Patent Publication Number: US-11647083-B2

Title: Cluster-aware multipath transmission control protocol (MPTCP) session load balancing

Description:
FIELD OF THE DISCLOSURE 
     The present application generally relates to load balancing, including but not limited to systems and methods for cluster-aware multipath transmission control protocol (MPTCP) session load balancing. 
     BACKGROUND 
     A client may establish a session or connection with a resource. In some instances, clients may attempt to establish multiple connections with a resource. For example, a client may establish a primary and secondary connection with the resource. The primary and secondary connection may together form a multipath connection. 
     SUMMARY 
     This Summary is provided to introduce a selection of concepts in a simplified form that is further described below in the Detailed Description. This Summary is not intended to identify key features or essential features, nor is it intended to limit the scope of the claims included herewith. 
     In some implementations, where multipath connections (such as multipath transmission control protocol MPTCP, RFC 6824, or other multipath connections) are processed by processors of a cluster of processors, the different connections (called subflows) of the same MPTCP session may be received by different processors or nodes of the cluster. However, the connections of the same MPTCP connection should be steered and aggregated into the same node for processing the in-sequence data and pass to the applications or resource to be accessed by the client. In various implementations, to steer the subflows to a single node, some systems may use a stateful session store, synchronization, lookup, packet encapsulation (such as generic routing encapsulation tunneling), and so forth. Such systems therefore result in a significant amount of processing just for steering packets to the proper node. For instance, due to the architecture of the cluster of processors (i.e., where the processors are horizontally scaled), the subflows may use two hop steering within the cluster of processors, which means that a large portion (such as two thirds) of the packets processed within the cluster would just be the steered packets. Such systems dramatically reduce the overall goodput of the clusters. Additionally, in the event of a cluster auto scale (i.e., adding or removing processors to a cluster, resetting one or more processors of the cluster, etc.), such systems may break the MPTCP sessions. This steering issue is more severe on the public cloud cluster deployments where steered packets are also considered as part of the ‘Packet Per Second’ (PPS) limit, which dramatically reduces the PPS limit for actual client/server packet processing capacity of each node or processor within a cluster. 
     The systems and methods described herein provide for associating multiple subflows of an MPTCP session using two-tier clustering deployment architecture. For example, a first tier of processors may act as an ingress to the second tier of processors. The first tier of processors may perform TCP level load balancing. Additionally, the first tier of processors may use or leverage tokens (or other code) for connection persistency for MPTCP connections. The first tier of processors may ensure that all the subflows belonging to the same MPTCP session are load balanced to the same node or processor within the second tier of processors. Such implementations and embodiments may avoid the packet steering requirement, session store and look-up requirement and requirement to encapsulate the packets while steering between nodes described above. By eliminating packet steering requirements, session store and look-up requirements, and encapsulation requirements, the systems and methods described herein may dramatically increase the overall goodput of the processors and reduces the cost to the customer. 
     According various implementations of the present disclosure, the systems and methods described herein may use a token derived from a key exchanged in the establishment of a primary connection as a way to associate and aggregate all the subsequent subflow connections associated to a single MPTCP session. In some implementations, clusters of processors may be configured in a two-tier mode. The processors of first tier may be configured on standalone, active-passive high availability or active-active clustering mode based on the scaling, availability and resiliency requirements for the customer. The processors of the first tier may load balance the primary subflow to one of the selected processors in the cluster of processors of the second tier (for example, using a load balancing algorithm, such as a least connection algorithm or a weighted round robin algorithm). The processors of the first tier may store key value information for the primary connection (such as a token or other identifier an information relating to the selected processor of the second tier) in a data structure (such as a distributed hash table). In some instances, the data structure may be accessible to all processors within the first tier. When a processor of the first cluster receive a secondary connection with a token (i.e., a token which matches a token in the data structure corresponding to a primary connection), the processor may perform a lookup in the data structure using the token to find or identify the processor of the second tier. The processor of the first tier may then route the second request for the secondary connection to processor of the second tier such that the secondary connections are load balanced to the same node in the second tier ADC which has the primary connection. On the selected processor of the second cluster, all the connections belonging to the same multipath connection are received on the same node or processor, thereby avoiding the steering requirement within the processors of the second cluster. 
