Section D describes embodiments of systems and methods for virtualizing an application delivery controller;   Section E describes embodiments of systems and methods for providing a multi-core architecture and environment;   Section F describes embodiments of systems and methods for distributing data packets across a multi-core architecture and environment; and   Section G describes embodiments of systems and methods for a para-virtualized driver.       

     A. Network and Computing Environment 
     Prior to discussing the specifics of embodiments of the systems and methods of an appliance and/or client, it may be helpful to discuss the network and computing environments in which such embodiments may be deployed. Referring now to  FIG. 1A , an embodiment of a network environment is depicted. In brief overview, the network environment comprises one or more clients  102   a - 102   n  (also generally referred to as local machine(s)  102 , or client(s)  102 ) in communication with one or more servers  106   a - 106   n  (also generally referred to as server(s)  106 , or remote machine(s)  106 ) via one or more networks  104 ,  104 ′ (generally referred to as network  104 ). In some embodiments, a client  102  communicates with a server  106  via an appliance  200 . 
     Although  FIG. 1A  shows a network  104  and a network  104 ′ between the clients  102  and the servers  106 , the clients  102  and the servers  106  may be on the same network  104 . The networks  104  and  104 ′ can be the same type of network or different types of networks. The network  104  and/or the network  104 ′ can be a local-area network (LAN), such as a company Intranet, a metropolitan area network (MAN), or a wide area network (WAN), such as the Internet or the World Wide Web. In one embodiment, network  104 ′ may be a private network and network  104  may be a public network. In some embodiments, network  104  may be a private network and network  104 ′ a public network. In another embodiment, networks  104  and  104 ′ may both be private networks. In some embodiments, clients  102  may be located at a branch office of a corporate enterprise communicating via a WAN connection over the network  104  to the servers  106  located at a corporate data center. 
     The network  104  and/or  104 ′ be any type and/or form of network and may include any of the following: a point to point network, a broadcast network, a wide area network, a local area network, a telecommunications network, a data communication network, a computer network, an ATM (Asynchronous Transfer Mode) network, a SONET (Synchronous Optical Network) network, a SDH (Synchronous Digital Hierarchy) network, a wireless network and a wireline network. In some embodiments, the network  104  may comprise a wireless link, such as an infrared channel or satellite band. The topology of the network  104  and/or  104 ′ may be a bus, star, or ring network topology. The network  104  and/or  104 ′ and network topology may be of any such network or network topology as known to those ordinarily skilled in the art capable of supporting the operations described herein. 
     As shown in  FIG. 1A , the appliance  200 , which also may be referred to as an interface unit  200  or gateway  200 , is shown between the networks  104  and  104 ′. In some embodiments, the appliance  200  may be located on network  104 . For example, a branch office of a corporate enterprise may deploy an appliance  200  at the branch office. In other embodiments, the appliance  200  may be located on network  104 ′. For example, an appliance  200  may be located at a corporate data center. In yet another embodiment, a plurality of appliances  200  may be deployed on network  104 . In some embodiments, a plurality of appliances  200  may be deployed on network  104 ′. In one embodiment, a first appliance  200  communicates with a second appliance  200 ′. In other embodiments, the appliance  200  could be a part of any client  102  or server  106  on the same or different network  104 , 104 ′ as the client  102 . One or more appliances  200  may be located at any point in the network or network communications path between a client  102  and a server  106 . 
     In some embodiments, the appliance  200  comprises any of the network devices manufactured by Citrix Systems, Inc. of Ft. Lauderdale Fla., referred to as Citrix NetScaler devices. In other embodiments, the appliance  200  includes any of the product embodiments referred to as WebAccelerator and BigIP manufactured by F5 Networks, Inc. of Seattle, Wash. In another embodiment, the appliance  205  includes any of the DX acceleration device platforms and/or the SSL VPN series of devices, such as SA  700 , SA  2000 , SA  4000 , and SA  6000  devices manufactured by Juniper Networks, Inc. of Sunnyvale, Calif. In yet another embodiment, the appliance  200  includes any application acceleration and/or security related appliances and/or software manufactured by Cisco Systems, Inc. of San Jose, Calif., such as the Cisco ACE Application Control Engine Module service software and network modules, and Cisco AVS Series Application Velocity System. 
     In one embodiment, the system may include multiple, logically-grouped servers  106 . In these embodiments, the logical group of servers may be referred to as a server farm  38 . In some of these embodiments, the serves  106  may be geographically dispersed. In some cases, a farm  38  may be administered as a single entity. In other embodiments, the server farm  38  comprises a plurality of server farms  38 . In one embodiment, the server farm executes one or more applications on behalf of one or more clients  102 . 
     The servers  106  within each farm  38  can be heterogeneous. One or more of the servers  106  can operate according to one type of operating system platform (e.g., WINDOWS NT, manufactured by Microsoft Corp. of Redmond, Wash.), while one or more of the other servers  106  can operate on according to another type of operating system platform (e.g., Unix or Linux). The servers  106  of each farm  38  do not need to be physically proximate to another server  106  in the same farm  38 . Thus, the group of servers  106  logically grouped as a farm  38  may be interconnected using a wide-area network (WAN) connection or medium-area network (MAN) connection. For example, a farm  38  may include servers  106  physically located in different continents or different regions of a continent, country, state, city, campus, or room. Data transmission speeds between servers  106  in the farm  38  can be increased if the servers  106  are connected using a local-area network (LAN) connection or some form of direct connection. 
     Servers  106  may be referred to as a file server, application server, web server, proxy server, or gateway server. In some embodiments, a server  106  may have the capacity to function as either an application server or as a master application server. In one embodiment, a server  106  may include an Active Directory. The clients  102  may also be referred to as client nodes or endpoints. In some embodiments, a client  102  has the capacity to function as both a client node seeking access to applications on a server and as an application server providing access to hosted applications for other clients  102   a - 102   n.    
     In some embodiments, a client  102  communicates with a server  106 . In one embodiment, the client  102  communicates directly with one of the servers  106  in a farm  38 . In another embodiment, the client  102  executes a program neighborhood application to communicate with a server  106  in a farm  38 . In still another embodiment, the server  106  provides the functionality of a master node. In some embodiments, the client  102  communicates with the server  106  in the farm  38  through a network  104 . Over the network  104 , the client  102  can, for example, request execution of various applications hosted by the servers  106   a - 106   n  in the farm  38  and receive output of the results of the application execution for display. In some embodiments, only the master node provides the functionality required to identify and provide address information associated with a server  106 ′ hosting a requested application. 
     In one embodiment, the server  106  provides functionality of a web server. In another embodiment, the server  106   a  receives requests from the client  102 , forwards the requests to a second server  106   b  and responds to the request by the client  102  with a response to the request from the server  106   b . In still another embodiment, the server  106  acquires an enumeration of applications available to the client  102  and address information associated with a server  106  hosting an application identified by the enumeration of applications. In yet another embodiment, the server  106  presents the response to the request to the client  102  using a web interface. In one embodiment, the client  102  communicates directly with the server  106  to access the identified application. In another embodiment, the client  102  receives application output data, such as display data, generated by an execution of the identified application on the server  106 . 
     Referring now to  FIG. 1B , an embodiment of a network environment deploying multiple appliances  200  is depicted. A first appliance  200  may be deployed on a first network  104  and a second appliance  200 ′ on a second network  104 ′. For example a corporate enterprise may deploy a first appliance  200  at a branch office and a second appliance  200 ′ at a data center. In another embodiment, the first appliance  200  and second appliance  200 ′ are deployed on the same network  104  or network  104 . For example, a first appliance  200  may be deployed for a first server farm  38 , and a second appliance  200  may be deployed for a second server farm  38 ′. In another example, a first appliance  200  may be deployed at a first branch office while the second appliance  200 ′ is deployed at a second branch office&#39;. In some embodiments, the first appliance  200  and second appliance  200 ′ work in cooperation or in conjunction with each other to accelerate network traffic or the delivery of application and data between a client and a server 
     Referring now to  FIG. 1C , another embodiment of a network environment deploying the appliance  200  with one or more other types of appliances, such as between one or more WAN optimization appliance  205 ,  205 ′ is depicted. For example a first WAN optimization appliance  205  is shown between networks  104  and  104 ′ and s second WAN optimization appliance  205 ′ may be deployed between the appliance  200  and one or more servers  106 . By way of example, a corporate enterprise may deploy a first WAN optimization appliance  205  at a branch office and a second WAN optimization appliance  205 ′ at a data center. In some embodiments, the appliance  205  may be located on network  104 ′. In other embodiments, the appliance  205 ′ may be located on network  104 . In some embodiments, the appliance  205 ′ may be located on network  104 ′ or network  104 ″. In one embodiment, the appliance  205  and  205 ′ are on the same network. In another embodiment, the appliance  205  and  205 ′ are on different networks. In another example, a first WAN optimization appliance  205  may be deployed for a first server farm  38  and a second WAN optimization appliance  205 ′ for a second server farm  38 ′ 
     In one embodiment, the appliance  205  is a device for accelerating, optimizing or otherwise improving the performance, operation, or quality of service of any type and form of network traffic, such as traffic to and/or from a WAN connection. In some embodiments, the appliance  205  is a performance enhancing proxy. In other embodiments, the appliance  205  is any type and form of WAN optimization or acceleration device, sometimes also referred to as a WAN optimization controller. In one embodiment, the appliance  205  is any of the product embodiments referred to as WANScaler manufactured by Citrix Systems, Inc. of Ft. Lauderdale, Fla. In other embodiments, the appliance  205  includes any of the product embodiments referred to as BIG-IP link controller and WANjet manufactured by F5 Networks, Inc. of Seattle, Wash. In another embodiment, the appliance  205  includes any of the WX and WXC WAN acceleration device platforms manufactured by Juniper Networks, Inc. of Sunnyvale, Calif. In some embodiments, the appliance  205  includes any of the steelhead line of WAN optimization appliances manufactured by Riverbed Technology of San Francisco, Calif. In other embodiments, the appliance  205  includes any of the WAN related devices manufactured by Expand Networks Inc. of Roseland, N.J. In one embodiment, the appliance  205  includes any of the WAN related appliances manufactured by Packeteer Inc. of Cupertino, Calif., such as the PacketShaper, iShared, and SkyX product embodiments provided by Packeteer. In yet another embodiment, the appliance  205  includes any WAN related appliances and/or software manufactured by Cisco Systems, Inc. of San Jose, Calif., such as the Cisco Wide Area Network Application Services software and network modules, and Wide Area Network engine appliances. 
     In one embodiment, the appliance  205  provides application and data acceleration services for branch-office or remote offices. In one embodiment, the appliance  205  includes optimization of Wide Area File Services (WAFS). In another embodiment, the appliance  205  accelerates the delivery of files, such as via the Common Internet File System (CIFS) protocol. In other embodiments, the appliance  205  provides caching in memory and/or storage to accelerate delivery of applications and data. In one embodiment, the appliance  205  provides compression of network traffic at any level of the network stack or at any protocol or network layer. In another embodiment, the appliance  205  provides transport layer protocol optimizations, flow control, performance enhancements or modifications and/or management to accelerate delivery of applications and data over a WAN connection. For example, in one embodiment, the appliance  205  provides Transport Control Protocol (TCP) optimizations. In other embodiments, the appliance  205  provides optimizations, flow control, performance enhancements or modifications and/or management for any session or application layer protocol. 
     In another embodiment, the appliance  205  encoded any type and form of data or information into custom or standard TCP and/or IP header fields or option fields of network packet to announce presence, functionality or capability to another appliance  205 ′. In another embodiment, an appliance  205 ′ may communicate with another appliance  205 ′ using data encoded in both TCP and/or IP header fields or options. For example, the appliance may use TCP option(s) or IP header fields or options to communicate one or more parameters to be used by the appliances  205 ,  205 ′ in performing functionality, such as WAN acceleration, or for working in conjunction with each other. 
     In some embodiments, the appliance  200  preserves any of the information encoded in TCP and/or IP header and/or option fields communicated between appliances  205  and  205 ′. For example, the appliance  200  may terminate a transport layer connection traversing the appliance  200 , such as a transport layer connection from between a client and a server traversing appliances  205  and  205 ′. In one embodiment, the appliance  200  identifies and preserves any encoded information in a transport layer packet transmitted by a first appliance  205  via a first transport layer connection and communicates a transport layer packet with the encoded information to a second appliance  205 ′ via a second transport layer connection. 
     Referring now to  FIG. 1D , a network environment for delivering and/or operating a computing environment on a client  102  is depicted. In some embodiments, a server  106  includes an application delivery system  190  for delivering a computing environment or an application and/or data file to one or more clients  102 . In brief overview, a client  10  is in communication with a server  106  via network  104 ,  104 ′ and appliance  200 . For example, the client  102  may reside in a remote office of a company, e.g., a branch office, and the server  106  may reside at a corporate data center. The client  102  comprises a client agent  120 , and a computing environment  15 . The computing environment  15  may execute or operate an application that accesses, processes or uses a data file. The computing environment  15 , application and/or data file may be delivered via the appliance  200  and/or the server  106 . 
     In some embodiments, the appliance  200  accelerates delivery of a computing environment  15 , or any portion thereof, to a client  102 . In one embodiment, the appliance  200  accelerates the delivery of the computing environment  15  by the application delivery system  190 . For example, the embodiments described herein may be used to accelerate delivery of a streaming application and data file processable by the application from a central corporate data center to a remote user location, such as a branch office of the company. In another embodiment, the appliance  200  accelerates transport layer traffic between a client  102  and a server  106 . The appliance  200  may provide acceleration techniques for accelerating any transport layer payload from a server  106  to a client  102 , such as: 1) transport layer connection pooling, 2) transport layer connection multiplexing, 3) transport control protocol buffering, 4) compression and 5) caching. In some embodiments, the appliance  200  provides load balancing of servers  106  in responding to requests from clients  102 . In other embodiments, the appliance  200  acts as a proxy or access server to provide access to the one or more servers  106 . In another embodiment, the appliance  200  provides a secure virtual private network connection from a first network  104  of the client  102  to the second network  104 ′ of the server  106 , such as an SSL VPN connection. It yet other embodiments, the appliance  200  provides application firewall security, control and management of the connection and communications between a client  102  and a server  106 . 
     In some embodiments, the application delivery management system  190  provides application delivery techniques to deliver a computing environment to a desktop of a user, remote or otherwise, based on a plurality of execution methods and based on any authentication and authorization policies applied via a policy engine  195 . With these techniques, a remote user may obtain a computing environment and access to server stored applications and data files from any network connected device  100 . In one embodiment, the application delivery system  190  may reside or execute on a server  106 . In another embodiment, the application delivery system  190  may reside or execute on a plurality of servers  106   a - 106   n . In some embodiments, the application delivery system  190  may execute in a server farm  38 . In one embodiment, the server  106  executing the application delivery system  190  may also store or provide the application and data file. In another embodiment, a first set of one or more servers  106  may execute the application delivery system  190 , and a different server  106   n  may store or provide the application and data file. In some embodiments, each of the application delivery system  190 , the application, and data file may reside or be located on different servers. In yet another embodiment, any portion of the application delivery system  190  may reside, execute or be stored on or distributed to the appliance  200 , or a plurality of appliances. 
     The client  102  may include a computing environment  15  for executing an application that uses or processes a data file. The client  102  via networks  104 ,  104 ′ and appliance  200  may request an application and data file from the server  106 . In one embodiment, the appliance  200  may forward a request from the client  102  to the server  106 . For example, the client  102  may not have the application and data file stored or accessible locally. In response to the request, the application delivery system  190  and/or server  106  may deliver the application and data file to the client  102 . For example, in one embodiment, the server  106  may transmit the application as an application stream to operate in computing environment  15  on client  102 . 
     In some embodiments, the application delivery system  190  comprises any portion of the Citrix Access Suite™ by Citrix Systems, Inc., such as the MetaFrame or Citrix Presentation Server™ and/or any of the Microsoft® Windows Terminal Services manufactured by the Microsoft Corporation. In one embodiment, the application delivery system  190  may deliver one or more applications to clients  102  or users via a remote-display protocol or otherwise via remote-based or server-based computing. In another embodiment, the application delivery system  190  may deliver one or more applications to clients or users via steaming of the application. 
     In one embodiment, the application delivery system  190  includes a policy engine  195  for controlling and managing the access to, selection of application execution methods and the delivery of applications. In some embodiments, the policy engine  195  determines the one or more applications a user or client  102  may access. In another embodiment, the policy engine  195  determines how the application should be delivered to the user or client  102 , e.g., the method of execution. In some embodiments, the application delivery system  190  provides a plurality of delivery techniques from which to select a method of application execution, such as a server-based computing, streaming or delivering the application locally to the client  120  for local execution. 
     In one embodiment, a client  102  requests execution of an application program and the application delivery system  190  comprising a server  106  selects a method of executing the application program. In some embodiments, the server  106  receives credentials from the client  102 . In another embodiment, the server  106  receives a request for an enumeration of available applications from the client  102 . In one embodiment, in response to the request or receipt of credentials, the application delivery system  190  enumerates a plurality of application programs available to the client  102 . The application delivery system  190  receives a request to execute an enumerated application. The application delivery system  190  selects one of a predetermined number of methods for executing the enumerated application, for example, responsive to a policy of a policy engine. The application delivery system  190  may select a method of execution of the application enabling the client  102  to receive application-output data generated by execution of the application program on a server  106 . The application delivery system  190  may select a method of execution of the application enabling the local machine  10  to execute the application program locally after retrieving a plurality of application files comprising the application. In yet another embodiment, the application delivery system  190  may select a method of execution of the application to stream the application via the network  104  to the client  102 . 
     A client  102  may execute, operate or otherwise provide an application, which can be any type and/or form of software, program, or executable instructions such as any type and/or form of web browser, web-based client, client-server application, a thin-client computing client, an ActiveX control, or a Java applet, or any other type and/or form of executable instructions capable of executing on client  102 . In some embodiments, the application may be a server-based or a remote-based application executed on behalf of the client  102  on a server  106 . In one embodiments the server  106  may display output to the client  102  using any thin-client or remote-display protocol, such as the Independent Computing Architecture (ICA) protocol manufactured by Citrix Systems, Inc. of Ft. Lauderdale, Fla. or the Remote Desktop Protocol (RDP) manufactured by the Microsoft Corporation of Redmond, Wash. The application can use any type of protocol and it can be, for example, an HTTP client, an FTP client, an Oscar client, or a Telnet client. In other embodiments, the application comprises any type of software related to VoIP communications, such as a soft IP telephone. In further embodiments, the application comprises any application related to real-time data communications, such as applications for streaming video and/or audio. 
     In some embodiments, the server  106  or a server farm  38  may be running one or more applications, such as an application providing a thin-client computing or remote display presentation application. In one embodiment, the server  106  or server farm  38  executes as an application, any portion of the Citrix Access Suite™ by Citrix Systems, Inc., such as the MetaFrame or Citrix Presentation Server™, and/or any of the Microsoft® Windows Terminal Services manufactured by the Microsoft Corporation. In one embodiment, the application is an ICA client, developed by Citrix Systems, Inc. of Fort Lauderdale, Fla. In other embodiments, the application includes a Remote Desktop (RDP) client, developed by Microsoft Corporation of Redmond, Wash. Also, the server  106  may run an application, which for example, may be an application server providing email services such as Microsoft Exchange manufactured by the Microsoft Corporation of Redmond, Wash., a web or Internet server, or a desktop sharing server, or a collaboration server. In some embodiments, any of the applications may comprise any type of hosted service or products, such as GoToMeeting™ provided by Citrix Online Division, Inc. of Santa Barbara, Calif., WebEx™ provided by WebEx, Inc. of Santa Clara, Calif., or Microsoft Office Live Meeting provided by Microsoft Corporation of Redmond, Wash. 
     Still referring to  FIG. 1D , an embodiment of the network environment may include a monitoring server  106 A. The monitoring server  106 A may include any type and form performance monitoring service  198 . The performance monitoring service  198  may include monitoring, measurement and/or management software and/or hardware, including data collection, aggregation, analysis, management and reporting. In one embodiment, the performance monitoring service  198  includes one or more monitoring agents  197 . The monitoring agent  197  includes any software, hardware or combination thereof for performing monitoring, measurement and data collection activities on a device, such as a client  102 , server  106  or an appliance  200 ,  205 . In some embodiments, the monitoring agent  197  includes any type and form of script, such as Visual Basic script, or Javascript. In one embodiment, the monitoring agent  197  executes transparently to any application and/or user of the device. In some embodiments, the monitoring agent  197  is installed and operated unobtrusively to the application or client. In yet another embodiment, the monitoring agent  197  is installed and operated without any instrumentation for the application or device. 
     In some embodiments, the monitoring agent  197  monitors, measures and collects data on a predetermined frequency. In other embodiments, the monitoring agent  197  monitors, measures and collects data based upon detection of any type and form of event. For example, the monitoring agent  197  may collect data upon detection of a request for a web page or receipt of an HTTP response. In another example, the monitoring agent  197  may collect data upon detection of any user input events, such as a mouse click. The monitoring agent  197  may report or provide any monitored, measured or collected data to the monitoring service  198 . In one embodiment, the monitoring agent  197  transmits information to the monitoring service  198  according to a schedule or a predetermined frequency. In another embodiment, the monitoring agent  197  transmits information to the monitoring service  198  upon detection of an event. 
     In some embodiments, the monitoring service  198  and/or monitoring agent  197  performs monitoring and performance measurement of any network resource or network infrastructure element, such as a client, server, server farm, appliance  200 , appliance  205 , or network connection. In one embodiment, the monitoring service  198  and/or monitoring agent  197  performs monitoring and performance measurement of any transport layer connection, such as a TCP or UDP connection. In another embodiment, the monitoring service  198  and/or monitoring agent  197  monitors and measures network latency. In yet one embodiment, the monitoring service  198  and/or monitoring agent  197  monitors and measures bandwidth utilization. 
     In other embodiments, the monitoring service  198  and/or monitoring agent  197  monitors and measures end-user response times. In some embodiments, the monitoring service  198  performs monitoring and performance measurement of an application. In another embodiment, the monitoring service  198  and/or monitoring agent  197  performs monitoring and performance measurement of any session or connection to the application. In one embodiment, the monitoring service  198  and/or monitoring agent  197  monitors and measures performance of a browser. In another embodiment, the monitoring service  198  and/or monitoring agent  197  monitors and measures performance of HTTP based transactions. In some embodiments, the monitoring service  198  and/or monitoring agent  197  monitors and measures performance of a Voice over IP (VoIP) application or session. In other embodiments, the monitoring service  198  and/or monitoring agent  197  monitors and measures performance of a remote display protocol application, such as an ICA client or RDP client. In yet another embodiment, the monitoring service  198  and/or monitoring agent  197  monitors and measures performance of any type and form of streaming media. In still a further embodiment, the monitoring service  198  and/or monitoring agent  197  monitors and measures performance of a hosted application or a Software-As-A-Service (SaaS) delivery model. 
     In some embodiments, the monitoring service  198  and/or monitoring agent  197  performs monitoring and performance measurement of one or more transactions, requests or responses related to application. In other embodiments, the monitoring service  198  and/or monitoring agent  197  monitors and measures any portion of an application layer stack, such as any .NET or J2EE calls. In one embodiment, the monitoring service  198  and/or monitoring agent  197  monitors and measures database or SQL transactions. In yet another embodiment, the monitoring service  198  and/or monitoring agent  197  monitors and measures any method, function or application programming interface (API) call. 
     In one embodiment, the monitoring service  198  and/or monitoring agent  197  performs monitoring and performance measurement of a delivery of application and/or data from a server to a client via one or more appliances, such as appliance  200  and/or appliance  205 . In some embodiments, the monitoring service  198  and/or monitoring agent  197  monitors and measures performance of delivery of a virtualized application. In other embodiments, the monitoring service  198  and/or monitoring agent  197  monitors and measures performance of delivery of a streaming application. In another embodiment, the monitoring service  198  and/or monitoring agent  197  monitors and measures performance of delivery of a desktop application to a client and/or the execution of the desktop application on the client. In another embodiment, the monitoring service  198  and/or monitoring agent  197  monitors and measures performance of a client/server application. 
     In one embodiment, the monitoring service  198  and/or monitoring agent  197  is designed and constructed to provide application performance management for the application delivery system  190 . For example, the monitoring service  198  and/or monitoring agent  197  may monitor, measure and manage the performance of the delivery of applications via the Citrix Presentation Server. In this example, the monitoring service  198  and/or monitoring agent  197  monitors individual ICA sessions. The monitoring service  198  and/or monitoring agent  197  may measure the total and per session system resource usage, as well as application and networking performance. The monitoring service  198  and/or monitoring agent  197  may identify the active servers for a given user and/or user session. In some embodiments, the monitoring service  198  and/or monitoring agent  197  monitors back-end connections between the application delivery system  190  and an application and/or database server. The monitoring service  198  and/or monitoring agent  197  may measure network latency, delay and volume per user-session or ICA session. 
     In some embodiments, the monitoring service  198  and/or monitoring agent  197  measures and monitors memory usage for the application delivery system  190 , such as total memory usage, per user session and/or per process. In other embodiments, the monitoring service  198  and/or monitoring agent  197  measures and monitors CPU usage the application delivery system  190 , such as total CPU usage, per user session and/or per process. In another embodiments, the monitoring service  198  and/or monitoring agent  197  measures and monitors the time required to log-in to an application, a server, or the application delivery system, such as Citrix Presentation Server. In one embodiment, the monitoring service  198  and/or monitoring agent  197  measures and monitors the duration a user is logged into an application, a server, or the application delivery system  190 . In some embodiments, the monitoring service  198  and/or monitoring agent  197  measures and monitors active and inactive session counts for an application, server or application delivery system session. In yet another embodiment, the monitoring service  198  and/or monitoring agent  197  measures and monitors user session latency. 
     In yet further embodiments, the monitoring service  198  and/or monitoring agent  197  measures and monitors measures and monitors any type and form of server metrics. In one embodiment, the monitoring service  198  and/or monitoring agent  197  measures and monitors metrics related to system memory, CPU usage, and disk storage. In another embodiment, the monitoring service  198  and/or monitoring agent  197  measures and monitors metrics related to page faults, such as page faults per second. In other embodiments, the monitoring service  198  and/or monitoring agent  197  measures and monitors round-trip time metrics. In yet another embodiment, the monitoring service  198  and/or monitoring agent  197  measures and monitors metrics related to application crashes, errors and/or hangs. 
     In some embodiments, the monitoring service  198  and monitoring agent  198  includes any of the product embodiments referred to as EdgeSight manufactured by Citrix Systems, Inc. of Ft. Lauderdale, Fla. In another embodiment, the performance monitoring service  198  and/or monitoring agent  198  includes any portion of the product embodiments referred to as the TrueView product suite manufactured by the Symphoniq Corporation of Palo Alto, California. In one embodiment, the performance monitoring service  198  and/or monitoring agent  198  includes any portion of the product embodiments referred to as the TeaLeaf CX product suite manufactured by the TeaLeaf Technology Inc. of San Francisco, Calif. In other embodiments, the performance monitoring service  198  and/or monitoring agent  198  includes any portion of the business service management products, such as the BMC Performance Manager and Patrol products, manufactured by BMC Software, Inc. of Houston, Tex. 
