Patent Publication Number: US-9413814-B2

Title: Systems and methods for providing quality of service via a flow controlled tunnel

Description:
RELATED APPLICATION 
     This application claims priority to and is a continuation of U.S. Non-provisional application Ser. No. 12/893,025, entitled “Systems And Methods For Providing Quality Of Service Via A Flow Controlled Tunnel”, filed on Sep. 29, 2010, and issued as U.S. Pat. No. 8,433,783 on Apr. 30, 2013, which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present disclosure generally relates to data communication networks. In particular, the present disclosure relates to systems and methods for providing quality of service of a plurality of applications via a flow controlled tunnel. 
     BACKGROUND OF THE INVENTION 
     In many systems executing a plurality of applications, it may be desirable to prioritize communications among the applications to meet performance, bandwidth and latency requirements. However, because of the different demands of these applications, and different server or endpoint destinations for application traffic, quality of service management may be complicated or require tedious per-link configuration. This may be particularly undesirable in mobile environments, such as smartphones or laptops, where users may connect from several different locations and over several different links in a week, or even in a single day. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is directed towards systems and methods for providing Quality of Service (QoS) via a flow controlled tunnel. Traffic from a plurality of applications may be directed into a single connection or flow-controlled tunnel and QoS policies may be applied across the plurality of applications without configuration of individual link speeds, enabling QoS scheduling to dynamically adjust traffic transmission and reception rates to ensure priority management of applications regardless of a final endpoint of the application communications. Accordingly, traffic of different types, including VPN, HTTP, Voice-over-IP (VoIP), remote desktop protocol traffic, or other traffic may be easily balanced and prioritized. In many embodiments, the tunnel may be transparent to applications, such that without any application configuration, application traffic may still be prioritized by QoS requirements. 
     In one aspect, the present invention features a method for providing quality of service of a plurality of applications via a flow controlled tunnel. The method includes an agent operating at a portion of a network stack of a client proxying a plurality of transport layer connections corresponding to each of a plurality of applications executing on the client. The method also includes the agent receiving data from each of the plurality of proxied transport layer connections in an order according to an assigned priority from classification of each of the plurality of applications. The method further includes the agent communicating to a first tunneling application executing on the client, in the order according to the assigned priority, a predetermined amount of data received from each of the plurality of proxied transport layer connections, the first tunneling application having an established transport layer connection to a second tunneling application. 
     In some embodiments, the method includes the first tunneling application on the client transmitting the predetermined amount of data from each of the applications to the second tunneling application executing on a device intermediary to the client and a plurality of a servers. In other embodiments, the agent executes in a kernel space of the client and the first tunneling application executes in a user space of the client. 
     In one embodiment, the method includes the agent proxying each of the plurality of transport layer connections back to the client. In another embodiment, the method includes the agent proxying each of the plurality of transport layer connections transparently to each of the plurality of applications. 
     In some embodiments, the method includes the agent classifying each of the plurality of applications according to a Quality of Service classification scheme. In other embodiments, the method includes the agent applying Quality of Service upon communicating the predetermined amount of data to the first tunneling application. In still other embodiments, the method includes the agent communicating each of the predetermined amount of data upon receipt to the first tunneling application. 
     In one embodiment, the method includes the agent receiving an indication from one or more of the plurality of proxied transport layer connections that data is available to be read. In a further embodiment, the method includes the agent propagating the indication to the first tunneling application and responsive to the indication, the first tunneling application accepting the predetermined amount of data from each of the one or more proxied transport layer connections in the order of the priority. 
     In another aspect, the present invention features a system for providing quality of service of a plurality of applications via a flow controlled tunnel. The system includes an agent operating at a portion of a network stack of a client and proxying a plurality of transport layer connections corresponding to each of a plurality of applications executing on the client. The system also includes a first tunneling application executing on the client and having an established transport layer connection to a second tunneling application. The system further includes the agent receiving data from each of the plurality of proxied transport layer connections in an order according to an assigned priority from classification of each of the plurality of applications and communicating to the first tunneling application, in the order according to the assigned priority, a predetermined amount of data received from each of the plurality of proxied transport layer connections. 
     In some embodiments, first tunneling application transmits the predetermined amount of data from each of the applications to the second tunneling application executing on a device intermediary to the client and a plurality of a servers. In other embodiments, the agent executes in a kernel space of the client and the first tunneling application executes in user space of the client. 
     In one embodiment, the agent proxies each of the plurality of proxied transport layer connections back to the client. In another embodiment, the agent proxies each of the plurality of proxied transport layer connections transparently to each of the plurality of applications. In still another embodiment, the agent classifies each of the plurality of applications according to a Quality of Service classification scheme. In yet still another embodiment, the agent applies Quality of Service scheduling as the predetermined amount of data is communicated to the first tunneling application. 
     In some embodiments, the agent communicates to the first tunneling application each of the predetermined amount of data upon receipt to the first tunneling application. In one embodiment, the agent receives an indication from one or more of the plurality of proxied transport layer connections that data is available to be read. In a further embodiment, the agent propagates the indication to the first tunneling application and responsive to the indication, the first tunneling application accepts the predetermined amount of data from each of the one or more proxied transport layer connections in the order of the priority. 
     The details of various embodiments of the invention 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 invention 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 one or more network optimization appliances; 
         FIG. 1B  is a block diagram of another embodiment of a network environment for a client to access a server via one or more network optimization appliances in conjunction with other network appliances; 
         FIG. 1C  is a block diagram of another embodiment of a network environment for a client to access a server via a single network optimization appliance deployed stand-alone or in conjunction with other network appliances; 
         FIGS. 1D and 1E  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 a client and/or server deploying the network optimization features of the appliance; 
         FIG. 3  is a block diagram of an embodiment of a client for communicating with a server using the network optimization feature; 
         FIG. 4A  is a block diagram of an embodiment of a system for providing quality of service of a plurality of applications via a flow controlled tunnel; 
         FIG. 4B  is a block diagram of an embodiment of a client or appliance for providing quality of service of a plurality of applications via a flow controlled tunnel; 
         FIG. 4C  is a block diagram of an embodiment of a queue tunnel proxy on a client or appliance; 
         FIG. 5A  is a block diagram of an embodiment of communication flow within a system providing quality of service of a plurality of applications via a flow controlled tunnel; 
         FIG. 5B  is a block diagram of an embodiment of communication flow within a system providing quality of service of a plurality of applications via an encryption gateway and a flow controlled tunnel; and 
         FIG. 6  is a flow chart of an embodiment of a method for providing quality of service of a plurality of applications via a flow controlled tunnel. 
     
    
    
     The features and advantages of the present invention 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 INVENTION 
     For purposes of reading the description of the various embodiments of the present invention 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 useful for practicing an embodiment of the present invention;   Section B describes embodiments of a system and appliance architecture for accelerating delivery of a computing environment to a remote user;   Section C describes embodiments of a client agent for accelerating communications between a client and a server; and   Section D describes embodiments of systems and methods for providing quality of service of a plurality of applications via a flow controlled tunnel.
 
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 has 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 ′,  104 ″. In some embodiments, a client  102  communicates with a server  106  via one or more network optimization appliances  200 ,  200 ′ (generally referred to as appliance  200 ). In one embodiment, the network optimization appliance  200  is designed, configured or adapted to optimize Wide Area Network (WAN) network traffic. In some embodiments, a first appliance  200  works in conjunction or cooperation with a second appliance  200 ′ to optimize network traffic. For example, a first appliance  200  may be located between a branch office and a WAN connection while the second appliance  200 ′ is located between the WAN and a corporate Local Area Network (LAN). The appliances  200  and  200 ′ may work together to optimize the WAN related network traffic between a client in the branch office and a server on the corporate LAN. 
     Although  FIG. 1A  shows a network  104 , network  104 ′ and network  104 ″ (generally referred to as network(s)  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 ,  104 ′,  104 ″ can be the same type of network or different types of networks. 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. The networks  104 ,  104 ′,  104 ″ can be a private or public network. In one embodiment, network  104 ′ or 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 ′ and/or network  104 ″ a public network. In another embodiment, networks  104 ,  104 ′,  104 ″ may 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 on a corporate LAN in a corporate data center. 
     The network  104  may 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  may be a bus, star, or ring network topology. The network  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 depicted in  FIG. 1A , a first network optimization appliance  200  is shown between networks  104  and  104 ′ and a second network optimization appliance  200 ′ is also between networks  104 ′ and  104 ″. In some embodiments, the appliance  200  may be located on network  104 . For example, a corporate enterprise may deploy an appliance  200  at the branch office. In other embodiments, the appliance  200  may be located on network  104 ′. In some embodiments, the appliance  200 ′ may be located on network  104 ′ or network  104 ″. For example, an appliance  200  may be located at a corporate data center. In one embodiment, the appliance  200  and  200 ′ are on the same network. In another embodiment, the appliance  200  and  200 ′ are on different networks. 
     In one embodiment, the appliance  200  is a device for accelerating, optimizing or otherwise improving the performance, operation, or quality of service of any type and form of network traffic. In some embodiments, the appliance  200  is a performance enhancing proxy. In other embodiments, the appliance  200  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  200  is any of the product embodiments referred to as WANScaler manufactured by Citrix Systems, Inc. of Ft. Lauderdale, Fla. In other embodiments, the appliance  200  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  200  includes any of the WX and WXC WAN acceleration device platforms manufactured by Juniper Networks, Inc. of Sunnyvale, Calif. In some embodiments, the appliance  200  includes any of the steelhead line of WAN optimization appliances manufactured by Riverbed Technology of San Francisco, Calif. In other embodiments, the appliance  200  includes any of the WAN related devices manufactured by Expand Networks Inc. of Roseland, N.J. In one embodiment, the appliance  200  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  200  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 some embodiments, the appliance  200  provides application and data acceleration services for branch-office or remote offices. In one embodiment, the appliance  200  includes optimization of Wide Area File Services (WAFS). In another embodiment, the appliance  200  accelerates the delivery of files, such as via the Common Internet File System (CIFS) protocol. In other embodiments, the appliance  200  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  200  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  200  provides Transport Control Protocol (TCP) optimizations. In other embodiments, the appliance  200  provides optimizations, flow control, performance enhancements or modifications and/or management for any session or application layer protocol. Further details of the optimization techniques, operations and architecture of the appliance  200  are discussed below in Section B. 
     Still referring to  FIG. 1A , the network environment 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 metropolitan-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 . 
     Deployed With Other Appliances. 
     Referring now to  FIG. 1B , another embodiment of a network environment is depicted in which the network optimization appliance  200  is deployed with one or more other appliances  205 ,  205 ′ (generally referred to as appliance  205  or second appliance  205 ) such as a gateway, firewall or acceleration appliance. For example, in one embodiment, the appliance  205  is a firewall or security appliance while appliance  205 ′ is a LAN acceleration device. In some embodiments, a client  102  may communicate to a server  106  via one or more of the first appliances  200  and one or more second appliances  205 . 
     One or more appliances  200  and  205  may be located at any point in the network or network communications path between a client  102  and a server  106 . In some embodiments, a second appliance  205  may be located on the same network  104  as the first appliance  200 . In other embodiments, the second appliance  205  may be located on a different network  104  as the first appliance  200 . In yet another embodiment, a first appliance  200  and second appliance  205  is on the same network, for example network  104 , while the first appliance  200 ′ and second appliance  205 ′ is on the same network, such as network  104 ″. 
     In one embodiment, the second appliance  205  includes any type and form of transport control protocol or transport later terminating device, such as a gateway or firewall device. In one embodiment, the appliance  205  terminates the transport control protocol by establishing a first transport control protocol connection with the client and a second transport control connection with the second appliance or server. In another embodiment, the appliance  205  terminates the transport control protocol by changing, managing or controlling the behavior of the transport control protocol connection between the client and the server or second appliance. For example, the appliance  205  may change, queue, forward or transmit network packets in manner to effectively terminate the transport control protocol connection or to act or simulate as terminating the connection. 
     In some embodiments, the second appliance  205  is a performance enhancing proxy. In one embodiment, the appliance  205  provides a virtual private network (VPN) connection. In some embodiments, the appliance  205  provides a Secure Socket Layer VPN (SSL VPN) connection. In other embodiments, the appliance  205  provides an IPsec (Internet Protocol Security) based VPN connection. In some embodiments, the appliance  205  provides any one or more of the following functionality: compression, acceleration, load-balancing, switching/routing, caching, and Transport Control Protocol (TCP) acceleration. 
     In one embodiment, the appliance  205  is any of the product embodiments referred to as Access Gateway, Application Firewall, Application Gateway, or NetScaler manufactured by Citrix Systems, Inc. of Ft. Lauderdale, Fla. As such, in some embodiments, the appliance  205  includes any logic, functions, rules, or operations to perform services or functionality such as SSL VPN connectivity, SSL offloading, switching/load balancing, Domain Name Service resolution, LAN acceleration and an application firewall. 
     In some embodiments, the appliance  205  provides a SSL VPN connection between a client  102  and a server  106 . For example, a client  102  on a first network  104  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, a client agent 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  205 . The appliance  205  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  205  receives the intercepted communication from the client agent, 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. In one embodiment, the appliance  205  provides an end-to-end secure transport layer connection for the client  102  between the two networks  104 ,  104 ′ 
     In one embodiments, the appliance  205  hosts an intranet internet protocol or intranetIP 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  205 , the appliance  205  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  205  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. In one embodiment, the appliance  205  acts as or on behalf of the client  102  on the second private network  104 . 
     In some embodiment, the appliance  205  has an encryption engine providing 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 encrypts and decrypts network packets, or any portion thereof, communicated via the appliance  205 . The encryption engine 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 ,  205 . As such, the encryption engine provides offloading and acceleration of SSL processing. In one embodiment, the encryption engine 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 uses an encryption processor. In other embodiments, the encryption engine includes executable instructions running on an encryption processor. 
     In some embodiments, the appliance  205  provides one or more of the following acceleration techniques 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  205  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  205 , 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  205  and the destination address is changed from that of appliance  205  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  205  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  205  provides switching or load-balancing functionality for communications between the client  102  and server  106 . In some embodiments, the appliance  205  distributes traffic and directs client requests to a server  106  based on layer 4 payload 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  205  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, a health monitoring program of the appliance  205  monitors the health of servers to determine the server  106  for which to distribute a client&#39;s request. In some embodiments, if the appliance  205  detects a server  106  is not available or has a load over a predetermined threshold, the appliance  205  can direct or distribute client requests to another server  106 . 