     In one or more embodiments, where a processor of the first cluster receives a request to establish a first (i.e., a primary) connection from a client to a server, the processor may select a processor of the second cluster to load balance the processors of the second cluster. The processor of the first cluster may select the processor of the second cluster and the processor of the second cluster may respond with an acknowledgement (i.e., a synchronization acknowledgement SYN-ACK) along with a key for the processor of the second cluster. The processor of the first cluster may receive a third acknowledgement of a three-way handshake (i.e., a TCP 3-way handshake between the client, the processor of the first cluster, and the processor of the second cluster). Upon receiving the third acknowledgement, the processor of the first cluster may derive a token for the primary connection using the key in the third acknowledgement. The processor of the first cluster may store the token in a data structure (such as a distributed hash table) along with information relating to the processor of the second cluster. In some implementations, the data structure may be distributed or otherwise accessible by other processors of the first engine, thereby providing persistency of the connection regardless of processor removal or addition events for the first cluster. The processor of the first cluster may forward the third acknowledgment to the selected processor of the second cluster. The processor of the first cluster may also allocate local load balancing session to forward subsequent packets on the primary connection to the selected processor of the second cluster. The processor of the second cluster may create a session or connection (such as a TCP socket session) for the primary connection between the client and the server. 
     When a processor of the first cluster receives a second request to establish a second connection (i.e., a secondary connection for a multipath connection), the processor of the first cluster may perform a lookup in the data structure using a token received with the second request to identify the selected processor of the second cluster. If the lookup returns data relating to the selected processor of the second cluster, the processor of the first cluster may forward the second request to the selected processor of the second cluster such that the secondary connection is load balanced to the same selected processor of the second cluster. If the lookup does not return data relating to a processor of the second cluster, the secondary connections for the token may be reset within the data structure (i.e., by sending a reset signal to the data structure). Upon receiving a connection closure signal from the client, the first processor may remove an entry corresponding to the token from the data structure. 
     According to one or more embodiments of the present solution, a first processor of a first cluster of processors that is intermediary to a client and a second cluster of processors and the second cluster of processors is intermediary to the first cluster of processors and a server, may forward, to a second processor of the second cluster, a first request from the client to establish a first connection with the server. A third processor of the first cluster of processors may receive a second request from the client to establish a multipath connection between the client and the server. The third processor may forward, responsive to determining that the second request is to establish a multipath connection, the second request to the second processor to establish the multipath connection that includes the first connection and a second connection used as paths of the multipath connection. 
     According to one or more embodiments of the present solution, a first processor intermediary to a server and a cluster of processors may receive, from a second processor of the cluster of processors that is intermediary to the first processor and a client, a first request to establish a first connection between the client and a server. The first processor may receive from a third processor of the cluster of processors, a second request to establish a second connection between the client and the server. The first processor may establish, between the client and the server, a multipath connection that includes the first connection and the second connection used as paths of the multipath connection. 
     The systems and methods of the present solution may increase or improve the overall goodput of the clusters by eliminating redundancies in identifying processors which are to receive multipath connections as well as routing of such packets. Additionally, the systems and methods of the present solution may increase persistency of multipath connections in the event of a cluster auto scale by maintaining tokens for the primary and secondary connections in a data structure. The systems and methods described herein may reduce the impact in PPS limits by reducing traffic between processors relating to routing or steering of connections, which may improve actual client/server packet processing capacity of each node or processor within a cluster. The systems and methods described herein may be used for multipath TCP connections, as well as other types of connections (such as QUIC protocol) which use connection identifiers (similar to tokens) for multipath support. 
     In one aspect, this disclosure is directed to a method. The method includes forwarding, by a first processor of a first cluster of processors that is intermediary to a client and a second cluster of processors and the second cluster of processors is intermediary to the first cluster of processors and a server, to a second processor of the second cluster, a first request from the client to establish a first connection with the server. The method includes receiving, by a third processor of the first cluster of processors, from the client, a second request to establish a multipath connection between the client and the server. The method includes forwarding, by the third processor, responsive to determining that the second request is to establish a multipath connection, the second request to the second processor to establish the multipath connection that includes the first connection and a second connection used as paths of the multipath connection. 