     The client  102 , server  106 , and appliance  200  may be deployed as and/or executed on any type and form of computing device, such as a computer, network device or appliance capable of communicating on any type and form of network and performing the operations described herein.  FIGS. 1E and 1F  depict block diagrams of a computing device  100  useful for practicing an embodiment of the client  102 , server  106  or appliance  200 . As shown in  FIGS. 1E and 1F , each computing device  100  includes a central processing unit  101 , and a main memory unit  122 . As shown in  FIG. 1E , a computing device  100  may include a visual display device  124 , a keyboard  126  and/or a pointing device  127 , such as a mouse. Each computing device  100  may also include additional optional elements, such as one or more input/output devices  130   a - 130   b  (generally referred to using reference numeral  130 ), and a cache memory  140  in communication with the central processing unit  101 . 
     The central processing unit  101  is any logic circuitry that responds to and processes instructions fetched from the main memory unit  122 . In many embodiments, the central processing unit is provided by a microprocessor unit, such as: those manufactured by Intel Corporation of Mountain View, Calif.; those manufactured by Motorola Corporation of Schaumburg, Ill.; those manufactured by Transmeta Corporation of Santa Clara, Calif.; the RS/6000 processor, those manufactured by International Business Machines of White Plains, N.Y.; or those manufactured by Advanced Micro Devices of Sunnyvale, Calif. The computing device  100  may be based on any of these processors, or any other processor capable of operating as described herein. 
     Main memory unit  122  may be one or more memory chips capable of storing data and allowing any storage location to be directly accessed by the microprocessor  101 , such as Static random access memory (SRAM), Burst SRAM or SynchBurst SRAM (BSRAM), Dynamic random access memory (DRAM), Fast Page Mode DRAM (FPM DRAM), Enhanced DRAM (EDRAM), Extended Data Output RAM (EDO RAM), Extended Data Output DRAM (EDO DRAM), Burst Extended Data Output DRAM (BEDO DRAM), Enhanced DRAM (EDRAM), synchronous DRAM (SDRAM), JEDEC SRAM, PC 100 SDRAM, Double Data Rate SDRAM (DDR SDRAM), Enhanced SDRAM (ESDRAM), SyncLink DRAM (SLDRAM), Direct Rambus DRAM (DRDRAM), or Ferroelectric RAM (FRAM). The main memory  122  may be based on any of the above described memory chips, or any other available memory chips capable of operating as described herein. In the embodiment shown in  FIG. 1E , the processor  101  communicates with main memory  122  via a system bus  150  (described in more detail below).  FIG. 1E  depicts an embodiment of a computing device  100  in which the processor communicates directly with main memory  122  via a memory port  103 . For example, in  FIG. 1F  the main memory  122  may be DRDRAM. 
       FIG. 1F  depicts an embodiment in which the main processor  101  communicates directly with cache memory  140  via a secondary bus, sometimes referred to as a backside bus. In other embodiments, the main processor  101  communicates with cache memory  140  using the system bus  150 . Cache memory  140  typically has a faster response time than main memory  122  and is typically provided by SRAM, BSRAM, or EDRAM. In the embodiment shown in  FIG. 1E , the processor  101  communicates with various I/O devices  130  via a local system bus  150 . Various busses may be used to connect the central processing unit  101  to any of the I/O devices  130 , including a VESA VL bus, an ISA bus, an EISA bus, a MicroChannel Architecture (MCA) bus, a PCI bus, a PCI-X bus, a PCI-Express bus, or a NuBus. For embodiments in which the I/O device is a video display  124 , the processor  101  may use an Advanced Graphics Port (AGP) to communicate with the display  124 .  FIG. 1F  depicts an embodiment of a computer  100  in which the main processor  101  communicates directly with I/O device  130  via HyperTransport, Rapid I/O, or InfiniBand.  FIG. 1F  also depicts an embodiment in which local busses and direct communication are mixed: the processor  101  communicates with I/O device  130  using a local interconnect bus while communicating with I/O device  130  directly. 
     The computing device  100  may support any suitable installation device  116 , such as a floppy disk drive for receiving floppy disks such as 3.5-inch, 5.25-inch disks or ZIP disks, a CD-ROM drive, a CD-R/RW drive, a DVD-ROM drive, tape drives of various formats, USB device, hard-drive or any other device suitable for installing software and programs such as any client agent  120 , or portion thereof. The computing device  100  may further comprise a storage device  128 , such as one or more hard disk drives or redundant arrays of independent disks, for storing an operating system and other related software, and for storing application software programs such as any program related to the client agent  120 . Optionally, any of the installation devices  116  could also be used as the storage device  128 . Additionally, the operating system and the software can be run from a bootable medium, for example, a bootable CD, such as KNOPPIX®, a bootable CD for GNU/Linux that is available as a GNU/Linux distribution from knoppix.net. 
     Furthermore, the computing device  100  may include a network interface  118  to interface to a Local Area Network (LAN), Wide Area Network (WAN) or the Internet through a variety of connections including, but not limited to, standard telephone lines, LAN or WAN links (e.g., 802.11, T1, T3, 56 kb, X.25), broadband connections (e.g., ISDN, Frame Relay, ATM), wireless connections, or some combination of any or all of the above. The network interface  118  may comprise a built-in network adapter, network interface card, PCMCIA network card, card bus network adapter, wireless network adapter, USB network adapter, modem or any other device suitable for interfacing the computing device  100  to any type of network capable of communication and performing the operations described herein. A wide variety of I/O devices  130   a - 130   n  may be present in the computing device  100 . Input devices include keyboards, mice, trackpads, trackballs, microphones, and drawing tablets. Output devices include video displays, speakers, inkjet printers, laser printers, and dye-sublimation printers. The I/O devices  130  may be controlled by an I/O controller  123  as shown in  FIG. 1E . The I/O controller may control one or more I/O devices such as a keyboard  126  and a pointing device  127 , e.g., a mouse or optical pen. Furthermore, an I/O device may also provide storage  128  and/or an installation medium  116  for the computing device  100 . In still other embodiments, the computing device  100  may provide USB connections to receive handheld USB storage devices such as the USB Flash Drive line of devices manufactured by Twintech Industry, Inc. of Los Alamitos, California. 
     In some embodiments, the computing device  100  may comprise or be connected to multiple display devices  124   a - 124   n , which each may be of the same or different type and/or form. As such, any of the I/O devices  130   a - 130   n  and/or the I/O controller  123  may comprise any type and/or form of suitable hardware, software, or combination of hardware and software to support, enable or provide for the connection and use of multiple display devices  124   a - 124   n  by the computing device  100 . For example, the computing device  100  may include any type and/or form of video adapter, video card, driver, and/or library to interface, communicate, connect or otherwise use the display devices  124   a - 124   n . In one embodiment, a video adapter may comprise multiple connectors to interface to multiple display devices  124   a - 124   n . In other embodiments, the computing device  100  may include multiple video adapters, with each video adapter connected to one or more of the display devices  124   a - 124   n . In some embodiments, any portion of the operating system of the computing device  100  may be configured for using multiple displays  124   a - 124   n . In other embodiments, one or more of the display devices  124   a - 124   n  may be provided by one or more other computing devices, such as computing devices  100   a  and  100   b  connected to the computing device  100 , for example, via a network. These embodiments may include any type of software designed and constructed to use another computer&#39;s display device as a second display device  124   a  for the computing device  100 . One ordinarily skilled in the art will recognize and appreciate the various ways and embodiments that a computing device  100  may be configured to have multiple display devices  124   a - 124   n.    
     In further embodiments, an I/O device  130  may be a bridge  170  between the system bus  150  and an external communication bus, such as a USB bus, an Apple Desktop Bus, an RS-232 serial connection, a SCSI bus, a FireWire bus, a FireWire 800 bus, an Ethernet bus, an AppleTalk bus, a Gigabit Ethernet bus, an Asynchronous Transfer Mode bus, a HIPPI bus, a Super HIPPI bus, a SerialPlus bus, a SCI/LAMP bus, a FibreChannel bus, or a Serial Attached small computer system interface bus. 
     A computing device  100  of the sort depicted in  FIGS. 1E and 1F  typically operate under the control of operating systems, which control scheduling of tasks and access to system resources. The computing device  100  can be running any operating system such as any of the versions of the Microsoft® Windows operating systems, the different releases of the Unix and Linux operating systems, any version of the Mac OS® for Macintosh computers, any embedded operating system, any real-time operating system, any open source operating system, any proprietary operating system, any operating systems for mobile computing devices, or any other operating system capable of running on the computing device and performing the operations described herein. Typical operating systems include: WINDOWS 3.x, WINDOWS 95, WINDOWS 98, WINDOWS 2000, WINDOWS NT 3.51, WINDOWS NT 4.0, WINDOWS CE, and WINDOWS XP, all of which are manufactured by Microsoft Corporation of Redmond, Wash.; MacOS, manufactured by Apple Computer of Cupertino, California; OS/2, manufactured by International Business Machines of Armonk, N.Y.; and Linux, a freely-available operating system distributed by Caldera Corp. of Salt Lake City, Utah, or any type and/or form of a Unix operating system, among others. 
     In other embodiments, the computing device  100  may have different processors, operating systems, and input devices consistent with the device. For example, in one embodiment the computer  100  is a Treo  180 ,  270 ,  1060 ,  600  or  650  smart phone manufactured by Palm, Inc. In this embodiment, the Treo smart phone is operated under the control of the PalmOS operating system and includes a stylus input device as well as a five-way navigator device. Moreover, the computing device  100  can be any workstation, desktop computer, laptop or notebook computer, server, handheld computer, mobile telephone, any other computer, or other form of computing or telecommunications device that is capable of communication and that has sufficient processor power and memory capacity to perform the operations described herein. 
     As shown in  FIG. 1G , the computing device  100  may comprise multiple processors and may provide functionality for simultaneous execution of instructions or for simultaneous execution of one instruction on more than one piece of data. In some embodiments, the computing device  100  may comprise a parallel processor with one or more cores. In one of these embodiments, the computing device  100  is a shared memory parallel device, with multiple processors and/or multiple processor cores, accessing all available memory as a single global address space. In another of these embodiments, the computing device  100  is a distributed memory parallel device with multiple processors each accessing local memory only. In still another of these embodiments, the computing device  100  has both some memory which is shared and some memory which can only be accessed by particular processors or subsets of processors. In still even another of these embodiments, the computing device  100 , such as a multi-core microprocessor, combines two or more independent processors into a single package, often a single integrated circuit (IC). In yet another of these embodiments, the computing device  100  includes a chip having a CELL BROADBAND ENGINE architecture and including a Power processor element and a plurality of synergistic processing elements, the Power processor element and the plurality of synergistic processing elements linked together by an internal high speed bus, which may be referred to as an element interconnect bus. 
     In some embodiments, the processors provide functionality for execution of a single instruction simultaneously on multiple pieces of data (SIMD). In other embodiments, the processors provide functionality for execution of multiple instructions simultaneously on multiple pieces of data (MIMD). In still other embodiments, the processor may use any combination of SIMD and MIMD cores in a single device. 
     In some embodiments, the computing device  100  may comprise a graphics processing unit. In one of these embodiments, depicted in  FIG. 1H , the computing device  100  includes at least one central processing unit  101  and at least one graphics processing unit. In another of these embodiments, the computing device  100  includes at least one parallel processing unit and at least one graphics processing unit. In still another of these embodiments, the computing device  100  includes a plurality of processing units of any type, one of the plurality of processing units comprising a graphics processing unit. 
     In some embodiments, a first computing device  100   a  executes an application on behalf of a user of a client computing device  100   b . In other embodiments, a computing device  100   a  executes a virtual machine, which provides an execution session within which applications execute on behalf of a user or a client computing devices  100   b . In one of these embodiments, the execution session is a hosted desktop session. In another of these embodiments, the computing device  100  executes a terminal services session. The terminal services session may provide a hosted desktop environment. In still another of these embodiments, the execution session provides access to a computing environment, which may comprise one or more of: an application, a plurality of applications, a desktop application, and a desktop session in which one or more applications may execute. 
     B. Appliance Architecture 
       FIG. 2A  illustrates an example embodiment of the appliance  200 . The architecture of the appliance  200  in  FIG. 2A  is provided by way of illustration only and is not intended to be limiting. As shown in  FIG. 2 , appliance  200  comprises a hardware layer  206  and a software layer divided into a user space  202  and a kernel space  204 . 
     Hardware layer  206  provides the hardware elements upon which programs and services within kernel space  204  and user space  202  are executed. Hardware layer  206  also provides the structures and elements which allow programs and services within kernel space  204  and user space  202  to communicate data both internally and externally with respect to appliance  200 . As shown in  FIG. 2 , the hardware layer  206  includes a processing unit  262  for executing software programs and services, a memory  264  for storing software and data, network ports  266  for transmitting and receiving data over a network, and an encryption processor  260  for performing functions related to Secure Sockets Layer processing of data transmitted and received over the network. In some embodiments, the central processing unit  262  may perform the functions of the encryption processor  260  in a single processor. Additionally, the hardware layer  206  may comprise multiple processors for each of the processing unit  262  and the encryption processor  260 . The processor  262  may include any of the processors  101  described above in connection with  FIGS. 1E and 1F . For example, in one embodiment, the appliance  200  comprises a first processor  262  and a second processor  262 ′. In other embodiments, the processor  262  or  262 ′ comprises a multi-core processor. 
     Although the hardware layer  206  of appliance  200  is generally illustrated with an encryption processor  260 , processor  260  may be a processor for performing functions related to any encryption protocol, such as the Secure Socket Layer (SSL) or Transport Layer Security (TLS) protocol. In some embodiments, the processor  260  may be a general purpose processor (GPP), and in further embodiments, may have executable instructions for performing processing of any security related protocol. 
     Although the hardware layer  206  of appliance  200  is illustrated with certain elements in  FIG. 2 , the hardware portions or components of appliance  200  may comprise any type and form of elements, hardware or software, of a computing device, such as the computing device  100  illustrated and discussed herein in conjunction with  FIGS. 1E and 1F . In some embodiments, the appliance  200  may comprise a server, gateway, router, switch, bridge or other type of computing or network device, and have any hardware and/or software elements associated therewith. 
     The operating system of appliance  200  allocates, manages, or otherwise segregates the available system memory into kernel space  204  and user space  204 . In example software architecture  200 , the operating system may be any type and/or form of Unix operating system although the methods and systems described herein are not so limited. As such, the appliance  200  can be running any operating system such as any of the versions of the Microsoft® Windows operating systems, the different releases of the Unix and Linux operating systems, any version of the Mac OS® for Macintosh computers, any embedded operating system, any network operating system, any real-time operating system, any open source operating system, any proprietary operating system, any operating systems for mobile computing devices or network devices, or any other operating system capable of running on the appliance  200  and performing the operations described herein. 
     The kernel space  204  is reserved for running the kernel  230 , including any device drivers, kernel extensions or other kernel related software. As known to those skilled in the art, the kernel  230  is the core of the operating system, and provides access, control, and management of resources and hardware-related elements of the application  104 . In accordance with an embodiment of the appliance  200 , the kernel space  204  also includes a number of network services or processes working in conjunction with a cache manager  232 , sometimes also referred to as the integrated cache, the benefits of which are described in detail further herein. Additionally, the embodiment of the kernel  230  will depend on the embodiment of the operating system installed, configured, or otherwise used by the device  200 . 
     In one embodiment, the device  200  comprises one network stack  267 , such as a TCP/IP based stack, for communicating with the client  102  and/or the server  106 . In one embodiment, the network stack  267  is used to communicate with a first network, such as network  108 , and a second network  110 . In some embodiments, the device  200  terminates a first transport layer connection, such as a TCP connection of a client  102 , and establishes a second transport layer connection to a server  106  for use by the client  102 , e.g., the second transport layer connection is terminated at the appliance  200  and the server  106 . The first and second transport layer connections may be established via a single network stack  267 . In other embodiments, the device  200  may comprise multiple network stacks, for example  267  and  267 ′, and the first transport layer connection may be established or terminated at one network stack  267 , and the second transport layer connection on the second network stack  267 ′. For example, one network stack may be for receiving and transmitting network packet on a first network, and another network stack for receiving and transmitting network packets on a second network. In one embodiment, the network stack  267  comprises a buffer  243  for queuing one or more network packets for transmission by the appliance  200 . 
     As shown in  FIG. 2 , the kernel space  204  includes the cache manager  232 , a high-speed layer  2 - 7  integrated packet engine  240 , an encryption engine  234 , a policy engine  236  and multi-protocol compression logic  238 . Running these components or processes  232 ,  240 ,  234 ,  236  and  238  in kernel space  204  or kernel mode instead of the user space  202  improves the performance of each of these components, alone and in combination. Kernel operation means that these components or processes  232 ,  240 ,  234 ,  236  and  238  run in the core address space of the operating system of the device  200 . For example, running the encryption engine  234  in kernel mode improves encryption performance by moving encryption and decryption operations to the kernel, thereby reducing the number of transitions between the memory space or a kernel thread in kernel mode and the memory space or a thread in user mode. For example, data obtained in kernel mode may not need to be passed or copied to a process or thread running in user mode, such as from a kernel level data structure to a user level data structure. In another aspect, the number of context switches between kernel mode and user mode are also reduced. Additionally, synchronization of and communications between any of the components or processes  232 ,  240 ,  235 ,  236  and  238  can be performed more efficiently in the kernel space  204 . 
     In some embodiments, any portion of the components  232 ,  240 ,  234 ,  236  and  238  may run or operate in the kernel space  204 , while other portions of these components  232 ,  240 ,  234 ,  236  and  238  may run or operate in user space  202 . In one embodiment, the appliance  200  uses a kernel-level data structure providing access to any portion of one or more network packets, for example, a network packet comprising a request from a client  102  or a response from a server  106 . In some embodiments, the kernel-level data structure may be obtained by the packet engine  240  via a transport layer driver interface or filter to the network stack  267 . The kernel-level data structure may comprise any interface and/or data accessible via the kernel space  204  related to the network stack  267 , network traffic or packets received or transmitted by the network stack  267 . In other embodiments, the kernel-level data structure may be used by any of the components or processes  232 ,  240 ,  234 ,  236  and  238  to perform the desired operation of the component or process. In one embodiment, a component  232 ,  240 ,  234 ,  236  and  238  is running in kernel mode  204  when using the kernel-level data structure, while in another embodiment, the component  232 ,  240 ,  234 ,  236  and  238  is running in user mode when using the kernel-level data structure. In some embodiments, the kernel-level data structure may be copied or passed to a second kernel-level data structure, or any desired user-level data structure. 
     The cache manager  232  may comprise software, hardware or any combination of software and hardware to provide cache access, control and management of any type and form of content, such as objects or dynamically generated objects served by the originating servers  106 . The data, objects or content processed and stored by the cache manager  232  may comprise data in any format, such as a markup language, or communicated via any protocol. In some embodiments, the cache manager  232  duplicates original data stored elsewhere or data previously computed, generated or transmitted, in which the original data may require longer access time to fetch, compute or otherwise obtain relative to reading a cache memory element. Once the data is stored in the cache memory element, future use can be made by accessing the cached copy rather than refetching or recomputing the original data, thereby reducing the access time. In some embodiments, the cache memory element may comprise a data object in memory  264  of device  200 . In other embodiments, the cache memory element may comprise memory having a faster access time than memory  264 . In another embodiment, the cache memory element may comprise any type and form of storage element of the device  200 , such as a portion of a hard disk. In some embodiments, the processing unit  262  may provide cache memory for use by the cache manager  232 . In yet further embodiments, the cache manager  232  may use any portion and combination of memory, storage, or the processing unit for caching data, objects, and other content. 
     Furthermore, the cache manager  232  includes any logic, functions, rules, or operations to perform any embodiments of the techniques of the appliance  200  described herein. For example, the cache manager  232  includes logic or functionality to invalidate objects based on the expiration of an invalidation time period or upon receipt of an invalidation command from a client  102  or server  106 . In some embodiments, the cache manager  232  may operate as a program, service, process or task executing in the kernel space  204 , and in other embodiments, in the user space  202 . In one embodiment, a first portion of the cache manager  232  executes in the user space  202  while a second portion executes in the kernel space  204 . In some embodiments, the cache manager  232  can comprise any type of general purpose processor (GPP), or any other type of integrated circuit, such as a Field Programmable Gate Array (FPGA), Programmable Logic Device (PLD), or Application Specific Integrated Circuit (ASIC). 
     The policy engine  236  may include, for example, an intelligent statistical engine or other programmable application(s). In one embodiment, the policy engine  236  provides a configuration mechanism to allow a user to identify, specify, define or configure a caching policy. Policy engine  236 , in some embodiments, also has access to memory to support data structures such as lookup tables or hash tables to enable user-selected caching policy decisions. In other embodiments, the policy engine  236  may comprise any logic, rules, functions or operations to determine and provide access, control and management of objects, data or content being cached by the appliance  200  in addition to access, control and management of security, network traffic, network access, compression or any other function or operation performed by the appliance  200 . Further examples of specific caching policies are further described herein. 
     The encryption engine  234  comprises any logic, business rules, functions or operations for handling the processing of any security related protocol, such as SSL or TLS, or any function related thereto. For example, the encryption engine  234  encrypts and decrypts network packets, or any portion thereof, communicated via the appliance  200 . The encryption engine  234  may also setup or establish SSL or TLS connections on behalf of the client  102   a - 102   n , server  106   a - 106   n , or appliance  200 . As such, the encryption engine  234  provides offloading and acceleration of SSL processing. In one embodiment, the encryption engine  234  uses a tunneling protocol to provide a virtual private network between a client  102   a - 102   n  and a server  106   a - 106   n . In some embodiments, the encryption engine  234  is in communication with the Encryption processor  260 . In other embodiments, the encryption engine  234  comprises executable instructions running on the Encryption processor  260 . 
     The multi-protocol compression engine  238  comprises any logic, business rules, function or operations for compressing one or more protocols of a network packet, such as any of the protocols used by the network stack  267  of the device  200 . In one embodiment, multi-protocol compression engine  238  compresses bi-directionally between clients  102   a - 102   n  and servers  106   a - 106   n  any TCP/IP based protocol, including Messaging Application Programming Interface (MAPI) (email), File Transfer Protocol (FTP), HyperText Transfer Protocol (HTTP), Common Internet File System (CIFS) protocol (file transfer), Independent Computing Architecture (ICA) protocol, Remote Desktop Protocol (RDP), Wireless Application Protocol (WAP), Mobile IP protocol, and Voice Over IP (VoIP) protocol. In other embodiments, multi-protocol compression engine  238  provides compression of Hypertext Markup Language (HTML) based protocols and in some embodiments, provides compression of any markup languages, such as the Extensible Markup Language (XML). In one embodiment, the multi-protocol compression engine  238  provides compression of any high-performance protocol, such as any protocol designed for appliance  200  to appliance  200  communications. In another embodiment, the multi-protocol compression engine  238  compresses any payload of or any communication using a modified transport control protocol, such as Transaction TCP (T/TCP), TCP with selection acknowledgements (TCP-SACK), TCP with large windows (TCP-LW), a congestion prediction protocol such as the TCP-Vegas protocol, and a TCP spoofing protocol. 
     As such, the multi-protocol compression engine  238  accelerates performance for users accessing applications via desktop clients, e.g., Microsoft Outlook and non-Web thin clients, such as any client launched by popular enterprise applications like Oracle, SAP and Siebel, and even mobile clients, such as the Pocket PC. In some embodiments, the multi-protocol compression engine  238  by executing in the kernel mode  204  and integrating with packet processing engine  240  accessing the network stack  267  is able to compress any of the protocols carried by the TCP/IP protocol, such as any application layer protocol. 
     High speed layer  2 - 7  integrated packet engine  240 , also generally referred to as a packet processing engine or packet engine, is responsible for managing the kernel-level processing of packets received and transmitted by appliance  200  via network ports  266 . The high speed layer  2 - 7  integrated packet engine  240  may comprise a buffer for queuing one or more network packets during processing, such as for receipt of a network packet or transmission of a network packet. Additionally, the high speed layer  2 - 7  integrated packet engine  240  is in communication with one or more network stacks  267  to send and receive network packets via network ports  266 . The high speed layer  2 - 7  integrated packet engine  240  works in conjunction with encryption engine  234 , cache manager  232 , policy engine  236  and multi-protocol compression logic  238 . In particular, encryption engine  234  is configured to perform SSL processing of packets, policy engine  236  is configured to perform functions related to traffic management such as request-level content switching and request-level cache redirection, and multi-protocol compression logic  238  is configured to perform functions related to compression and decompression of data. 
     The high speed layer  2 - 7  integrated packet engine  240  includes a packet processing timer  242 . In one embodiment, the packet processing timer  242  provides one or more time intervals to trigger the processing of incoming, i.e., received, or outgoing, i.e., transmitted, network packets. In some embodiments, the high speed layer  2 - 7  integrated packet engine  240  processes network packets responsive to the timer  242 . The packet processing timer  242  provides any type and form of signal to the packet engine  240  to notify, trigger, or communicate a time related event, interval or occurrence. In many embodiments, the packet processing timer  242  operates in the order of milliseconds, such as for example 100 ms, 50 ms or 25 ms. For example, in some embodiments, the packet processing timer  242  provides time intervals or otherwise causes a network packet to be processed by the high speed layer  2 - 7  integrated packet engine  240  at a 10 ms time interval, while in other embodiments, at a 5 ms time interval, and still yet in further embodiments, as short as a 3, 2, or 1 ms time interval. The high speed layer  2 - 7  integrated packet engine  240  may be interfaced, integrated or in communication with the encryption engine  234 , cache manager  232 , policy engine  236  and multi-protocol compression engine  238  during operation. As such, any of the logic, functions, or operations of the encryption engine  234 , cache manager  232 , policy engine  236  and multi-protocol compression logic  238  may be performed responsive to the packet processing timer  242  and/or the packet engine  240 . Therefore, any of the logic, functions, or operations of the encryption engine  234 , cache manager  232 , policy engine  236  and multi-protocol compression logic  238  may be performed at the granularity of time intervals provided via the packet processing timer  242 , for example, at a time interval of less than or equal to 10 ms. For example, in one embodiment, the cache manager  232  may perform invalidation of any cached objects responsive to the high speed layer  2 - 7  integrated packet engine  240  and/or the packet processing timer  242 . In another embodiment, the expiry or invalidation time of a cached object can be set to the same order of granularity as the time interval of the packet processing timer  242 , such as at every 10 ms. 