     In some embodiments, the appliance  205  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  205  responds to a client&#39;s DNS request with an IP address of or hosted by the appliance  205 . 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  205  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  205  provides application firewall functionality for communications between the client  102  and server  106 . In one embodiment, a policy engine  295 ′ provides rules for detecting and blocking illegitimate requests. In some embodiments, the application firewall 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 includes 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 of the appliance 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 of the appliance  205  ensures cookies are not modified. In other embodiments, the appliance  205  protects against forceful browsing by enforcing legal URLs. 
     In still yet other embodiments, the application firewall appliance  205  protects any confidential information contained in the network communication. The appliance  205  may inspect or analyze any network communication in accordance with the rules or polices of the policy engine to identify any confidential information in any field of the network packet. In some embodiments, the application firewall 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 include these occurrences or the confidential information. Based on these occurrences, in one embodiment, the application firewall may take a policy action on the network communication, such as prevent transmission of the network communication. In another embodiment, the application firewall may rewrite, remove or otherwise mask such identified occurrence or confidential information. 
     Although generally referred to as a network optimization or first appliance  200  and a second appliance  205 , the first appliance  200  and second appliance  205  may be the same type and form of appliance. In one embodiment, the second appliance  205  may perform the same functionality, or portion thereof, as the first appliance  200 , and vice-versa. For example, the first appliance  200  and second appliance  205  may both provide acceleration techniques. In one embodiment, the first appliance may perform LAN acceleration while the second appliance performs WAN acceleration, or vice-versa. In another example, the first appliance  200  may also be a transport control protocol terminating device as with the second appliance  205 . Furthermore, although appliances  200  and  205  are shown as separate devices on the network, the appliance  200  and/or  205  could be a part of any client  102  or server  106 . 
     Referring now to  FIG. 1C , other embodiments of a network environment for deploying the appliance  200  are depicted. In another embodiment as depicted on the top of  FIG. 1C , the appliance  200  may be deployed as a single appliance or single proxy on the network  104 . For example, the appliance  200  may be designed, constructed or adapted to perform WAN optimization techniques discussed herein without a second cooperating appliance  200 ′. In other embodiments as depicted on the bottom of  FIG. 1C , a single appliance  200  may be deployed with one or more second appliances  205 . For example, a WAN acceleration first appliance  200 , such as a Citrix WANScaler appliance, may be deployed with a LAN accelerating or Application Firewall second appliance  205 , such as a Citrix NetScaler appliance. 
     Computing Device 
     The client  102 , server  106 , and appliance  200  and  205  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. 1D and 1E  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. 1D and 1E , each computing device  100  includes a central processing unit  101 , and a main memory unit  122 . As shown in  FIG. 1D , 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. 1D , 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. 1E  the main memory  122  may be DRDRAM. 
       FIG. 1E  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. 1C , 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. 1E  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. 1E  also depicts an embodiment in which local busses and direct communication are mixed: the processor  101  communicates with I/O device  130   a  using a local interconnect bus while communicating with I/O device  130   b  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. 1D . 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, Calif. 
     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. 1D and 1E  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, Calif.; 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. 
     B. System and Appliance Architecture 
     Referring now to  FIG. 2A , an embodiment of a system environment and architecture of an appliance  200  for delivering and/or operating a computing environment on a client is depicted. In some embodiments, a server  106  includes an application delivery system  290  for delivering a computing environment or an application and/or data file to one or more clients  102 . In brief overview, a client  102  is in communication with a server  106  via network  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  has a client agent  120 , and a computing environment  215 . The computing environment  215  may execute or operate an application that accesses, processes or uses a data file. The computing environment  215 , 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  215 , or any portion thereof, to a client  102 . In one embodiment, the appliance  200  accelerates the delivery of the computing environment  215  by the application delivery system  290 . 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 . In another embodiment, the appliance  200  controls, manages, or adjusts the transport layer protocol to accelerate delivery of the computing environment. In some embodiments, the appliance  200  uses caching and/or compression techniques to accelerate delivery of a computing environment. 
     In some embodiments, the application delivery management system  290  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  295 . 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  290  may reside or execute on a server  106 . In another embodiment, the application delivery system  290  may reside or execute on a plurality of servers  106   a - 106   n . In some embodiments, the application delivery system  290  may execute in a server farm  38 . In one embodiment, the server  106  executing the application delivery system  290  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  290 , and a different server  106   n  may store or provide the application and data file. In some embodiments, each of the application delivery system  290 , the application, and data file may reside or be located on different servers. In yet another embodiment, any portion of the application delivery system  290  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  215  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  290  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  215  on client  102 . 
     In some embodiments, the application delivery system  290  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  290  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  290  may deliver one or more applications to clients or users via steaming of the application. 
     In one embodiment, the application delivery system  290  includes a policy engine  295  for controlling and managing the access to, selection of application execution methods and the delivery of applications. In some embodiments, the policy engine  295  determines the one or more applications a user or client  102  may access. In another embodiment, the policy engine  295  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  290  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  290  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  290  enumerates a plurality of application programs available to the client  102 . The application delivery system  290  receives a request to execute an enumerated application. The application delivery system  290  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  290  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  290  may select a method of execution of the application enabling the client or local machine  102  to execute the application program locally after retrieving a plurality of application files comprising the application. In yet another embodiment, the application delivery system  290  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 embodiment 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. 
     Example Appliance Architecture 
       FIG. 2A  also 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 in any manner. The appliance  200  may include any type and form of computing device  100 , such as any element or portion described in conjunction with FIGS.  1 D and  1 E above. In brief overview, the appliance  200  has one or more network ports  266 A- 226 N and one or more networks stacks  267 A- 267 N for receiving and/or transmitting communications via networks  104 . The appliance  200  also has a network optimization engine  250  for optimizing, accelerating or otherwise improving the performance, operation, or quality of any network traffic or communications traversing the appliance  200 . 
     The appliance  200  includes or is under the control of an operating system. The operating system of the appliance  200  may be any type and/or form of Unix operating system although the invention is 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 operating system of appliance  200  allocates, manages, or otherwise segregates the available system memory into what is referred to as kernel or system space, and user or application space. The kernel space is typically reserved for running the kernel, including any device drivers, kernel extensions or other kernel related software. As known to those skilled in the art, the kernel is the core of the operating system, and provides access, control, and management of resources and hardware-related elements of the appliance  200 . In accordance with an embodiment of the appliance  200 , the kernel space also includes a number of network services or processes working in conjunction with the network optimization engine  250 , or any portion thereof. Additionally, the embodiment of the kernel will depend on the embodiment of the operating system installed, configured, or otherwise used by the device  200 . In contrast to kernel space, user space 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 directly and uses service calls in order to access kernel services. The operating system uses the user or application space for executing or running applications and provisioning of user level programs, services, processes and/or tasks. 
     The appliance  200  has one or more network ports  266  for transmitting and receiving data over a network  104 . The network port  266  provides a physical and/or logical interface between the computing device and a network  104  or another device  100  for transmitting and receiving network communications. The type and form of network port  266  depends on the type and form of network and type of medium for connecting to the network. Furthermore, any software of, provisioned for or used by the network port  266  and network stack  267  may run in either kernel space or user space. 
     In one embodiment, the appliance  200  has one network stack  267 , such as a TCP/IP based stack, for communicating on a network  105 , such 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  104 , and also with a second network  104 ′. In another embodiment, the appliance  200  has two or more network stacks, such as first network stack  267 A and a second network stack  267 N. The first network stack  267 A may be used in conjunction with a first port  266 A to communicate on a first network  104 . The second network stack  267 N may be used in conjunction with a second port  266 N to communicate on a second network  104 ′. In one embodiment, the network stack(s)  267  has one or more buffers for queuing one or more network packets for transmission by the appliance  200 . 
     The network stack  267  includes 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  267  includes a software implementation for a network protocol suite. The network stack  267  may have 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  267  may have 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  267  includes 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  267  has 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  267  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  267 , such as for voice communications or real-time data communications. 
     Furthermore, the network stack  267  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  267  may be customized, modified or adapted to provide a custom or modified portion of the network stack  267  in support of any of the techniques described herein. 
     In one embodiment, the appliance  200  provides for or maintains a transport layer connection between a client  102  and server  106  using a single network stack  267 . In some embodiments, the appliance  200  effectively terminates the transport layer connection by changing, managing or controlling the behavior of the transport control protocol connection between the client and the server. In these embodiments, the appliance  200  may use a single network stack  267 . In other embodiments, the appliance  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 or on behalf of 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 appliance  200  may use multiple network stacks, for example  267 A and  267 N. In these embodiments, the first transport layer connection may be established or terminated at one network stack  267 A, and the second transport layer connection may be established or terminated on the second network stack  267 N. For example, one network stack may be for receiving and transmitting network packets on a first network, and another network stack for receiving and transmitting network packets on a second network. 
     As shown in  FIG. 2A , the network optimization engine  250  includes one or more of the following elements, components or modules: network packet processing engine  240 , LAN/WAN detector  210 , flow controller  220 , QoS engine  236 , protocol accelerator  234 , compression engine  238 , cache manager  232  and policy engine  295 ′. The network optimization engine  250 , or any portion thereof, may include software, hardware or any combination of software and hardware. Furthermore, any software of, provisioned for or used by the network optimization engine  250  may run in either kernel space or user space. For example, in one embodiment, the network optimization engine  250  may run in kernel space. In another embodiment, the network optimization engine  250  may run in user space. In yet another embodiment, a first portion of the network optimization engine  250  runs in kernel space while a second portion of the network optimization engine  250  runs in user space. 
     Network Packet Processing Engine 
     The network packet engine  240 , also generally referred to as a packet processing engine or packet engine, is responsible for controlling and managing the processing of packets received and transmitted by appliance  200  via network ports  266  and network stack(s)  267 . The network packet engine  240  may operate at any layer of the network stack  267 . In one embodiment, the network packet engine  240  operates at layer 2 or layer 3 of the network stack  267 . In some embodiments, the packet engine  240  intercepts or otherwise receives packets at the network layer, such as the IP layer in a TCP/IP embodiment. In another embodiment, the packet engine  240  operates at layer 4 of the network stack  267 . For example, in some embodiments, the packet engine  240  intercepts or otherwise receives packets at the transport layer, such as intercepting packets as the TCP layer in a TCP/IP embodiment. In other embodiments, the packet engine  240  operates at any session or application layer above layer 4. For example, in one embodiment, the packet engine  240  intercepts or otherwise receives network packets above the transport layer protocol layer, such as the payload of a TCP packet in a TCP embodiment. 
     The packet engine  240  may include 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 packet engine  240  is in communication with one or more network stacks  267  to send and receive network packets via network ports  266 . The packet engine  240  may include a packet processing timer. In one embodiment, the packet processing timer 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 packet engine  240  processes network packets responsive to the timer. The packet processing timer 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 operates in the order of milliseconds, such as for example 100 ms, 50 ms, 25 ms, 10 ms, 5 ms or 1 ms. 
     During operations, the packet engine  240  may be interfaced, integrated or be in communication with any portion of the network optimization engine  250 , such as the LAN/WAN detector  210 , flow controller  220 , QoS engine  236 , protocol accelerator  234 , compression engine  238 , cache manager  232  and/or policy engine  295 ′. As such, any of the logic, functions, or operations of the LAN/WAN detector  210 , flow controller  220 , QoS engine  236 , protocol accelerator  234 , compression engine  238 , cache manager  232  and policy engine  295 ′ may be performed responsive to the packet processing timer and/or the packet engine  240 . In some embodiments, 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, 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 expiration of any cached objects responsive to the 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, such as at every 10 ms. 
     Cache Manager 
     The cache manager  232  may include software, hardware or any combination of software and hardware to store data, information and objects to a cache in memory or storage, provide cache access, and control and manage the cache. The data, objects or content processed and stored by the cache manager  232  may include data in any format, such as a markup language, or any type of data 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 or storage element. Once the data is stored in the cache, 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 may comprise a data object in memory of the appliance  200 . In another embodiment, the cache may comprise any type and form of storage element of the appliance  200 , such as a portion of a hard disk. In some embodiments, the processing unit of the device 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 caching techniques of the appliance  200 . In some embodiments, the cache manager  232  may operate as an application, library, program, service, process, thread or task. 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). 
     Policy Engine 
     The policy engine  295 ′ includes any logic, function or operations for providing and applying one or more policies or rules to the function, operation or configuration of any portion of the appliance  200 . The policy engine  295 ′ may include, for example, an intelligent statistical engine or other programmable application(s). In one embodiment, the policy engine  295  provides a configuration mechanism to allow a user to identify, specify, define or configure a policy for the network optimization engine  250 , or any portion thereof. For example, the policy engine  295  may provide policies for what data to cache, when to cache the data, for whom to cache the data, when to expire an object in cache or refresh the cache. In other embodiments, the policy engine  236  may include 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 . 
     In some embodiments, the policy engine  295 ′ provides and applies one or more policies based on any one or more of the following: a user, identification of the client, identification of the server, the type of connection, the time of the connection, the type of network, or the contents of the network traffic. In one embodiment, the policy engine  295 ′ provides and applies a policy based on any field or header at any protocol layer of a network packet. In another embodiment, the policy engine  295 ′ provides and applies a policy based on any payload of a network packet. For example, in one embodiment, the policy engine  295 ′ applies a policy based on identifying a certain portion of content of an application layer protocol carried as a payload of a transport layer packet. In another example, the policy engine  295 ′ applies a policy based on any information identified by a client, server or user certificate. In yet another embodiment, the policy engine  295 ′ applies a policy based on any attributes or characteristics obtained about a client  102 , such as via any type and form of endpoint detection (see for example the collection agent of the client agent discussed below). 
     In one embodiment, the policy engine  295 ′ works in conjunction or cooperation with the policy engine  295  of the application delivery system  290 . In some embodiments, the policy engine  295 ′ is a distributed portion of the policy engine  295  of the application delivery system  290 . In another embodiment, the policy engine  295  of the application delivery system  290  is deployed on or executed on the appliance  200 . In some embodiments, the policy engines  295 ,  295 ′ both operate on the appliance  200 . In yet another embodiment, the policy engine  295 ′, or a portion thereof, of the appliance  200  operates on a server  106 . 