     In some embodiments, the method further includes receiving, by the first processor from the second processor, a response to the first request including a key. The method may further include generating, by the first processor, a token based on the key from the response to the first request. The method may further include storing, by the first processor, the token in a data structure maintained by the first processor accessible by the first cluster of processors. In some embodiments, the third processor determines that the second request is to establish a multipath connection responsive to the second request including the token. In some embodiments, the token is a first token, the second request includes a second token. The method may further include identifying, by the third processor, the second processor to which the third processor is to forward the second request based on the second token matching the first token from the data structure. In some embodiments, the data structure comprises a table including a plurality of tokens for a plurality of connections between clients and processors of the second cluster. In some embodiments, the token is a first token, the client generates a second token which matches the first token using the key received from the response, the second request includes the second token, and the method further includes querying, by the third processor, the table maintained by the first processor using the second token to identify the second processor. In some embodiments, the data structure comprises a first data structure, and the method further includes transmitting, by the first processor to the third processor of the first cluster, the token for storage in a second data structure maintained by the third processor. In some embodiments, the method further includes receiving, by the first processor, the first request from a client, and selecting, by the first processor, the second processor from the cluster of second processors to which to forward the first request based on a load of the second processor. 
     In another aspect, this disclosure is directed to a method. The method includes receiving, by a first processor intermediary to a server and a cluster of processors, from a second processor of the cluster of processors that is intermediary to the first processor and a client, a first request to establish a first connection between the client and a server. The method includes receiving, by the first processor, from a third processor of the cluster of processors, a second request to establish a second connection between the client and the server. The method includes establishing, by the first processor, between the client and the server, a multipath connection that includes the first connection and the second connection used as paths of the multipath connection. 
     In some embodiments, the method includes providing, by the first processor to the second processor, a response to the first request including a key, the key used to generate a token by the second processor to be shared across the processors of the cluster. In some embodiments, the second request includes a token generated by the client based on a key included in a response to the first request, and the first processor establishes the multipath connection responsive to the token included in the second request. In some embodiments, the third processor identifies a token from the second request received from the client, the token generated using a key provided by the first processor to the second processor responsive to the first request, the third processor forwarding the second request to the first processor responsive to determining. 
     In another aspect, this disclosure is directed to a device. The device includes a first processor of a first cluster of processors that is intermediary to a client and a second cluster of processors and the second cluster of processors is intermediary to the first cluster of processors and a server. The first processor is configured to transmit, to a second processor of the second cluster, a first request from the client to establish a first connection with the server. The first processor is configured to receive, from the client, a second request to establish a multipath connection between the client and the server. The first processor is configured to forward, responsive to determining that the second request is to establish a multipath connection, the second request to the second processor to establish the multipath connection that includes the first connection and a second connection used as paths of the multipath connection. 
     In some embodiments, the first processor is further configured to receive, from the second processor, a response to the first request including a key generate a token based on the key from the response to the first request, and store the token in a data structure maintained by the first processor accessible by the first cluster of processors. In some embodiments, the first processor determines that the second request is to establish a multipath connection responsive to the second request including the token. In some embodiments, the token is a first token, the second request includes a second token, and the first processor is further configured to identify the second processor to which the first processor is to forward the second request based on the second token matching the first token from the data structure. In some embodiments, the data structure comprises a table including a plurality of tokens for a plurality of connections between clients and processors of the second cluster. In some embodiments, the token is a first token, the client generates a second token which matches the first token using the key received from the response, the second request includes the second token, and the first processor is further configured to query the table maintained by the first processor using the second token to identify the second processor. In some embodiments, the data structure comprises a first data structure, and the first processor is further configured to transmit, to a third processor of the first cluster, the token for storage in a second data structure maintained by the third processor. In some embodiments, the first processor is further configured to receive the first request from a client, and select the second processor from the cluster of second processors to which to forward the first request based on a load of the second processor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       Objects, aspects, features, and advantages of embodiments disclosed herein will become more fully apparent from the following detailed description, the appended claims, and the accompanying drawing figures in which like reference numerals identify similar or identical elements. Reference numerals that are introduced in the specification in association with a drawing figure may be repeated in one or more subsequent figures without additional description in the specification in order to provide context for other features, and not every element may be labeled in every figure. The drawing figures are not necessarily to scale, with emphasis instead being placed upon illustrating embodiments, principles, and concepts. The drawings are not intended to limit the scope of the claims included herewith. 