     In contrast to kernel space  204 , user space  202  is the memory area or portion of the operating system used by user mode applications or programs otherwise running in user mode. A user mode application may not access kernel space  204  directly and uses service calls in order to access kernel services. As shown in  FIG. 2 , user space  202  of appliance  200  includes a graphical user interface (GUI)  210 , a command line interface (CLI)  212 , shell services  214 , health monitoring program  216 , and daemon services  218 . GUI  210  and CLI  212  provide a means by which a system administrator or other user can interact with and control the operation of appliance  200 , such as via the operating system of the appliance  200 . The GUI  210  or CLI  212  can comprise code running in user space  202  or kernel space  204 . The GUI  210  may be any type and form of graphical user interface and may be presented via text, graphical or otherwise, by any type of program or application, such as a browser. The CLI  212  may be any type and form of command line or text-based interface, such as a command line provided by the operating system. For example, the CLI  212  may comprise a shell, which is a tool to enable users to interact with the operating system. In some embodiments, the CLI  212  may be provided via a bash, csh, tcsh, or ksh type shell. The shell services  214  comprises the programs, services, tasks, processes or executable instructions to support interaction with the appliance  200  or operating system by a user via the GUI  210  and/or CLI  212 . 
     Health monitoring program  216  is used to monitor, check, report and ensure that network systems are functioning properly and that users are receiving requested content over a network. Health monitoring program  216  comprises one or more programs, services, tasks, processes or executable instructions to provide logic, rules, functions or operations for monitoring any activity of the appliance  200 . In some embodiments, the health monitoring program  216  intercepts and inspects any network traffic passed via the appliance  200 . In other embodiments, the health monitoring program  216  interfaces by any suitable means and/or mechanisms with one or more of the following: the encryption engine  234 , cache manager  232 , policy engine  236 , multi-protocol compression logic  238 , packet engine  240 , daemon services  218 , and shell services  214 . As such, the health monitoring program  216  may call any application programming interface (API) to determine a state, status, or health of any portion of the appliance  200 . For example, the health monitoring program  216  may ping or send a status inquiry on a periodic basis to check if a program, process, service or task is active and currently running. In another example, the health monitoring program  216  may check any status, error or history logs provided by any program, process, service or task to determine any condition, status or error with any portion of the appliance  200 . 
     Daemon services  218  are programs that run continuously or in the background and handle periodic service requests received by appliance  200 . In some embodiments, a daemon service may forward the requests to other programs or processes, such as another daemon service  218  as appropriate. As known to those skilled in the art, a daemon service  218  may run unattended to perform continuous or periodic system wide functions, such as network control, or to perform any desired task. In some embodiments, one or more daemon services  218  run in the user space  202 , while in other embodiments, one or more daemon services  218  run in the kernel space. 
     Referring now to  FIG. 2B , another embodiment of the appliance  200  is depicted. In brief overview, the appliance  200  provides one or more of the following services, functionality or operations: SSL VPN connectivity  280 , switching/load balancing  284 , Domain Name Service resolution  286 , acceleration  288  and an application firewall  290  for communications between one or more clients  102  and one or more servers  106 . Each of the servers  106  may provide one or more network related services  270   a - 270   n  (referred to as services  270 ). For example, a server  106  may provide an http service  270 . The appliance  200  comprises one or more virtual servers or virtual internet protocol servers, referred to as a vServer, VIP server, or just VIP  275   a - 275   n  (also referred herein as vServer  275 ). The vServer  275  receives, intercepts or otherwise processes communications between a client  102  and a server  106  in accordance with the configuration and operations of the appliance  200 . 
     The vServer  275  may comprise software, hardware or any combination of software and hardware. The vServer  275  may comprise any type and form of program, service, task, process or executable instructions operating in user mode  202 , kernel mode  204  or any combination thereof in the appliance  200 . The vServer  275  includes any logic, functions, rules, or operations to perform any embodiments of the techniques described herein, such as SSL VPN  280 , switching/load balancing  284 , Domain Name Service resolution  286 , acceleration  288  and an application firewall  290 . In some embodiments, the vServer  275  establishes a connection to a service  270  of a server  106 . The service  275  may comprise any program, application, process, task or set of executable instructions capable of connecting to and communicating to the appliance  200 , client  102  or vServer  275 . For example, the service  275  may comprise a web server, http server, ftp, email or database server. In some embodiments, the service  270  is a daemon process or network driver for listening, receiving and/or sending communications for an application, such as email, database or an enterprise application. In some embodiments, the service  270  may communicate on a specific IP address, or IP address and port. 
     In some embodiments, the vServer  275  applies one or more policies of the policy engine  236  to network communications between the client  102  and server  106 . In one embodiment, the policies are associated with a VServer  275 . In another embodiment, the policies are based on a user, or a group of users. In yet another embodiment, a policy is global and applies to one or more vServers  275   a - 275   n , and any user or group of users communicating via the appliance  200 . In some embodiments, the policies of the policy engine have conditions upon which the policy is applied based on any content of the communication, such as internet protocol address, port, protocol type, header or fields in a packet, or the context of the communication, such as user, group of the user, vServer  275 , transport layer connection, and/or identification or attributes of the client  102  or server  106 . 
     In other embodiments, the appliance  200  communicates or interfaces with the policy engine  236  to determine authentication and/or authorization of a remote user or a remote client  102  to access the computing environment  15 , application, and/or data file from a server  106 . In another embodiment, the appliance  200  communicates or interfaces with the policy engine  236  to determine authentication and/or authorization of a remote user or a remote client  102  to have the application delivery system  190  deliver one or more of the computing environment  15 , application, and/or data file. In yet another embodiment, the appliance  200  establishes a VPN or SSL VPN connection based on the policy engine&#39;s  236  authentication and/or authorization of a remote user or a remote client  102  In one embodiment, the appliance  200  controls the flow of network traffic and communication sessions based on policies of the policy engine  236 . For example, the appliance  200  may control the access to a computing environment  15 , application or data file based on the policy engine  236 . 
     In some embodiments, the vServer  275  establishes a transport layer connection, such as a TCP or UDP connection with a client  102  via the client agent  120 . In one embodiment, the vServer  275  listens for and receives communications from the client  102 . In other embodiments, the vServer  275  establishes a transport layer connection, such as a TCP or UDP connection with a client server  106 . In one embodiment, the vServer  275  establishes the transport layer connection to an internet protocol address and port of a server  270  running on the server  106 . In another embodiment, the vServer  275  associates a first transport layer connection to a client  102  with a second transport layer connection to the server  106 . In some embodiments, a vServer  275  establishes a pool of transport layer connections to a server  106  and multiplexes client requests via the pooled transport layer connections. 
     In some embodiments, the appliance  200  provides a SSL VPN connection  280  between a client  102  and a server  106 . For example, a client  102  on a first network  102  requests to establish a connection to a server  106  on a second network  104 ′. In some embodiments, the second network  104 ′ is not routable from the first network  104 . In other embodiments, the client  102  is on a public network  104  and the server  106  is on a private network  104 ′, such as a corporate network. In one embodiment, the client agent  120  intercepts communications of the client  102  on the first network  104 , encrypts the communications, and transmits the communications via a first transport layer connection to the appliance  200 . The appliance  200  associates the first transport layer connection on the first network  104  to a second transport layer connection to the server  106  on the second network  104 . The appliance  200  receives the intercepted communication from the client agent  102 , decrypts the communications, and transmits the communication to the server  106  on the second network  104  via the second transport layer connection. The second transport layer connection may be a pooled transport layer connection. As such, the appliance  200  provides an end-to-end secure transport layer connection for the client  102  between the two networks  104 ,  104 ′. 
     In one embodiment, the appliance  200  hosts an intranet internet protocol or intranetIP  282  address of the client  102  on the virtual private network  104 . The client  102  has a local network identifier, such as an internet protocol (IP) address and/or host name on the first network  104 . When connected to the second network  104 ′ via the appliance  200 , the appliance  200  establishes, assigns or otherwise provides an IntranetIP, which is network identifier, such as IP address and/or host name, for the client  102  on the second network  104 ′. The appliance  200  listens for and receives on the second or private network  104 ′ for any communications directed towards the client  102  using the client&#39;s established IntranetIP  282 . In one embodiment, the appliance  200  acts as or on behalf of the client  102  on the second private network  104 . For example, in another embodiment, a vServer  275  listens for and responds to communications to the IntranetIP  282  of the client  102 . In some embodiments, if a computing device  100  on the second network  104 ′ transmits a request, the appliance  200  processes the request as if it were the client  102 . For example, the appliance  200  may respond to a ping to the client&#39;s IntranetIP  282 . In another example, the appliance may establish a connection, such as a TCP or UDP connection, with computing device  100  on the second network  104  requesting a connection with the client&#39;s IntranetIP  282 . 
     In some embodiments, the appliance  200  provides one or more of the following acceleration techniques  288  to communications between the client  102  and server  106 : 1) compression; 2) decompression; 3) Transmission Control Protocol pooling; 4) Transmission Control Protocol multiplexing; 5) Transmission Control Protocol buffering; and 6) caching. In one embodiment, the appliance  200  relieves servers  106  of much of the processing load caused by repeatedly opening and closing transport layers connections to clients  102  by opening one or more transport layer connections with each server  106  and maintaining these connections to allow repeated data accesses by clients via the Internet. This technique is referred to herein as “connection pooling”. 
     In some embodiments, in order to seamlessly splice communications from a client  102  to a server  106  via a pooled transport layer connection, the appliance  200  translates or multiplexes communications by modifying sequence number and acknowledgment numbers at the transport layer protocol level. This is referred to as “connection multiplexing”. In some embodiments, no application layer protocol interaction is required. For example, in the case of an in-bound packet (that is, a packet received from a client  102 ), the source network address of the packet is changed to that of an output port of appliance  200 , and the destination network address is changed to that of the intended server. In the case of an outbound packet (that is, one received from a server  106 ), the source network address is changed from that of the server  106  to that of an output port of appliance  200  and the destination address is changed from that of appliance  200  to that of the requesting client  102 . The sequence numbers and acknowledgment numbers of the packet are also translated to sequence numbers and acknowledgement expected by the client  102  on the appliance&#39;s  200  transport layer connection to the client  102 . In some embodiments, the packet checksum of the transport layer protocol is recalculated to account for these translations. 
     In another embodiment, the appliance  200  provides switching or load-balancing functionality  284  for communications between the client  102  and server  106 . In some embodiments, the appliance  200  distributes traffic and directs client requests to a server  106  based on layer  4  or application-layer request data. In one embodiment, although the network layer or layer  2  of the network packet identifies a destination server  106 , the appliance  200  determines the server  106  to distribute the network packet by application information and data carried as payload of the transport layer packet. In one embodiment, the health monitoring programs  216  of the appliance  200  monitor the health of servers to determine the server  106  for which to distribute a client&#39;s request. In some embodiments, if the appliance  200  detects a server  106  is not available or has a load over a predetermined threshold, the appliance  200  can direct or distribute client requests to another server  106 . 
     In some embodiments, the appliance  200  acts as a Domain Name Service (DNS) resolver or otherwise provides resolution of a DNS request from clients  102 . In some embodiments, the appliance intercepts&#39; a DNS request transmitted by the client  102 . In one embodiment, the appliance  200  responds to a client&#39;s DNS request with an IP address of or hosted by the appliance  200 . In this embodiment, the client  102  transmits network communication for the domain name to the appliance  200 . In another embodiment, the appliance  200  responds to a client&#39;s DNS request with an IP address of or hosted by a second appliance  200 ′. In some embodiments, the appliance  200  responds to a client&#39;s DNS request with an IP address of a server  106  determined by the appliance  200 . 
     In yet another embodiment, the appliance  200  provides application firewall functionality  290  for communications between the client  102  and server  106 . In one embodiment, the policy engine  236  provides rules for detecting and blocking illegitimate requests. In some embodiments, the application firewall  290  protects against denial of service (DoS) attacks. In other embodiments, the appliance inspects the content of intercepted requests to identify and block application-based attacks. In some embodiments, the rules/policy engine  236  comprises one or more application firewall or security control policies for providing protections against various classes and types of web or Internet based vulnerabilities, such as one or more of the following: 1) buffer overflow, 2) CGI-BIN parameter manipulation, 3) form/hidden field manipulation, 4) forceful browsing, 5) cookie or session poisoning, 6) broken access control list (ACLs) or weak passwords, 7) cross-site scripting (XSS), 8) command injection, 9) SQL injection, 10) error triggering sensitive information leak, 11) insecure use of cryptography, 12) server misconfiguration, 13) back doors and debug options, 14) website defacement, 15) platform or operating systems vulnerabilities, and 16) zero-day exploits. In an embodiment, the application firewall  290  provides HTML form field protection in the form of inspecting or analyzing the network communication for one or more of the following: 1) required fields are returned, 2) no added field allowed, 3) read-only and hidden field enforcement, 4) drop-down list and radio button field conformance, and 5) form-field max-length enforcement. In some embodiments, the application firewall  290  ensures cookies are not modified. In other embodiments, the application firewall  290  protects against forceful browsing by enforcing legal URLs. 
     In still yet other embodiments, the application firewall  290  protects any confidential information contained in the network communication. The application firewall  290  may inspect or analyze any network communication in accordance with the rules or polices of the engine  236  to identify any confidential information in any field of the network packet. In some embodiments, the application firewall  290  identifies in the network communication one or more occurrences of a credit card number, password, social security number, name, patient code, contact information, and age. The encoded portion of the network communication may comprise these occurrences or the confidential information. Based on these occurrences, in one embodiment, the application firewall  290  may take a policy action on the network communication, such as prevent transmission of the network communication. In another embodiment, the application firewall  290  may rewrite, remove or otherwise mask such identified occurrence or confidential information. 
     Still referring to  FIG. 2B , the appliance  200  may include a performance monitoring agent  197  as discussed above in conjunction with  FIG. 1D . In one embodiment, the appliance  200  receives the monitoring agent  197  from the monitoring service  198  or monitoring server  106  as depicted in  FIG. 1D . In some embodiments, the appliance  200  stores the monitoring agent  197  in storage, such as disk, for delivery to any client or server in communication with the appliance  200 . For example, in one embodiment, the appliance  200  transmits the monitoring agent  197  to a client upon receiving a request to establish a transport layer connection. In other embodiments, the appliance  200  transmits the monitoring agent  197  upon establishing the transport layer connection with the client  102 . In another embodiment, the appliance  200  transmits the monitoring agent  197  to the client upon intercepting or detecting a request for a web page. In yet another embodiment, the appliance  200  transmits the monitoring agent  197  to a client or a server in response to a request from the monitoring server  198 . In one embodiment, the appliance  200  transmits the monitoring agent  197  to a second appliance  200 ′ or appliance  205 . 
     In other embodiments, the appliance  200  executes the monitoring agent  197 . In one embodiment, the monitoring agent  197  measures and monitors the performance of any application, program, process, service, task or thread executing on the appliance  200 . For example, the monitoring agent  197  may monitor and measure performance and operation of vServers  275 A- 275 N. In another embodiment, the monitoring agent  197  measures and monitors the performance of any transport layer connections of the appliance  200 . In some embodiments, the monitoring agent  197  measures and monitors the performance of any user sessions traversing the appliance  200 . In one embodiment, the monitoring agent  197  measures and monitors the performance of any virtual private network connections and/or sessions traversing the appliance  200 , such an SSL VPN session. In still further embodiments, the monitoring agent  197  measures and monitors the memory, CPU and disk usage and performance of the appliance  200 . In yet another embodiment, the monitoring agent  197  measures and monitors the performance of any acceleration technique  288  performed by the appliance  200 , such as SSL offloading, connection pooling and multiplexing, caching, and compression. In some embodiments, the monitoring agent  197  measures and monitors the performance of any load balancing and/or content switching  284  performed by the appliance  200 . In other embodiments, the monitoring agent  197  measures and monitors the performance of application firewall  290  protection and processing performed by the appliance  200 . 
     C. Client Agent 
     Referring now to  FIG. 3 , an embodiment of the client agent  120  is depicted. The client  102  includes a client agent  120  for establishing and exchanging communications with the appliance  200  and/or server  106  via a network  104 . In brief overview, the client  102  operates on computing device  100  having an operating system with a kernel mode  302  and a user mode  303 , and a network stack  310  with one or more layers  310   a - 310   b . The client  102  may have installed and/or execute one or more applications. In some embodiments, one or more applications may communicate via the network stack  310  to a network  104 . One of the applications, such as a web browser, may also include a first program  322 . For example, the first program  322  may be used in some embodiments to install and/or execute the client agent  120 , or any portion thereof. The client agent  120  includes an interception mechanism, or interceptor  350 , for intercepting network communications from the network stack  310  from the one or more applications. 
     The network stack  310  of the client  102  may comprise any type and form of software, or hardware, or any combinations thereof, for providing connectivity to and communications with a network. In one embodiment, the network stack  310  comprises a software implementation for a network protocol suite. The network stack  310  may comprise one or more network layers, such as any networks layers of the Open Systems Interconnection (OSI) communications model as those skilled in the art recognize and appreciate. As such, the network stack  310  may comprise any type and form of protocols for any of the following layers of the OSI model: 1) physical link layer, 2) data link layer, 3) network layer, 4) transport layer, 5) session layer, 6) presentation layer, and 7) application layer. In one embodiment, the network stack  310  may comprise a transport control protocol (TCP) over the network layer protocol of the internet protocol (IP), generally referred to as TCP/IP. In some embodiments, the TCP/IP protocol may be carried over the Ethernet protocol, which may comprise any of the family of IEEE wide-area-network (WAN) or local-area-network (LAN) protocols, such as those protocols covered by the IEEE 802.3. In some embodiments, the network stack  310  comprises any type and form of a wireless protocol, such as IEEE 802.11 and/or mobile internet protocol. 
     In view of a TCP/IP based network, any TCP/IP based protocol may be used, including Messaging Application Programming Interface (MAPI) (email), File Transfer Protocol (FTP), HyperText Transfer Protocol (HTTP), Common Internet File System (CIFS) protocol (file transfer), Independent Computing Architecture (ICA) protocol, Remote Desktop Protocol (RDP), Wireless Application Protocol (WAP), Mobile IP protocol, and Voice Over IP (VoIP) protocol. In another embodiment, the network stack  310  comprises any type and form of transport control protocol, such as a modified transport control protocol, for example a Transaction TCP (T/TCP), TCP with selection acknowledgements (TCP-SACK), TCP with large windows (TCP-LW), a congestion prediction protocol such as the TCP-Vegas protocol, and a TCP spoofing protocol. In other embodiments, any type and form of user datagram protocol (UDP), such as UDP over IP, may be used by the network stack  310 , such as for voice communications or real-time data communications. 
     Furthermore, the network stack  310  may include one or more network drivers supporting the one or more layers, such as a TCP driver or a network layer driver. The network drivers may be included as part of the operating system of the computing device  100  or as part of any network interface cards or other network access components of the computing device  100 . In some embodiments, any of the network drivers of the network stack  310  may be customized, modified or adapted to provide a custom or modified portion of the network stack  310  in support of any of the techniques described herein. In other embodiments, the acceleration program  120  is designed and constructed to operate with or work in conjunction with the network stack  310  installed or otherwise provided by the operating system of the client  102 . 
     The network stack  310  comprises any type and form of interfaces for receiving, obtaining, providing or otherwise accessing any information and data related to network communications of the client  102 . In one embodiment, an interface to the network stack  310  comprises an application programming interface (API). The interface may also comprise any function call, hooking or filtering mechanism, event or call back mechanism, or any type of interfacing technique. The network stack  310  via the interface may receive or provide any type and form of data structure, such as an object, related to functionality or operation of the network stack  310 . For example, the data structure may comprise information and data related to a network packet or one or more network packets. In some embodiments, the data structure comprises a portion of the network packet processed at a protocol layer of the network stack  310 , such as a network packet of the transport layer. In some embodiments, the data structure  325  comprises a kernel-level data structure, while in other embodiments, the data structure  325  comprises a user-mode data structure. A kernel-level data structure may comprise a data structure obtained or related to a portion of the network stack  310  operating in kernel-mode  302 , or a network driver or other software running in kernel-mode  302 , or any data structure obtained or received by a service, process, task, thread or other executable instructions running or operating in kernel-mode of the operating system. 
     Additionally, some portions of the network stack  310  may execute or operate in kernel-mode  302 , for example, the data link or network layer, while other portions execute or operate in user-mode  303 , such as an application layer of the network stack  310 . For example, a first portion  310   a  of the network stack may provide user-mode access to the network stack  310  to an application while a second portion  310   a  of the network stack  310  provides access to a network. In some embodiments, a first portion  310   a  of the network stack may comprise one or more upper layers of the network stack  310 , such as any of layers  5 - 7 . In other embodiments, a second portion  310   b  of the network stack  310  comprises one or more lower layers, such as any of layers  1 - 4 . Each of the first portion  310   a  and second portion  310   b  of the network stack  310  may comprise any portion of the network stack  310 , at any one or more network layers, in user-mode  203 , kernel-mode,  202 , or combinations thereof, or at any portion of a network layer or interface point to a network layer or any portion of or interface point to the user-mode  203  and kernel-mode  203 . 
     The interceptor  350  may comprise software, hardware, or any combination of software and hardware. In one embodiment, the interceptor  350  intercept a network communication at any point in the network stack  310 , and redirects or transmits the network communication to a destination desired, managed or controlled by the interceptor  350  or client agent  120 . For example, the interceptor  350  may intercept a network communication of a network stack  310  of a first network and transmit the network communication to the appliance  200  for transmission on a second network  104 . In some embodiments, the interceptor  350  comprises any type interceptor  350  comprises a driver, such as a network driver constructed and designed to interface and work with the network stack  310 . In some embodiments, the client agent  120  and/or interceptor  350  operates at one or more layers of the network stack  310 , such as at the transport layer. In one embodiment, the interceptor  350  comprises a filter driver, hooking mechanism, or any form and type of suitable network driver interface that interfaces to the transport layer of the network stack, such as via the transport driver interface (TDI). In some embodiments, the interceptor  350  interfaces to a first protocol layer, such as the transport layer and another protocol layer, such as any layer above the transport protocol layer, for example, an application protocol layer. In one embodiment, the interceptor  350  may comprise a driver complying with the Network Driver Interface Specification (NDIS), or a NDIS driver. In another embodiment, the interceptor  350  may comprise a min-filter or a mini-port driver. In one embodiment, the interceptor  350 , or portion thereof, operates in kernel-mode  202 . In another embodiment, the interceptor  350 , or portion thereof, operates in user-mode  203 . In some embodiments, a portion of the interceptor  350  operates in kernel-mode  202  while another portion of the interceptor  350  operates in user-mode  203 . In other embodiments, the client agent  120  operates in user-mode  203  but interfaces via the interceptor  350  to a kernel-mode driver, process, service, task or portion of the operating system, such as to obtain a kernel-level data structure  225 . In further embodiments, the interceptor  350  is a user-mode application or program, such as application. 
     In one embodiment, the interceptor  350  intercepts any transport layer connection requests. In these embodiments, the interceptor  350  execute transport layer application programming interface (API) calls to set the destination information, such as destination IP address and/or port to a desired location for the location. In this manner, the interceptor  350  intercepts and redirects the transport layer connection to a IP address and port controlled or managed by the interceptor  350  or client agent  120 . In one embodiment, the interceptor  350  sets the destination information for the connection to a local IP address and port of the client  102  on which the client agent  120  is listening. For example, the client agent  120  may comprise a proxy service listening on a local IP address and port for redirected transport layer communications. In some embodiments, the client agent  120  then communicates the redirected transport layer communication to the appliance  200 . 
     In some embodiments, the interceptor  350  intercepts a Domain Name Service (DNS) request. In one embodiment, the client agent  120  and/or interceptor  350  resolves the DNS request. In another embodiment, the interceptor transmits the intercepted DNS request to the appliance  200  for DNS resolution. In one embodiment, the appliance  200  resolves the DNS request and communicates the DNS response to the client agent  120 . In some embodiments, the appliance  200  resolves the DNS request via another appliance  200 ′ or a DNS server  106 . 
     In yet another embodiment, the client agent  120  may comprise two agents  120  and  120 ′. In one embodiment, a first agent  120  may comprise an interceptor  350  operating at the network layer of the network stack  310 . In some embodiments, the first agent  120  intercepts network layer requests such as Internet Control Message Protocol (ICMP) requests (e.g., ping and traceroute). In other embodiments, the second agent  120 ′ may operate at the transport layer and intercept transport layer communications. In some embodiments, the first agent  120  intercepts communications at one layer of the network stack  210  and interfaces with or communicates the intercepted communication to the second agent  120 ′. 
     The client agent  120  and/or interceptor  350  may operate at or interface with a protocol layer in a manner transparent to any other protocol layer of the network stack  310 . For example, in one embodiment, the interceptor  350  operates or interfaces with the transport layer of the network stack  310  transparently to any protocol layer below the transport layer, such as the network layer, and any protocol layer above the transport layer, such as the session, presentation or application layer protocols. This allows the other protocol layers of the network stack  310  to operate as desired and without modification for using the interceptor  350 . As such, the client agent  120  and/or interceptor  350  can interface with the transport layer to secure, optimize, accelerate, route or load-balance any communications provided via any protocol carried by the transport layer, such as any application layer protocol over TCP/IP. 
     Furthermore, the client agent  120  and/or interceptor may operate at or interface with the network stack  310  in a manner transparent to any application, a user of the client  102 , and any other computing device, such as a server, in communications with the client  102 . The client agent  120  and/or interceptor  350  may be installed and/or executed on the client  102  in a manner without modification of an application. In some embodiments, the user of the client  102  or a computing device in communications with the client  102  are not aware of the existence, execution or operation of the client agent  120  and/or interceptor  350 . As such, in some embodiments, the client agent  120  and/or interceptor  350  is installed, executed, and/or operated transparently to an application, user of the client  102 , another computing device, such as a server, or any of the protocol layers above and/or below the protocol layer interfaced to by the interceptor  350 . 
     The client agent  120  includes an acceleration program  302 , a streaming client  306 , a collection agent  304 , and/or monitoring agent  197 . In one embodiment, the client agent  120  comprises an Independent Computing Architecture (ICA) client, or any portion thereof, developed by Citrix Systems, Inc. of Fort Lauderdale, Fla., and is also referred to as an ICA client. In some embodiments, the client  120  comprises an application streaming client  306  for streaming an application from a server  106  to a client  102 . In some embodiments, the client agent  120  comprises an acceleration program  302  for accelerating communications between client  102  and server  106 . In another embodiment, the client agent  120  includes a collection agent  304  for performing end-point detection/scanning and collecting end-point information for the appliance  200  and/or server  106 . 