     Multi-Protocol and Multi-Layer Compression Engine 
     The compression engine  238  includes 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 appliance  200 . The compression engine  238  may also be referred to as a multi-protocol compression engine  238  in that it may be designed, constructed or capable of compressing a plurality of protocols. In one embodiment, the compression engine  238  applies context insensitive compression, which is compression applied to data without knowledge of the type of data. In another embodiment, the compression engine  238  applies context-sensitive compression. In this embodiment, the compression engine  238  utilizes knowledge of the data type to select a specific compression algorithm from a suite of suitable algorithms. In some embodiments, knowledge of the specific protocol is used to perform context-sensitive compression. In one embodiment, the appliance  200  or compression engine  238  can use port numbers (e.g., well-known ports), as well as data from the connection itself to determine the appropriate compression algorithm to use. Some protocols use only a single type of data, requiring only a single compression algorithm that can be selected when the connection is established. Other protocols contain different types of data at different times. For example, POP, IMAP, SMTP, and HTTP all move files of arbitrary types interspersed with other protocol data. 
     In one embodiment, the compression engine  238  uses a delta-type compression algorithm. In another embodiment, the compression engine  238  uses first site compression as well as searching for repeated patterns among data stored in cache, memory or disk. In some embodiments, the compression engine  238  uses a lossless compression algorithm. In other embodiments, the compression engine uses a lossy compression algorithm. In some cases, knowledge of the data type and, sometimes, permission from the user are required to use a lossy compression algorithm. Compression is not limited to the protocol payload. The control fields of the protocol itself may be compressed. In some embodiments, the compression engine  238  uses a different algorithm than that used for the payload. 
     In some embodiments, the compression engine  238  compresses at one or more layers of the network stack  267 . In one embodiment, the compression engine  238  compresses at a transport layer protocol. In another embodiment, the compression engine  238  compresses at an application layer protocol. In some embodiments, the compression engine  238  compresses at a layer 2-4 protocol. In other embodiments, the compression engine  238  compresses at a layer 5-7 protocol. In yet another embodiment, the compression engine compresses a transport layer protocol and an application layer protocol. In some embodiments, the compression engine  238  compresses a layer 2-4 protocol and a layer 5-7 protocol. 
     In some embodiments, the compression engine  238  uses memory-based compression, cache-based compression or disk-based compression or any combination thereof. As such, the compression engine  238  may be referred to as a multi-layer compression engine. In one embodiment, the compression engine  238  uses a history of data stored in memory, such as RAM. In another embodiment, the compression engine  238  uses a history of data stored in a cache, such as L2 cache of the processor. In other embodiments, the compression engine  238  uses a history of data stored to a disk or storage location. In some embodiments, the compression engine  238  uses a hierarchy of cache-based, memory-based and disk-based data history. The compression engine  238  may first use the cache-based data to determine one or more data matches for compression, and then may check the memory-based data to determine one or more data matches for compression. In another case, the compression engine  238  may check disk storage for data matches for compression after checking either the cache-based and/or memory-based data history. 
     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 by integrating with packet processing engine  240  accessing the network stack  267  is able to compress any of the protocols carried by a transport layer protocol, such as any application layer protocol. 
     LAN/WAN Detector 
     The LAN/WAN detector  238  includes any logic, business rules, function or operations for automatically detecting a slow side connection (e.g., a wide area network (WAN) connection such as an Intranet) and associated port  267 , and a fast side connection (e.g., a local area network (LAN) connection) and an associated port  267 . In some embodiments, the LAN/WAN detector  238  monitors network traffic on the network ports  267  of the appliance  200  to detect a synchronization packet, sometimes referred to as a “tagged” network packet. The synchronization packet identifies a type or speed of the network traffic. In one embodiment, the synchronization packet identifies a WAN speed or WAN type connection. The LAN/WAN detector  238  also identifies receipt of an acknowledgement packet to a tagged synchronization packet and on which port it is received. The appliance  200  then configures itself to operate the identified port on which the tagged synchronization packet arrived so that the speed on that port is set to be the speed associated with the network connected to that port. The other port is then set to the speed associated with the network connected to that port. 
     For ease of discussion herein, reference to “fast” side will be made with respect to connection with a wide area network (WAN), e.g., the Internet, and operating at a network speed of the WAN. Likewise, reference to “slow” side will be made with respect to connection with a local area network (LAN) and operating at a network speed the LAN. However, it is noted that “fast” and “slow” sides in a network can change on a per-connection basis and are relative terms to the speed of the network connections or to the type of network topology. Such configurations are useful in complex network topologies, where a network is “fast” or “slow” only when compared to adjacent networks and not in any absolute sense. 
     In one embodiment, the LAN/WAN detector  238  may be used to allow for auto-discovery by an appliance  200  of a network to which it connects. In another embodiment, the LAN/WAN detector  238  may be used to detect the existence or presence of a second appliance  200 ′ deployed in the network  104 . For example, an auto-discovery mechanism in operation in accordance with  FIG. 1A  functions as follows: appliance  200  and  200 ′ are placed in line with the connection linking client  102  and server  106 . The appliances  200  and  200 ′ are at the ends of a low-speed link, e.g., Internet, connecting two LANs. In one example embodiment, appliances  200  and  200 ′ each include two ports—one to connect with the “lower” speed link and the other to connect with a “higher” speed link, e.g., a LAN. Any packet arriving at one port is copied to the other port. Thus, appliance  200  and  200 ′ are each configured to function as a bridge between the two networks  104 . 
     When an end node, such as the client  102 , opens a new TCP connection with another end node, such as the server  106 , the client  102  sends a TCP packet with a synchronization (SYN) header bit set, or a SYN packet, to the server  106 . In the present example, client  102  opens a transport layer connection to server  106 . When the SYN packet passes through appliance  200 , the appliance  200  inserts, attaches or otherwise provides a characteristic TCP header option to the packet, which announces its presence. If the packet passes through a second appliance, in this example appliance  200 ′ the second appliance notes the header option on the SYN packet. The server  106  responds to the SYN packet with a synchronization acknowledgment (SYN-ACK) packet. When the SYN-ACK packet passes through appliance  200 ′, a TCP header option is tagged (e.g., attached, inserted or added) to the SYN-ACK packet to announce appliance  200 ′ presence to appliance  200 . When appliance  200  receives this packet, both appliances  200 ,  200 ′ are now aware of each other and the connection can be appropriately accelerated. 
     Further to the operations of the LAN/WAN detector  238 , a method or process for detecting “fast” and “slow” sides of a network using a SYN packet is described. During a transport layer connection establishment between a client  102  and a server  106 , the appliance  200  via the LAN/WAN detector  238  determines whether the SYN packet is tagged with an acknowledgement (ACK). If it is tagged, the appliance  200  identifies or configures the port receiving the tagged SYN packet (SYN-ACK) as the “slow” side. In one embodiment, the appliance  200  optionally removes the ACK tag from the packet before copying the packet to the other port. If the LAN/WAN detector  238  determines that the packet is not tagged, the appliance  200  identifies or configure the port receiving the untagged packet as the “fast” side. The appliance  200  then tags the SYN packet with an ACK and copies the packet to the other port. 
     In another embodiment, the LAN/WAN detector  238  detects fast and slow sides of a network using a SYN-ACK packet. The appliance  200  via the LAN/WAN detector  238  determines whether the SYN-ACK packet is tagged with an acknowledgement (ACK). If it is tagged, the appliance  200  identifies or configures the port receiving the tagged SYN packet (SYN-ACK) as the “slow” side. In one embodiment, the appliance  200  optionally removes the ACK tag from the packet before copying the packet to the other port. If the LAN/WAN detector  238  determines that the packet is not tagged, the appliance  200  identifies or configures the port receiving the untagged packet as the “fast” side. The LAN/WAN detector  238  determines whether the SYN packet was tagged. If the SYN packet was not tagged, the appliance  200  copied the packet to the other port. If the SYN packet was tagged, the appliance tags the SYN-ACK packet before copying it to the other port. 
     The appliance  200 ,  200 ′ may add, insert, modify, attach or otherwise provide any information or data in the TCP option header to provide any information, data or characteristics about the network connection, network traffic flow, or the configuration or operation of the appliance  200 . In this manner, not only does an appliance  200  announce its presence to another appliance  200 ′ or tag a higher or lower speed connection, the appliance  200  provides additional information and data via the TCP option headers about the appliance or the connection. The TCP option header information may be useful to or used by an appliance in controlling, managing, optimizing, acceleration or improving the network traffic flow traversing the appliance  200 , or to otherwise configure itself or operation of a network port. 
     Although generally described in conjunction with detecting speeds of network connections or the presence of appliances, the LAN/WAN detector  238  can be used for applying any type of function, logic or operation of the appliance  200  to a port, connection or flow of network traffic. In particular, automated assignment of ports can occur whenever a device performs different functions on different ports, where the assignment of a port to a task can be made during the unit&#39;s operation, and/or the nature of the network segment on each port is discoverable by the appliance  200 . 
     Flow Control 
     The flow controller  220  includes any logic, business rules, function or operations for optimizing, accelerating or otherwise improving the performance, operation or quality of service of transport layer communications of network packets or the delivery of packets at the transport layer. A flow controller, also sometimes referred to as a flow control module, regulates, manages and controls data transfer rates. In some embodiments, the flow controller  220  is deployed at or connected at a bandwidth bottleneck in the network  104 . In one embodiment, the flow controller  220  effectively regulates, manages and controls bandwidth usage or utilization. In other embodiments, the flow control modules may also be deployed at points on the network of latency transitions (low latency to high latency) and on links with media losses (such as wireless or satellite links). 
     In some embodiments, a flow controller  220  may include a receiver-side flow control module for controlling the rate of receipt of network transmissions and a sender-side flow control module for the controlling the rate of transmissions of network packets. In other embodiments, a first flow controller  220  includes a receiver-side flow control module and a second flow controller  220 ′ includes a sender-side flow control module. In some embodiments, a first flow controller  220  is deployed on a first appliance  200  and a second flow controller  220 ′ is deployed on a second appliance  200 ′. As such, in some embodiments, a first appliance  200  controls the flow of data on the receiver side and a second appliance  200 ′ controls the data flow from the sender side. In yet another embodiment, a single appliance  200  includes flow control for both the receiver-side and sender-side of network communications traversing the appliance  200 . 
     In one embodiment, a flow control module  220  is configured to allow bandwidth at the bottleneck to be more fully utilized, and in some embodiments, not overutilized. In some embodiments, the flow control module  220  transparently buffers (or rebuffers data already buffered by, for example, the sender) network sessions that pass between nodes having associated flow control modules  220 . When a session passes through two or more flow control modules  220 , one or more of the flow control modules controls a rate of the session(s). 
     In one embodiment, the flow control module  200  is configured with predetermined data relating to bottleneck bandwidth. In another embodiment, the flow control module  220  may be configured to detect the bottleneck bandwidth or data associated therewith. Unlike conventional network protocols such as TCP, a receiver-side flow control module  220  controls the data transmission rate. The receiver-side flow control module controls  220  the sender-side flow control module, e.g.,  220 , data transmission rate by forwarding transmission rate limits to the sender-side flow control module  220 . In one embodiment, the receiver-side flow control module  220  piggybacks these transmission rate limits on acknowledgement (ACK) packets (or signals) sent to the sender, e.g., client  102 , by the receiver, e.g., server  106 . The receiver-side flow control module  220  does this in response to rate control requests that are sent by the sender side flow control module  220 ′. The requests from the sender-side flow control module  220 ′ may be “piggybacked” on data packets sent by the sender  106 . 
     In some embodiments, the flow controller  220  manipulates, adjusts, simulates, changes, improves or otherwise adapts the behavior of the transport layer protocol to provide improved performance or operations of delivery, data rates and/or bandwidth utilization of the transport layer. The flow controller  220  may implement a plurality of data flow control techniques at the transport layer, including but not limited to 1) pre-acknowledgements, 2) window virtualization, 3) recongestion techniques, 3) local retransmission techniques, 4) wavefront detection and disambiguation, 5) transport control protocol selective acknowledgements, 6) transaction boundary detection techniques and 7) repacketization. 
     Although a sender may be generally described herein as a client  102  and a receiver as a server  106 , a sender may be any end point such as a server  106  or any computing device  100  on the network  104 . Likewise, a receiver may be a client  102  or any other computing device on the network  104 . 
     Pre-Acknowledgements 
     In brief overview of a pre-acknowledgement flow control technique, the flow controller  220 , in some embodiments, handles the acknowledgements and retransmits for a sender, effectively terminating the sender&#39;s connection with the downstream portion of a network connection. In reference to  FIG. 1B , one possible deployment of an appliance  200  into a network architecture to implement this feature is depicted. In this example environment, a sending computer or client  102  transmits data on network  104 , for example, via a switch, which determines that the data is destined for VPN appliance  205 . Because of the chosen network topology, all data destined for VPN appliance  205  traverses appliance  200 , so the appliance  200  can apply any necessary algorithms to this data. 
     Continuing further with the example, the client  102  transmits a packet, which is received by the appliance  200 . When the appliance  200  receives the packet, which is transmitted from the client  102  to a recipient via the VPN appliance  205  the appliance  200  retains a copy of the packet and forwards the packet downstream to the VPN appliance  205 . The appliance  200  then generates an acknowledgement packet (ACK) and sends the ACK packet back to the client  102  or sending endpoint. This ACK, a pre-acknowledgment, causes the sender  102  to believe that the packet has been delivered successfully, freeing the sender&#39;s resources for subsequent processing. The appliance  200  retains the copy of the packet data in the event that a retransmission of the packet is required, so that the sender  102  does not have to handle retransmissions of the data. This early generation of acknowledgements may be called “preacking.” 
     If a retransmission of the packet is required, the appliance  200  retransmits the packet to the sender. The appliance  200  may determine whether retransmission is required as a sender would in a traditional system, for example, determining that a packet is lost if an acknowledgement has not been received for the packet after a predetermined amount of time. To this end, the appliance  200  monitors acknowledgements generated by the receiving endpoint, e.g., server  106  (or any other downstream network entity) so that it can determine whether the packet has been successfully delivered or needs to be retransmitted. If the appliance  200  determines that the packet has been successfully delivered, the appliance  200  is free to discard the saved packet data. The appliance  200  may also inhibit forwarding acknowledgements for packets that have already been received by the sending endpoint. 
     In the embodiment described above, the appliance  200  via the flow controller  220  controls the sender  102  through the delivery of pre-acknowledgements, also referred to as “preacks”, as though the appliance  200  was a receiving endpoint itself. Since the appliance  200  is not an endpoint and does not actually consume the data, the appliance  200  includes a mechanism for providing overflow control to the sending endpoint. Without overflow control, the appliance  200  could run out of memory because the appliance  200  stores packets that have been preacked to the sending endpoint but not yet acknowledged as received by the receiving endpoint. Therefore, in a situation in which the sender  102  transmits packets to the appliance  200  faster than the appliance  200  can forward the packets downstream, the memory available in the appliance  200  to store unacknowledged packet data can quickly fill. A mechanism for overflow control allows the appliance  200  to control transmission of the packets from the sender  102  to avoid this problem. 