         FIG.  1 A  is a block diagram of a network computing system, in accordance with an illustrative embodiment; 
         FIG.  1 B  is a block diagram of a network computing system for delivering a computing environment from a server to a client via an appliance, in accordance with an illustrative embodiment; 
         FIG.  1 C  is a block diagram of a computing device, in accordance with an illustrative embodiment; 
         FIG.  2    is a block diagram of an appliance for processing communications between a client and a server, in accordance with an illustrative embodiment; 
         FIG.  3    is a block diagram of a virtualization environment, in accordance with an illustrative embodiment; 
         FIG.  4    is a block diagram of a cluster system, in accordance with an illustrative embodiment; 
         FIG.  5    is a block diagram of a system for cluster-aware multipath transmission control protocol (MPTCP) session load balancing, in accordance with an illustrative embodiment; 
         FIG.  6    is a block diagram of a processing system which may be included in the system of  FIG.  5   , in accordance with an illustrative embodiment; 
         FIG.  7 A  is a flow diagram showing an example method of establishing a primary connection, in accordance with an illustrative embodiment; 
         FIG.  7 B  is a flow diagram showing an example method of establishing a secondary connection, in accordance with an illustrative embodiment; 
         FIG.  8 A  is a flow diagram showing a method for establishing a multipath connection, in accordance with an illustrative embodiment; and 
         FIG.  8 B  is a flow diagram showing a method for establishing a multipath connection, in accordance with an illustrative embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In some implementations, where multipath connections (such as multipath transmission control protocol MPTCP, RFC 6824, or other multipath connections) are processed by processors of a cluster of processors, the different connections (called subflows) of the same MPTCP session may be received by different processors or nodes of the cluster. However, the connections of the same MPTCP connection should be steered and aggregated into the same node for processing the in-sequence data and pass to the applications or resource to be accessed by the client. In various implementations, to steer the subflows to a single node, some systems may use a stateful session store, synchronization, lookup, packet encapsulation (such as generic routing encapsulation tunneling), and so forth. Such systems therefore result in a significant amount of processing just for steering packets to the proper node. For instance, due to the architecture of the cluster of processors (i.e., where the processors are horizontally scaled), the subflows may use two hop steering within the cluster of processors, which means that a large portion (such as two thirds) of the packets processed within the cluster would just be the steered packets. Such systems dramatically reduce the overall goodput of the clusters. Additionally, in the event of a cluster auto scale (i.e., adding or removing processors to a cluster, resetting one or more processors of the cluster, etc.), such systems may break the MPTCP sessions. This steering issue is more severe on the public cloud cluster deployments where steered packets are also considered as part of the ‘Packet Per Second’ (PPS) limit, which dramatically reduces the PPS limit for actual client/server packet processing capacity of each node or processor within a cluster. 
     The systems and methods described herein provide for associating multiple subflows of an MPTCP session using two-tier clustering deployment architecture. For example, a first tier of processors may act as an ingress to the second tier of processors. The first tier of processors may perform TCP level load balancing. Additionally, the first tier of processors may use or leverage tokens (or other code) for connection persistency for MPTCP connections. The first tier of processors may ensure that all the subflows belonging to the same MPTCP session are load balanced to the same node or processor within the second tier of processors. Such implementations and embodiments may avoid the packet steering requirement, session store and look-up requirement and requirement to encapsulate the packets while steering between nodes described above. By eliminating packet steering requirements, session store and look-up requirements, and encapsulation requirements, the systems and methods described herein may dramatically increase the overall goodput of the processors and reduces the cost to the customer. 
     According various implementations of the present disclosure, the systems and methods described herein may use a token derived from a key exchanged in the establishment of a primary connection as a way to associate and aggregate all the subsequent subflow connections associated to a single MPTCP session. In some implementations, clusters of processors may be configured in a two-tier mode. The processors of first tier may be configured on standalone, active-passive high availability or active-active clustering mode based on the scaling, availability and resiliency requirements for the customer. The processors of the first tier may load balance the primary subflow to one of the selected processors in the cluster of processors of the second tier (for example, using a load balancing algorithm, such as a least connection algorithm or a weighted round robin algorithm). The processors of the first tier may store key value information for the primary connection (such as a token or other identifier an information relating to the selected processor of the second tier) in a data structure (such as a distributed hash table). In some instances, the data structure may be accessible to all processors within the first tier. When a processor of the first cluster receive a secondary connection with a token (i.e., a token which matches a token in the data structure corresponding to a primary connection), the processor may perform a lookup in the data structure using the token to find or identify the processor of the second tier. The processor of the first tier may then route the second request for the secondary connection to processor of the second tier such that the secondary connections are load balanced to the same node in the second tier ADC which has the primary connection. On the selected processor of the second cluster, all the connections belonging to the same multipath connection are received on the same node or processor, thereby avoiding the steering requirement within the processors of the second cluster. 