     In some embodiments, the acceleration program  302  comprises a client-side acceleration program for performing one or more acceleration techniques to accelerate, enhance or otherwise improve a client&#39;s communications with and/or access to a server  106 , such as accessing an application provided by a server  106 . The logic, functions, and/or operations of the executable instructions of the acceleration program  302  may perform one or more of the following acceleration techniques: 1) multi-protocol compression, 2) transport control protocol pooling, 3) transport control protocol multiplexing, 4) transport control protocol buffering, and 5) caching via a cache manager. Additionally, the acceleration program  302  may perform encryption and/or decryption of any communications received and/or transmitted by the client  102 . In some embodiments, the acceleration program  302  performs one or more of the acceleration techniques in an integrated manner or fashion. Additionally, the acceleration program  302  can perform compression on any of the protocols, or multiple-protocols, carried as a payload of a network packet of the transport layer protocol. 
     The streaming client  306  comprises an application, program, process, service, task or executable instructions for receiving and executing a streamed application from a server  106 . A server  106  may stream one or more application data files to the streaming client  306  for playing, executing or otherwise causing to be executed the application on the client  102 . In some embodiments, the server  106  transmits a set of compressed or packaged application data files to the streaming client  306 . In some embodiments, the plurality of application files are compressed and stored on a file server within an archive file such as a CAB, ZIP, SIT, TAR, JAR or other archive. In one embodiment, the server  106  decompresses, unpackages or unarchives the application files and transmits the files to the client  102 . In another embodiment, the client  102  decompresses, unpackages or unarchives the application files. The streaming client  306  dynamically installs the application, or portion thereof, and executes the application. In one embodiment, the streaming client  306  may be an executable program. In some embodiments, the streaming client  306  may be able to launch another executable program. 
     The collection agent  304  comprises an application, program, process, service, task or executable instructions for identifying, obtaining and/or collecting information about the client  102 . In some embodiments, the appliance  200  transmits the collection agent  304  to the client  102  or client agent  120 . The collection agent  304  may be configured according to one or more policies of the policy engine  236  of the appliance. In other embodiments, the collection agent  304  transmits collected information on the client  102  to the appliance  200 . 
     In one embodiment, the policy engine  236  of the appliance  200  uses the collected information to determine and provide access, authentication and authorization control of the client&#39;s connection to a network  104 . 
     In one embodiment, the collection agent  304  comprises an end-point detection and scanning mechanism, which identifies and determines one or more attributes or characteristics of the client. For example, the collection agent  304  may identify and determine any one or more of the following client-side attributes: 1) the operating system an/or a version of an operating system, 2) a service pack of the operating system, 3) a running service, 4) a running process, and 5) a file. The collection agent  304  may also identify and determine the presence or versions of any one or more of the following on the client: 1) antivirus software, 2) personal firewall software, 3) anti-spam software, and 4) internet security software. The policy engine  236  may have one or more policies based on any one or more of the attributes or characteristics of the client or client-side attributes. 
     In some embodiments, the client agent  120  includes a monitoring agent  197  as discussed in conjunction with  FIGS. 1D and 2B . The monitoring agent  197  may be any type and form of script, such as Visual Basic or Java script. In one embodiment, the monitoring agent  129  monitors and measures performance of any portion of the client agent  120 . For example, in some embodiments, the monitoring agent  129  monitors and measures performance of the acceleration program  302 . In another embodiment, the monitoring agent  129  monitors and measures performance of the streaming client  306 . In other embodiments, the monitoring agent  129  monitors and measures performance of the collection agent  304 . In still another embodiment, the monitoring agent  129  monitors and measures performance of the interceptor  350 . In some embodiments, the monitoring agent  129  monitors and measures any resource of the client  102 , such as memory, CPU and disk. 
     The monitoring agent  197  may monitor and measure performance of any application of the client. In one embodiment, the monitoring agent  129  monitors and measures performance of a browser on the client  102 . In some embodiments, the monitoring agent  197  monitors and measures performance of any application delivered via the client agent  120 . In other embodiments, the monitoring agent  197  measures and monitors end user response times for an application, such as web-based or HTTP response times. The monitoring agent  197  may monitor and measure performance of an ICA or RDP client. In another embodiment, the monitoring agent  197  measures and monitors metrics for a user session or application session. In some embodiments, monitoring agent  197  measures and monitors an ICA or RDP session. In one embodiment, the monitoring agent  197  measures and monitors the performance of the appliance  200  in accelerating delivery of an application and/or data to the client  102 . 
     In some embodiments and still referring to  FIG. 3 , a first program  322  may be used to install and/or execute the client agent  120 , or portion thereof, such as the interceptor  350 , automatically, silently, transparently, or otherwise. In one embodiment, the first program  322  comprises a plugin component, such an ActiveX control or Java control or script that is loaded into and executed by an application. For example, the first program comprises an ActiveX control loaded and run by a web browser application, such as in the memory space or context of the application. In another embodiment, the first program  322  comprises a set of executable instructions loaded into and run by the application, such as a browser. In one embodiment, the first program  322  comprises a designed and constructed program to install the client agent  120 . In some embodiments, the first program  322  obtains, downloads, or receives the client agent  120  via the network from another computing device. In another embodiment, the first program  322  is an installer program or a plug and play manager for installing programs, such as network drivers, on the operating system of the client  102 . 
     D. Systems and Methods for Providing Virtualized Application Delivery Controller 
     Referring now to  FIG. 4A , a block diagram depicts one embodiment of a virtualization environment  400 . In brief overview, a computing device  100  includes a hypervisor layer, a virtualization layer, and a hardware layer. The hypervisor layer includes a hypervisor  401  (also referred to as a virtualization manager) that allocates and manages access to a number of physical resources in the hardware layer (e.g., the processor(s)  421 , and disk(s)  428 ) by at least one virtual machine executing in the virtualization layer. The virtualization layer includes at least one operating system  410  and a plurality of virtual resources allocated to the at least one operating system  410 . Virtual resources may include, without limitation, a plurality of virtual processors  432   a ,  432   b ,  432   c  (generally  432 ), and virtual disks  442   a ,  442   b ,  442   c  (generally  442 ), as well as virtual resources such as virtual memory and virtual network interfaces. The plurality of virtual resources and the operating system  410  may be referred to as a virtual machine  406 . A virtual machine  406  may include a control operating system  405  in communication with the hypervisor  401  and used to execute applications for managing and configuring other virtual machines on the computing device  100 . 
     In greater detail, a hypervisor  401  may provide virtual resources to an operating system in any manner which simulates the operating system having access to a physical device. A hypervisor  401  may provide virtual resources to any number of guest operating systems  410   a ,  410   b  (generally  410 ). In some embodiments, a computing device  100  executes one or more types of hypervisors. In these embodiments, hypervisors may be used to emulate virtual hardware, partition physical hardware, virtualize physical hardware, and execute virtual machines that provide access to computing environments. Hypervisors may include those manufactured by VMWare, Inc., of Palo Alto, Calif.; the XEN hypervisor, an open source product whose development is overseen by the open source Xen.org community; HyperV, VirtualServer or virtual PC hypervisors provided by Microsoft, or others. In some embodiments, a computing device  100  executing a hypervisor that creates a virtual machine platform on which guest operating systems may execute is referred to as a host server. In one of these embodiments, for example, the computing device  100  is a XEN SERVER provided by Citrix Systems, Inc., of Fort Lauderdale, Fla. 
     In some embodiments, a hypervisor  401  executes within an operating system executing on a computing device. In one of these embodiments, a computing device executing an operating system and a hypervisor  401  may be said to have a host operating system (the operating system executing on the computing device), and a guest operating system (an operating system executing within a computing resource partition provided by the hypervisor  401 ). In other embodiments, a hypervisor  401  interacts directly with hardware on a computing device, instead of executing on a host operating system. In one of these embodiments, the hypervisor  401  may be said to be executing on “bare metal,” referring to the hardware comprising the computing device. 
     In some embodiments, a hypervisor  401  may create a virtual machine  406   a - c  (generally  406 ) in which an operating system  410  executes. In one of these embodiments, for example, the hypervisor  401  loads a virtual machine image to create a virtual machine  406 . In another of these embodiments, the hypervisor  401  executes an operating system  410  within the virtual machine  406 . In still another of these embodiments, the virtual machine  406  executes an operating system  410 . 
     In some embodiments, the hypervisor  401  controls processor scheduling and memory partitioning for a virtual machine  406  executing on the computing device  100 . In one of these embodiments, the hypervisor  401  controls the execution of at least one virtual machine  406 . In another of these embodiments, the hypervisor  401  presents at least one virtual machine  406  with an abstraction of at least one hardware resource provided by the computing device  100 . In other embodiments, the hypervisor  401  controls whether and how physical processor capabilities are presented to the virtual machine  406 . 
     A control operating system  405  may execute at least one application for managing and configuring the guest operating systems. In one embodiment, the control operating system  405  may execute an administrative application, such as an application including a user interface providing administrators with access to functionality for managing the execution of a virtual machine, including functionality for executing a virtual machine, terminating an execution of a virtual machine, or identifying a type of physical resource for allocation to the virtual machine. In another embodiment, the hypervisor  401  executes the control operating system  405  within a virtual machine  406  created by the hypervisor  401 . In still another embodiment, the control operating system  405  executes in a virtual machine  406  that is authorized to directly access physical resources on the computing device  100 . In some embodiments, a control operating system  405   a  on a computing device  100   a  may exchange data with a control operating system  405   b  on a computing device  100   b , via communications between a hypervisor  401   a  and a hypervisor  401   b . In this way, one or more computing devices  100  may exchange data with one or more of the other computing devices  100  regarding processors and other physical resources available in a pool of resources. In one of these embodiments, this functionality allows a hypervisor to manage a pool of resources distributed across a plurality of physical computing devices. In another of these embodiments, multiple hypervisors manage one or more of the guest operating systems executed on one of the computing devices  100 . 
     In one embodiment, the control operating system  405  executes in a virtual machine  406  that is authorized to interact with at least one guest operating system  410 . In another embodiment, a guest operating system  410  communicates with the control operating system  405  via the hypervisor  401  in order to request access to a disk or a network. In still another embodiment, the guest operating system  410  and the control operating system  405  may communicate via a communication channel established by the hypervisor  401 , such as, for example, via a plurality of shared memory pages made available by the hypervisor  401 . 
     In some embodiments, the control operating system  405  includes a network back-end driver for communicating directly with networking hardware provided by the computing device  100 . In one of these embodiments, the network back-end driver processes at least one virtual machine request from at least one guest operating system  110 . In other embodiments, the control operating system  405  includes a block back-end driver for communicating with a storage element on the computing device  100 . In one of these embodiments, the block back-end driver reads and writes data from the storage element based upon at least one request received from a guest operating system  410 . 
     In one embodiment, the control operating system  405  includes a tools stack  404 . In another embodiment, a tools stack  404  provides functionality for interacting with the hypervisor  401 , communicating with other control operating systems  405  (for example, on a second computing device  100   b ), or managing virtual machines  406   b ,  406   c  on the computing device  100 . In another embodiment, the tools stack  404  includes customized applications for providing improved management functionality to an administrator of a virtual machine farm. In some embodiments, at least one of the tools stack  404  and the control operating system  405  include a management API that provides an interface for remotely configuring and controlling virtual machines  406  running on a computing device  100 . In other embodiments, the control operating system  405  communicates with the hypervisor  401  through the tools stack  104 . 
     In one embodiment, the hypervisor  401  executes a guest operating system  410  within a virtual machine  406  created by the hypervisor  401 . In another embodiment, the guest operating system  410  provides a user of the computing device  100  with access to resources within a computing environment. In still another embodiment, a resource includes a program, an application, a document, a file, a plurality of applications, a plurality of files, an executable program file, a desktop environment, a computing environment, or other resource made available to a user of the computing device  100 . In yet another embodiment, the resource may be delivered to the computing device  100  via a plurality of access methods including, but not limited to, conventional installation directly on the computing device  100 , delivery to the computing device  100  via a method for application streaming, delivery to the computing device  100  of output data generated by an execution of the resource on a second computing device  100 ′ and communicated to the computing device  100  via a presentation layer protocol, delivery to the computing device  100  of output data generated by an execution of the resource via a virtual machine executing on a second computing device  100 ′, or execution from a removable storage device connected to the computing device  100 , such as a USB device, or via a virtual machine executing on the computing device  100  and generating output data. In some embodiments, the computing device  100  transmits output data generated by the execution of the resource to another computing device  100 ′. 
     In one embodiment, the guest operating system  410 , in conjunction with the virtual machine on which it executes, forms a fully-virtualized virtual machine which is not aware that it is a virtual machine; such a machine may be referred to as a “Domain U HVM (Hardware Virtual Machine) virtual machine”. In another embodiment, a fully-virtualized machine includes software emulating a Basic Input/Output System (BIOS) in order to execute an operating system within the fully-virtualized machine. In still another embodiment, a fully-virtualized machine may include a driver that provides functionality by communicating with the hypervisor  401 . In such an embodiment, the driver may be aware that it executes within a virtualized environment. In another embodiment, the guest operating system  410 , in conjunction with the virtual machine on which it executes, forms a paravirtualized virtual machine, which is aware that it is a virtual machine; such a machine may be referred to as a “Domain U PV virtual machine”. In another embodiment, a paravirtualized machine includes additional drivers that a fully-virtualized machine does not include. In still another embodiment, the paravirtualized machine includes the network back-end driver and the block back-end driver included in a control operating system  405 , as described above. 
     Referring now to  FIG. 4B , a block diagram depicts one embodiment of a plurality of networked computing devices in a system in which at least one physical host executes a virtual machine. In brief overview, the system includes a management component  404  and a hypervisor  401 . The system includes a plurality of computing devices  100 , a plurality of virtual machines  406 , a plurality of hypervisors  401 , a plurality of management components referred to as tools stacks  404 , and a physical resource  421 ,  428 . The plurality of physical machines  100  may each be provided as computing devices  100 , described above in connection with  FIGS. 1E-1H  and  4 A. 
     In greater detail, a physical disk  428  is provided by a computing device  100  and stores at least a portion of a virtual disk  442 . In some embodiments, a virtual disk  442  is associated with a plurality of physical disks  428 . In one of these embodiments, one or more computing devices  100  may exchange data with one or more of the other computing devices  100  regarding processors and other physical resources available in a pool of resources, allowing a hypervisor to manage a pool of resources distributed across a plurality of physical computing devices. In some embodiments, a computing device  100  on which a virtual machine  406  executes is referred to as a physical host  100  or as a host machine  100 . 
     The hypervisor executes on a processor on the computing device  100 . The hypervisor allocates, to a virtual disk, an amount of access to the physical disk. In one embodiment, the hypervisor  401  allocates an amount of space on the physical disk. In another embodiment, the hypervisor  401  allocates a plurality of pages on the physical disk. In some embodiments, the hypervisor provisions the virtual disk  442  as part of a process of initializing and executing a virtual machine  450 . 
     In one embodiment, the management component  404   a  is referred to as a pool management component  404   a . In another embodiment, a management operating system  405   a , which may be referred to as a control operating system  405   a , includes the management component. In some embodiments, the management component is referred to as a tools stack. In one of these embodiments, the management component is the tools stack  404  described above in connection with  FIG. 4A . In other embodiments, the management component  404  provides a user interface for receiving, from a user such as an administrator, an identification of a virtual machine  406  to provision and/or execute. In still other embodiments, the management component  404  provides a user interface for receiving, from a user such as an administrator, the request for migration of a virtual machine  406   b  from one physical machine  100  to another. In further embodiments, the management component  404   a  identifies a computing device  100   b  on which to execute a requested virtual machine  406   d  and instructs the hypervisor  401   b  on the identified computing device  100   b  to execute the identified virtual machine; such a management component may be referred to as a pool management component. 
     Referring now to  FIG. 4C , embodiments of a virtual application delivery controller or virtual appliance  450  are depicted. In brief overview, any of the functionality and/or embodiments of the appliance  200  (e.g., an application delivery controller) described above in connection with  FIGS. 2A and 2B  may be deployed in any embodiment of the virtualized environment described above in connection with  FIGS. 4A and 4B . Instead of the functionality of the application delivery controller being deployed in the form of an appliance  200 , such functionality may be deployed in a virtualized environment  400  on any computing device  100 , such as a client  102 , server  106  or appliance  200 . 
     Referring now to  FIG. 4C , a diagram of an embodiment of a virtual appliance  450  operating on a hypervisor  401  of a server  106  is depicted. As with the appliance  200  of  FIGS. 2A and 2B , the virtual appliance  450  may provide functionality for availability, performance, offload and security. For availability, the virtual appliance may perform load balancing between layers  4  and  7  of the network and may also perform intelligent service health monitoring. For performance increases via network traffic acceleration, the virtual appliance may perform caching and compression. To offload processing of any servers, the virtual appliance may perform connection multiplexing and pooling and/or SSL processing. For security, the virtual appliance may perform any of the application firewall functionality and SSL VPN function of appliance  200 . 
     Any of the modules of the appliance  200  as described in connection with  FIG. 2A  may be packaged, combined, designed or constructed in a form of the virtualized appliance delivery controller  450  deployable as one or more software modules or components executable in a virtualized environment  300  or non-virtualized environment on any server, such as an off the shelf server. For example, the virtual appliance may be provided in the form of an installation package to install on a computing device. With reference to  FIG. 2A , any of the cache manager  232 , policy engine  236 , compression  238 , encryption engine  234 , packet engine  240 , GUI  210 , CLI  212 , shell services  214  and health monitoring programs  216  may be designed and constructed as a software component or module to run on any operating system of a computing device and/or of a virtualized environment  300 . Instead of using the encryption processor  260 , processor  262 , memory  264  and network stack  267  of the appliance  200 , the virtualized appliance  400  may use any of these resources as provided by the virtualized environment  400  or as otherwise available on the server  106 . 
     Still referring to  FIG. 4C , and in brief overview, any one or more vServers  275 A- 275 N may be in operation or executed in a virtualized environment  400  of any type of computing device  100 , such as any server  106 . Any of the modules or functionality of the appliance  200  described in connection with  FIG. 2B  may be designed and constructed to operate in either a virtualized or non-virtualized environment of a server. Any of the vServer  275 , SSL VPN  280 , Intranet UP  282 , Switching  284 , DNS  286 , acceleration  288 , App FW  280  and monitoring agent may be packaged, combined, designed or constructed in a form of application delivery controller  450  deployable as one or more software modules or components executable on a device and/or virtualized environment  400 . 
     In some embodiments, a server may execute multiple virtual machines  406   a - 406   n  in the virtualization environment with each virtual machine running the same or different embodiments of the virtual application delivery controller  450 . In some embodiments, the server may execute one or more virtual appliances  450  on one or more virtual machines on a core of a multi-core processing system. In some embodiments, the server may execute one or more virtual appliances  450  on one or more virtual machines on each processor of a multiple processor device. 
     E. Systems and Methods for Providing a Multi-Core Architecture 
     In accordance with Moore&#39;s Law, the number of transistors that may be placed on an integrated circuit may double approximately every two years. However, CPU speed increases may reach plateaus, for example CPU speed has been around 3.5-4 GHz range since 2005. In some cases, CPU manufacturers may not rely on CPU speed increases to gain additional performance. Some CPU manufacturers may add additional cores to their processors to provide additional performance. Products, such as those of software and networking vendors, that rely on CPUs for performance gains may improve their performance by leveraging these multi-core CPUs. The software designed and constructed for a single CPU may be redesigned and/or rewritten to take advantage of a multi-threaded, parallel architecture or otherwise a multi-core architecture. 
     A multi-core architecture of the appliance  200 , referred to as nCore or multi-core technology, allows the appliance in some embodiments to break the single core performance barrier and to leverage the power of multi-core CPUs. In the previous architecture described in connection with  FIG. 2A , a single network or packet engine is run. The multiple cores of the nCore technology and architecture allow multiple packet engines to run concurrently and/or in parallel. With a packet engine running on each core, the appliance architecture leverages the processing capacity of additional cores. In some embodiments, this provides up to a 7× increase in performance and scalability. 
     Illustrated in  FIG. 5A  are some embodiments of work, task, load or network traffic distribution across one or more processor cores according to a type of parallelism or parallel computing scheme, such as functional parallelism, data parallelism or flow-based data parallelism. In brief overview,  FIG. 5A  illustrates embodiments of a multi-core system such as an appliance  200 ′ with n-cores, a total of cores numbers 1 through N. In one embodiment, work, load or network traffic can be distributed among a first core  505 A, a second core  505 B, a third core  505 C, a fourth core  505 D, a fifth core  505 E, a sixth core  505 F, a seventh core  505 G, and so on such that distribution is across all or two or more of the n cores  505 N (hereinafter referred to collectively as cores  505 .) There may be multiple VIPs  275  each running on a respective core of the plurality of cores. There may be multiple packet engines  240  each running on a respective core of the plurality of cores. Any of the approaches used may lead to different, varying or similar work load or performance level  515  across any of the cores. For a functional parallelism approach, each core may run a different function of the functionalities provided by the packet engine, a VIP  275  or appliance  200 . In a data parallelism approach, data may be paralleled or distributed across the cores based on the Network Interface Card (NIC) or VIP  275  receiving the data. In another data parallelism approach, processing may be distributed across the cores by distributing data flows to each core. 
     In further detail to  FIG. 5A , in some embodiments, load, work or network traffic can be distributed among cores  505  according to functional parallelism  500 . Functional parallelism may be based on each core performing one or more respective functions. In some embodiments, a first core may perform a first function while a second core performs a second function. In functional parallelism approach, the functions to be performed by the multi-core system are divided and distributed to each core according to functionality. In some embodiments, functional parallelism may be referred to as task parallelism and may be achieved when each processor or core executes a different process or function on the same or different data. The core or processor may execute the same or different code. In some cases, different execution threads or code may communicate with one another as they work. Communication may take place to pass data from one thread to the next as part of a workflow. 
     In some embodiments, distributing work across the cores  505  according to functional parallelism  500 , can comprise distributing network traffic according to a particular function such as network input/output management (NW I/O)  510 A, secure sockets layer (SSL) encryption and decryption  510 B and transmission control protocol (TCP) functions  510 C. This may lead to a work, performance or computing load  515  based on a volume or level of functionality being used. In some embodiments, distributing work across the cores  505  according to data parallelism  540 , can comprise distributing an amount of work  515  based on distributing data associated with a particular hardware or software component. In some embodiments, distributing work across the cores  505  according to flow-based data parallelism  520 , can comprise distributing data based on a context or flow such that the amount of work  515 A-N on each core may be similar, substantially equal or relatively evenly distributed. 
     In the case of the functional parallelism approach, each core may be configured to run one or more functionalities of the plurality of functionalities provided by the packet engine or VIP of the appliance. For example, core  1  may perform network I/O processing for the appliance  200 ′ while core  2  performs TCP connection management for the appliance. Likewise, core  3  may perform SSL offloading while core  4  may perform layer  7  or application layer processing and traffic management. Each of the cores may perform the same function or different functions. Each of the cores may perform more than one function. Any of the cores may run any of the functionality or portions thereof identified and/or described in conjunction with  FIGS. 2A and 2B . In this the approach, the work across the cores may be divided by function in either a coarse-grained or fine-grained manner. In some cases, as illustrated in  FIG. 5A , division by function may lead to different cores running at different levels of performance or load  515 . 
     In the case of the functional parallelism approach, each core may be configured to run one or more functionalities of the plurality of functionalities provided by the packet engine of the appliance. For example, core  1  may perform network I/O processing for the appliance  200 ′ while core  2  performs TCP connection management for the appliance. Likewise, core  3  may perform SSL offloading while core  4  may perform layer  7  or application layer processing and traffic management. Each of the cores may perform the same function or different functions. Each of the cores may perform more than one function. Any of the cores may run any of the functionality or portions thereof identified and/or described in conjunction with  FIGS. 2A and 2B . In this the approach, the work across the cores may be divided by function in either a coarse-grained or fine-grained manner. In some cases, as illustrated in  FIG. 5A  division by function may lead to different cores running at different levels of load or performance. 
     The functionality or tasks may be distributed in any arrangement and scheme. For example,  FIG. 5B  illustrates a first core, Core  1   505 A, processing applications and processes associated with network I/O functionality  510 A. Network traffic associated with network I/O, in some embodiments, can be associated with a particular port number. Thus, outgoing and incoming packets having a port destination associated with NW I/O  510 A will be directed towards Core  1   505 A which is dedicated to handling all network traffic associated with the NW I/O port. Similarly, Core  2   505 B is dedicated to handling functionality associated with SSL processing and Core  4   505 D may be dedicated handling all TCP level processing and functionality. 
     While  FIG. 5A  illustrates functions such as network I/O, SSL and TCP, other functions can be assigned to cores. These other functions can include any one or more of the functions or operations described herein. For example, any of the functions described in conjunction with  FIGS. 2A and 2B  may be distributed across the cores on a functionality basis. In some cases, a first VIP  275 A may run on a first core while a second VIP  275 B with a different configuration may run on a second core. In some embodiments, each core  505  can handle a particular functionality such that each core  505  can handle the processing associated with that particular function. For example, Core  2   505 B may handle SSL offloading while Core  4   505 D may handle application layer processing and traffic management. 
     In other embodiments, work, load or network traffic may be distributed among cores  505  according to any type and form of data parallelism  540 . In some embodiments, data parallelism may be achieved in a multi-core system by each core performing the same task or functionally on different pieces of distributed data. In some embodiments, a single execution thread or code controls operations on all pieces of data. In other embodiments, different threads or instructions control the operation, but may execute the same code. In some embodiments, data parallelism is achieved from the perspective of a packet engine, vServers (VIPs)  275 A-C, network interface cards (NIC)  542 D-E and/or any other networking hardware or software included on or associated with an appliance  200 . For example, each core may run the same packet engine or VIP code or configuration but operate on different sets of distributed data. Each networking hardware or software construct can receive different, varying or substantially the same amount of data, and as a result may have varying, different or relatively the same amount of load  515   
     In the case of a data parallelism approach, the work may be divided up and distributed based on VIPs, NICs and/or data flows of the VIPs or NICs. In one of these approaches, the work of the multi-core system may be divided or distributed among the VIPs by having each VIP work on a distributed set of data. For example, each core may be configured to run one or more VIPs. Network traffic may be distributed to the core for each VIP handling that traffic. In another of these approaches, the work of the appliance may be divided or distributed among the cores based on which NIC receives the network traffic. For example, network traffic of a first NIC may be distributed to a first core while network traffic of a second NIC may be distributed to a second core. In some cases, a core may process data from multiple NICs. 
     While  FIG. 5A  illustrates a single vServer associated with a single core  505 , as is the case for VIP 1   275 A, VIP 2   275 B and VIP 3   275 C. In some embodiments, a single vServer can be associated with one or more cores  505 . In contrast, one or more vServers can be associated with a single core  505 . Associating a vServer with a core  505  may include that core  505  to process all functions associated with that particular vServer. In some embodiments, each core executes a VIP having the same code and configuration. In other embodiments, each core executes a VIP having the same code but different configuration. In some embodiments, each core executes a VIP having different code and the same or different configuration. 