     In one embodiment, the appliance  200  or flow controller  220  includes an inherent “self-clocking” overflow control mechanism. This self-clocking is due to the order in which the appliance  200  may be designed to transmit packets downstream and send ACKs to the sender  102  or  106 . In some embodiments, the appliance  200  does not preack the packet until after it transmits the packet downstream. In this way, the sender  102  will receive the ACKs at the rate at which the appliance  200  is able to transmit packets rather than the rate at which the appliance  200  receives packets from the sender  100 . This helps to regulate the transmission of packets from a sender  102 . 
     Window Virtualization 
     Another overflow control mechanism that the appliance  200  may implement is to use the TCP window size parameter, which tells a sender how much buffer the receiver is permitting the sender to fill up. A nonzero window size (e.g., a size of at least one Maximum Segment Size (MSS)) in a preack permits the sending endpoint to continue to deliver data to the appliance, whereas a zero window size inhibits further data transmission. Accordingly, the appliance  200  may regulate the flow of packets from the sender, for example when the appliance&#39;s  200  buffer is becoming full, by appropriately setting the TCP window size in each preack. 
     Another technique to reduce this additional overhead is to apply hysteresis. When the appliance  200  delivers data to the slower side, the overflow control mechanism in the appliance  200  can require that a minimum amount of space be available before sending a nonzero window advertisement to the sender. In one embodiment, the appliance  200  waits until there is a minimum of a predetermined number of packets, such as four packets, of space available before sending a nonzero window packet, such as a window size of four packet). This reduces the overhead by approximately a factor four, since only two ACK packets are sent for each group of four data packets, instead of eight ACK packets for four data packets. 
     Another technique the appliance  200  or flow controller  220  may use for overflow control is the TCP delayed ACK mechanism, which skips ACKs to reduce network traffic. The TCP delayed ACKs automatically delay the sending of an ACK, either until two packets are received or until a fixed timeout has occurred. This mechanism alone can result in cutting the overhead in half; moreover, by increasing the numbers of packets above two, additional overhead reduction is realized. But merely delaying the ACK itself may be insufficient to control overflow, and the appliance  200  may also use the advertised window mechanism on the ACKs to control the sender. When doing this, the appliance  200  in one embodiment avoids triggering the timeout mechanism of the sender by delaying the ACK too long. 
     In one embodiment, the flow controller  220  does not preack the last packet of a group of packets. By not preacking the last packet, or at least one of the packets in the group, the appliance avoids a false acknowledgement for a group of packets. For example, if the appliance were to send a preack for a last packet and the packet were subsequently lost, the sender would have been tricked into thinking that the packet is delivered when it was not. Thinking that the packet had been delivered, the sender could discard that data. If the appliance also lost the packet, there would be no way to retransmit the packet to the recipient. By not preacking the last packet of a group of packets, the sender will not discard the packet until it has been delivered. 
     In another embodiment, the flow controller  220  may use a window virtualization technique to control the rate of flow or bandwidth utilization of a network connection. Though it may not immediately be apparent from examining conventional literature such as RFC 1323, there is effectively a send window for transport layer protocols such as TCP. The send window is similar to the receive window, in that it consumes buffer space (though on the sender). The sender&#39;s send window consists of all data sent by the application that has not been acknowledged by the receiver. This data must be retained in memory in case retransmission is required. Since memory is a shared resource, some TCP stack implementations limit the size of this data. When the send window is full, an attempt by an application program to send more data results in blocking the application program until space is available. Subsequent reception of acknowledgements will free send-window memory and unblock the application program. In some embodiments, this window size is known as the socket buffer size in some TCP implementations. 
     In one embodiment, the flow control module  220  is configured to provide access to increased window (or buffer) sizes. This configuration may also be referenced to as window virtualization. In the embodiment of TCP as the transport layer protocol, the TCP header includes a bit string corresponding to a window scale. In one embodiment, “window” may be referenced in a context of send, receive, or both. 
     One embodiment of window virtualization is to insert a preacking appliance  200  into a TCP session. In reference to any of the environments of  FIG. 1A or 1B , initiation of a data communication session between a source node, e.g., client  102  (for ease of discussion, now referenced as source node  102 ), and a destination node, e.g., server  106  (for ease of discussion, now referenced as destination node  106 ) is established. For TCP communications, the source node  102  initially transmits a synchronization signal (“SYN”) through its local area network  104  to first flow control module  220 . The first flow control module  220  inserts a configuration identifier into the TCP header options area. The configuration identifier identifies this point in the data path as a flow control module. 
     The appliances  200  via a flow control module  220  provide window (or buffer) to allow increasing data buffering capabilities within a session despite having end nodes with small buffer sizes, e.g., typically 16 k bytes. However, RFC 1323 requires window scaling for any buffer sizes greater than 64 k bytes, which must be set at the time of session initialization (SYN, SYN-ACK signals). Moreover, the window scaling corresponds to the lowest common denominator in the data path, often an end node with small buffer size. This window scale often is a scale of 0 or 1, which corresponds to a buffer size of up to 64 k or 128 k bytes. Note that because the window size is defined as the window field in each packet shifted over by the window scale, the window scale establishes an upper limit for the buffer, but does not guarantee the buffer is actually that large. Each packet indicates the current available buffer space at the receiver in the window field. 
     In one embodiment of scaling using the window virtualization technique, during connection establishment (i.e., initialization of a session) when the first flow control module  220  receives from the source node  102  the SYN signal (or packet), the flow control module  220  stores the windows scale of the source node  102  (which is the previous node) or stores a 0 for window scale if the scale of the previous node is missing. The first flow control module  220  also modifies the scale, e.g., increases the scale to 4 from 0 or 1, in the SYN-FCM signal. When the second flow control module  220  receives the SYN signal, it stores the increased scale from the first flow control signal and resets the scale in the SYN signal back to the source node  103  scale value for transmission to the destination node  106 . When the second flow controller  220  receives the SYN-ACK signal from the destination node  106 , it stores the scale from the destination node  106  scale, e.g., 0 or 1, and modifies it to an increased scale that is sent with the SYN-ACK-FCM signal. The first flow control node  220  receives and notes the received window scale and revises the windows scale sent back to the source node  102  back down to the original scale, e.g., 0 or 1. Based on the above window shift conversation during connection establishment, the window field in every subsequent packet, e.g., TCP packet, of the session must be shifted according to the window shift conversion. 
     The window scale, as described above, expresses buffer sizes of over 64 k and may not be required for window virtualization. Thus, shifts for window scale may be used to express increased buffer capacity in each flow control module  220 . This increase in buffer capacity in may be referenced as window (or buffer) virtualization. The increase in buffer size allows greater packet through put from and to the respective end nodes  102  and  106 . Note that buffer sizes in TCP are typically expressed in terms of bytes, but for ease of discussion “packets” may be used in the description herein as it relates to virtualization. 
     By way of example, a window (or buffer) virtualization performed by the flow controller  220  is described. In this example, the source node  102  and the destination node  106  are configured similar to conventional end nodes having a limited buffer capacity of 16 k bytes, which equals approximately 10 packets of data. Typically, an end node  102 ,  106  must wait until the packet is transmitted and confirmation is received before a next group of packets can be transmitted. In one embodiment, using increased buffer capacity in the flow control modules  220 , when the source node  103  transmits its data packets, the first flow control module  220  receives the packets, stores it in its larger capacity buffer, e.g., 512 packet capacity, and immediately sends back an acknowledgement signal indicating receipt of the packets (“REC-ACK”) back to the source node  102 . The source node  102  can then “flush” its current buffer, load it with 10 new data packets, and transmit those onto the first flow control module  220 . Again, the first flow control module  220  transmits a REC-ACK signal back to the source node  102  and the source node  102  flushes its buffer and loads it with 10 more new packets for transmission. 
     As the first flow control module  220  receives the data packets from the source nodes, it loads up its buffer accordingly. When it is ready the first flow control module  220  can begin transmitting the data packets to the second flow control module  230 , which also has an increased buffer size, for example, to receive 512 packets. The second flow control module  220 ′ receives the data packets and begins to transmit 10 packets at a time to the destination node  106 . Each REC-ACK received at the second flow control node  220  from the destination node  106  results in 10 more packets being transmitted to the destination node  106  until all the data packets are transferred. Hence, the present invention is able to increase data transmission throughput between the source node (sender)  102  and the destination node (receiver)  106  by taking advantage of the larger buffer in the flow control modules  220 ,  220 ′ between the devices. 
     It is noted that by “preacking” the transmission of data as described previously, a sender (or source node  102 ) is allowed to transmit more data than is possible without the preacks, thus affecting a larger window size. For example, in one embodiment this technique is effective when the flow control module  220 ,  220 ′ is located “near” a node (e.g., source node  102  or destination node  106 ) that lacks large windows. 
     Recongestion 
     Another technique or algorithm of the flow controller  220  is referred to as recongestion. The standard TCP congestion avoidance algorithms are known to perform poorly in the face of certain network conditions, including: large RTTs (round trip times), high packet loss rates, and others. When the appliance  200  detects a congestion condition such as long round trip times or high packet loss, the appliance  200  intervenes, substituting an alternate congestion avoidance algorithm that better suits the particular network condition. In one embodiment, the recongestion algorithm uses preacks to effectively terminate the connection between the sender and the receiver. The appliance  200  then resends the packets from itself to the receiver, using a different congestion avoidance algorithm. Recongestion algorithms may be dependent on the characteristics of the TCP connection. The appliance  200  monitors each TCP connection, characterizing it with respect to the different dimensions, selecting a recongestion algorithm that is appropriate for the current characterization. 
     In one embodiment, upon detecting a TCP connection that is limited by round trip times (RTT), a recongestion algorithm is applied which behaves as multiple TCP connections. Each TCP connection operates within its own performance limit but the aggregate bandwidth achieves a higher performance level. One parameter in this mechanism is the number of parallel connections that are applied (N). Too large a value of N and the connection bundle achieves more than its fair share of bandwidth. Too small a value of N and the connection bundle achieves less than its fair share of bandwidth. One method of establishing “N” relies on the appliance  200  monitoring the packet loss rate, RTT, and packet size of the actual connection. These numbers are plugged into a TCP response curve formula to provide an upper limit on the performance of a single TCP connection in the present configuration. If each connection within the connection bundle is achieving substantially the same performance as that computed to be the upper limit, then additional parallel connections are applied. If the current bundle is achieving less performance than the upper limit, the number of parallel connections is reduced. In this manner, the overall fairness of the system is maintained since individual connection bundles contain no more parallelism than is required to eliminate the restrictions imposed by the protocol itself. Furthermore, each individual connection retains TCP compliance. 
     Another method of establishing “N” is to utilize a parallel flow control algorithm such as the TCP “Vegas” algorithm or its improved version “Stabilized Vegas.” In this method, the network information associated with the connections in the connection bundle (e.g., RTT, loss rate, average packet size, etc.) is aggregated and applied to the alternate flow control algorithm. The results of this algorithm are in turn distributed among the connections of the bundle controlling their number (i.e., N). Optionally, each connection within the bundle continues using the standard TCP congestion avoidance algorithm. 
     In another embodiment, the individual connections within a parallel bundle are virtualized, i.e., actual individual TCP connections are not established. Instead the congestion avoidance algorithm is modified to behave as though there were N parallel connections. This method has the advantage of appearing to transiting network nodes as a single connection. Thus the QOS, security and other monitoring methods of these nodes are unaffected by the recongestion algorithm. In yet another embodiment, the individual connections within a parallel bundle are real, i.e., a separate. TCP connection is established for each of the parallel connections within a bundle. The congestion avoidance algorithm for each TCP connection need not be modified. 
     Retransmission 
     In some embodiments, the flow controller  220  may apply a local retransmission technique. One reason for implementing preacks is to prepare to transit a high-loss link (e.g., wireless). In these embodiments, the preacking appliance  200  or flow control module  220  is located most beneficially “before” the wireless link. This allows retransmissions to be performed closer to the high loss link, removing the retransmission burden from the remainder of the network. The appliance  200  may provide local retransmission, in which case, packets dropped due to failures of the link are retransmitted directly by the appliance  200 . This is advantageous because it eliminates the retransmission burden upon an end node, such as server  106 , and infrastructure of any of the networks  104 . With appliance  200  providing local retransmissions, the dropped packet can be retransmitted across the high loss link without necessitating a retransmit by an end node and a corresponding decrease in the rate of data transmission from the end node. 
     Another reason for implementing preacks is to avoid a receive time out (RTO) penalty. In standard TCP there are many situations that result in an RTO, even though a large percentage of the packets in flight were successfully received. With standard TCP algorithms, dropping more than one packet within an RTT window would likely result in a timeout. Additionally, most TCP connections experience a timeout if a retransmitted packet is dropped. In a network with a high bandwidth delay product, even a relatively small packet loss rate will cause frequent Retransmission timeouts (RTOs). In one embodiment, the appliance  200  uses a retransmit and timeout algorithm is avoid premature RTOs. The appliance  200  or flow controller  220  maintains a count of retransmissions is maintained on a per-packet basis. Each time that a packet is retransmitted, the count is incremented by one and the appliance  200  continues to transmit packets. In some embodiments, only if a packet has been retransmitted a predetermined number of times is an RTO declared. 
     Wavefront Detection and Disambiguation 
     In some embodiments, the appliance  200  or flow controller  220  uses wavefront detection and disambiguation techniques in managing and controlling flow of network traffic. In this technique, the flow controller  220  uses transmit identifiers or numbers to determine whether particular data packets need to be retransmitted. By way of example, a sender transmits data packets over a network, where each instance of a transmitted data packet is associated with a transmit number. It can be appreciated that the transmit number for a packet is not the same as the packet&#39;s sequence number, since a sequence number references the data in the packet while the transmit number references an instance of a transmission of that data. The transmit number can be any information usable for this purpose, including a timestamp associated with a packet or simply an increasing number (similar to a sequence number or a packet number). Because a data segment may be retransmitted, different transmit numbers may be associated with a particular sequence number. 