     In one or more embodiments, where a processor of the first cluster receives a request to establish a first (i.e., a primary) connection from a client to a server, the processor may select a processor of the second cluster to load balance the processors of the second cluster. The processor of the first cluster may select the processor of the second cluster and the processor of the second cluster may respond with an acknowledgement (i.e., a synchronization acknowledgement SYN-ACK) along with a key for the processor of the second cluster. The processor of the first cluster may receive a third acknowledgement of a three-way handshake (i.e., a TCP 3-way handshake between the client, the processor of the first cluster, and the processor of the second cluster). Upon receiving the third acknowledgement, the processor of the first cluster may derive a token for the primary connection using the key in the third acknowledgement. The processor of the first cluster may store the token in a data structure (such as a distributed hash table) along with information relating to the processor of the second cluster. In some implementations, the data structure may be distributed or otherwise accessible by other processors of the first engine, thereby providing persistency of the connection regardless of processor removal or addition events for the first cluster. The processor of the first cluster may forward the third acknowledgment to the selected processor of the second cluster. The processor of the first cluster may also allocate local load balancing session to forward subsequent packets on the primary connection to the selected processor of the second cluster. The processor of the second cluster may create a session or connection (such as a TCP socket session) for the primary connection between the client and the server. 
     When a processor of the first cluster receives a second request to establish a second connection (i.e., a secondary connection for a multipath connection), the processor of the first cluster may perform a lookup in the data structure using a token received with the second request to identify the selected processor of the second cluster. If the lookup returns data relating to the selected processor of the second cluster, the processor of the first cluster may forward the second request to the selected processor of the second cluster such that the secondary connection is load balanced to the same selected processor of the second cluster. If the lookup does not return data relating to a processor of the second cluster, the secondary connections for the token may be reset within the data structure (i.e., by sending a reset signal to the data structure). Upon receiving a connection closure signal from the client, the first processor may remove an entry corresponding to the token from the data structure. 
     According to one or more embodiments of the present solution, a first processor of a first cluster of processors that is intermediary to a client and a second cluster of processors and the second cluster of processors is intermediary to the first cluster of processors and a server, may forward, to a second processor of the second cluster, a first request from the client to establish a first connection with the server. A third processor of the first cluster of processors may receive a second request from the client to establish a multipath connection between the client and the server. The third processor may forward, responsive to determining that the second request is to establish a multipath connection, the second request to the second processor to establish the multipath connection that includes the first connection and a second connection used as paths of the multipath connection. 
     According to one or more embodiments of the present solution, a first processor intermediary to a server and a cluster of processors may receive, from a second processor of the cluster of processors that is intermediary to the first processor and a client, a first request to establish a first connection between the client and a server. The first processor may receive from a third processor of the cluster of processors, a second request to establish a second connection between the client and the server. The first processor may establish, between the client and the server, a multipath connection that includes the first connection and the second connection used as paths of the multipath connection. 
     The systems and methods of the present solution may increase or improve the overall goodput of the clusters by eliminating redundancies in identifying processors which are to receive multipath connections as well as routing of such packets. Additionally, the systems and methods of the present solution may increase persistency of multipath connections in the event of a cluster auto scale by maintaining tokens for the primary and secondary connections in a data structure. The systems and methods described herein may reduce the impact in PPS limits by reducing traffic between processors relating to routing or steering of connections, which may improve actual client/server packet processing capacity of each node or processor within a cluster. The systems and methods described herein may be used for multipath TCP connections, as well as other types of connections (such as QUIC protocol) which use connection identifiers (similar to tokens) for multipath support. 
     For purposes of reading the description of the various embodiments below, the following descriptions of the sections of the specification and their respective contents may be helpful: 
     Section A describes a network environment and computing environment which may be useful for practicing embodiments described herein; 
     Section B describes embodiments of systems and methods for delivering a computing environment to a remote user; 
     Section C describes embodiments of systems and methods for providing a clustered appliance architecture environment; 
     Section D describes embodiments of systems and methods for providing a clustered appliance architecture environment; and 
     Section E describes embodiments of systems and methods for cluster-aware multipath transmission control protocol (MPTCP) session load balancing. 