     Like vServers, NICs can also be associated with particular cores  505 . In many embodiments, NICs can be connected to one or more cores  505  such that when a NIC receives or transmits data packets, a particular core  505  handles the processing involved with receiving and transmitting the data packets. In one embodiment, a single NIC can be associated with a single core  505 , as is the case with NIC 1   542 D and NIC 2   542 E. In other embodiments, one or more NICs can be associated with a single core  505 . In other embodiments, a single NIC can be associated with one or more cores  505 . In these embodiments, load could be distributed amongst the one or more cores  505  such that each core  505  processes a substantially similar amount of load. A core  505  associated with a NIC may process all functions and/or data associated with that particular NIC. 
     While distributing work across cores based on data of VIPs or NICs may have a level of independency, in some embodiments, this may lead to unbalanced use of cores as illustrated by the varying loads  515  of  FIG. 5A . 
     In some embodiments, load, work or network traffic can be distributed among cores  505  based on any type and form of data flow. In another of these approaches, the work may be divided or distributed among cores based on data flows. For example, network traffic between a client and a server traversing the appliance may be distributed to and processed by one core of the plurality of cores. In some cases, the core initially establishing the session or connection may be the core for which network traffic for that session or connection is distributed. In some embodiments, the data flow is based on any unit or portion of network traffic, such as a transaction, a request/response communication or traffic originating from an application on a client. In this manner and in some embodiments, data flows between clients and servers traversing the appliance  200 ′ may be distributed in a more balanced manner than the other approaches. 
     In flow-based data parallelism  520 , distribution of data is related to any type of flow of data, such as request/response pairings, transactions, sessions, connections or application communications. For example, network traffic between a client and a server traversing the appliance may be distributed to and processed by one core of the plurality of cores. In some cases, the core initially establishing the session or connection may be the core for which network traffic for that session or connection is distributed. The distribution of data flow may be such that each core  505  carries a substantially equal or relatively evenly distributed amount of load, data or network traffic. 
     In some embodiments, the data flow is based on any unit or portion of network traffic, such as a transaction, a request/response communication or traffic originating from an application on a client. In this manner and in some embodiments, data flows between clients and servers traversing the appliance  200 ′ may be distributed in a more balanced manner than the other approached. In one embodiment, data flow can be distributed based on a transaction or a series of transactions. This transaction, in some embodiments, can be between a client and a server and can be characterized by an IP address or other packet identifier. For example, Core  1   505 A can be dedicated to transactions between a particular client and a particular server, therefore the load  536 A on Core  1   505 A may be comprised of the network traffic associated with the transactions between the particular client and server. Allocating the network traffic to Core  1   505 A can be accomplished by routing all data packets originating from either the particular client or server to Core  1   505 A. 
     While work or load can be distributed to the cores based in part on transactions, in other embodiments load or work can be allocated on a per packet basis. In these embodiments, the appliance  200  can intercept data packets and allocate them to a core  505  having the least amount of load. For example, the appliance  200  could allocate a first incoming data packet to Core  1   505 A because the load  536 A on Core  1  is less than the load  536 B-N on the rest of the cores  505 B-N. Once the first data packet is allocated to Core  1   505 A, the amount of load  536 A on Core  1   505 A is increased proportional to the amount of processing resources needed to process the first data packet. When the appliance  200  intercepts a second data packet, the appliance  200  will allocate the load to Core  4   505 D because Core  4   505 D has the second least amount of load. Allocating data packets to the core with the least amount of load can, in some embodiments, ensure that the load  536 A-N distributed to each core  505  remains substantially equal. 
     In other embodiments, load can be allocated on a per unit basis where a section of network traffic is allocated to a particular core  505 . The above-mentioned example illustrates load balancing on a per/packet basis. In other embodiments, load can be allocated based on a number of packets such that every 10, 100 or 1000 packets are allocated to the core  505  having the least amount of load. The number of packets allocated to a core  505  can be a number determined by an application, user or administrator and can be any number greater than zero. In still other embodiments, load can be allocated based on a time metric such that packets are distributed to a particular core  505  for a predetermined amount of time. In these embodiments, packets can be distributed to a particular core  505  for five milliseconds or for any period of time determined by a user, program, system, administrator or otherwise. After the predetermined time period elapses, data packets are transmitted to a different core  505  for the predetermined period of time. 
     Flow-based data parallelism methods for distributing work, load or network traffic among the one or more cores  505  can comprise any combination of the above-mentioned embodiments. These methods can be carried out by any part of the appliance  200 , by an application or set of executable instructions executing on one of the cores  505 , such as the packet engine, or by any application, program or agent executing on a computing device in communication with the appliance  200 . 
     The functional and data parallelism computing schemes illustrated in  FIG. 5A  can be combined in any manner to generate a hybrid parallelism or distributed processing scheme that encompasses function parallelism  500 , data parallelism  540 , flow-based data parallelism  520  or any portions thereof. In some cases, the multi-core system may use any type and form of load balancing schemes to distribute load among the one or more cores  505 . The load balancing scheme may be used in any combination with any of the functional and data parallelism schemes or combinations thereof. 
     Illustrated in  FIG. 5B  is an embodiment of a multi-core system  545 , which may be any type and form of one or more systems, appliances, devices or components. This system  545 , in some embodiments, can be included within an appliance  200  having one or more processing cores  505 A-N. The system  545  can further include one or more packet engines (PE) or packet processing engines (PPE)  548 A-N communicating with a memory bus  556 . The memory bus may be used to communicate with the one or more processing cores  505 A-N. Also included within the system  545  can be one or more network interface cards (NIC)  552  and a flow distributor  550  which can further communicate with the one or more processing cores  505 A-N. The flow distributor  550  can comprise a Receive Side Scaler (RSS) or Receive Side Scaling (RSS) module  560 . 
     Further referring to  FIG. 5B , and in more detail, in one embodiment the packet engine(s)  548 A-N can comprise any portion of the appliance  200  described herein, such as any portion of the appliance described in  FIGS. 2A and 2B . The packet engine(s)  548 A-N can, in some embodiments, comprise any of the following elements: the packet engine  240 , a network stack  267 ; a cache manager  232 ; a policy engine  236 ; a compression engine  238 ; an encryption engine  234 ; a GUI  210 ; a CLI  212 ; shell services  214 ; monitoring programs  216 ; and any other software or hardware element able to receive data packets from one of either the memory bus  556  or the one of more cores  505 A-N. In some embodiments, the packet engine(s)  548 A-N can comprise one or more vServers  275 A-N, or any portion thereof. In other embodiments, the packet engine(s)  548 A-N can provide any combination of the following functionalities: SSL VPN  280 ; Intranet UP  282 ; switching  284 ; DNS  286 ; packet acceleration  288 ; App FW  280 ; monitoring such as the monitoring provided by a monitoring agent  197 ; functionalities associated with functioning as a TCP stack; load balancing; SSL offloading and processing; content switching; policy evaluation; caching; compression; encoding; decompression; decoding; application firewall functionalities; XML processing and acceleration; and SSL VPN connectivity. 
     The packet engine(s)  548 A-N can, in some embodiments, be associated with a particular server, user, client or network. When a packet engine  548  becomes associated with a particular entity, that packet engine  548  can process data packets associated with that entity. For example, should a packet engine  548  be associated with a first user, that packet engine  548  will process and operate on packets generated by the first user, or packets having a destination address associated with the first user. Similarly, the packet engine  548  may choose not to be associated with a particular entity such that the packet engine  548  can process and otherwise operate on any data packets not generated by that entity or destined for that entity. 
     In some instances, the packet engine(s)  548 A-N can be configured to carry out the any of the functional and/or data parallelism schemes illustrated in  FIG. 5A . In these instances, the packet engine(s)  548 A-N can distribute functions or data among the processing cores  505 A-N so that the distribution is according to the parallelism or distribution scheme. In some embodiments, a single packet engine(s)  548 A-N carries out a load balancing scheme, while in other embodiments one or more packet engine(s)  548 A-N carry out a load balancing scheme. Each core  505 A-N, in one embodiment, can be associated with a particular packet engine  505  such that load balancing can be carried out by the packet engine  505 . Load balancing may in this embodiment, require that each packet engine  505  associated with a core  505  communicate with the other packet engines  505  associated with cores  505  so that the packet engines  505  can collectively determine where to distribute load. One embodiment of this process can include an arbiter that receives votes from each packet engine  505  for load. The arbiter can distribute load to each packet engine  505  based in part on the age of the engine&#39;s vote and in some cases a priority value associated with the current amount of load on an engine&#39;s associated core  505 . 
     Any of the packet engines running on the cores may run in user mode, kernel or any combination thereof. In some embodiments, the packet engine operates as an application or program running is user or application space. In these embodiments, the packet engine may use any type and form of interface to access any functionality provided by the kernel. In some embodiments, the packet engine operates in kernel mode or as part of the kernel. In some embodiments, a first portion of the packet engine operates in user mode while a second portion of the packet engine operates in kernel mode. In some embodiments, a first packet engine on a first core executes in kernel mode while a second packet engine on a second core executes in user mode. In some embodiments, the packet engine or any portions thereof operates on or in conjunction with the NIC or any drivers thereof. 
     In some embodiments the memory bus  556  can be any type and form of memory or computer bus. While a single memory bus  556  is depicted in  FIG. 5B , the system  545  can comprise any number of memory buses  556 . In one embodiment, each packet engine  548  can be associated with one or more individual memory buses  556 . 
     The NIC  552  can in some embodiments be any of the network interface cards or mechanisms described herein. The NIC  552  can have any number of ports. The NIC can be designed and constructed to connect to any type and form of network  104 . While a single NIC  552  is illustrated, the system  545  can comprise any number of NICs  552 . In some embodiments, each core  505 A-N can be associated with one or more single NICs  552 . Thus, each core  505  can be associated with a single NIC  552  dedicated to a particular core  505 . The cores  505 A-N can comprise any of the processors described herein. Further, the cores  505 A-N can be configured according to any of the core  505  configurations described herein. 
     Still further, the cores  505 A-N can have any of the core  505  functionalities described herein. While  FIG. 5B  illustrates seven cores  505 A-G, any number of cores  505  can be included within the system  545 . In particular, the system  545  can comprise “N” cores, where “N” is a whole number greater than zero. 
     A core may have or use memory that is allocated or assigned for use to that core. The memory may be considered private or local memory of that core and only accessible by that core. A core may have or use memory that is shared or assigned to multiple cores. The memory may be considered public or shared memory that is accessible by more than one core. A core may use any combination of private and public memory. With separate address spaces for each core, some level of coordination is eliminated from the case of using the same address space. With a separate address space, a core can perform work on information and data in the core&#39;s own address space without worrying about conflicts with other cores. Each packet engine may have a separate memory pool for TCP and/or SSL connections. 
     Further referring to  FIG. 5B , any of the functionality and/or embodiments of the cores  505  described above in connection with  FIG. 5A  can be deployed in any embodiment of the virtualized environment described above in connection with  FIGS. 4A and 4B . Instead of the functionality of the cores  505  being deployed in the form of a physical processor  505 , such functionality may be deployed in a virtualized environment  400  on any computing device  100 , such as a client  102 , server  106  or appliance  200 . In other embodiments, instead of the functionality of the cores  505  being deployed in the form of an appliance or a single device, the functionality may be deployed across multiple devices in any arrangement. For example, one device may comprise two or more cores and another device may comprise two or more cores. For example, a multi-core system may include a cluster of computing devices, a server farm or network of computing devices. In some embodiments, instead of the functionality of the cores  505  being deployed in the form of cores, the functionality may be deployed on a plurality of processors, such as a plurality of single core processors. 
     In one embodiment, the cores  505  may be any type and form of processor. In some embodiments, a core can function substantially similar to any processor or central processing unit described herein. In some embodiment, the cores  505  may comprise any portion of any processor described herein. While  FIG. 5A  illustrates seven cores, there can exist any “N” number of cores within an appliance  200 , where “N” is any whole number greater than one. In some embodiments, the cores  505  can be installed within a common appliance  200 , while in other embodiments the cores  505  can be installed within one or more appliance(s)  200  communicatively connected to one another. The cores  505  can in some embodiments comprise graphics processing software, while in other embodiments the cores  505  provide general processing capabilities. The cores  505  can be installed physically near each other and/or can be communicatively connected to each other. The cores may be connected by any type and form of bus or subsystem physically and/or communicatively coupled to the cores for transferring data between to, from and/or between the cores. 
     While each core  505  can comprise software for communicating with other cores, in some embodiments a core manager (Not Shown) can facilitate communication between each core  505 . In some embodiments, the kernel may provide core management. The cores may interface or communicate with each other using a variety of interface mechanisms. In some embodiments, core to core messaging may be used to communicate between cores, such as a first core sending a message or data to a second core via a bus or subsystem connecting the cores. In some embodiments, cores may communicate via any type and form of shared memory interface. In one embodiment, there may be one or more memory locations shared among all the cores. In some embodiments, each core may have separate memory locations shared with each other core. For example, a first core may have a first shared memory with a second core and a second share memory with a third core. In some embodiments, cores may communicate via any type of programming or API, such as function calls via the kernel. In some embodiments, the operating system may recognize and support multiple core devices and provide interfaces and API for inter-core communications. 
     The flow distributor  550  can be any application, program, library, script, task, service, process or any type and form of executable instructions executing on any type and form of hardware. In some embodiments, the flow distributor  550  may any design and construction of circuitry to perform any of the operations and functions described herein. In some embodiments, the flow distributor distribute, forwards, routes, controls and/ors manage the distribution of data packets among the cores  505  and/or packet engine or VIPs running on the cores. The flow distributor  550 , in some embodiments, can be referred to as an interface master. In one embodiment, the flow distributor  550  comprises a set of executable instructions executing on a core or processor of the appliance  200 . In another embodiment, the flow distributor  550  comprises a set of executable instructions executing on a computing machine in communication with the appliance  200 . In some embodiments, the flow distributor  550  comprises a set of executable instructions executing on a NIC, such as firmware. In still other embodiments, the flow distributor  550  comprises any combination of software and hardware to distribute data packets among cores or processors. In one embodiment, the flow distributor  550  executes on at least one of the cores  505 A-N, while in other embodiments a separate flow distributor  550  assigned to each core  505 A-N executes on an associated core  505 A-N. The flow distributor may use any type and form of statistical or probabilistic algorithms or decision making to balance the flows across the cores. The hardware of the appliance, such as a NIC, or the kernel may be designed and constructed to support sequential operations across the NICs and/or cores. 
     In embodiments where the system  545  comprises one or more flow distributors  550 , each flow distributor  550  can be associated with a processor  505  or a packet engine  548 . The flow distributors  550  can comprise an interface mechanism that allows each flow distributor  550  to communicate with the other flow distributors  550  executing within the system  545 . In one instance, the one or more flow distributors  550  can determine how to balance load by communicating with each other. This process can operate substantially similarly to the process described above for submitting votes to an arbiter which then determines which flow distributor  550  should receive the load. In other embodiments, a first flow distributor  550 ′ can identify the load on an associated core and determine whether to forward a first data packet to the associated core based on any of the following criteria: the load on the associated core is above a predetermined threshold; the load on the associated core is below a predetermined threshold; the load on the associated core is less than the load on the other cores; or any other metric that can be used to determine where to forward data packets based in part on the amount of load on a processor. 
     The flow distributor  550  can distribute network traffic among the cores  505  according to a distribution, computing or load balancing scheme such as those described herein. In one embodiment, the flow distributor can distribute network traffic or; pad according to any one of a functional parallelism distribution scheme  550 , a data parallelism load distribution scheme  540 , a flow-based data parallelism distribution scheme  520 , or any combination of these distribution scheme or any load balancing scheme for distributing load among multiple processors. The flow distributor  550  can therefore act as a load distributor by taking in data packets and distributing them across the processors according to an operative load balancing or distribution scheme. In one embodiment, the flow distributor  550  can comprise one or more operations, functions or logic to determine how to distribute packers, work or load accordingly. In still other embodiments, the flow distributor  550  can comprise one or more sub operations, functions or logic that can identify a source address and a destination address associated with a data packet, and distribute packets accordingly. 
     In some embodiments, the flow distributor  550  can comprise a receive-side scaling (RSS) network driver, module  560  or any type and form of executable instructions which distribute data packets among the one or more cores  505 . The RSS module  560  can comprise any combination of hardware and software, In some embodiments, the RSS module  560  works in conjunction with the flow distributor  550  to distribute data packets across the cores  505 A-N or among multiple processors in a multi-processor network. The RSS module  560  can execute within the NIC  552  in some embodiments, and in other embodiments can execute on any one of the cores  505 . 
     In some embodiments, the RSS module  560  uses the MICROSOFT receive-side-scaling (RSS) scheme. In one embodiment, RSS is a Microsoft Scalable Networking initiative technology that enables receive processing to be balanced across multiple processors in the system while maintaining in-order delivery of the data. The RSS may use any type and form of hashing scheme to determine a core or processor for processing a network packet. 
     The RSS module  560  can apply any type and form hash function such as the Toeplitz hash function. The hash function may be applied to the hash type or any the sequence of values. The hash function may be a secure hash of any security level or is otherwise cryptographically secure. The has function may use a hash key. The size of the key is dependent upon the hash function. For the Toeplitz hash, the size may be 40 bytes for IPv6 and 16 bytes for IPv4. 
     The hash function may be designed and constructed based on any one or more criteria or design goals. In some embodiments, a hash function may be used that provides an even distribution of hash result for different hash inputs and different hash types, including TCP/IPv4, TCP/IPv6, IPv4, and IPv6 headers. In some embodiments, a hash function may be used that provides a hash result that is evenly distributed when a small number of buckets are present (for example, two or four). In some embodiments, hash function may be used that provides a hash result that is randomly distributed when a large number of buckets were present (for example, 64 buckets). In some embodiments, the hash function is determined based on a level of computational or resource usage. In some embodiments, the hash function is determined based on ease or difficulty of implementing the hash in hardware. In some embodiments, the hash function is determined bases on the ease or difficulty of a malicious remote host to send packets that would all hash to the same bucket. 
     The RSS may generate hashes from any type and form of input, such as a sequence of values. This sequence of values can include any portion of the network packet, such as any header, field or payload of network packet, or portions thereof. In some embodiments, the input to the hash may be referred to as a hash type and include any tuples of information associated with a network packet or data flow, such as any of the following: a four tuple comprising at least two IP addresses and two ports; a four tuple comprising any four sets of values; a six tuple; a two tuple; and/or any other sequence of numbers or values. The following are example of hash types that may be used by RSS:
         4-tuple of source TCP Port, source IP version 4 (IPv4) address, destination TCP Port, and destination IPv4 address. This is the only required hash type to support.   4-tuple of source TCP Port, source IP version 6 (IPv6) address, destination TCP Port, and destination IPv6 address.   2-tuple of source IPv4 address, and destination IPv4 address.   2-tuple of source IPv6 address, and destination IPv6 address.   2-tuple of source IPv6 address, and destination IPv6 address, including support for parsing IPv6 extension headers.       

     The hash result or any portion thereof may used to identify a core or entity, such as a packet engine or VIP, for distributing a network packet. In some embodiments, one or more hash bits or mask are applied to the hash result. The hash bit or mask may be any number of bits or bytes. A NIC may support any number of bits, such as seven bits. The network stack may set the actual number of bits to be used during initialization. The number will be between 1 and 7, inclusive. 
     The hash result may be used to identify the core or entity via any type and form of table, such as a bucket table or indirection table. In some embodiments, the number of hash-result bits are used to index into the table. The range of the hash mask may effectively define the size of the indirection table. Any portion of the hash result or the hast result itself may be used to index the indirection table. The values in the table may identify any of the cores or processor, such as by a core or processor identifier. In some embodiments, all of the cores of the multi-core system are identified in the table. In other embodiments, a port of the cores of the multi-core system are identified in the table. The indirection table may comprise any number of buckets for example 2 to 128 buckets that may be indexed by a hash mask. Each bucket may comprise a range of index values that identify a core or processor. In some embodiments, the flow controller and/or RSS module may rebalance the network rebalance the network load by changing the indirection table. 
     In some embodiments, the multi-core system  575  does not include a RSS driver or RSS module  560 . In some of these embodiments, a software steering module (Not Shown) or a software embodiment of the RSS module within the system can operate in conjunction with or as part of the flow distributor  550  to steer packets to cores  505  within the multi-core system  575 . 
     The flow distributor  550 , in some embodiments, executes within any module or program on the appliance  200 , on any one of the cores  505  and on any one of the devices or components included within the multi-core system  575 . In some embodiments, the flow distributor  550 ′ can execute on the first core  505 A, while in other embodiments the flow distributor  550 ″ can execute on the NIC  552 . In still other embodiments, an instance of the flow distributor  550 ′ can execute on each core  505  included in the multi-core system  575 . In this embodiment, each instance of the flow distributor  550 ′ can communicate with other instances of the flow distributor  550 ′ to forward packets back and forth across the cores  505 . There exist situations where a response to a request packet may not be processed by the same core, i.e. the first core processes the request while the second core processes the response. In these situations, the instances of the flow distributor  550 ′ can intercept the packet and forward it to the desired or correct core  505 , i.e. a flow distributor instance  550 ′ can forward the response to the first core. Multiple instances of the flow distributor  550 ′ can execute on any number of cores  505  and any combination of cores  505 . 
     The flow distributor may operate responsive to any one or more rules or policies. The rules may identify a core or packet processing engine to receive a network packet, data or data flow. The rules may identify any type and form of tuple information related to a network packet, such as a 4-tuple of source and destination IP address and source and destination ports. Based on a received packet matching the tuple specified by the rule, the flow distributor may forward the packet to a core or packet engine. In some embodiments, the packet is forwarded to a core via shared memory and/or core to core messaging. 
     Although  FIG. 5B  illustrates the flow distributor  550  as executing within the multi-core system  575 , in some embodiments the flow distributor  550  can execute on a computing device or appliance remotely located from the multi-core system  575 . In such an embodiment, the flow distributor  550  can communicate with the multi-core system  575  to take in data packets and distribute the packets across the one or more cores  505 . The flow distributor  550  can, in one embodiment, receive data packets destined for the appliance  200 , apply a distribution scheme to the received data packets and distribute the data packets to the one or more cores  505  of the multi-core system  575 . In one embodiment, the flow distributor  550  can be included in a router or other appliance such that the router can target particular cores  505  by altering meta data associated with each packet so that each packet is targeted towards a sub-node of the multi-core system  575 . In such an embodiment, CISCO&#39;s vn-tag mechanism can be used to alter or tag each packet with the appropriate meta data. 
     Illustrated in  FIG. 5C  is an embodiment of a multi-core system  575  comprising one or more processing cores  505 A-N. In brief overview, one of the cores  505  can be designated as a control core  505 A and can be used as a control plane  570  for the other cores  505 . The other cores may be secondary cores which operate in a data plane while the control core provides the control plane. The cores  505 A-N may share a global cache  580 . While the control core provides a control plane, the other cores in the multi-core system form or provide a data plane. These cores perform data processing functionality on network traffic while the control provides initialization, configuration and control of the multi-core system. 
     Further referring to  FIG. 5C , and in more detail, the cores  505 A-N as well as the control core  505 A can be any processor described herein. Furthermore, the cores  505 A-N and the control core  505 A can be any processor able to function within the system  575  described in  FIG. 5C . Still further, the cores  505 A-N and the control core  505 A can be any core or group of cores described herein. The control core may be a different type of core or processor than the other cores. In some embodiments, the control may operate a different packet engine or have a packet engine configured differently than the packet engines of the other cores. 
     Any portion of the memory of each of the cores may be allocated to or used for a global cache that is shared by the cores. In brief overview, a predetermined percentage or predetermined amount of each of the memory of each core may be used for the global cache. For example, 50% of each memory of each code may be dedicated or allocated to the shared global cache. That is, in the illustrated embodiment, 2 GB of each core excluding the control plane core or core  1  may be used to form a 28 GB shared global cache. The configuration of the control plane such as via the configuration services may determine the amount of memory used for the shared global cache. In some embodiments, each core may provide a different amount of memory for use by the global cache. In other embodiments, any one core may not provide any memory or use the global cache. In some embodiments, any of the cores may also have a local cache in memory not allocated to the global shared memory. Each of the cores may store any portion of network traffic to the global shared cache. Each of the cores may check the cache for any content to use in a request or response. Any of the cores may obtain content from the global shared cache to use in a data flow, request or response. 
     The global cache  580  can be any type and form of memory or storage element, such as any memory or storage element described herein. In some embodiments, the cores  505  may have access to a predetermined amount of memory (i.e. 32 GB or any other memory amount commensurate with the system  575 .) The global cache  580  can be allocated from that predetermined amount of memory while the rest of the available memory can be allocated among the cores  505 . In other embodiments, each core  505  can have a predetermined amount of memory. The global cache  580  can comprise an amount of the memory allocated to each core  505 . This memory amount can be measured in bytes, or can be measured as a percentage of the memory allocated to each core  505 . Thus, the global cache  580  can comprise 1 GB of memory from the memory associated with each core  505 , or can comprise 20 percent or one-half of the memory associated with each core  505 . In some embodiments, only a portion of the cores  505  provide memory to the global cache  580 , while in other embodiments the global cache  580  can comprise memory not allocated to the cores  505 . 
     Each core  505  can use the global cache  580  to store network traffic or cache data. In some embodiments, the packet engines of the core use the global cache to cache and use data stored by the plurality of packet engines. For example, the cache manager of  FIG. 2A  and cache functionality of  FIG. 2B  may use the global cache to share data for acceleration. For example, each of the packet engines may store responses, such as HTML data, to the global cache. Any of the cache managers operating on a core may access the global cache to server caches responses to client requests. 
     In some embodiments, the cores  505  can use the global cache  580  to store a port allocation table which can be used to determine data flow based in part on ports. In other embodiments, the cores  505  can use the global cache  580  to store an address lookup table or any other table or list that can be used by the flow distributor to determine where to direct incoming and outgoing data packets. The cores  505  can, in some embodiments read from and write to cache  580 , while in other embodiments the cores  505  can only read from or write to cache  580 . The cores may use the global cache to perform core to core communications. 
     The global cache  580  may be sectioned into individual memory sections where each section can be dedicated to a particular core  505 . In one embodiment, the control core  505 A can receive a greater amount of available cache, while the other cores  505  can receiving varying amounts or access to the global cache  580 . 
     In some embodiments, the system  575  can comprise a control core  505 A. While  FIG. 5C  illustrates core  1   505 A as the control core, the control core can be any core within the appliance  200  or multi-core system. Further, while only a single control core is depicted, the system  575  can comprise one or more control cores each having a level of control over the system. In some embodiments, one or more control cores can each control a particular aspect of the system  575 . For example, one core can control deciding which distribution scheme to use, while another core can determine the size of the global cache  580 . 