     As the sender transmits data packets, the sender maintains a data structure of acknowledged instances of data packet transmissions. Each instance of a data packet transmission is referenced by its sequence number and transmit number. By maintaining a transmit number for each packet, the sender retains the ordering of the transmission of data packets. When the sender receives an ACK or a SACK, the sender determines the highest transmit number associated with packets that the receiver indicated has arrived (in the received acknowledgement). Any outstanding unacknowledged packets with lower transmit numbers are presumed lost. 
     In some embodiments, the sender is presented with an ambiguous situation when the arriving packet has been retransmitted: a standard ACK/SACK does not contain enough information to allow the sender to determine which transmission of the arriving packet has triggered the acknowledgement. After receiving an ambiguous acknowledgement, therefore, the sender disambiguates the acknowledgement to associate it with a transmit number. In various embodiments, one or a combination of several techniques may be used to resolve this ambiguity. 
     In one embodiment, the sender includes an identifier with a transmitted data packet, and the receiver returns that identifier or a function thereof with the acknowledgement. The identifier may be a timestamp (e.g., a TCP timestamp as described in RFC 1323), a sequential number, or any other information that can be used to resolve between two or more instances of a packet&#39;s transmission. In an embodiment in which the TCP timestamp option is used to disambiguate the acknowledgement, each packet is tagged with up to 32-bits of unique information. Upon receipt of the data packet, the receiver echoes this unique information back to the sender with the acknowledgement. The sender ensures that the originally sent packet and its retransmitted version or versions contain different values for the timestamp option, allowing it to unambiguously eliminate the ACK ambiguity. The sender may maintain this unique information, for example, in the data structure in which it stores the status of sent data packets. This technique is advantageous because it complies with industry standards and is thus likely to encounter little or no interoperability issues. However, this technique may require ten bytes of TCP header space in some implementations, reducing the effective throughput rate on the network and reducing space available for other TCP options. 
     In another embodiment, another field in the packet, such as the IP ID field, is used to disambiguate in a way similar to the TCP timestamp option described above. The sender arranges for the ID field values of the original and the retransmitted version or versions of the packet to have different ID fields in the IP header. Upon reception of the data packet at the receiver, or a proxy device thereof, the receiver sets the ID field of the ACK packet to a function of the ID field of the packet that triggers the ACK. This method is advantageous, as it requires no additional data to be sent, preserving the efficiency of the network and TCP header space. The function chosen should provide a high degree of likelihood of providing disambiguation. In a preferred embodiment, the sender selects IP ID values with the most significant bit set to 0. When the receiver responds, the IP ID value is set to the same IP ID value with the most significant bit set to a one. 
     In another embodiment, the transmit numbers associated with non-ambiguous acknowledgements are used to disambiguate an ambiguous acknowledgement. This technique is based on the principle that acknowledgements for two packets will tend to be received closer in time as the packets are transmitted closer in time. Packets that are not retransmitted will not result in ambiguity, as the acknowledgements received for such packets can be readily associated with a transmit number. Therefore, these known transmit numbers are compared to the possible transmit numbers for an ambiguous acknowledgement received near in time to the known acknowledgement. The sender compares the transmit numbers of the ambiguous acknowledgement against the last known received transmit number, selecting the one closest to the known received transmit number. For example, if an acknowledgement for data packet  1  is received and the last received acknowledgement was for data packet  5 , the sender resolves the ambiguity by assuming that the third instance of data packet  1  caused the acknowledgement. 
     Selective Acknowledgements 
     Another technique of the appliance  200  or flow controller  220  is to implement an embodiment of transport control protocol selective acknowledgements, or TCP SACK, to determine what packets have or have not been received. This technique allows the sender to determine unambiguously a list of packets that have been received by the receiver as well as an accurate list of packets not received. This functionality may be implemented by modifying the sender and/or receiver, or by inserting sender- and receiver-side flow control modules  220  in the network path between the sender and receiver. In reference to  FIG. 1A  or  FIG. 1B , a sender, e.g., client  102 , is configured to transmit data packets to the receiver, e.g., server  106 , over the network  104 . In response, the receiver returns a TCP Selective Acknowledgment option, referred to as SACK packet to the sender. In one embodiment, the communication is bi-directional, although only one direction of communication is discussed here for simplicity. The receiver maintains a list, or other suitable data structure, that contains a group of ranges of sequence numbers for data packets that the receiver has actually received. In some embodiments, the list is sorted by sequence number in an ascending or descending order. The receiver also maintains a left-off pointer, which comprises a reference into the list and indicates the left-off point from the previously generated SACK packet. 
     Upon reception of a data packet, the receiver generates and transmits a SACK packet back to the sender. In some embodiments, the SACK packet includes a number of fields, each of which can hold a range of sequence numbers to indicate a set of received data packets. The receiver fills this first field of the SACK packet with a range of sequence numbers that includes the landing packet that triggered the SACK packet. The remaining available SACK fields are filled with ranges of sequence numbers from the list of received packets. As there are more ranges in the list than can be loaded into the SACK packet, the receiver uses the left-off pointer to determine which ranges are loaded into the SACK packet. The receiver inserts the SACK ranges consecutively from the sorted list, starting from the range referenced by the pointer and continuing down the list until the available SACK range space in the TCP header of the SACK packet is consumed. The receiver wraps around to the start of the list if it reaches the end. In some embodiments, two or three additional SACK ranges can be added to the SACK range information. 
     Once the receiver generates the SACK packet, the receiver sends the acknowledgement back to the sender. The receiver then advances the left-off pointer by one or more SACK range entries in the list. If the receiver inserts four SACK ranges, for example, the left-off pointer may be advanced two SACK ranges in the list. When the advanced left-off pointer reaches at the end of the list, the pointer is reset to the start of the list, effectively wrapping around the list of known received ranges. Wrapping around the list enables the system to perform well, even in the presence of large losses of SACK packets, since the SACK information that is not communicated due to a lost SACK packet will eventually be communicated once the list is wrapped around. 
     It can be appreciated, therefore, that a SACK packet may communicate several details about the condition of the receiver. First, the SACK packet indicates that, upon generation of the SACK packet, the receiver had just received a data packet that is within the first field of the SACK information. Secondly, the second and subsequent fields of the SACK information indicate that the receiver has received the data packets within those ranges. The SACK information also implies that the receiver had not, at the time of the SACK packet&#39;s generation, received any of the data packets that fall between the second and subsequent fields of the SACK information. In essence, the ranges between the second and subsequent ranges in the SACK information are “holes” in the received data, the data therein known not to have been delivered. Using this method, therefore, when a SACK packet has sufficient space to include more than two SACK ranges, the receiver may indicate to the sender a range of data packets that have not yet been received by the receiver. 
     In another embodiment, the sender uses the SACK packet described above in combination with the retransmit technique described above to make assumptions about which data packets have been delivered to the receiver. For example, when the retransmit algorithm (using the transmit numbers) declares a packet lost, the sender considers the packet to be only conditionally lost, as it is possible that the SACK packet identifying the reception of this packet was lost rather than the data packet itself. The sender thus adds this packet to a list of potentially lost packets, called the presumed lost list. Each time a SACK packet arrives, the known missing ranges of data from the SACK packet are compared to the packets in the presumed lost list. Packets that contain data known to be missing are declared actually lost and are subsequently retransmitted. In this way, the two schemes are combined to give the sender better information about which packets have been lost and need to be retransmitted. 
     Transaction Boundary Detection 
     In some embodiments, the appliance  200  or flow controller  220  applies a technique referred to as transaction boundary detection. In one embodiment, the technique pertains to ping-pong behaved connections. At the TCP layer, ping-pong behavior is when one communicant—a sender-sends data and then waits for a response from the other communicant—the receiver. Examples of ping-pong behavior include remote procedure call, HTTP and others. The algorithms described above use retransmission timeout (RTO) to recover from the dropping of the last packet or packets associated with the transaction. Since the TCP RTO mechanism is extremely coarse in some embodiments, for example requiring a minimum one second value in all cases), poor application behavior may be seen in these situations. 
     In one embodiment, the sender of data or a flow control module  220  coupled to the sender detects a transaction boundary in the data being sent. Upon detecting a transaction boundary, the sender or a flow control module  220  sends additional packets, whose reception generates additional ACK or SACK responses from the receiver. Insertion of the additional packets is preferably limited to balance between improved application response time and network capacity utilization. The number of additional packets that is inserted may be selected according to the current loss rate associated with that connection, with more packets selected for connections having a higher loss rate. 
     One method of detecting a transaction boundary is time based. If the sender has been sending data and ceases, then after a period of time the sender or flow control module  200  declares a transaction boundary. This may be combined with other techniques. For example, the setting of the PSH (TCP Push) bit by the sender in the TCP header may indicate a transaction boundary. Accordingly, combining the time-based approach with these additional heuristics can provide for more accurate detection of a transaction boundary. In another technique, if the sender or flow control module  220  understands the application protocol, it can parse the protocol data stream and directly determine transaction boundaries. In some embodiment, this last behavior can be used independent of any time-based mechanism. 
     Responsive to detecting a transaction boundary, the sender or flow control module  220  transmits additional data packets to the receiver to cause acknowledgements therefrom. The additional data packets should therefore be such that the receiver will at least generate an ACK or SACK in response to receiving the data packet. In one embodiment, the last packet or packets of the transaction are simply retransmitted. This has the added benefit of retransmitting needed data if the last packet or packets had been dropped, as compared to merely sending dummy data packets. In another embodiment, fractions of the last packet or packets are sent, allowing the sender to disambiguate the arrival of these packets from their original packets. This allows the receiver to avoid falsely confusing any reordering adaptation algorithms. In another embodiment, any of a number of well-known forward error correction techniques can be used to generate additional data for the inserted packets, allowing for the reconstruction of dropped or otherwise missing data at the receiver. 
     In some embodiments, the boundary detection technique described herein helps to avoid a timeout when the acknowledgements for the last data packets in a transaction are dropped. When the sender or flow control module  220  receives the acknowledgements for these additional data packets, the sender can determine from these additional acknowledgements whether the last data packets have been received or need to be retransmitted, thus avoiding a timeout. In one embodiment, if the last packets have been received but their acknowledgements were dropped, a flow control module  220  generates an acknowledgement for the data packets and sends the acknowledgement to the sender, thus communicating to the sender that the data packets have been delivered. In another embodiment, if the last packets have not been received, a flow control module  200  sends a packet to the sender to cause the sender to retransmit the dropped data packets. 
     Repacketization 
     In yet another embodiment, the appliance  200  or flow controller  220  applies a repacketization technique for improving the flow of transport layer network traffic. In some embodiments, performance of TCP is proportional to packet size. Thus increasing packet sizes improves performance unless it causes substantially increased packet loss rates or other nonlinear effects, like IP fragmentation. In general, wired media (such as copper or fibre optics) have extremely low bit-error rates, low enough that these can be ignored. For these media, it is advantageous for the packet size to be the maximum possible before fragmentation occurs (the maximum packet size is limited by the protocols of the underlying transmission media). Whereas for transmission media with higher loss rates (e.g., wireless technologies such as WiFi, etc., or high-loss environments such as power-line networking, etc.), increasing the packet size may lead to lower transmission rates, as media-induced errors cause an entire packet to be dropped (i.e., media-induced errors beyond the capability of the standard error correcting code for that media), increasing the packet loss rate. A sufficiently large increase in the packet loss rate will actually negate any performance benefit of increasing packet size. In some cases, it may be difficult for a TCP endpoint to choose an optimal packet size. For example, the optimal packet size may vary across the transmission path, depending on the nature of each link. 
     By inserting an appliance  200  or flow control module  220  into the transmission path, the flow controller  220  monitors characteristics of the link and repacketizes according to determined link characteristics. In one embodiment, an appliance  200  or flow controller  220  repacketizes packets with sequential data into a smaller number of larger packets. In another embodiment, an appliance  200  or flow controller  220  repacketizes packets by breaking part a sequence of large packets into a larger number of smaller packets. In other embodiments, an appliance  200  or flow controller  220  monitors the link characteristics and adjusts the packet sizes through recombination to improve throughput. 
     QoS 
     Still referring to  FIG. 2A , the flow controller  220 , in some embodiments, may include a QoS Engine  236 , also referred to as a QoS controller. In another embodiment, the appliance  200  and/or network optimization engine  250  includes the QoS engine  236 , for example, separately but in communication with the flow controller  220 . The QoS Engine  236  includes any logic, business rules, function or operations for performing one or more Quality of Service (QoS) techniques improving the performance, operation or quality of service of any of the network connections. In some embodiments, the QoS engine  236  includes network traffic control and management mechanisms that provide different priorities to different users, applications, data flows or connections. In other embodiments, the QoS engine  236  controls, maintains, or assures a certain level of performance to a user, application, data flow or connection. In one embodiment, the QoS engine  236  controls, maintains or assures a certain portion of bandwidth or network capacity for a user, application, data flow or connection. In some embodiments, the QoS engine  236  monitors the achieved level of performance or the quality of service corresponding to a user, application, data flow or connection, for example, the data rate and delay. In response to monitoring, the QoS engine  236  dynamically controls or adjusts scheduling priorities of network packets to achieve the desired level of performance or quality of service. 
     In some embodiments, the QoS engine  236  prioritizes, schedules and transmits network packets according to one or more classes or levels of services. In some embodiments, the class or level service may include: 1) best efforts, 2) controlled load, 3) guaranteed or 4) qualitative. For a best efforts class of service, the appliance  200  makes reasonable effort to deliver packets (a standard service level). For a controlled load class of service, the appliance  200  or QoS engine  236  approximates the standard packet error loss of the transmission medium or approximates the behavior of best-effort service in lightly loaded network conditions. For a guaranteed class of service, the appliance  200  or QoS engine  236  guarantees the ability to transmit data at a determined rate for the duration of the connection. For a qualitative class of service, the appliance  200  or QoS engine  236  the qualitative service class is used for applications, users, data flows or connection that require or desire prioritized traffic but cannot quantify resource needs or level of service. In these cases, the appliance  200  or QoS engine  236  determines the class of service or prioritization based on any logic or configuration of the QoS engine  236  or based on business rules or policies. For example, in one embodiment, the QoS engine  236  prioritizes, schedules and transmits network packets according to one or more policies as specified by the policy engine  295 ,  295 ′. 