     The control plane of the multi-core system may be the designation and configuration of a core as the dedicated management core or as a master core. This control plane core may provide control, management and coordination of operation and functionality the plurality of cores in the multi-core system. This control plane core may provide control, management and coordination of allocation and use of memory of the system among the plurality of cores in the multi-core system, including initialization and configuration of the same. In some embodiments, the control plane includes the flow distributor for controlling the assignment of data flows to cores and the distribution of network packets to cores based on data flows. In some embodiments, the control plane core runs a packet engine and in other embodiments, the control plane core is dedicated to management and control of the other cores of the system. 
     The control core  505 A can exercise a level of control over the other cores  505  such as determining how much memory should be allocated to each core  505  or determining which core  505  should be assigned to handle a particular function or hardware/software entity. The control core  505 A, in some embodiments, can exercise control over those cores  505  within the control plan  570 . Thus, there can exist processors outside of the control plane  570  which are not controlled by the control core  505 A. Determining the boundaries of the control plane  570  can include maintaining, by the control core  505 A or agent executing within the system  575 , a list of those cores  505  controlled by the control core  505 A. The control core  505 A can control any of the following: initialization of a core; determining when a core is unavailable; re-distributing load to other cores  505  when one core fails; determining which distribution scheme to implement; determining which core should receive network traffic; determining how much cache should be allocated to each core; determining whether to assign a particular function or element to a particular core; determining whether to permit cores to communicate with one another; determining the size of the global cache  580 ; and any other determination of a function, configuration or operation of the cores within the system  575 . 
     F. Systems and Methods for Distributing Data Packets Across a Multi-Core Architecture and System 
     Illustrated in  FIG. 6  is one embodiment of a multi-core system  545 . This system  545  can include, in most embodiments, one or more network interface cards (NIC)  552  which can execute or include a RSS module  560 . The NIC  552  can communicate with one or more cores  505  where each core can execute a packet engine  548  and/or a flow distributor  550 . In some embodiments, the NIC  552  can store one or more port allocation tables  604  and can comprise one or more ports  632  and one or more internet protocol (IP) addresses  630 . 
     Further referring to  FIG. 6  and in more detail, in one embodiment, the multi-core system  545  can be any of the multi-core systems  545  described herein. In particular, the multi-core system  545  can be any of the multi-core systems  545  described in  FIGS. 5B-5C . The multi-core system  545  can execute on an appliance  200 , a client, a server or any other computing machine that executes the multi-core system  545  described herein. While the multi-core system  545  illustrated in  FIG. 6  includes a plurality of cores  505  and a NIC  552 , in some embodiments the multi-core system  545  can comprise additional devices and can execute additional programs, clients and modules. 
     In one embodiment, the multi-core system  545  can comprise a NIC  552  such as any of the NICs described herein. Although the multi-core system  545  illustrated in  FIG. 6  depicts a multi-core system  545  having a single NIC  552 , in some embodiments, the multi-core system  545  can have one or more NICs  552 . These NICs  552  can be the same type of NIC  552 , and in other embodiments can be different types of NICs  552 . The NIC(s)  552  can communicate with one or more of the processing cores  505  in the multi-core system  545 . For example, the NIC  552  can communicate with each of a first core  505 A, a second core  505 B, a third core  505 C, a fourth core  505 D, a fifth core  505 E, a sixth core  505 F, a seventh core  505 G, and any “N” number of cores  505 N, where “N” is a whole number greater than zero. In other embodiments, the NIC  552  can communicate with a single core  505  or a subset of cores  505 . For example, the NIC  552  may communicate with a first core  505 A, or cores one through 4  505 A- 505 D. In embodiments where multiple NICs  552  are included within the multi-core system  545 , each NIC  552  can communicate with one or more cores  505 . For example, a first NIC  552  can communicate with cores one through 4  505 A- 505 D, while a second NIC  552  can communicate with cores five through seven  505 E- 505 G. In other embodiments where multiple NICs  552  are included within the multi-core system  545 , one or more NICs  552  can communicate with the cores  505  while the other NICs  552  can perform an alternative function, communication with other systems or devices within the multi-core system  545 , or can function as redundant NICs  552  that are used as backup when a primary NIC  552  fails. 
     In some embodiments, the NIC  552  executes a RSS module  560  such as any of the RSS module  560  described herein. The RSS module  560  applies a hash function to a tuple or sequence of values comprising any combination of the following: a client IP address; a client port; a destination IP address; a destination port; or any other value associated with the source or destination of a data packet. In some embodiments, the value that results from the application of the hash function to the tuple, identifies a core  505  within the multi-core system  545 . The RSS module  560  can use this property of the hash function to distribute packets across cores  505  in a multi-core system  545 . By distributing packets across the cores  505  of the multi-core system  545 , the RSS module  560  can symmetrically distribute network traffic across the cores  505  in a manner substantially similar to flow-based data parallelism. 
     The cores  505  within the multi-core system  545  can be any of the cores  505  described herein. In one embodiment, the multi-core system  545  can include any “N” number of cores where “N” is a whole number greater than zero. In other embodiments, the multi-core system  545  can include eight cores. Cores  505  can be dedicated to process programs or services performing certain functions, and in some embodiments, can be dedicated to process data packets received or transmitted by certain devices or program modules. In some embodiments, each core  505  can execute any of the following: a packet engine  548  such as any of the packet engines  548  described herein or a flow distributor  550  such as any of the flow distributors  550  described herein. In other embodiments, each core  505  stores, in an associated storage repository, any of the following: a port allocation table; a listing of ports of the core  505 ; or a listing of IP addresses of the core  505 . 
     In one embodiment, each core  505  executes a packet engine  548 A-N such as any of the vServers described herein. A packet engine  548 A-N can be included in each core  505 , and collectively the packet engines  548 A-N can be referred to as a packet engine  548 . Packet engines  548 , in some embodiments, alter or modify tuples of data packets according to flow distribution rules executed by each packet engine  548 . In one embodiment, a packet engine  548  replaces a client IP address in a tuple of a data packet received by the packet engine  548 , with an IP address  630 A-B of the core  505  on which the packet engine  548  executes. The packet engine  548 , in another embodiment, replaces a client port in a tuple of a data packet received by the packet engine  548 , with a port  632 A-B selected from a plurality of ports  632 A-B of the core  505  on which the packet engine  548  executes. In still other embodiments, the packet engine  548  maintains all aspects of a data packet including the contents of a tuple of the data packet. The packet engine  548 , in some embodiments, communicates with one or more servers  106  to forward servers  106  received data packets that are destined for those servers  106 . Similarly, the packet engine  548 , in some embodiments, communicates with one or more clients  102  to forward clients  102  received data packets that are destined for those clients  102 . 
     Each core  505 , in some embodiments, accesses a storage repository allocated to each core  505  or a shared storage repository available to all cores  505  in a multi-core system  545 . In one embodiment, a port allocation  604 A-N is stored in a storage repository either shared or allocated to a specific core  505 . A single core  505  can have one or more port allocation tables  604 A-N (referred to generally as port allocation table  604 ,) where each port allocation table  604  lists both available and un-available ports on a particular core  505 A. In one embodiment, a core  505  can have one port allocation table  604 , while in other embodiments a core  505  can have 64 or 256 port allocation tables  604 . For example, Port Allocation Table A  604 A on Core  1   505 A can store entries indicating the status of each port  632 A-B on Core  1   505 A. The status of each port  632 A-B can include any of the following characteristics: whether the port is open or closed; whether the port has been assigned, i.e. whether the port is available or un-available; whether the port is within a pre-assigned range; and any other pertinent characteristic of the port. Thus, if Packet Engine A  548 A on Core  1   505 A wants to determine whether a particular port is open and/or available, Packet Engine A  548 A can query Port Allocation Table A  604 A to determine whether the desired port is open and/or available. 
     In instances where a core  505  has multiple port allocation tables  604 , each port allocation table can be associated with a value or other unique identifier. Each port allocation table  604 , in one embodiment, has an identifying value that can be determined by applying a hash function to a portion of a tuple of a data packet. Therefore any of the hashes described herein can be applied by a packet engine  548  or flow distributor  550  to any combination of a client IP address, a client port, a destination IP address and/or a destination port to determine a unique value for that data packet. This unique value further identifies a port allocation table  604  on the core  505 . For example, if a packet engine  548 B on Core  2   505 B wants to assign a port to a received data packet, the packet engine  548 B first applies a hash to a client IP address and a destination IP address identified in the data packet. Based on the result of the hash, the packet engine  548 B selects a port allocation table  604  from amongst one or more port allocation tables  604  on Core  2   505 B, and selects a port  632 C-D based on a review of the selected port allocation table  604 . 
     Port allocation tables  604 , in some embodiments, can be dynamically altered by a packet engine  548 , flow distributor  550  or other program, service or device based on changes made to ports  632  of a core  505 , or based on allocation of ports  632  to a data packet or transaction. In one embodiment, when a section of ports is assigned to a particular port allocation table  604  in a core  505  or to a particular core  505 , the port allocation table  604  is updated to reflect the assignment. The update can either be an update the entries of the affected ports  632  to reflect the assignment, or an update of the affected ports  632  to list the ports  632  within the section of ports  632  as open and all other ports  632  as closed. In other embodiments, once a port is assigned to a data packet or transaction between two computing machines, the port allocation table  604  is updated to reflect the assignment by listing the assigned port as closed or unavailable and in some cases by identifying the data packet or transaction. 
     In some embodiments, each core  505  can have one or more ports  632  (referred to generally as ports  632 .) While  FIG. 6  illustrates each core  505  as having two ports  632 , each core  505  has multiple ports  632 , i.e. hundreds and in some cases thousands or millions of ports  632 . Ports  632 , in most embodiments, are identified by unique values or numbers. Assigning a data packet or transaction to a port  632  can comprise updating a header of the data packet or data packets of the transaction to reflect the unique value or number associated with the assigned port  632 . Ports  632 , in many embodiments, are tracked within port allocation tables  604  on each core  505 . While each core  505  has its own set of ports  632 , the values or number associated with each port  632  can repeat on each core  505 . For example, Core  3   505 C can have ports one through three-thousand, while Core  5   505 E can also have ports one through three-thousand. The uniqueness of each port in Core  3   505 C and Core  5   505  E comes from the fact that Core  3   505 C ports are associated with one or more IP addresses specific to Core  3   505 C, and Core  5   505 E ports are associated with one or more IP addresses specific to Core  5   505 E. 
     Similarly, each core  505 , in most embodiments, has one or more IP addresses  630 A-B. While  FIG. 6  illustrates each core  505  as having two IP addresses  630  (referred to generally as IP addresses  630 ,) each core  505  can have any “N” number of IP addresses  630  where “N” is a whole number greater than zero. In some embodiments, the IP addresses  630  of a core  505  are pre-assigned by an administrator, application or other service or program executing in the multi-core system  545 . In other embodiments, a group or range of IP addresses  630  are assigned to each core  505 . In still other embodiments, the same IP address  630  is assigned to each core  505 . This IP address  630 , in most embodiments, is an IP address of the multi-core system  545 . 
     In one embodiment, a first core  505  can execute a flow distributor  550 . The flow distributor  550  can be any of the flow distributors  550  described herein. While  FIG. 6  illustrates a multi-core system  545  where the flow distributor  550  executes on a first core  505 , each core  505  can execute an instance of the flow distributor  550  specific to that core  505 . In instances where the flow distributor  550  executes on a single core  505 , that core can be considered the control core. In some embodiments where a RSS module  560  is included in the multi-core system  545 , the system  545  may not include a flow distributor  550 . 
     Distribution of data packets, network traffic or requests and responses can be accomplished by any of the parallel computing schemes described herein. In one embodiment, the distribution of network traffic can be based on a symmetric flow distribution. Symmetric flow distribution can be accomplished using the Toeplitz hash or any comparable hash to determine a destination core for each data packet received by the multi-core system  545 . In some embodiments, the symmetric flow distribution hash, or the symmetric hash distribution (SHD) has is substantially the same as the hash used by the RSS module  560 . The hash operates by inputting a byte stream, such as a tuple or sequence of values, and supplying the RSS driver within the RSS module  560  with a key that can be used within the hash calculation. Thus, when an array of “N” bytes is inputted into the hash function, the byte stream can be identified as input[0] input[1] input[2] input[N−1]; where the leftmost byte is input[0] and the leftmost bit is the most significant bit of input [0], and where the rightmost byte is input [N−1] and the rightmost bit is the least significant bit of input [N−1]. The hash can, in some embodiments, operate according to the following relationship:
         For all inputs up to the value “N”, calculate the following: for each bit “B” in input[ ] from left to right, if “B” is equal to one then (“Result” ̂=(leftmost 32 bits of K)) and shift K left 1 bit position, then return “Result.”
 
The hash, in some embodiments, is distributed over a XOR operation according to the following equation or relationship, Hash(A xor B)=Hash(A) xor Hash(B). In other embodiments, the hash can be distributed over any logical operation such as: NAND; NOR, OR, AND or any other logical operation functional in the methods and systems described herein.
       

     The tuple or sequence of values inputted into the hash can be a concatenation of any of the following values: client IP address; source IP address; destination IP address; local IP address; dummy IP address; assigned IP address; appliance IP address; client port; source port; destination port; local port; dummy port; assigned port; appliance port; or any other IP address or port. In some embodiments, the order of the tuple is maintained such that the tuple is a concatenation of client IP address, client port, destination IP address and destination port. The tuple can comprise two, four, six or any number of values. Additionally, the tuple can comprise any type of value, i.e. numeric, binary, trinary, alphabetic, or alpha-numeric. 
     Included below are examples of how the hash is applied in different versions of the internet protocol and when TCP or UDP is used. These examples are meant to be illustrative of applying the hash and are not meant to limit the scope of the 
     Example 1 
     IPV4: TCP/UDP 
     In this example, the tuple comprises a concatenation of the following values: source address; destination address; source port; and destination port. The tuple, or input string, can therefore be characterized by the following relationship: INPUT[12]=@12-15, @16-19, @20-21, @22-23. The entries @n-m identify a byte range, i.e. n=12, m=15, @12-15. The application of the hash to this input string is characterized by following equation: 
       Hash Result=ComputeHash(Input,12) 
     Example 2 
     IPV4: Others 
     In this example, the tuple comprises a concatenation of the following values: source address; and destination address. The tuple, or input string, can therefore be characterized by the following relationship: INPUT[8]=@12-15, @16-19. The entries @n-m identify a byte range, i.e. n=12, m=15, @12-15. The application of the hash to this input string is characterized by following equation: 
       Hash Result=ComputeHash(Input,8) 
     Example 3 
     IPV6: TCP/UDP 
     In this example, the tuple comprises a concatenation of the following values: source address; destination address; source port; and destination port. The tuple, or input string, can therefore be characterized by the following relationship: INPUT[36]=@8-23, @24-39, @40-41, @42-43. The entries @n-m identify a byte range, i.e. n=8, m=23, @8-23. The application of the hash to this input string is characterized by following equation: 
       Hash Result=ComputeHash(Input,36) 
     Example 4 
     IPV6: Others 
     In this example, the tuple comprises a concatenation of the following values: source address; and destination address. The tuple, or input string, can therefore be characterized by 
     the following relationship: INPUT[32]=@8-23, @24-39. The entries @n-m identify a byte range, i.e. n=8, m=23, @8-23. The application of the hash to this input string is characterized by following equation: 
       Hash Result=ComputeHash(Input,32) 
     In some embodiments, when the multi-core system  545  intercepts or otherwise processes data packets and/or network traffic that does not use the internet protocol, no hash is calculated. In this embodiment, the non-IP packets or traffic can be routed to a default core  505 . This core  505  can be dedicated to handling non-IP packets or can allocate a certain amount of resources to the handling and processing of non-IP network traffic. 
     Illustrated in  FIGS. 7A-7D  are block diagrams illustrating embodiments of the multi-core system  545  for distributing data packets to logical queues on the cores  505  and to transmit/receive queues on the NIC  552 . Included in the multi-core system  545  are one or more processing cores  505 A- 505 N (referred to generally as cores  505 ) executing one or more packet engines  548 A- 548 N (referred to generally as packet engines  548 .) In some aspects, each packet engine comprises at least one logical receive queue  604  and at least one logical transmit queue  602 . Included within the multi-core system  545  are a flow distributor  550  and a network interface card (NIC)  552 . In some instances, the flow distributor  550  can comprise an interface module  612 , while the NIC  552  can comprise a RSS module  560  and at least one receive queue  622  and at least one transmit queue  620 . 
     Further referring to  FIGS. 7A-7D , in some embodiments the multi-core system  545  can be any of the multi-core system  545  described herein. Thus, the multi-core system  545  can be established on an appliance  200  or a computing machine. In some embodiments, a portion of the multi-core system  545  can execute on an appliance  200 , while the other portion of the multi-core system  545  can execute on a computing machine. Execution of the multi-core system  545 , in some embodiments, can be spread out over one or more appliances  200  and/or one or more computing machines. 
     The cores  505  included in the multi-core system  545  can be any of the cores  505  described herein. Each core  505  can communicate with the other cores  505  in the multi-core system  545  via any type of inter-core communication system. The cores  505  can communicate by transmitting data packets over a multi-core system  545  network, or by copying data into a global cache or storage repository and messaging other cores download instructions. There can be any “N” number of cores  505  included in the multi-core system  545  where “N” is a whole number greater than zero. Further, the cores  505  can be any processor and any processor described herein. In some embodiments, each core  505  is an identical type of core and an identical size, however in other embodiments the core type and core size vary amongst the cores  505  included in the multi-core system  545 . 
     Executing on each core  505  can be one or more packet engines  548 . In some embodiments each core  505  executes at least one packet engine  548 , while in other embodiments a portion of the cores  505  execute at least one packet engine  548 . A packet engine  548  can be any packet engine  548  described herein. Packet engines  548  can be used to apply any of the above-described hashes to tuples of data packets received by the packet engines  548 . In some embodiments, packet engines  548  can manage the transmission of data packets from the core  505 , and receive of data packets transmitted or distributed to the core  505 . 
     In one embodiment, each packet engine  548  can comprise at least one logical receive queue  604  and at least one logical transmit queue  602 . A logical receive queue  604 , in some embodiments, can be any storage repository or memory element that stores data packets or communication data. In some instances, the logical receive queue  604  can be an array, list, matrix or other data construct for orderly storing information. The logical receive queue  604 , in one embodiment, is a table or database storing pointers to data packets stored in a memory element, or storing data packets. 
     In most embodiments, the logical receive queue  604  stores data packets received by the packet engine  548 . The received data packets can be stored in the logical receive queue  604  according to a first-in-first-out ordering, or a last-in-first-out ordering. While in most embodiments the logical receive queue  604  comprises a storage repository or memory element, in other embodiments the logical receive queue  604  can comprise a client or module that receives data packets and orders them in the logical receive queue  604  storage element according to any of the following criteria: the type of data packet; when the data packet was receives; the source IP address or port of the data packet; the destination IP address or port of the data packet; the size of the data packet; or any other criteria useful to order data packets in a logical receive queue  604 . In some embodiments, entries in a table are updated each time a data packet is written or copied into the logical receive queue  604  and each time a data packet is drained or obtained from the logical receive queue  604 . 
     The logical receive queue  604  can receive data packets from the packet engine  548  or from any other communication module executing on the core  505 . In some embodiments, the packet engine  548  writes all data packets received from the system  545  into the logical receive queue  604 . In other embodiments, each core  505  or packet engine  548  further comprises a logical NIC (Not Shown) that receives data packets transmitted to a core  505  and further stores those data packets in a logical receive queue  604 . In this embodiment, the logical receive queue  604  can be included within the logical NIC. Similarly, in some embodiments the logical NIC can be included within the packet engine  548 . 
     A logical transmit queue  602 , in some embodiments, can be any storage repository or memory element that stores data packets or communication data. In some instances, the logical transmit queue  602  can be an array, list, matrix or other data construct for orderly storing information. The logical transmit queue  602 , in one embodiment, is a table or database storing pointers to data packets stored in a memory element, or storing data packets. 
     In most embodiments, the logical transmit queue  602  stores data packets transmitted by the packet engine  548  or the core  505 . The transmitted data packets can be stored in the logical transmit queue  602  according to a first-in-first-out ordering, or a last-in-first-out ordering. While in most embodiments the logical transmit queue  602  comprises a storage repository or memory element, in other embodiments the logical transmit queue  602  can comprise a client or module that receives transmitted data packets and orders them in the logical transmit queue  602  storage element according to any of the following criteria: the type of data packet; when the data packet was receives; the source IP address or port of the data packet; the destination IP address or port of the data packet; the size of the data packet; or any other criteria useful to order data packets in a logical transmit queue  602 . In some embodiments, entries in a table are updated each time a data packet is written or copied into the logical transmit queue  602  and each time a data packet is drained or obtained from the logical transmit queue  602 . 
     The logical transmit queue  602  can receive data packets from the packet engine  548  or from any other communication module executing on the core  505 . In some embodiments, the packet engine  548  writes all data packets received from the core  505  or applications executing on the core  505  into the logical transmit queue  602 . In other embodiments, each core  505  or packet engine  548  further comprises a logical NIC (Not Shown) that receives data packets generated or modified by the core  505  and/or the packet engine  548  that are transmitted by the packet engine  548  or the core  505  to a destination and stores those data packets in a logical transmit queue  602 . In this embodiment, the logical transmit queue  602  can be included within the logical NIC. Similarly, in some embodiments the logical NIC can be included within the packet engine  548 . 
     Also included within the multi-core system  545  can be a flow distributor  550  which, in some embodiments, can be any flow distribution program, module or object. In some embodiments, the flow distributor  550  can be any of the flow distributors described herein. The flow distributor, in most cases, receives network traffic received by the multi-core system  545  and distributes the network traffic amongst the cores  505  in the multi-core system  545 . Distributing the network traffic can include extracting from a data packet information that can be concatenated to form a string or tuple. The result of applying any of the above-described hashes to the tuple tends to identify a core  505  in the multi-core system  545 . Based on the application of the hash to the tuple, the flow distributor  550  distributes data packets amongst the cores  505  in the multi-core system  545 . 
     In some embodiments, the flow distributor  550  can comprise an interface module  612  that interfaces with the logical receive queues  604  and the logical transmit queues  602  within each packet engine  548 . The interface module  612  can also interface with the transmit queue  620  and the receive queue  622  on the NIC  552 . When a data packet is received by the NIC  552 , the data packet can be stored into the NIC receive queue  622 . The interface module  612  in turn can drain the NIC receive queue  622  and distribute the received data packets to the logical receive queues  604  within each core&#39;s packet engine  548 . Thus, the interface module  612 , in some embodiments, operates in conjunction with the flow distributor  550  to obtain, drain or otherwise pull information from the NIC receive queue  622  and the logical transmit queues  602 , and write or otherwise store information into the logical receive queues  604  and the NIC transmit queue  620 . Stored and obtained information can comprise data packets and any other information stored within a transmit or receive queue. 
     While  FIGS. 7A-7D  illustrate a flow distributor  550  comprising an interface module  612 , in some embodiments, the flow distributor  550  does not contain an interface module  612 . Rather, the above-described functionality of the interface module  612  is carried out by the flow distributor  550 . Thus, the flow distributor  550  obtains, drains or otherwise pulls information from the NIC receive queue  622  and the logical transmit queues  602 , and writes or otherwise stores information into the logical receive queues  604  and the NIC transmit queue  620 . 
     Also included within the multi-core system  545  can be a NIC  552 . The NIC  552 , in most embodiments, can be any network interface card and can further be any NIC described herein. While  FIGS. 7A-7D  illustrate a multi-core system  545  having a single NIC  552 , in some embodiments, the multi-core system  545  comprises multiple NICs. Each NIC  552  can, in some embodiments, include a transmit queue  620  and a receive queue  622 . The transmit queue  620 , in some embodiments, can receive data packets from the multi-core system  545  and transmit those data packets over a network to a destination. In other embodiments, the transmit queue  620  can communicate directly with the flow distributor  550 , the RSS module  650  the interface module  612  or any other core communication module in the multi-core system  545  to receive data packets generated, modified or processed by the multi-core system  545 . 
     The receive queue  622 , in some embodiments, can receive data packets from remote computing machines and/or appliances transmitting information over the network to the multi-core system  545 . In some embodiments, when the NIC  552  receives network traffic or data packets from a network connected to the multi-core system  545 , the received data packets are stored into the NIC receive queue  622 . This receive queue  622  can be accessed or drained by the flow distributor  550 , the RSS module  560  and the interface module  612 . 
     In some embodiments, the NIC  552  further includes a RSS module  560 . The RSS module  560  can be any RSS module  560  and can further be any RSS module  560  described herein. The RSS module  560 , in some embodiments, operates substantially similar to the flow distributor  550  in that the RSS module  560  distributes data packets amongst cores  505  in the multi-core system  545  based on a hash applied to each data packet tuple. The hash can be any of the above-described hashes, and the result of the hash identifies at least one core in the multi-core system  545 . 
     While  FIGS. 7A-7D  illustrate a multi-core system  545  comprising both a NIC  552  having a RSS module  560  and a flow distributor  550 , in some embodiments, the multi-core system  545  can comprise only a flow distributor  550 . In these embodiments, the flow distributor  550  handles distributing data packets according to the application of a hash. In other embodiments, the multi-core system  545  can comprise only a NIC  552  having a RSS module  560 . In these embodiments, the RSS module  560  handles distributing data packets according to the application of a hash. 
     In some embodiments, each logical transmit queue  602  and logical receive queue  603  can have a buffer (Not Shown) storing values that can contain the check sum offload, the physical address of the packet engine  548  and similar information within which each logical queue operates. These values can be called addresses, descriptors or pseudo descriptors, and can in most embodiments be stored within the buffer associated with each logical queue. Thus, a first logical transmit queue can be associated with a buffer storing descriptors about the packet engine  548  within which the first logical transmit queue executes, and in some cases the core  505  on which the first logical transmit queue executes. These descriptors can be used by the interface module  612  to configure the NIC  552 . 
     In other embodiments, each packet engine  548  can be given both read and write access to a buffer (Not Shown) available to each component and module within the multi-core system  545 . In some embodiments, each packet engine  548  can write information that can be accessed by the interface module  612  and used to configure the NIC  552 . This information can comprise the check sum offload, the physical address of the packet engine and other similar information. 
     Further referring to  FIG. 7A , and in more detail, in one embodiment the flow distributor  550  can be included within the NIC  552 . In these embodiments, each core communicates directly with the NIC  552 . The flow distributor  550 , in some instances, facilitates communication between the cores  505  and the NIC  552  by relaying communication from the cores  505  to modules within the NIC  552 . In other instances, packet engines  548  executing on the cores  505  communicate directly with either the NIC  552 , the flow distributor  550  or the RSS module  560 . 
     Further referring to  FIG. 7B , and in more detail, in some embodiments the flow distributor  550  can be included on a single core  505  and not within the NIC  552 . This core  505  can be a first core  505 , or in some embodiments can be a control core within the multi-core system  545 . When the flow distributor  550  is included within a first core  505 , the flow distributor can interface with any of the cores  505  in the multi-core system  545  along with the NIC  552  and the RSS module  560 . 
     Further referring to  FIG. 7C , and in more detail, in some embodiments a separate flow distributor  550 A- 550 N or instance of a flow distributor  550 A- 550 N can be included on each core  505  in the multi-core system  545 . In this embodiment, each flow distributor  550 A- 550 N can interact with the packet engine  548  in that particular core  505 , as well as the NIC  552  and the RSS module  560 . 
     Further referring to  FIG. 7D , and in more detail, in some embodiments the flow distributor  550  can be included in a kernel  610  in the multi-core system  545 . The kernel  610  can be any operating system kernel. Further, the flow distributor  550  can communicate with each core  505  and each packet engine  548 , as well as the NIC  552  and the RSS module  560 . 
     Illustrated in  FIG. 8  is one embodiment of a method  800  for distributing data packets amongst cores  505  in a multi-core system  545 . In one embodiment, the multi-core system  545  receives a data packet (Step  802 ) and a flow distributor  550  or RSS module  560  identifies a tuple of the data packet (Step  804 ). After identifying the tuple, the above-described hash is applied to the identified tuple (Step  806 ) to generate a resultant value. The resultant value, in most embodiments, identifies a core  505  in the multi-core system  545 . The RSS module  560  or the flow distributor  550  transmits the data packet to the core  505  identified by the resultant hash value (Step  808 ). In some embodiments, a packet engine  548  on the selected core  505  receives the data packet and selects an IP address and port of the selected core  505  (Step  810 ). The packet engine  548  can then determine whether a hash of the selected IP address, the selected port and a portion of the tuple generates a value that identifies the selected core  505 . When it is determined that the value generated by the hash applied to the above-mentioned tuple identifies the selected core  505 , the packet engine  548  modifies the tuple with the selected IP address and port (Step  814 ). Upon modifying the tuple, the packet engine  548  or another module executing on the selected core  505  forwards the modified data packet to a remote computing machine (Step  816 ). 
     Further referring to  FIG. 8 , and in more detail, in one embodiment a NIC  552  in the multi-core system  545  receives one or more data packets transmitted to the multi-core system  545  over a network  104  (Step  802 ). In one embodiment, a flow distributor obtains data packets from the NIC  552 . In other embodiments, a RSS module  560 , packet engine  548  or other distribution module or program drains or otherwise obtains data packets from the NIC  552 . The flow distributor can drain or obtain data packets from a receive queue on the NIC  552 . 
     Once the flow distributor  550  receives the data packets, the flow distributor or a distribution module can identify a tuple of the data packet (Step  804 ). The tuple, in some embodiments, can comprise any combination of the following values: a client IP address; a destination IP address; a client port; a destination port; or any other IP address, port or other source or destination identifying value. The client IP address, in some embodiments, can be a source IP address. Similarly, the client port, in some embodiments, can be a source port. Identifying a tuple of the data packet can, in some embodiments, comprise generating the tuple by concatenating any of the above-mentioned values to create a string. The tuple, in some embodiments, is a string or array of values. 
     A hash or hash value is, in some embodiments, calculated by applying the above-described hash to the identified tuple (Step  806 ). The hash value can be referred to by any of the following designations: hash; hash value; result value; result; or value. The hash can be applied by the flow distributor  550  or by any other distribution module executing within the multi-core system  545 . 
     After applying the hash, a determination can be made as to whether the resultant value identifies a core  505  in the multi-core system  545 . When the hash result identifies a particular core  505 , the data packet is forwarded to the identified core  505  by the flow distributor  550  or by any other flow distribution module (Step  808 ). In some embodiments, the hash result may not identify a core  505  within the multi-core system  545 . In these embodiments, the data packet can be forwarded to a default core  505  in the multi-core system  545 . In still other embodiments, the data packet may not have an associated tuple. In those embodiments, the data packet can be forwarded to a default core  505  in the multi-core system  545 . 
     Upon forwarding the data packet to the identified core  505 , a packet engine  548  or other module or engine executing on the identified core  505  can receive the forwarded data packet. In some embodiments, a communication module executing on the identified core  505  receives the data packet and forwards the data packet to a packet engine  548  on the identified core  505 . Once the packet engine  548  receives the forwarded packet, the packet engine  548  can select an IP address of the core  505  and a port of the core (Step  810 ). This IP address, in some embodiments, can be an IP address of the multi-core system  545  or an IP address of the appliance  200 . In other embodiments, the IP address can be an IP address of the core  505 . The core  505  can have one or more IP addresses, therefore in some embodiments the packet engine  548  can select an IP address based on a determination as to whether the IP address combined with a selected port and a portion of the first tuple identifies the identified core  505 . 
     Selecting a port of the core  505  can include searching through ports associated with the selected core  505  to identify a port that when included in the first tuple, identifies the selected core  505 . In some embodiments, the packet engine  548  can iterate through each IP address of the core  505  and each port of the core  505  to identify an IP address/port combination that identifies the selected core  505 . For example, the selected core  505  can be a first core  505  having a tuple comprising a client IP address, a client port, a destination IP address and a destination port. The packet engine  548  can modify the tuple to include a selected IP address, a selected port, the destination IP address and the destination port. Before permanently modifying the data packet, the packet engine  548  first applies the above-described hash to the modified tuple (Step  812 ). If the resultant hash value identifies the first core  505 , then the packet engine  548  permanently modifies the data packet to replace or change the client IP address to the selected IP address, and replace or change the client port to the selected port. If the resultant hash value does not identify the first core  505 , then the packet engine  548  changes either or both of the selected IP address and the selected port, and applies the hash again. 
     After applying the above-described hash (Step  812 ) to verify that the selected IP address and the selected port, when combined with the destination IP address and the destination port, identify the selected core  505 , the packet engine can then modify the data packet so that the tuple comprises: the selected IP address; the destination IP address; the selected port; and the destination port (Step  814 ). In this embodiment, the client IP address and the client port are not longer included within the tuple. Rather, these values have been replaced by the selected IP address and the selected port. 
     The packet engine  548 , in many embodiments, transmits the updated data packet and tuple to a remote computing device (Step  816 ) after modifying the data packet and tuple. In some embodiments, the remote computing device can be a client, a server or another computing machine or appliance located remote from the multi-core system  545 . In other embodiments, the packet engine  548  can transmit the modified data packet to an intermediary device which forwards the data packet to a destination location. The destination location, in some embodiments, is identified by the destination IP address and/or the destination port. 
     Illustrated in  FIG. 9  is an embodiment of a method  900  for distributing network traffic amongst cores  505  in a multi-core system  545 . The method  900  described in  FIG. 9  illustrates how a packet engine  548  on a core  505  handles a received data packet. The packet engine  548  receives an allocated data packet (Step  902 ) and selects an IP address of the core  505  on which the packet engine  548  executes (Step  904 ). The packet engine  548  also selects a port from a plurality of ports on or of the core  505  (Step  906 ). Once an IP address and port are selected, the packet engine  548  then determines whether a hash of the selected IP address and the selected port together with a destination IP address and destination port, will identify the current core  505 . In particular, the packet engine  548  determines whether the selected port will identify the current core  505  (Step  908 ). When it is determined that the selected port will not identify the current core  505 , the packet engine  548  selects the next port from amongst the ports associated with the core  505  (Step  906 ). When it is determined that the selected port will identify the current core  505 , the packet engine  548  next determines whether the selected port is open or otherwise available (Step  910 ). When it is determined that the selected port is not open, the packet engine  548  selects the next port from amongst the ports associated with the core  505  (Step  906 ). When it is determined that the selected port is open or otherwise available, the packet engine  548  modifies the data packet with the selected IP address and the selected port (Step  912 ) and forwards the data packet and its modified tuple to a remote computing machine (Step  914 ). 
     Further referring to  FIG. 9 , and in more detail, in one embodiment the method  900  can be carried out by a packet engine  548  executing on a core  505 . In another embodiment, the method  900  can be carried out by a flow distributor  550  or instance of a flow distributor executing on the core  505 . In still other embodiments, the method  900  can be carried out by any flow distribution module or agent that may execute on the core  505 . While  FIG. 9  contemplates processing a data packet that can be modified in part on a particular core  505 , modification of the data packet can be handled, in some embodiments, by a control core in the multi-core system  545 . 
     The packet engine  548  carrying out the steps of the method  900  described in  FIG. 9  can execute on a particular core  505 . The core  505 , in most embodiments, is selected ahead of time by the method  800  illustrated in  FIG. 8 . Therefore in most instances, the data packet received by the packet engine  548  has been allocated to the core  505  based on the application of an above-described hash to a tuple of the data packet. This tuple, in most cases, comprises at least a client IP address, a destination IP address, a client port and a destination port. In some embodiments, the tuple can be any of the above described tuples and can comprise any number of source or destination identifying values. In still other embodiments, the client IP address can be a source IP address identifying the machine from which the data packet originated. Similarly, the client port can be a source port. 
     In one embodiment, a packet engine  548  executing on a particular core  505  in the multi-core system  545 , receives data packets allocated to that particular core  505  (Step  902 ). The packet engine  548  can directly receive data packets, or in some embodiments, a communication module executing on the core  505  can receive and transmit data packets. In other embodiments, a virtual NIC (Not Shown) executing on the core  505  can receive and transmit data packets. Receiving data packets, in some embodiments, can further comprise draining data packets from a logical receive queue on the core  505 . A logical receive queue can store data packets transmitted to a core  505 . The packet engine  548  can access data packets in the logical receive queue by draining or otherwise obtaining the data packets from the receive queue according to a first-in-first-out method of access. Another possible method of access can be first-in-last-out. 
     When a packet engine  548  obtains a data packet, the packet engine  548  can in some embodiments determine whether the data packet can be modified. The packet engine  548 , after determining what portions of the data packet can be modified, can modify the data packet. In some embodiments, the multi-core system  545  can be configured to instruct packet engines  548  executing within the multi-core system  545  to modify only certain portions of the data packet. 
     In some embodiments, the packet engine  548  can select an IP address of the core  505  from amongst one or more IP addresses associated with the core  505  (Step  904 ). The core  505  can have multiple IP addresses, and in some embodiments can have a range of IP addresses. In other embodiments, the core  505  can have a single IP address. While in some embodiments the packet engine  548  selects an IP address of the core  505 , in other embodiments the packet engine  548  can select an IP address of the multi-core system  545  or an appliance  200  in the multi-core system  545 . 
     Once the IP address is selected, the packet engine  548  can then select a port from amongst a plurality of ports of the core  505  (Step  906 ). The core  505  can have one or more ports, and in some embodiments can store in a port allocation table a listing of each of the ports  505  of a multi-core system  545 . Selecting a port can comprise cycling through the entries of a port allocation table listing each of the ports of a core  505  and selecting a port number. The ports can be cycled through numerically based on port number or based on the order in which the ports are listed in the port allocation table. In other embodiments, the packet engine  548  can select a port by cycling through a range of numbers or values corresponding to possible port numbers on the core  505 . 
     In some embodiments, the packet engine  548  can select a first port (step  906 ) and then determine whether that port is the correct port (Step  908 ) and whether that port is available or open (step  910 ). If the selected first port is either not the correct port or not available or open, the packet engine  548  can select the a next port, i.e. a second port of the core  505 , and again determine whether that port is the correct port (Step  908 ) and whether that port is available or open (Step  910 ). In most embodiments, the packet engine  548  cycles through all possible ports until the packet engine  548  identifies a port that is both the correct port and an open port. 
     Once the packet engine  548  selects a port, the packet engine first determines whether the selected port is the correct port by determining whether the selected port will cause a response packet to return to the selected core (Step  908 ). This determination can be made by applying the above-described hash to a tuple comprised of a concatenation of the following values: the selected IP address; the destination address; the selected port; and the destination port. Applying the above-described hash to this tuple generates a resultant hash value that either identifies or does not identify the core  505  on which the packet engine  548  is currently executing. Concatenating the tuple values to generate the tuple can be carried out by the packet engine  548  or by an instance of a flow distributor  550  executing on the core  505 . Similarly, applying the hash to the tuple can be carried out by the packet engine  548  or by an instance of a flow distributor. When the resultant hash value identifies the current or selected core  505 , the selected port is the correct port because it will cause a response packet to return to the current core  505 . When the resultant hash value does not identify the current or selected core  505 , the selected port is not the correct port because it will not cause a response packet to return to the current core  505 . In this situation, the packet engine  548  will select another port (Step  906 ) and reiterate the process of determining whether the port is the correct port (Step  910 ). 
     When it is determined that a selected port is the correct port (Step  908 ), a determination is then made as to whether the port is available or open (Step  912 ). In most embodiments, a port is open or available when any of the following is true: the port is not being used; or the port is available for use. In contrast, a port is not open or available when any of the following is true: the port has been assigned to another transaction, service or data packet; or the port has been closed either by a network administrator or by the multi-core system  545 . Whether a port is available or open, is a characteristic that in many embodiments is tracked by a port allocation table. The port allocation table can any of the above-mentioned port allocation tables and can be stored in any of the above-mentioned locations that a port table can be stored. In some embodiments, after the packet engine  548  determines that the port is the correct port, the packet engine  548  can determine that the port is available by querying a port allocation table for the details, attributes or characteristics of a particular port. When the response indicates both that the port is open and that the port has not been assigned to any other data packet, transaction, or server, then the packet engine  548  modifies the tuple with the selected IP address and the selected port. However, when the response indicates that the port is either not available or not open, the packet engine  548  selects another port (Step  906 ) and repeats the process of determining whether the port is the correct port (Step  908 ) and whether the port is open and available (Step  910 ). 
     When a correct, open and available port is selected by the packet engine  548 , the packet engine  548  then updates the data packet and therefore the tuple of the data packet to include the selected IP address and the selected port (Step  912 ). Modifying or updating the tuple can comprise making any modification necessary to cause the tuple to comprise: the selected IP address; the destination IP address; the selected port; and the destination port. Thus, the client IP address and the client port information can be replaced by the selected IP address and the selected port. 
     After modifying the data packet, the packet engine  548  can transmit the modified data packet to a remote computing machine (Step  914 ). Transmitting the modified data packet to a remote computing machine can comprise transmitting the modified data packet to a client, server, appliance, or computing machine identified by the destination IP address and/or the destination port. In some embodiments, the modified data packet is transmitted to a proxy server or appliance before the data packet is transmitted to its destination computing machine or appliance. In other embodiments, the modified data packet is stored in a memory element within the multi-core system  545  before the data packet is transmitted to its destination computing machine or appliance. The memory element, in some embodiments, can be a global cache or other memory element shared by all cores and devices in the multi-core system  545 . In other embodiments, the memory element can be a cache or other storage repository accessible by the current core  505 . 
     While  FIGS. 8 and 9  describe methods where the client IP address and the client port are modified or replaced by an IP address and port selected by a packet engine  548  on a particular core  505 ,  FIG. 10A  describes a system where the client IP address and the client port are maintained. In some systems, the owner of a server farm or the administrator of a network within which the multi-core system  545  executes can desire that each data packet retain its original source IP address and source port. An administrator may want to do this for any number of reasons, some of those reasons can include for security purposes, for marketing purposes, to track network access, to restrict network access, or for any other reason. By permitting each data packet to retain its source IP address or source port, each data packet can be tracked and controlled. For example, knowing the source of a data packet can permit the system to prevent particular IP addresses or domains from accessing a network. Similarly, knowing the source of a data packet can permit the system to track the geographic location of users accessing the network or domain. In most cases, knowing the source IP address and source port allows a system to identify the location of where a packet originates and to further control whether or not a particular data packet is processed by a system. 
     Illustrated in  FIG. 10  is one embodiment of a method  1000  for obtaining data packets from logical transmit queues and forwarding them to a logical transmit queue on the NIC  552 . Although the method  1000  is described as being carried out by the interface module  612 , the method  1000  can also be carried out by either the RSS module  560  or the flow distributor  550 . A first data packet is obtained from a first logical transmit queue on a first core (Step  1002 ) and a second data packet is obtained from a second logical transmit queue on a second core (Step  1004 ). The interface module  612  can store the first data packet in the NIC transmit queue  620  (Step  1006 ) after which the NIC  552  can transmit the first data packet from the multi-core system  545  over the network (Step  1008 ). The interface module  612  can then store the second data packet in the NIC transmit queue  620  (Step  1010 ) after which the NIC  552  can transmit the second data packet from the multi-core system  545  over the network (Step  1012 ). 
     Further referring to  FIG. 10 , and in more detail, in one embodiment the interface module  612  can obtain a first data packet from a first logical queue of a first core (Step  1002 ) and can obtain a second data packet from a second logical queue of a second core (Step  1004 ). Obtaining data packets from logical transmit queues on the cores  505  can comprise polling each logical transmit queue for data. In one embodiment, the interface module  612  polls each logical queue in a round-robin fashion for a predetermined period of time. For example, every millisecond the interface module  612  can poll a first logical transmit queue by determining whether a storage repository or buffer associated with the first logical transmit queue is empty. Making this determination can, in some embodiments, comprise copying the contents of the first logical transmit queue into a temporary buffer and determining whether the elements of the buffer are equal to null. In other embodiments, the interface module  612  can search through an array or buffer of the first logical transmit queue to determine whether the buffer is null. When the interface module  612  determines that the first logical queue has data, the interface module  612  downloads the stored data packets and forwards them to the NIC transmit queue  620  (Step  1006 ) after which they are transmitted out to a destination address in the data packet. After the interface module  612  downloads the data packets from the first logical queue, and/or when the interface module  12  determines that the first logical queue is empty, the interface module  612  moves to a second logical transmit queue and repeats the above-described process. 
     In some embodiments, the logical transmit queues are polled round-robin. In other embodiments, the logical transmit queues are polled batch round-robin. In still other embodiments, the logical transmit queues are polled according to an order dictated by any one of the following sets of criteria: a priority associated with each core, or logical transmit queue; the amount of time since the last time data packet packets were drained from a logical transmit queue; a function associated with the core on which the logical transmit queue is stored; a level of activity associated with each core  505 ; a probability distribution indicating logical transmit queues most likely to have stored data packets; votes submitted by packet engines when a data packet is stored in a logical transmit queue, where the votes can be addressed in a first-in-first-out, or last-in-first-out manner; or an arbitrary order dictated by an administrator, user, program or system. 
     While the method  1000  describes obtaining a first and second data packet, the method  1000  can comprise obtaining any number of data packets. Further, the method  1000  can obtain any number of data packets at a time. In one embodiment, the interface module  612  can drain the entire logical transmit queue, while in other embodiments the interface module  612  can drain a predetermined amount of data or number of data packets. Still further, in some embodiments, the interface module  612  can drain the logical transmit queue for a predetermined period of time. 
     Once a data packet is obtained from a logical transmit queue, that data packet is stored in the NIC transmit queue  620  (Step  1006 ,  1010 ). Storing a data packet in a NIC transmit queue  620  can comprise writing the data packet to a memory element in the NIC  552  corresponding to the NIC transmit queue  620 . The NIC transmit queue  620  can comprise any of the following: a memory element; a buffer; an array; a list; a cache; or any other storage repository. Thus, when storing a data packet into the NIC transmit queue  620 , the interface module  612  stores the data packet into a storage repository associated with the NIC transmit queue  620 . 
     In some embodiments, storing a data packet into the NIC transmit queue  620  can further comprise downloading configuration information associated with a packet engine, and using the configuration information to configure the NIC  552  prior to storing one or more data packets in the NIC transmit queue  620 . Obtaining or otherwise downloading the configuration information can include draining the configuration information from a buffer associated with the logical transmit queue from which the data packet is obtained. In other embodiments, obtaining the configuration information can include copying the configuration information from a common buffer accessible by both the interface module  612  and each of the packet engines  548 . The configuration information can comprise a check sum offload, a physical address of the packet engine, and any other such information. 
     When the interface module  612  obtains the configuration information, the interface module  612  can configure the NIC  552  prior to copying a data packet into the NIC transmit queue  620 . Configuration information can vary from packet engine to packet engine, therefore the interface module  612  reconfigures the NIC  552  each time the interface module  612  copies in information from a new core  505 . For example, the interface module  612  can obtain configuration information associated with the first logical transmit queue, configure the NIC using that information and then store the first data packet into the NIC transmit queue  620  (Step  1006 ). Similarly, the interface module  612  can obtain configuration information associated with the second logical transmit queue, configure the NIC using that information and then store the second data packet into the NIC transmit queue  620  (Step  1010 ). 
     After a NIC  552  has been configured and a data packet has been stored into the transmit queue  620 , the NIC  552  can transmit the data packet to a destination (Step  1008 , Step  1012 ). The destination address or location can be obtained by the NIC  552  from the data packet header or from metadata associated with the data packet. Upon obtaining this information, the NIC  552  can transmit the data packet from the multi-core system  545  over the network. 
     Illustrated in  FIG. 11  is an embodiment of a method  1100  for distributing data packets received by a multi-core system  545  to logical receive queues  602  within the multi-core system  545 . A NIC  552  can receive both a first data packet and a second data packet (Step  1102 ) and store both data packets in a NIC receive queue  622  (Step  1104 ). After the data packets are stored in the NIC receive queue  622 , they are obtained by the interface module  612  which applies a hash to a tuple of each data packet (Step  1106 , Step  1110 ). The results of applying this hash identify a core  505  in the multi-core system  545 , i.e. a hash of a tuple of the first data packet identifies the first core (Step  1106 ), while a hash of a tuple of the second data packet identifies the second core (Step  1110 ). Once the cores have been identified, the data packets are stored in their respective logical receive queue, i.e. the first data packet is stored in the first logical receive queue of the first core (Step  1108 ), and the second data packet is stored in the second logical receive queue of the second core (Step  1112 ). 
     Further referring to  FIG. 11 , and in more detail, in one embodiment the NIC receives a first data packet and a second data packet (Step  1102 ) and stores them in a NIC receive queue  622  (Step  1104 ). Storing the first and second data packet in a NIC receive queue  622  can comprise writing the data packets to a memory element in the NIC  552  corresponding to the NIC receive queue  622 . The NIC transmit queue  622  can comprise any of the following: a memory element; a buffer; an array; a list; a cache; or any other storage repository. Thus, when storing a data packet into the NIC receive queue  622 , the data packet can be stored into a storage repository associated with the NIC receive queue  622 . 
     Once the data packets have been stored in the NIC receive queue  622 , the interface module  612  can drain the data packets from the NIC receive queue  622 . In other embodiments, any one of the following modules or objects can drain the receive queue  622  of stored data packets: the flow distributor  550 ; the RSS module  560 ; or any of the packet engines  548 . The interface module  612  can periodically drain the receive queue  622 , or can drain the receive queue  622  when the interface module  612  determines that the receive queue  622  contains data packets. In some embodiments, when a first data packet is written into the NIC receive queue  622 , a flag can be set indicating that the receive queue  622  contains un-allocated data packets. In other embodiments, the interface module  612  can drain the receive queue  622  on a continual basis. In still other embodiments, a module on the NIC  522  can transmit received data packets to the interface module  612  as they are received by the NIC  522  and stored into the NIC receive queue  622 . 
     In one embodiment, the interface module  612  can obtain data packets from the NIC receive queue  622  one at a time. In other embodiments, the interface module  612  can drain a predetermined number of data packet, a predetermined amount of data, or can continued to drain data packets from the NIC receive queue  622  for a predetermined length of time. In still other embodiments, the interface module  612  can drain all data packets corresponding to a common tuple, wait a predetermined period of time, and drain a second set of data packets corresponding to a second common tuple. 
     Once the interface module  612  obtains the data packets from the NIC receive queue  622 , either the interface module  612 , the flow distributor  550  or the RSS module  560  can apply any of the above-described hashes to a tuple of the first data packet (Step  1106 ) and a tuple of the second data packet (Step  1110 ). The tuples can be any of the above-described tuples and in some embodiments can comprise: a source IP address; a destination IP address; a source port; and a destination port. The result of applying the hash to a tuple of the first data packet can identify the first core (Step  1106 ). Similarly, the result of applying the hash to a tuple of the second data packet can identify the second core (Step  1110 ). 
     Once a core has been identified, the interface module  612  can store each data packet in their respective logical queue. The first data packet can be stored in a first logical receive queue on the first core (Step  1108 ), while the second data packet can be stored in a second logical receive queue on the second core (Step  1112 ). Storing the data packets in a logical receive queue can comprise copying, storing or otherwise writing the data packet data to a buffer, cache, array, list or other storage repository comprising the logical receive queue. 
     G. Systems and Methods for A Para-virtualized NIC Drivers 
     Referring now to  FIGS. 12A and 12B , systems and methods for providing or implementing para-virtualized drivers are depicted. As described above in connection with at least  FIGS. 4A-4C , embodiments of the packet engine or application delivery controller may operate in a virtual environment, such as the virtualized ADC  450  in the virtualized environment  400  of  FIG. 4C . A packet engine or ADC operating in a virtualized environment may be referred to as a VPX. Furthermore, in view of the embodiments of the multi-core systems of  FIGS. 5A-5C , any of the virtualized embodiments may operate on a plurality of cores. This environment may be referred to a multi-core or nCore VPX. Embodiments of network communication management and processing in such a multi-core system are described above in connection with  FIGS. 7-11 . 
     Referring to  FIG. 12A , an embodiment of a VPX operating on a processor or core of a device is depicted. In some embodiments, the VPX may operate on an hypervisor, such as a Xen Hypervisor, using para-virtualized NIC drivers to receive and transmit packets. In some embodiments, the para-virtualized driver is designed and constructed to support a multi-core VPX environment. In certain embodiments, a portion of the driver operates in a virtualization domain executing on a core and another portion of the driver operates in another virtualization domain executing on another core. In other embodiments, one of a plurality of drivers may operate in a virtualization domain executing on a core while another driver operates in another virtualization domain executing on another core. The drivers or portions of the drivers may interoperate directly or indirectly to facilitate reception and/or transmission of packets. 
     In brief overview, a hypervisor may operate on a processor of a device, such as any single processor appliance described herein. The hypervisor may provide one or more guest operating systems, referred to as domains or virtualization domains. A first domain, such as Domain  0  referred to as Dom 0  may provide a privileged domain which can access hardware (e.g., network interface hardware, such as a NIC) directly. DomU may be a guest domain with less privileges. The VPX operating on the DomU domain may provide a packet processing engine (e.g., application). 
     I/O operations on non-virtualized platforms are generally performed on a hardware device (e.g., NIC or GPU). A software stack may interface with the hardware device using some form of memory-mapped I/O (MMIO) mechanism, whereby a processor may issue instructions to read and write specific memory or port address ranges. Such a software stack may comprise a driver executing in an OS of the platform. The values read and written may correspond to direct functions in the hardware device. Dom 0  may provide or host a TCP/IP stack with access to real NIC drivers. The TCP/IP stack may provide routing and bridging functionality. DomU may provide or host an application with a TCP/IP stack, such as any embodiment of a packet processing engine (PPE) described herein. 