     Protocol Acceleration 
     The protocol accelerator  234  includes any logic, business rules, function or operations for optimizing, accelerating, or otherwise improving the performance, operation or quality of service of one or more protocols. In one embodiment, the protocol accelerator  234  accelerates any application layer protocol or protocols at layers 5-7 of the network stack. In other embodiments, the protocol accelerator  234  accelerates a transport layer or a layer 4 protocol. In one embodiment, the protocol accelerator  234  accelerates layer 2 or layer 3 protocols. In some embodiments, the protocol accelerator  234  is configured, constructed or designed to optimize or accelerate each of one or more protocols according to the type of data, characteristics and/or behavior of the protocol. In another embodiment, the protocol accelerator  234  is configured, constructed or designed to improve a user experience, response times, network or computer load, and/or network or bandwidth utilization with respect to a protocol. 
     In one embodiment, the protocol accelerator  234  is configured, constructed or designed to minimize the effect of WAN latency on file system access. In some embodiments, the protocol accelerator  234  optimizes or accelerates the use of the CIFS (Common Internet File System) protocol to improve file system access times or access times to data and files. In some embodiments, the protocol accelerator  234  optimizes or accelerates the use of the NFS (Network File System) protocol. In another embodiment, the protocol accelerator  234  optimizes or accelerates the use of the File Transfer protocol (FTP). 
     In one embodiment, the protocol accelerator  234  is configured, constructed or designed to optimize or accelerate a protocol carrying as a payload or using any type and form of markup language. In other embodiments, the protocol accelerator  234  is configured, constructed or designed to optimize or accelerate a HyperText Transfer Protocol (HTTP). In another embodiment, the protocol accelerator  234  is configured, constructed or designed to optimize or accelerate a protocol carrying as a payload or otherwise using XML (eXtensible Markup Language). 
     Transparency and Multiple Deployment Configuration 
     In some embodiments, the appliance  200  and/or network optimization engine  250  is transparent to any data flowing across a network connection or link, such as a WAN link. In one embodiment, the appliance  200  and/or network optimization engine  250  operates in such a manner that the data flow across the WAN is recognizable by any network monitoring, QOS management or network analysis tools. In some embodiments, the appliance  200  and/or network optimization engine  250  does not create any tunnels or streams for transmitting data that may hide, obscure or otherwise make the network traffic not transparent. In other embodiments, the appliance  200  operates transparently in that the appliance does not change any of the source and/or destination address information or port information of a network packet, such as internet protocol addresses or port numbers. In other embodiments, the appliance  200  and/or network optimization engine  250  is considered to operate or behave transparently to the network, an application, client, server or other appliances or computing device in the network infrastructure. That is, in some embodiments, the appliance is transparent in that network related configuration of any device or appliance on the network does not need to be modified to support the appliance  200 . 
     The appliance  200  may be deployed in any of the following deployment configurations: 1) in-line of traffic, 2) in proxy mode, or 3) in a virtual in-line mode. In some embodiments, the appliance  200  may be deployed inline to one or more of the following: a router, a client, a server or another network device or appliance. In other embodiments, the appliance  200  may be deployed in parallel to one or more of the following: a router, a client, a server or another network device or appliance. In parallel deployments, a client, server, router or other network appliance may be configured to forward, transfer or transit networks to or via the appliance  200 . 
     In the embodiment of in-line, the appliance  200  is deployed inline with a WAN link of a router. In this way, all traffic from the WAN passes through the appliance before arriving at a destination of a LAN. 
     In the embodiment of a proxy mode, the appliance  200  is deployed as a proxy device between a client and a server. In some embodiments, the appliance  200  allows clients to make indirect connections to a resource on a network. For example, a client connects to a resource via the appliance  200 , and the appliance provides the resource either by connecting to the resource, a different resource, or by serving the resource from a cache. In some cases, the appliance may alter the client&#39;s request or the server&#39;s response for various purposes, such as for any of the optimization techniques discussed herein. In other embodiments, the appliance  200  behaves as a transparent proxy, by intercepting and forwarding requests and responses transparently to a client and/or server. Without client-side configuration, the appliance  200  may redirect client requests to different servers or networks. In some embodiments, the appliance  200  may perform any type and form of network address translation, referred to as NAT, on any network traffic traversing the appliance. 
     In some embodiments, the appliance  200  is deployed in a virtual in-line mode configuration. In this embodiment, a router or a network device with routing or switching functionality is configured to forward, reroute or otherwise provide network packets destined to a network to the appliance  200 . The appliance  200  then performs any desired processing on the network packets, such as any of the WAN optimization techniques discussed herein. Upon completion of processing, the appliance  200  forwards the processed network packet to the router to transmit to the destination on the network. In this way, the appliance  200  can be coupled to the router in parallel but still operate as it if the appliance  200  were inline. This deployment mode also provides transparency in that the source and destination addresses and port information are preserved as the packet is processed and transmitted via the appliance through the network. 
     End Node Deployment 
     Although the network optimization engine  250  is generally described above in conjunction with an appliance  200 , the network optimization engine  250 , or any portion thereof, may be deployed, distributed or otherwise operated on any end node, such as a client  102  and/or server  106 . As such, a client or server may provide any of the systems and methods of the network optimization engine  250  described herein in conjunction with one or more appliances  200  or without an appliance  200 . 
     Referring now to  FIG. 2B , an example embodiment of the network optimization engine  250  deployed on one or more end nodes is depicted. In brief overview, the client  102  may include a first network optimization engine  250 ′ and the server  106  may include a second network optimization engine  250 ″. The client  102  and server  106  may establish a transport layer connection and exchange communications with or without traversing an appliance  200 . 
     In one embodiment, the network optimization engine  250 ′ of the client  102  performs the techniques described herein to optimize, accelerate or otherwise improve the performance, operation or quality of service of network traffic communicated with the server  106 . In another embodiment, the network optimization engine  250 ″ of the server  106  performs the techniques described herein to optimize, accelerate or otherwise improve the performance, operation or quality of service of network traffic communicated with the client  102 . In some embodiments, the network optimization engine  250 ′ of the client  102  and the network optimization engine  250 ″ of the server  106  perform the techniques described herein to optimize, accelerate or otherwise improve the performance, operation or quality of service of network traffic communicated between the client  102  and the server  106 . In yet another embodiment, the network optimization engine  250 ′ of the client  102  performs the techniques described herein in conjunction with an appliance  200  to optimize, accelerate or otherwise improve the performance, operation or quality of service of network traffic communicated with the client  102 . In still another embodiment, the network optimization engine  250 ″ of the server  106  performs the techniques described herein in conjunction with an appliance  200  to optimize, accelerate or otherwise improve the performance, operation or quality of service of network traffic communicated with the server  106 . 
     C. Client Agent 
     Referring now to  FIG. 3 , an embodiment of a client agent  120  is depicted. The client  102  has a client agent  120  for establishing, exchanging, managing or controlling communications with the appliance  200 , appliance  205  and/or server  106  via a network  104 . In some embodiments, the client agent  120 , which may also be referred to as a WAN client, accelerates WAN network communications and/or is used to communicate via appliance  200  on a network. 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  267  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  267  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  267  from the one or more applications. 
     As with the appliance  200 , the client has a network stack  267  including any type and form of software, hardware, or any combinations thereof, for providing connectivity to and communications with a network  104 . The network stack  267  of the client  102  includes any of the network stack embodiments described above in conjunction with the appliance  200 . In some embodiments, the client agent  120 , or any portion thereof, is designed and constructed to operate with or work in conjunction with the network stack  267  installed or otherwise provided by the operating system of the client  102 . 
     In further details, the network stack  267  of the client  102  or appliance  200  (or  205 ) may include 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  267  includes an application programming interface (API). The interface may also have any function call, hooking or filtering mechanism, event or call back mechanism, or any type of interfacing technique. The network stack  267  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  267 . For example, the data structure may include information and data related to a network packet or one or more network packets. In some embodiments, the data structure includes, references or identifies a portion of the network packet processed at a protocol layer of the network stack  267 , such as a network packet of the transport layer. In some embodiments, the data structure  325  is a kernel-level data structure, while in other embodiments, the data structure  325  is a user-mode data structure. A kernel-level data structure may have a data structure obtained or related to a portion of the network stack  267  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  267  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  267 . For example, a first portion  310   a  of the network stack may provide user-mode access to the network stack  267  to an application while a second portion  310   a  of the network stack  267  provides access to a network. In some embodiments, a first portion  310   a  of the network stack has one or more upper layers of the network stack  267 , such as any of layers 5-7. In other embodiments, a second portion  310   b  of the network stack  267  includes 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  267  may include any portion of the network stack  267 , at any one or more network layers, in user-mode  303 , kernel-mode,  302 , 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  302  and kernel-mode  203 . 
     The interceptor  350  may include software, hardware, or any combination of software and hardware. In one embodiment, the interceptor  350  intercepts or otherwise receives a network communication at any point in the network stack  267 , 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  267  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  includes or is a driver, such as a network driver constructed and designed to interface and work with the network stack  267 . In some embodiments, the client agent  120  and/or interceptor  350  operates at one or more layers of the network stack  267 , such as at the transport layer. In one embodiment, the interceptor  350  includes 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  includes a driver complying with the Network Driver Interface Specification (NDIS), or a NDIS driver. In another embodiment, the interceptor  350  may be 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 or receives any transport layer connection requests. In these embodiments, the interceptor  350  executes 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 an 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 include two agents  120  and  120 ′. In one embodiment, a first agent  120  may include an interceptor  350  operating at the network layer of the network stack  267 . 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  267 . For example, in one embodiment, the interceptor  350  operates or interfaces with the transport layer of the network stack  267  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  267  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  350  may operate at or interface with the network stack  267  in a manner transparent to any application, a user of the client  102 , the client  102  and/or any other computing device  100 , such as a server or appliance  200 ,  206 , in communications with the client  102 . The client agent  120 , or any portion thereof, may be installed and/or executed on the client  102  in a manner without modification of an application. In one embodiment, the client agent  120 , or any portion thereof, is installed and/or executed in a manner transparent to any network configuration of the client  102 , appliance  200 ,  205  or server  106 . In some embodiments, the client agent  120 , or any portion thereof, is installed and/or executed with modification to any network configuration of the client  102 , appliance  200 ,  205  or server  106 . In one embodiment, 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  12 , or any portion thereof. 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 , the client  102 , another computing device, such as a server or appliance  200 ,  2005 , or any of the protocol layers above and/or below the protocol layer interfaced to by the interceptor  350 . 
     The client agent  120  includes a streaming client  306 , a collection agent  304 , SSL VPN agent  308 , a network optimization engine  250 , and/or acceleration program  302 . In one embodiment, the client agent  120  is 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 agent  120  has an application streaming client  306  for streaming an application from a server  106  to a client  102 . 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 client agent  120  has one or more network accelerating or optimizing programs or agents, such as an network optimization engine  250  and an acceleration program  302 . In one embodiment, the acceleration program  302  accelerates communications between client  102  and server  106  via appliance  205 ′. In some embodiments, the network optimization engine  250  provides WAN optimization techniques as discussed herein. 
     The streaming client  306  is an application, program, process, service, task or set of 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  is an application, program, process, service, task or set of 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  is an end-point detection and scanning program, 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 version 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. 
     The SSL VPN agent  308  is an application, program, process, service, task or set of executable instructions for establishing a Secure Socket Layer (SSL) virtual private network (VPN) connection from a first network  104  to a second network  104 ′,  104 ″, or a SSL VPN connection from a client  102  to a server  106 . In one embodiment, the SSL VPN agent  308  establishes a SSL VPN connection from a public network  104  to a private network  104 ′ or  104 ″. In some embodiments, the SSL VPN agent  308  works in conjunction with appliance  205  to provide the SSL VPN connection. In one embodiment, the SSL VPN agent  308  establishes a first transport layer connection with appliance  205 . In some embodiment, the appliance  205  establishes a second transport layer connection with a server  106 . In another embodiment, the SSL VPN agent  308  establishes a first transport layer connection with an application on the client, and a second transport layer connection with the appliance  205 . In other embodiments, the SSL VPN agent  308  works in conjunction with WAN optimization appliance  200  to provide SSL VPN connectivity. 
     In some embodiments, the acceleration program  302  is 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. 
     In one embodiment, the acceleration program  302  is designed, constructed or configured to work with appliance  205  to provide LAN side acceleration or to provide acceleration techniques provided via appliance  205 . For example, in one embodiment of a NetScaler appliance  205  manufactured by Citrix Systems, Inc., the acceleration program  302  includes a NetScaler client. In some embodiments, the acceleration program  302  provides NetScaler acceleration techniques stand-alone in a remote device, such as in a branch office. In other embodiments, the acceleration program  302  works in conjunction with one or more NetScaler appliances  205 . In one embodiment, the acceleration program  302  provides LAN-side or LAN based acceleration or optimization of network traffic. 
     In some embodiments, the network optimization engine  250  may be designed, constructed or configured to work with WAN optimization appliance  200 . In other embodiments, network optimization engine  250  may be designed, constructed or configured to provide the WAN optimization techniques of appliance  200 , with or without an appliance  200 . For example, in one embodiment of a WANScaler appliance  200  manufactured by Citrix Systems, Inc. the network optimization engine  250  includes the WANscaler client. In some embodiments, the network optimization engine  250  provides WANScaler acceleration techniques stand-alone in a remote location, such as a branch office. In other embodiments, the network optimization engine  250  works in conjunction with one or more WANScaler appliances  200 . 
     In another embodiment, the network optimization engine  250  includes the acceleration program  302 , or the function, operations and logic of the acceleration program  302 . In some embodiments, the acceleration program  302  includes the network optimization engine  250  or the function, operations and logic of the network optimization engine  250 . In yet another embodiment, the network optimization engine  250  is provided or installed as a separate program or set of executable instructions from the acceleration program  302 . In other embodiments, the network optimization engine  250  and acceleration program  302  are included in the same program or same set of executable instructions. 
     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 any portion thereof, automatically, silently, transparently, or otherwise. In one embodiment, the first program  322  is 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  is 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 and the client agent  120 , or any portion thereof, on the operating system of the client  102 . 
     In some embodiments, each or any of the portions of the client agent  120 —a streaming client  306 , a collection agent  304 , SSL VPN agent  308 , a network optimization engine  250 , acceleration program  302 , and interceptor  350 —may be installed, executed, configured or operated as a separate application, program, process, service, task or set of executable instructions. In other embodiments, each or any of the portions of the client agent  120  may be installed, executed, configured or operated together as a single client agent  120 . 