     A paravirtualized driver may be selected for operation in place of an original or real device driver corresponding to a hardware. In some embodiments, the paravirtualized driver  377  is a modified version of the device driver. With paravirtualization, the para-virtualized driver may interact directly with a domain  0  (e.g., Dom 0 ) entity, for example, at a higher abstraction level than the normal hardware/software interface. The domain  0  entity may expose an I/O type-specific API, for example, to send and receive packets. The paravirtualized driver executing on a guest OS may then use this I/O interface instead of interacting directly with a hardware device interface. 
     Dom 0  and DomU may employ or support a split device driver model. One portion of the driver (e.g., paravirtualized driver), such as a front end and another portion of the driver, such as a back end of the driver may be isolated from each other in separate domains, and may communicate by mechanisms provided by the hypervisor. DomO may operate one portion of the split driver while DomU operates the other portion of the driver. In some embodiments, each of the domains or Dom operates the same driver or type of driver. The split driver may communicate via a shared memory interface or other mechanism(s) provided by the hypervisor. The driver may comprise any embodiments of the packet engine described herein or any components thereof. In some embodiments, one of the mechanisms may include an I/O ring or queue built on top of a shared memory segment provided by the hypervisor. 
       FIG. 12A  illustrates embodiments of the flow of the receive and transmit, RX/TX, communications of the VPX PPE executing or running in DomU. DomU may communicate with Dom 0  for packet transfers. DomU may share the RX and/or TX rings with Dom 0 . The communication between the domains may occur via the Hypervisor through Hypervisor calls. During initialization of the driver, DomU may fill the RX ring with data from memory buffers, sometimes referred to as mbufs. DomU may establish a notification channel with Dom 0 . The domains may use this notification channel to indicate the reception and/or transmission of the packets on the shared RX and TX rings. 
     In further details of a packet reception (RX), packets may be received by Dom 0  from one or more physical NICs. Dom 0  may transmit these packets to DomU via a shared RX ring. A ring may sometimes be referred to as a queue, buffer, cache or other memory structure. A ring or queue may be allocated a certain amount memory and may comprise one or more data structures for holding information or packets. Dom 0  may send a notification to DomU indicating the packet is on the shared ring. A ring may comprise any buffering, storage or conveyance structure and/or mechanism, such as a queue. DomU may get this notification. In some embodiments, DomU may not take any action with respect to this notification and may simply return (e.g., a response or the notification). During the DomU&#39;s net I/O loop (e.g., netio loop), DomU may poll, via the split-driver, the RX ring to check for reception of any packets. This check may be based on a detection of a difference in read and write descriptors. For example, the number of read descriptors allocated, used or processed may be compared to the number of write descriptors allocated, used or processed. In some embodiments, DomU detects if there is a difference between the number of packets read from the RX ring and the number of packets written onto the RX ring. If the difference shows the presence of some packets, DomU, via the split driver model, may read some or all of the packets from the RX ring. DomU may then convert these packets from memory buffer (e.g., mbuf data structure) to packet engine data structures, sometimes referred to as nsb. DomU may move the nsb to the stack for processing. DomU, via the split driver model, may also fill the RX ring with fresh mbufs. 
     In further details on packet transmission, the Packet Processing Engine may transfer or hand over the nsbs to the driver. The driver may then convert them to mbufs and transmit the mbufs to Dom 0  through a shared TX ring. DomU may also send a notification to Dom 0  indicating the presence of a (e.g., new or unread) packet on the shared ring. Dom 0  may reclaim or obtain the transmitted mbufs by calling a function (e.g., txproc function or routine) in its netio (e.g., net, network or network interface input/output) loop. Referring now to  FIG. 12B , the RX/TX communication of VPX in a multi-core environment is depicted. Each of a plurality of packet engines may be running in a corresponding domain, such as DomUl-N. In some embodiments, a plurality of PPEs may execute in one or more domains. The interaction between DomU and Dom 0  may occur via Hypercalls or hypervisor system API calls, which may only be made in the kernel memory space. In some embodiments, the packet transfer between DomU and Dom 0  happens in substantially the same way as explained in  FIG. 12A . 
     In further details, packets may be received by Dom 0  from physical NICs which may be transmitted to DomU via a shared RX ring. Dom 0  may also send a notification to DomU indicating that the packet is on the shared ring. DomU may receive this notification, such as in kernel space. The notification handler may compute the RSS hash on the incoming packet and may put these mbufs on corresponding PPE ring maintained in the kernel space. During a netio loop, the PPE may poll the RX ring to check for the reception of any packet. This check may be based on the difference in shared read and write descriptors. If the difference shows the presence of some packets, the PPE may make a system call to draw out or obtain the packets from the RX ring. Before making a system call, the PPE may allocate (e.g., read index—write index) number of nsbs and may copy the mbufs into nsbs in kernel memory space. The PPE may also fill the RX ring with fresh mbufs. Each PPE may have its own queue inside the kernel memory space and may access the queues independently without applying locks (e.g., to prevent access). 
     For transmission, a PPE may accumulate the transmitted nsbs in the TX ring maintained in user memory space. During the PPE&#39;s netio loop, if there are nsbs present in the TX ring, the PPE may make a system call to send some or all of these packets to Dom 0 . Inside the kernel space, these nsbs may be converted to mbufs and transmitted to Dom 0  through a shared TX ring. The PPE/DomU may also send a notification to Dom 0  indicating the presence of packets on the shared ring. In some embodiments, both RX and TX are done in the same system call. 
     Illustrated in  FIG. 12C  is one embodiment of a method for communicating packets between network interface hardware of a multi-core device and a plurality of virtualized packet processors executed by one or more cores of the multi-core device. A first virtualization domain executing on a multi-core device may receive a packet via a network interface hardware (e.g., NIC) of the multi-core device (Step  1201 ). The first virtualization domain may comprise a privileged domain having direct access to the network interface hardware. The system may communicate the packet to a queue for a virtualized packet processor (Step  1203 ). The virtualized packet processor may be from a plurality of virtualized packet processors and executing within a second virtualization domain on a core of the multi-core device. The second virtualization domain may be from a plurality of virtualization domains not having direct access to the network interface hardware. The virtualized packet processor may determine that the queue includes a difference between a number of packets read from the queue and a number of packets written to the queue (Step  1205 ). The virtualized packet processor may process the packet from the queue responsive to the determination (Step  1207 ). 
     Further referring to  FIG. 12C , and in more detail, a first virtualization domain executing on a multi-core device may receive a packet via a network interface hardware of the multi-core device (Step  1201 ). The virtualization domain (e.g., first virtualization domain) may comprise a privileged domain having direct access to the network interface hardware. This virtualization domain may sometimes be generally referred to as Dom 0 , a Domain  0  entity, a control domain, or a control virtual machine. This virtualization domain may include or execute a control program and/or drivers for accessing the network interface hardware (e.g., a network interface card). In some embodiments, the virtualization domain executes one or more para-virtualized drivers or portions thereof. The virtualization domain may execute the one or more drivers in user memory space of the multi-core device. 
     The virtualization domain may support or employ a split driver model for communicating and/or processing packets. In these embodiments, the virtualization domain may execute a split driver that interacts with the network interface hardware (e.g., via the TCP/IP stack). The split driver may intercept, receive, redirect and/or communicate packets received by the network interface hardware. In some embodiments, the split driver intercepts or receives a packet at a layer of the TCP/IP protocol stack. The virtualization domain may have privileged access to the network interface hardware in part because of the split driver. In certain embodiments, the split driver in the virtualization domain is configured to have access to the network interface hardware and/or the TCP/IP stack for conveying a received packet. The virtualization domain, or any domain  0  entity (e.g., the hypervisor, or a control program) may establish and/or configure the split driver in conjunction with one or more other split drivers in one or more other virtualization domains (e.g., DomU). A set of split drivers may have complementary functions, and in some cases, may each comprise portions or features of a device driver. In certain embodiments, a set of two or more split drivers may be collectively referred to as a single driver. 
     The system may communicate the packet to a queue for a virtualized packet processor (Step  1203 ). The split driver may communicate the packet to kernel space for forwarding to a virtualized packet processor executing in another virtualized domain. Thus virtualized packet processor may be from a plurality of virtualized packet processors. The virtualized packet processor may be executing within another (e.g., a second) virtualization domain on another core of the multi-core device. In some embodiments, the virtualization domain (e.g., DomU or second virtualization domain) executes the virtualized packet processor in user memory space of the multi-core device. In other embodiments, the virtualization domain may execute the virtualized packet processor in kernel space or any memory space of the multi-core device. The virtualized packet processor may sometimes be referred to as a packet processing engine, packet engine or PPE. 
     In some embodiments, the virtualization domain may not have direct access to network interface hardware. For example, the virtualization domain may not have memory-mapped access to the network interface hardware of the multi-core system. This virtualization domain may be from a plurality of virtualization domains not having direct access to the network interface hardware. In some embodiments, the network interface hardware may be allocated or assigned to another virtualization domain (e.g., DomU), and may not be accessible to the present virtualization domain. In certain embodiments, the present virtualization domain is prevented from, or not configured for direct network input/output operations. This virtualization domain may execute a driver, such as a split driver, that does not have access to the network interface hardware. 
     The privileged or first virtualization domain (e.g., Dom 0 ), its control program, and/or its split driver may direct or place a received packet into a shared queue. The virtualization domain may issue or generate a function call, system call or hypervisor call to initiate transfer of the received packet from user memory space to the shared queue. The shared queue may reside in kernel space of the multi-core device. The privileged virtualization domain may place the packet into the shared queue in shared memory space prior to communicating the packet to a queue of the virtualized packet processor. The shared queue may reside in a shared memory segment of the multi-core system. The multi-core system and/or a domain  0  entity may establish the shared queue and/or the queue of the virtualized packet processor in kernel memory space of the multi-core device accessible by the privileged (e.g., first) virtualization domain and the non-privileged (e.g., second) virtualization domain. 
     The multi-core system and/or a domain  0  entity may assign a RX queue residing in kernel space to the virtualized packet processor. In some embodiments, the multi-core system and/or a domain  0  entity may assign each of the plurality of virtualized packet processors with a corresponding, dedicated queue in kernel memory space of the multi-core device. The multi-core system may include a receive side scaling (RSS) module and/or flow distributor, such as embodiments of RSS modules and/or flow distributors described above in connection with at least  FIGS. 5A-C ,  6  and  7 A-D, for redirecting packets on the shared queue. The RSS module or flow distributor may direct packets on the shared queue to a queue assigned to DomU or the virtualized packet processor. In certain embodiments, a RSS module or flow distributor directly assigns a received packet from Dom 0  to an assigned RX queue. The RSS module or flow distributor may direct or assign packets to a particular queue based on any type or form of policy or rule, for example, as described above in connection with at least  FIGS. 5A-C ,  6  and  7 A-D. As an example, the RSS module may recognize that a received packet is associated with a plurality of packets handled by a particular virtualized packet processor. 
     In some embodiments, the split driver in Dom 0  may generate and/or send a notification upon receiving and/or placing the packet on the shared queue. The split driver may send this notification to the RSS module, flow distributor and/or virtualized packet processor. In certain embodiments, the RSS module, flow distributor and/or virtualized packet processor may receive or intercept the notification. A RSS module may, for example, include a notification handler that computes or determines a RSS hash on the received packet. Based on the RSS hash, the RSS module may direct or assign the received packet to a corresponding queue assigned to a virtualized packet processor. 
     In some embodiments, the assigned queue comprises a locking mechanism for restricting or limiting its access to the corresponding virtualization domain and/or virtualized packet processor. In other embodiments, the assigned queue does not include, or may not require, a locking mechanism. The RSS module or flow distributor may populate or store the received packet in an available mbuf data structure of the queue. In some embodiments, if no mbufs are available, or if the queue is filled, the RSS module or flow distributor may send a notification to the corresponding virtualized packet processor. In certain embodiments, the RSS module, flow distributor and/or queue may send a notification to the corresponding virtualized packet processor responsive to receiving and/or storing the packet in the queue. In some embodiments, the virtualized packet processor may or may not initiate a read of the queue responsive to the notification. 
     The virtualized packet processor may determine that the queue includes a difference between a number of packets read from the queue and a number of packets written to the queue (Step  1205 ). The virtualized packet processor may determine that the queue includes a difference between a read indicator and a write indicator of the queue. Such a read indicator may identify or indicate how recently the queue was read (e.g., a timestamp) and/or the extent of the queue read (e.g., a position indicator or locator of an entry most-recently read, the number of packets read or unread, or the number of fresh/vacant mbufs available). The write indicator may identify or indicate how recently the queue was written into (e.g., a timestamp) and/or the extent of the queue populated with received packets (e.g., a position locator of a entry most-recently written into, the number of queue position populated, or the number of mbufs used or available). 
     The virtualized packet processor may compare the read and write indicators, or may compare the number of packets read from and written into the queue. In some embodiments, the virtualized packet processor may determine that there is no difference, as no new packet was received since the last read. The virtualized packet processor may receive a notification that a received packet was written into the queue. In some embodiments, the queue may notify the virtualized packet processor that packets are available for reading, and/or identify the packets to read. In certain embodiments, the virtualized packet processor may poll the queue for new packets. The virtualized packet processor may poll the queue for the number of available mbufs, and may read from the queue if the number of available mbufs falls below a predetermined threshold. 
     In some embodiments, the virtualized packet processor may poll the queue at specific times and/or intervals to determine if there are new packets to be read. By way of example and in one embodiment, the virtualized packet processor may poll the assigned queue at a predetermined point within the virtualized packet processor&#39;s netio loop or routine. The virtualized packet processor may poll or otherwise communicate with the queue via a hypercall or system call. In some embodiments, the system call is translated into a hypercall in kernel space. The hypervisor may receive and/or process the translated hypercall in response. 
     The virtualized packet processor may process the packet from the queue responsive to the determination (Step  1207 ). The virtualized packet processor or split driver may initiate a read or a packet transfer from the queue via a hypercall or system call. The virtualized packet processor or split driver may generate a system call or hypervisor call to transfer the packet from the queue to DomU or the second virtualization domain responsive to identifying a difference between the number of packets read from the queue and the number of packets written to the queue. In some embodiments, the virtualized packet processor may transfer or initiate a transfer of a plurality of packets from the queue to DomU or the second virtualization domain. The split driver or virtualized packet processor may transfer or initiate a transfer of one or more packets to a receive queue of the virtualized packet processor residing in user memory space. The hypervisor may, in response to the generated hypercall for example, communicate the packet from the queue in kernel memory space to a receive queue of DomU in user memory space of the multi-core device. 
     In some embodiments, the hypervisor, split driver and/or DomU may convert the packet from a first format in the queue to a second format for processing by the virtualized packet processor. By way of illustration and not intended to be limiting in any way, a split driver may transfer or copy one or more mbuf data structures in the assigned queue to DomU. The split driver may convert, modify or translate packet information from one format (e.g., mbuf format) to another format that the virtualized packet processor can understand or process. For example and in one embodiment, the split driver may convert, modify or translate packet information from a mbuf format into a nsb format. 
     In some embodiments, the split driver and/or virtualized packet processor may allocate a number of nsb data structures (e.g., corresponding to “read index—write index” of the queue) to kernel space. The split driver and/or virtualized packet processor may allocate the nsb data structures prior to issuing a system call to initiate the read. In some embodiments, the split driver and/or virtualized packet processor may copy the mbufs into nsbs in kernel space. The split driver and/or virtualized packet processor may buffer the nsbs in the RX queue of the virtualized packet processor. In some embodiments, a packet may comprise or occupy one or more mbufs (e.g., mbuf data structures) in the assigned queue, and may be translated into one or more nsbs (e.g., nsb data structures) in the RX queue. 
     In some embodiments, the split driver and/or the hypervisor may allocate fresh or new mbufs (e.g., memory or any other data structure or container for buffering packet information) to the assigned queue responsive to a packet transfer or read. The split driver and/or the hypervisor may vacate, release or overwrite an existing data structure (e.g., mbufs) in the assigned queue responsive to a read. The split driver may allocate memory or data structures (e.g., nsbs) in the RX queue for buffering a packet read or transferred from the assigned queue in kernel space. 
     In some embodiments, a first portion of a driver (e.g., para-virtualized driver) executes in Dom 0  (e.g., the first virtualization domain) for receiving a packet from the network interface hardware and/or communicating the packet to kernel space. A second portion of the driver may execute in DomU (e.g., the second virtualization domain) for receiving the packet from the assigned queue and/or formatting the packet for processing by the virtualized packet processor. The first and second portions of the driver may operate independently or in a coordinated fashion (e.g., in communication with each other via notifications or messages). The first and/or second portions of the driver may operate as respective interfaces with the shared queue and assigned queue in kernel space. In some embodiments, the first and second portions of the driver each comprise one or more para-virtualized drivers. 
     Illustrated in  FIG. 12D  is one embodiment of a method for communicating packets between network interface hardware of a multi-core device and a plurality of virtualized packet processors executed by one or more cores of the multi-core device. A virtualized packet processor executing within a first virtualization domain on a core of a multi-core device may buffer a packet in a transmit queue of the first virtualization domain (Step  1251 ). The first virtualization domain (e.g., DomU) may be from a plurality of virtualization domains not having direct access to a network interface hardware of the multi-core device. The virtualized packet processor may determine that the transmit queue has one or more packets to transmit (Step  1253 ). The system may transmit, responsive to the determination, the one or more packets from the transmit queue to a shared queue residing on kernel memory space of the multi-core device accessible by another virtualization domain (e.g., second virtualization domain or Dom 0 ) (Step  1255 ). The second virtualization domain may comprise a privileged domain having direct access to the network interface hardware. The second virtualization domain may access the one or more packets from the shared queue in response to a notification from DomU or the first virtualization domain, for transmission via the network interface hardware to a network (Step  1257 ). 
     Further referring to  FIG. 12D , and in more detail, a virtualized packet processor executing within a virtualization domain (e.g., DomU) on a core of a multi-core device may buffer a packet in a transmit queue of the first virtualization domain (Step  1251 ). This virtualization domain may be from a plurality of virtualization domains not having direct access to a network interface hardware of the multi-core device. The virtualized packet processor may convey, generate or otherwise provide a packet for transmission onto a network (e.g., an outbound packet). The virtualized packet processor may maintain, operate and/or provide input to a TX queue in user memory space (e.g., in DomU). In some embodiments, the virtualized packet processor manages or allocates memory in the TX queue for buffering or queuing outbound packets. The virtualized packet processor may accumulate or queue packets (e.g., in nsbs form) in the TX queue over a period of time, and/or as the packets become available. 
     The virtualized packet processor may determine that the transmit queue has one or more packets to transmit (Step  1253 ). In some embodiments, the virtualized packet processor keeps track of outbound packets that it provides to the TX queue for transmission. In certain embodiments, the virtualized packet processor may not keep track of outbound packets that it provides to the TX queue for transmission. In some embodiments, the virtualized packet processor comprises a para-virtualized driver or a split driver. A para-virtualized driver or a split driver executing in that virtualization domain (e.g., DomU) may, in certain embodiments, be an independent entity from the virtualized packet processor. The virtualized packet processor and/or a split driver may determine if the transmit queue has any packets to transmit. The virtualized packet processor and/or a split driver may poll the transmit queue for new transmit packets. In some embodiments, the virtualized packet processor and/or the split driver may determine if there are packets for transmission, during a predefined time or based on a predefined event (e.g., during a netio loop). 
     In some embodiments, the virtualized packet processor and/or the split driver may determine that the TX queue is populated or occupied beyond a particular threshold. In other embodiments, the virtualized packet processor and/or the split driver may determine that the TX queue is full, that allocated memory has been used up, and/or that there is no available data structures (e.g., fsbs) for buffering additional packets for transmission. The virtualized packet processor and/or the split driver may allocate additional memory or data structures and/or initiate transmission of one or more queued packets based on the determination. 
     The system may transmit, responsive to the determination, the one or more packets from the transmit queue to a shared queue residing on kernel memory space of the multi-core device accessible by Dom 0  (e.g., a second virtualization domain) (Step  1255 ). Dom 0  may comprise a privileged domain having direct access to the network interface hardware. The hypervisor may establish the shared queue in kernel space of the multi-core device accessible by one or more virtualization domains (e.g., DomU or the first virtualization domain, and Dom 0  or the second virtualization domain). In some embodiments, the shared queue resides in kernel space of a hypervisor providing or executing the first and second virtualization domains. In some embodiments, one or more packets from the transmit queue are transmitted to an assigned or dedicated TX queue, e.g., of the virtualized packet processor. The hypervisor may interleave or move packets from one or more dedicated TX queues (e.g., in kernel space, of one or more virtualized packet processors) onto a shared queue. 
     The virtualized packet processor and/or the split driver may issue or generate a system call responsive to the determination that the transmit queue has one or more packets to transmit. The virtualized packet processor and/or the split driver may issue or generate the system call as part of a netio loop or routine. The virtualized packet processor and/or the split driver may generate a system call to initiate communication of the one or more packets on the transmit queue to a shared queue in kernel memory space, e.g., responsive to the buffering of the one or more packets on the transmit queue. The virtualized packet processor and/or the split driver may generate or trigger a hypervisor call in kernel memory space to initiate communication of the one or more packets on the transmit queue to the shared queue in kernel memory space. The split driver and/or the hypervisor may convert or translate the system call into a hypercall for processing in the kernel space. The system call and/or hypercall may be generated responsive to the determination. The system call and/or hypercall may be generated responsive to the buffering of the one or more packets on the transmit queue. 
     In some embodiments, the hypervisor, split driver and/or DomU may convert the packet from a first format associated with the virtualized packet processor to a second format prior to buffering the packet in the transmit queue. The hypervisor, split driver and/or DomU may convert the packet from a first format in the user space TX queue to a second format for the shared TX queue in kernel space. By way of illustration and not intended to be limiting in any way, a split driver may transfer or copy one or more nsb data structures in the TX queue to kernel space. The split driver may convert, modify or translate packet information from one format (e.g., nsb format) to another format (e.g., msb format) for transmission to Dom 0 . In some embodiments, the split driver and/or the virtualized packet processor may allocate or release memory or any other data structure to the user space TX queue responsive to a packet transfer or transmission to kernel space. 
     In some embodiments, the split driver and/or hypervisor may allocate a number of nsb data structures (e.g., corresponding to “read index—write index” in the user space TX queue) to the shared TX queue in kernel space. In certain embodiments, this allocation may occur responsive to the generated system call or hypercall. The split driver and/or hypervisor may allocate msb data structures prior to issuing a system call to initiate the transmission to kernel space. In some embodiments, the split driver and/or hypervisor may copy, convert or translate the nsbs into mbufs in kernel space. The split driver and/or hypervisor may buffer, store or queue the mbufs in the shared TX queue for access by Dom 0 . In some embodiments, a packet may comprise one or more nsbs (e.g., nsb data structures) in the user space TX queue, and may be translated into one or more mbufs (e.g., mbuf data structures) in the shared TX queue. 
     In some embodiments, the virtualized packet processor and/or the split driver may generate a notification to Dom 0  (e.g., to Dom 0 &#39;s split driver) responsive to the packet transmission to kernel space. The notification may indicate to Dom 0  that an outbound packet is present in the shared queue or has been transferred to the shared queue. In some embodiments, a notification is generated and/or sent as part of, or responsive to the system call or hypercall. In certain embodiments, a received packet and an outbound packet may be transferred between queues and/or different types of memory space in parallel or at substantially the same time. For example and in one embodiment, a received packet and an outbound packet may be processed or transferred using a single system call. 
     Dom 0  (e.g., the second virtualization domain) may access the one or more packets from the shared queue in response to a notification from DomU (e.g., the first virtualization domain), for transmission via the network interface hardware to a network (Step  1257 ). A split driver of Dom 0  may access one or more packets from the shared TX queue responsive to a netio loop event, responsive to a received notification, or responsive to a system call or hypercall. In certain embodiments, the split driver may generate a system call to initiate transfer of one or more packets from the shared TX queue to Dom 0 . The split driver may convert an outbound packet from one format (e.g., mbuf format) to another format in preparation for transmission via network interface hardware. The split driver may maintain a TX queue in Dom 0  for buffering the outbound packets. In certain embodiments, the split driver may direct an outbound packet through a stack (e.g., TCP/IP stack), for transmission onto a network via the network interface hardware. 
     In certain embodiments, the hypervisor or one of the split drivers may reclaim memory or data structures previously occupied by a transmitted or received packet (e.g., a mbuf). The hypervisor or one of the split drivers may trigger or initiate reclamation by using a system call, a hypercall, or a function call. By way of illustration and in one embodiment, a txproc function may be called in a netio loop to reclaim or release previously occupied memory space or data structures. Reclaimed memory or data structures may be re-allocated, for example, as mbufs or nsbs. 
     In some embodiments, a first portion of a driver executes in DomU (e.g., the first virtualization domain) to buffer an outbound packet from a virtualized packet processor in the user space transmit queue and/or initiate transfer of the packet to the kernel space shared queue. A second portion of the driver may execute in Dom 0  (e.g., in the second virtualization domain) to access the packet from the shared queue. The first and second portions of the driver may operate independently or in a coordinated fashion (e.g., in communication with each other via notifications or messages). The first and/or second portions of the driver may operate as respective interfaces with the shared queue and assigned queue in kernel space. In some embodiments, the first and second portions of the driver each comprises one or more para-virtualized drivers. 
     It should be understood that the systems described above may provide multiple ones of any or each of those components and these components may be provided on either a standalone machine or, in some embodiments, on multiple machines in a distributed system. The systems and methods described above may be implemented as a method, apparatus or article of manufacture using programming and/or engineering techniques to produce software, firmware, hardware, or any combination thereof. In addition, the systems and methods described above may be provided as one or more computer-readable programs embodied on or in one or more articles of manufacture. The term “article of manufacture” as used herein is intended to encompass code or logic accessible from and embedded in one or more computer-readable devices, firmware, programmable logic, memory devices (e.g., EEPROMs, ROMs, PROMs, RAMs, SRAMs, etc.), hardware (e.g., integrated circuit chip, Field Programmable Gate Array (FPGA), Application Specific Integrated Circuit (ASIC), etc.), electronic devices, a computer readable non-volatile storage unit (e.g., CD-ROM, floppy disk, hard disk drive, etc.). The article of manufacture may be accessible from a file server providing access to the computer-readable programs via a network transmission line, wireless transmission media, signals propagating through space, radio waves, infrared signals, etc. The article of manufacture may be a flash memory card or a magnetic tape. The article of manufacture includes hardware logic as well as software or programmable code embedded in a computer readable medium that is executed by a processor. In general, the computer-readable programs may be implemented in any programming language, such as LISP, PERL, C, C++, C#, PROLOG, or in any byte code language such as JAVA. The software programs may be stored on or in one or more articles of manufacture as object code. 
     Having described certain embodiments of these methods and systems, it will now become apparent to one of skill in the art that other embodiments incorporating the concepts of the disclosure may be used.