     D. Systems and Methods for Providing Quality of Service of a Plurality of Applications via a Flow Controlled Tunnel 
     In many systems executing a plurality of applications, each application may create or utilize a link to one or more servers, and the speed or bandwidth of these links may be independently negotiated between each application and server. Accordingly, applying QoS techniques in these systems may require complicated management to avoid inadvertent effects. For example, congestion on a high-priority link may result in delaying communications on uncongested low-priority links. Furthermore, QoS policies frequently are required to be individually configured per link. When a client moves to a new location, such as a mobile client connecting from a hotel room, previously configured QoS policies may be unable to be utilized. 
     Accordingly, by directing traffic from a plurality of applications into a single connection or flow-controlled tunnel, QoS policies may be applied across the plurality of applications without link speed configuration. In some embodiments, this tunnel enables QoS scheduling to dynamically adjust traffic transmission and reception rates to ensure priority management of applications regardless of a final endpoint of the application communications. Accordingly, traffic of different types, including VPN, HTTP, Voice-over-IP (VoIP), remote desktop protocol traffic, or other traffic may be easily balanced and prioritized. Additionally, by utilizing a flow controlled tunnel, the QoS provider may not need to know the rate of the connection between the client and server. Thus, QoS prioritization and scheduling can be maintained even if the connection rate changes dynamically, for example due to interference or station hand-off on a cellular or wireless WAN connection, congestion on a LAN or WAN, bandwidth allotment being divided among a plurality of users on a network as they connect and disconnect (such as bandwidth availability to a hotel room changing depending on the number of guests using the network), or any other reason. For example, in a Wireless WAN connection, such as 35, bandwidth can go from megabits per second to zero and back in a matter of seconds. Due to slow start algorithms used in TCP that drop throughput exponentially on congestion, but increase linearly, brief drops in bandwidth cause large efficiency losses and wastes bandwidth when the link recovers. However, by using a tunnel for flow control, this waste can be avoided. Furthermore, in many embodiments, the tunnel may be transparent to applications, such that without any application configuration, application traffic may still be prioritized by QoS requirements. 
     Many embodiments of systems applying QoS prioritization have system or application-specific options. For example, the Independent Computing Architecture (ICA) protocol may use TCP options for setting or applying QoS prioritization, but some intermediary devices between a client and server may not support these options. By passing traffic of these sorts through a tunnel connection, the options may be preserved and processed by the end destination, regardless of the capabilities of any intermediaries. 
     Referring now to  FIG. 4A , shown is a block diagram of an embodiment of a system for providing quality of service of a plurality of applications via a flow controlled tunnel. In brief overview, in some embodiments, a client  102  communicates via a tunnel protocol  408  over network  104  to an intermediary appliance  200 . Although illustrated as an intermediary device, such as an appliance, in many embodiments, client  102  may communicate via the tunnel protocol to another client, a server, or any other device acting as a tunnel endpoint. Similarly, although illustrated as a client  102 , in may embodiments, the first endpoint of the tunnel may be another client, a server, an appliance, or any other device. Endpoints of the tunnel, such as client  102  and appliance  200 , may include a queue tunneler client  400   a  or host  400   b , referred to generally as a queue tunneler  400 . Although referred to as client and host, in many embodiments, a queue tunneler  400  includes both a client and server process, allowing two-way tunneled communications. 
     In some embodiments, a client or device at one end of the tunnel may execute or communicate with other clients executing one or more applications. Traffic from these applications may be prioritized or processed using any of the other techniques discussed herein, including compression, acceleration, and encryption. The queue tunneler client  400   a  may encapsulate this traffic with the tunnel protocol and direct the traffic into the tunnel to the other endpoint. At the opposite tunnel endpoint, appliance  200  in the embodiment shown in  FIG. 4A , the traffic may be de-encapsulated by queue tunneler  400   b  and forwarded to sockets corresponding to connections to one or more application servers  106 , appliances, or clients, via one or more networks. Thus, in some embodiments, the appliance or other endpoint may provide both QoS prioritization functions and rerouting of received traffic to various service providers. 
     Return traffic from application servers, appliances or other clients may be similarly compressed, accelerated, encrypted, or otherwise processed and prioritized in QoS order by the tunnel endpoint, appliance  200  in  FIG. 4A . This traffic may then be encapsulated and forwarded to client  102  via the tunnel connection. Upon receipt, the queue tunneler  400  on client  102  may de-encapsulate the traffic, perform other processing including decryption and decompression, and pass application-specific responses to each of the one or more applications in QoS priority. 
     Referring now to  FIG. 4B , illustrated is a block diagram of an embodiment of a computing device for providing quality of service of a plurality of applications via a flow controlled tunnel. In brief overview, the computing device may comprise a client, appliance, server, or other computing device, and may include a QoS and redirection module  404 , queue tunnel proxy module  406 , and tunnel protocol  408 , collectively referred to in some embodiments as queue tunneler  400 . As shown, the computing device may also include a plurality of applications or servers, a transport and network protocol  402 , and a client agent  120 . 
     Referring to  FIG. 4B  and in more detail, in some embodiments, one or more applications send traffic via a transport and network protocol  402 . In many embodiments, transport and network protocol  402  may comprise a transport protocol such as TCP, UDP, stream control transmission protocol (SCTP), FAST TCP, reliable datagram sockets (RDS) protocol, Generic Routing Encapsulation (GRE), or any other transport protocol, and a network protocol such as IPv4, IPv6, or IPSec. In some embodiments, transport and network protocol  402  may comprise a combined protocol such as AppleTalk or IPX. 
     Application traffic passed through or encapsulated by transport and network protocol  402  may be intercepted or otherwise received by QoS and redirection module  404 . QoS and redirection module  404  may comprise hardware, software, or a combination of hardware and software, and may comprise an application, server, process, service, daemon, or other executable logic for intercepting or receiving communications, prioritizing traffic according to QoS, and redirecting traffic to a tunnel proxy module. In one embodiment in which the tunnel operates transparently to applications, traffic from transport and network protocol  402  may include destination addresses and ports that are external to the computing device. In other embodiments, traffic from transport and network protocol  402  may be directed to a local address hosted by QoS and redirection module  404  serving as a proxy server. In these embodiments, QoS and redirection module  404  may perform address translation or lookup or parse application traffic to determine destination addresses and/or ports. 
     In some embodiments, QoS and redirection module  404  may comprise one or more filters and redirectors. Filters may be used to parse outgoing vs. incoming traffic, for example, or for redirecting traffic to or from another application. For example, as shown in  FIG. 4B  with applications  1  and  2 , in many embodiments some application traffic may be redirected to a client agent  120 , discussed in detail above. This may be done for purposes such as acceleration, compression, encryption, or other features provided by the client agent  120 . Other traffic, such as that associated with application  3  shown in  FIG. 4B , may be redirected to queue tunnel proxy module  406  without passing through the client agent. This may be done, for example, to separate VoIP traffic from ICA, RDP or other traffic, for processing using lower latency techniques. Similarly, traffic from the client agent or directed to the client agent from an external device may also be filtered and redirected via the queue tunnel proxy module  406 . This avoids traffic from the client agent  120  being redirected back to the client agent endlessly. 
     QoS and redirection module  404  may also comprise one or more buffers or queues for performing QoS prioritization. In some embodiments, QoS and redirection module  404  may include a single queue or buffer and move data packets within the buffer according to priority. In other embodiments, QoS and redirection module  404  may include multiple buffers or queues, each corresponding to a priority class, such as background, very-low, low, medium, high, or very-high. Similarly, queues may be associated with a priority class by value, such as 10, 20, 30, 40, 50, 60, or 70, or any other value. In other embodiments, QoS and redirection module  404  may include a buffer, queue, or other similar structure for priority processing of traffic. For example, some traffic may be designated for immediate transmission, and QoS and redirection module  404  may transmit this traffic immediately, regardless of waiting items in even a high-priority queue. QoS and redirection module  404  may comprise one or more counters or timers for processing packets from buffers or queues in order of priority. 
     In some embodiments, QoS and redirection module  404  may direct packets and other network traffic into a tunnel provided by queue tunnel proxy module  406  in order of QoS priority. Similarly, packets and network traffic received over the tunnel provided by the queue tunnel proxy module  406  may be redirected by QoS and redirection module back to client agent  120  or one or more applications via the transport and network protocol. Because packets may be provided over the tunnel from the remote side in order of priority, in some embodiments, QoS and redirection module  404  may not need to perform buffering or queuing on packets received from queue tunnel proxy module  406 . In other embodiments, QoS and redirection module  404  may buffer packets until requested by an application or until the application otherwise prepared to receive them. This may allow for multi-threaded operation. 
     In some embodiments, QoS and redirection module  404  may further include one or more classifiers to parse and classify application traffic. For example, traffic may be classified as corresponding to an application, such as Microsoft Outlook; a group of applications, such as “email” or “games”; a destination; a source; a protocol, such as HTTP or TCP; an application or service provided by another application, such as Google web applications or a Flash-based media player; or any other classifier. Classifying traffic may allow QoS and redirection module  404  to apply QoS prioritization at different levels or according to different policies, based on any identified characteristics. 
     Queue tunnel proxy module  406  may comprise hardware, software, or a combination of hardware and software, for processing packets for transmission and receipt via a tunnel protocol  408 . Queue tunnel proxy module  406  may comprise an application, server, process, service, daemon, or other executable logic for encapsulating and de-encapsulating traffic with a tunnel protocol. Various tunnel protocols may be utilized, including generic routing encapsulation (GRE); layer 2 tunneling protocol (L2TP); UDP-based Data Transport, sometimes referred to as UDP-based Data Transfer Protocol (UDT); SCTP; TCP; or any other protocol. Additionally, the tunnel protocol may use SSL or a similar security protocol for encryption. In some embodiments, packets encapsulated by the transport and network protocol  402  may be further encapsulated as a payload to the tunnel protocol, which may comprise a UDP or TCP-based tunnel. In some embodiments, queue tunnel proxy module  406  may include functionality for establishing various tunnels. For example, if the infrastructure does not support a UDP-based tunneling protocol, in these embodiments, the queue tunnel proxy module  406  may establish a different tunnel, such as a TCP-based tunnel. UDP-based tunnels may be more efficient in some embodiments, due to the reduction in the number of acknowledgements. Flow control may be provided by the queue tunnel proxy module  406 , for example, using a real-time control protocol (RTCP) or similar feedback mechanisms. 
     In some embodiments, incoming traffic via tunnel protocol  408  may be processed and de-encapsulated by tunnel proxy module  406  and passed to QoS and redirection module  404 . In one embodiment, as shown in  FIG. 4B , QoS and redirection module  404  may then route the incoming traffic via transport and network protocol  402  to the corresponding application or client agent  120 . Applications external to the computing device may also be targets of incoming traffic communication. For example, as shown in  FIG. 4A , an intermediary device receiving tunnel traffic may de-encapsulate and route traffic to one or more destinations external to the device. In some embodiments, the traffic may include a destination IP address and/or port which the computing device may use for redirection. In other embodiments, the traffic may correspond to a predetermined application, such as VoIP, and the intermediary may have a table or other information indicating a destination for traffic corresponding to the predetermined application, such as a VoIP server. In still other embodiments, the intermediary device may parse the traffic for identifiers indicating a destination for re-direction, such as a URL, MAC address, VLAN, or other indicator. 
     Redirection of tunneled traffic by an intermediary may be easier in client-initiated connections than in server-initiated connections, because the initiation request may implicitly or explicitly identify a destination. However, in one embodiment, the client agent  120  may communicate with the intermediary, during establishment of the tunnel or at some other point, to identify the client, an application of the client, a capability of the client, or other identifying information including IP addresses, virtual IP addresses, ports, virtual ports, VLANs, MAC addresses, or other identifiers. When a server attempts to initiate a connection to the client via the intermediary, in this embodiment, the intermediary may use this information provided by the client agent to achieve proper routing and identify destinations on the client. 
     In some embodiments, the queue tunnel proxy module  406  may transmit data provided by sockets from QoS and redirection module  404  in an order of priority, using a weighted fair queuing (WFQ), fair queuing, round robin, or other algorithm. For example, in one embodiment, queue tunnel proxy module  406  may process data from one or more sockets corresponding to a first traffic class, then one or more sockets corresponding to the next traffic class, then one or more sockets corresponding to a following traffic class, etc. In other embodiments, prioritization may be handled by the QoS and redirection module, with traffic provided in an order of priority to the queue tunnel proxy module  406 . 
     In one embodiment, queue tunnel proxy module  406  may use a variant of weighted fair queuing, WFQ2+, by Zhang et al., which includes capabilities for rate control and regulation in addition to queuing and scheduling capabilities of WFQ. In one embodiment applying WFQ, the algorithm assumes a sender that transmits at the rate of the link, and use the link speed to cause downward pressure on a WFQ tree, the tree comprising multiple traffic classes per link, and multiple sockets per class, as discussed above. In another embodiment, however, the algorithm may be rate-agnostic. Instead, the algorithm may use the presence of congestion or rate and number of congestion notifications on the link to provide back pressure to the tree. 
     In some embodiments, a tunnel that includes congestion control and flow control for QoS will appear to be a single link to the WFQ algorithm. Thus, QoS scheduling can be applied from tunnel end to tunnel end, regardless of the number or type of intermediary links. This may result in better performance and efficiency over other non-tunneled QoS implementations, where each intermediary router or node only has capability of scheduling outgoing packets for the next link or hop, providing only local QoS. As more traffic is merged along the route, packet order may change from the order in which they were transmitted by the first node. However, a tunnel implementation can provide full end-to-end QoS, without knowing individual link speeds or types, and regardless of a dynamically changing end-to-end data path. In a further embodiment, a client and server may establish a plurality of tunnel connections, and may balance traffic loads across the tunnels, consolidating flow and congestion control at the endpoints. This may provide enhanced scalability and enable use of additional acceleration techniques and processing on a per-tunnel basis, and optimal route balancing for the plurality of tunnels. 
     Referring now to  FIG. 4C , illustrated is a block diagram of an embodiment of a queue tunnel proxy on a client, appliance or server, showing distribution of components between kernel mode  302  and user mode  303 . In the embodiment shown, QoS and redirection module  404  may execute in kernel mode  302 , while queue tunnel proxy module  406  may execute in user mode  303 . In some embodiments, queue tunnel proxy module  406  may include a socket interface  410 , a tunnel interface  412 , a connection manager  414 , and a data manager  416 . Queue tunnel proxy module  406  may also communicate with a configuration manager  418 . In some embodiments, one of either QoS and redirection module  404  or queue tunnel proxy module  406  may be responsible for encapsulating packets for the tunnel, or rewriting destination IP addresses and ports to control redirection. 
     Still referring to  FIG. 4C  and in more detail, in some embodiments, queue tunnel proxy module  406  may include a socket interface  410 . Socket Interface  410  may include any type and form of socket based programming interface or API, such as TCP or UDP socket API. Socket interface  410  may comprise an API or other interface for communicating packets to and from QoS and redirection module  404 . In some embodiments, socket interface  410  or QoS and redirection module  404  may also include functions or commands for controlling congestion, such as commands to cause socket interface  410  or QoS and redirection module  404  to pause or resume transmitting data via the communications channel. In other embodiments, socket interface  410  or QoS and redirection module  404  may include functions or commands for getting connection information about a new connection, including information relating to a destination of a connection to be established discussed above. 
     In some embodiments, the socket interface may listen on one or more local addresses and ports for communications from a QoS and redirection module  404 , or other application communicating directly to the tunnel proxy module. The socket interface may, in some embodiments, include functions or methods for opening a TCP or UDP connection; sending data from one or more buffers to the tunnel interface via the data manager; returning received data from the tunnel interface to QoS and redirection module or another application; and notifying QoS and redirection module or other application of an established, open, or closed connection. The socket interface may also include functions or methods for requesting resending of data when a previously attempted transmission of data has failed. 
     In some embodiments, queue tunnel proxy module  406  may include a tunnel interface  412 . Tunnel interface  412  may comprise an API or other interface for communicating packets to and from a network interface  118 . In some embodiments, tunnel interface  412  may also comprise functionality for establishing and closing tunnel connections to a tunnel peer, such as an intermediary appliance, server, or client, as discussed above. In some embodiments, tunnel interface  412  may provide support for encapsulation and communication of packets via several tunnel protocols, including UDT, SCTP, and TCP, or any other protocol. In some embodiments, a protocol such as UDT may provide maximum response time with minimal latency, but in an environment where a network firewall prevents a UDT connection, then other protocols, such as TCP Reno, may be used. In some embodiments, tunnel interface  412  may select a protocol for optimal performance responsive to security requirements, error rate, and latency. Accordingly, tunnel interface  412  may also include functionality for determining error rate, latency, bandwidth, jitter, round trip time, and other features of a connection, for benchmarking purposes. 
     In some embodiments, tunnel interface  412  may also provide encryption and decryption features, such as SSL or TLS encryption protocols. Tunnel interface  412  may also provide one or more callbacks to network interface  118 , connection manager  414  or data manager  416 , including messages to indicate the state of a peer, such as connected, disconnected, stalled, paused, ready, or any other state; messages to indicate a send buffer is available; messages to indicate one or more data packets have been received; messages to indicate to create a new buffer; or other management or setup functions. Although shown in  FIGS. 4A and 4B  as a single tunnel, in many embodiments, tunnel interface  412  may include functionality for establishing a plurality of tunnels to a corresponding plurality of remote devices and/or a plurality of network interfaces on a single remote device. 
     In some embodiments, tunnel interface may include API functions or methods for establishing or connecting a tunnel; responding to a remote request to establish a tunnel; disconnect a tunnel; open or close a TCP, UDP, or other protocol connection to an destination address and port, and including an initiator connection handle; transmit a receiver connection handle responsive to a request to open a connection; pause or resume a connection; and send data of a specified length via a connection. In one embodiment, to avoid delays of TCP slow start and similar techniques, open opening a connection, a tunnel client may immediately begin sending data. If the device on the opposite side of the tunnel fails to open a connection, a close message may send back, and all data in transit discarded. In some embodiments, upon receipt of a request to open a connection including an initiator connection handle, the receiver may send an acknowledgement with a receiver connection handle. In some embodiments, these handles may be unique, may be incremented with each established connection, may be determined randomly, or may be a function of each other. For example, in one embodiment, a receiver connection handle may be an inverse of an initiator connection handle or may include the initiator handle with a predetermined bit flipped. Commands to pause, resume, or close a connection, and commands to send data via a connection may include the initiator or receiver connection handle. The tunnel interface may also include local API functions or methods for connecting or disconnecting a tunnel connection, open or close a TCP or UDP connection via the tunnel, or send a pause or resume command to a remote computing device via the tunnel to control congestion. These commands may include a local handle to identify the tunnel. The tunnel interface may also include commands for sending data from one or more buffers via a tunnel identified by a local handle, and may include an identification of the one or more send buffers. The tunnel interface may also include one or more callbacks for passing received messages to the data manager, requesting resending of data in case of connection or communication failure, sending notification of a peer status change to the connection manager, notifying a connection manager of a newly requested or established connection, or indicating a pause or resume command has been received from a remote computing device. 
     In some embodiments, queue tunnel proxy module  406  may include a connection manager  414 . Connection manager  414  may comprise a service, daemon, application, routine or subroutine, or other executable code or logic for managing connections via both the socket interface  410  and tunnel interface  412 . Connection manager  414  may include functionality for creating and deleting connections, and maintaining a connection table. In some embodiments, connection manager  414  may include one or more timers for maintaining the connection table by marking dormant connections as having timed out. When congestion is detected or when a pause command is received via the socket or tunnel interface, the connection manager  414  may mark a connection as halted and may send a pause command to the corresponding other interface. The connection manager  414  may manage a list of peers; a list of connections; a list of non-paused connections, which may be used to service local sockets; a list of paused connections which may be paused when the tunnel interface detects congestion or due to back pressure from the local socket&#39;s buffer; a list of connections with data received from the tunnel that needs to be delivered locally, which may be buffered until the local connection can accept the data; a list of new local connections that need to be established to the peer; and a list a new peer connections that need to be established locally. In some embodiments, these lists may be combined into one or more lists. For example, in one such embodiment, a connection list may include a flag to indicate a paused or non-paused connection. Similar flags may be used to denote other connection statuses. 
     The connection manager  414  may include functionality for creating a new connection responsive to a request received via the socket interface. In some embodiments, the connection manager  414  may determine a peer unit using a list of peers, and may include load balancing or other functions for selecting a peer from the list of peers. The connection manager may send to the peer, via the tunnel interface, a request to establish a new connection. Similarly, in some embodiments, the connection manager may receive a request to open a connection via the tunnel interface from a peer, and may initialize the connection and route incoming packets to the proper destination. In some embodiments, this may comprise directing packets to QoS and redirection module  404  for forwarding to client agent  120  or an application. In other embodiments, this may comprise directing packets to a network interface for transmission to a remote server, intermediary, or client. In some embodiments, the connection manager may redirect a tunneled packet to the original destination IP address of the payload packet. 
     Connection manager  414  may also include functionality for closing a connection. The connection manager may close a connection responsive to receiving a request to close the connection, a failure of transmission in the tunnel, a timeout, or other reasons. Closing a connection may comprise cleaning up resources allocated to the connection, resetting buffers, or performing other maintenance tasks. In some embodiments, connection manager  414  may also include functionality for pausing or resuming connections responsive to congestion. For example, if a local socket cannot accept all of the data provided to the socket, the connection manager may pause the connection and send a pause command to the peer unit. Similarly, if the connection manager receives a pause command from the peer unit, the connection manager may pause the connection and direct the QoS and redirection module to pause. Connection manager may also include functionality for explicit congestion notifications. In some embodiments, the connection manager may also perform periodic housekeeping functions, including maintaining connection timeouts and a list of closed connections. 
     In some embodiments, queue tunnel proxy module  406  may also include a data manager  416 . Data manager  416  may comprise an application, service, daemon, routine or subroutine, or other executable code or logic for managing and moving data between the socket interface and tunnel interface. As shown, in some embodiments, receive and send paths may be separate and may be controlled in parallel. 
     In one embodiment, when the socket interface receives data from the QoS and redirection module or other application, it may call a data receive function provided by data manager  416 . Responsive to this function, the data manager may attempt to pass the data to a peer unit via the tunnel interface. If the tunnel interfaces refuses the data, for example due to congestion or a paused connection, data manager  416  may store the data in a per-connection queue or buffer. This queue or buffer may then be added to a pending send list. When the amount of pending data to be send passes a predetermined threshold, in some embodiments, the data manager may send a request to pause the connection to the QoS and redirection module  404  or other application via the socket interface. Upon receipt of a connection resume message from the tunnel interface, data manager  416  will attempt to send data from the queues or buffers in the pending send list. In some embodiments, the data manager may attempt to send as much data as possible, avoiding delays caused by slow start techniques. 
     Similarly, in one embodiment, when the tunnel interface receives data from the peer via the network interface, it may send a callback function to the data manager. Responsive to this callback, the data manager may attempt to pass the data to QoS and redirection module or another application via the socket interface. If the socket interface refuses the data, for example due to congestion or execution delays, the data manager may store the data in a local buffer or queue. When the amount of data to send to local buffers or queues passes a predetermined threshold, in some embodiments, the data manager may send a request to pause the connection to the remote peer via the tunnel interface. Data manager may similarly send a resume command when local sockets are again able to accept data. 
     Referring now to  FIGS. 5A and 5B , illustrated are block diagrams of embodiments of communications flow within a system providing quality of service of a plurality of applications via a flow controlled tunnel. In brief overview,  FIG. 5A  shows an embodiment providing direct QoS and redirection, while  FIG. 5B  shows an embodiment providing redirection via an encryption gateway. In the embodiment shown in  FIG. 5A , traffic flow from an application travels to QoS module  420  of QoS and redirection module  404 , which intercepts, classifies and prioritizes the traffic, using any of the techniques described above. The traffic is then passed to the traffic redirector  422  of QoS and redirection module  404 , which passes the traffic to the queue tunnel proxy module  406  via a socket interface, discussed above. When receiving traffic, this is performed in reverse: data is received through the tunnel and split back into individual streams and delivered to the application. 
     In the embodiment shown in  FIG. 5B , however, a client agent  120  is placed in the flow of traffic to perform additional functions, such as encryption, compression, or other acceleration techniques. In this embodiment, traffic from the application is intercepted and prioritized by the QoS module  420  and then sent to a client agent redirector  424 . Similar to the traffic redirector  422 , the client agent redirector  424  sends traffic to a socket of client agent  120  for further processing using any of the encryption, compression, acceleration, or other techniques described above. For example, the client agent  120  may encapsulate the stream in an SSL stream for sending to a remote decryption module. The client agent may then transmit the processed traffic back through the network stack, where it may be intercepted again by QoS and redirection module  404 . Having already been prioritized, the traffic may be redirected by traffic redirector  422  to the queue tunnel proxy module  406  for transmission via the tunnel. When receiving encrypted, compressed, or otherwise accelerated traffic, the traffic redirector  422  may direct the traffic back to the client agent  120  for processing. After processing, in some embodiments, the traffic may be passed to client agent redirector  424 , and then returned to QoS and redirection module  404  for sending to its original destination. In other embodiments, received traffic processed by the client agent  120  may be sent to the network stack by the client agent and intercepted by QoS and redirection module  404  for sending to its original destination. 
     Referring briefly back to  FIG. 4B  in connection with  FIGS. 5A and 5B , both embodiments of communications flow may be utilized simultaneously. For example, as shown in  FIG. 4B , some traffic (corresponding to applications  1  and  2 ) may be redirected through client agent  120 , while other traffic (corresponding to application  3 , and processed traffic from client agent  120 ) may be redirected to the queue tunnel proxy module  406 . This may be done, for example, when the additional processing performed by client agent  120  is unnecessary or undesirable for an application, such as a low-latency VoIP connection. 
       FIG. 6  is a flow chart of an embodiment of a method for providing quality of service of a plurality of applications via a flow controlled tunnel. In brief overview, at step  602 , an agent operating at a portion of a network stack of a client may proxy a plurality of transport layer connections corresponding to each of a plurality of applications executing on the client. At step  604 , the agent may receive data from each of the plurality of proxied transport layer connections in an order according to an assigned priority from classification of each of the plurality of applications. At step  606 , the agent may communicate, in the order according to the assigned priority, a predetermined amount of data received from each of the plurality of proxied transport layer connections via an established transport layer connection. 
     Still referring to  FIG. 6  and in more detail, at step  602 , an agent executing or operating at a portion of a network stack of a client may proxy a plurality of transport layer connections corresponding to each of a plurality of applications executing on the client. In some embodiments, proxying a connection may comprise acting as an intermediary for the transport layer connection, and may include encapsulating data of one or more packets from the plurality of transport layer connections as payloads of one or more tunneling protocol packets. In some embodiments, the agent may proxy each of the plurality of transport layer connections back to the client. In many embodiments, the agent may proxy each of the plurality of transport layer connections transparently to each of the plurality of applications. For example, the agent may rewrite destination and/or source addresses or ports or encapsulate and de-encapsulate packets transparently to the applications. 
     At step  604 , in some embodiments, the agent may receive data from each of the plurality of proxied transport layer connections in an order according to an assigned priority from classification of each of the plurality of applications. In some embodiments, the agent may classify each of the plurality of applications according to a Quality of Service classification scheme. Such classification may be performed using any identifiable information in the connections, data, or applications, including by destination, by application, by application group or type, by protocol, by data size, or any other information. 
     At step  606 , in some embodiments, the agent may communicate, to a first tunneling application executing on the client, a predetermined amount of data received from each of the plurality of proxied transport layer connections. The first tunneling application may, in some embodiments, have an established transport layer connection to a second tunneling application, which may execute on a remote computing device, such as a remote client, intermediary, or server. In many embodiments, the agent may communicate the predetermined data in the order according to the assigned priority. In some embodiments, the agent may apply Quality of Service upon communicating the predetermined amount of data to the first tunneling application. In other embodiments, the agent may communicate each of the predetermined amount of data upon receipt to the first tunneling application. 
     In some embodiments, the first tunneling application on the client may transmit the predetermined amount of data from each of the applications to a second tunneling application executing on a device intermediary to the client and a plurality of servers. In many embodiments, the agent may execute in a kernel space or kernel mode of the client and the first tunneling application may execute in a user space or user mode of the client. In some embodiments, the agent may receive an indication from one or more of the plurality of proxied transport layer connections that data is available to be read. In a further embodiment, the agent may propagate the indication to the first tunneling application and, responsive to the indication, the first tunneling application may accept the predetermined amount of data from each of the one or more proxied transport layer connections in the order of priority. 
     While various embodiments of the methods and systems have been described, these embodiments are exemplary and in no way limit the scope of the described methods or systems. Those having skill in the relevant art can effect changes to form and details of the described methods and systems without departing from the broadest scope of the described methods and systems. Thus, the scope of the methods and systems described herein should not be limited by any of the exemplary embodiments and should be defined in accordance with the accompanying claims and their equivalents.