     A. Network and Computing Environment 
     Referring to  FIG.  1 A , an illustrative network environment  100  is depicted. Network environment  100  may include one or more clients  102 ( 1 )- 102 ( n ) (also generally referred to as local machine(s)  102  or client(s)  102 ) in communication with one or more servers  106 ( 1 )- 106 ( n ) (also generally referred to as remote machine(s)  106  or server(s)  106 ) via one or more networks  104 ( 1 )- 104   n  (generally referred to as network(s)  104 ). In some embodiments, a client  102  may communicate with a server  106  via one or more appliances  200 ( 1 )- 200   n  (generally referred to as appliance(s)  200  or gateway(s)  200 ). 
     Although the embodiment shown in  FIG.  1 A  shows one or more networks  104  between clients  102  and servers  106 , in other embodiments, clients  102  and servers  106  may be on the same network  104 . The various networks  104  may be the same type of network or different types of networks. For example, in some embodiments, network  104 ( 1 ) may be a private network such as a local area network (LAN) or a company Intranet, while network  104 ( 2 ) and/or network  104 ( n ) may be a public network, such as a wide area network (WAN) or the Internet. In other embodiments, both network  104 ( 1 ) and network  104 ( n ) may be private networks. Networks  104  may employ one or more types of physical networks and/or network topologies, such as wired and/or wireless networks, and may employ one or more communication transport protocols, such as transmission control protocol (TCP), internet protocol (IP), user datagram protocol (UDP) or other similar protocols. 
     As shown in  FIG.  1 A , one or more appliances  200  may be located at various points or in various communication paths of network environment  100 . For example, appliance  200  may be deployed between two networks  104 ( 1 ) and  104 ( 2 ), and appliances  200  may communicate with one another to work in conjunction to, for example, accelerate network traffic between clients  102  and servers  106 . In other embodiments, the appliance  200  may be located on a network  104 . For example, appliance  200  may be implemented as part of one of clients  102  and/or servers  106 . In an embodiment, appliance  200  may be implemented as a network device such as Citrix networking (formerly NetScaler®) products sold by Citrix Systems, Inc. of Fort Lauderdale, Fla. 
     As shown in  FIG.  1 A , one or more servers  106  may operate as a server farm  38 . Servers  106  of server farm  38  may be logically grouped, and may either be geographically co-located (e.g., on premises) or geographically dispersed (e.g., cloud based) from clients  102  and/or other servers  106 . In an embodiment, server farm  38  executes one or more applications on behalf of one or more of clients  102  (e.g., as an application server), although other uses are possible, such as a file server, gateway server, proxy server, or other similar server uses. Clients  102  may seek access to hosted applications on servers  106 . 
     As shown in  FIG.  1 A , in some embodiments, appliances  200  may include, be replaced by, or be in communication with, one or more additional appliances, such as WAN optimization appliances  205 ( 1 )- 205 ( n ), referred to generally as WAN optimization appliance(s)  205 . For example, WAN optimization appliance  205  may accelerate, cache, compress or otherwise optimize or improve performance, operation, flow control, or quality of service of network traffic, such as traffic to and/or from a WAN connection, such as optimizing Wide Area File Services (WAFS), accelerating Server Message Block (SMB) or Common Internet File System (CIFS). In some embodiments, appliance  205  may be a performance enhancing proxy or a WAN optimization controller. In one embodiment, appliance  205  may be implemented as Citrix SD-WAN products sold by Citrix Systems, Inc. of Fort Lauderdale, Fla. 
     Referring to  FIG.  1 B , an example network environment,  100 ′, for delivering and/or operating a computing network environment on a client  102  is shown. As shown in  FIG.  1 B , a server  106  may include an application delivery system  190  for delivering a computing environment, application, and/or data files to one or more clients  102 . Client  102  may include client agent  120  and computing environment  15 . Computing environment  15  may execute or operate an application,  16 , that accesses, processes or uses a data file  17 . Computing environment  15 , application  16  and/or data file  17  may be delivered via appliance  200  and/or the server  106 . 
     Appliance  200  may accelerate delivery of all or a portion of computing environment  15  to a client  102 , for example by the application delivery system  190 . For example, appliance  200  may accelerate delivery of a streaming application and data file processable by the application from a data center to a remote user location by accelerating transport layer traffic between a client  102  and a server  106 . Such acceleration may be provided by one or more techniques, such as: 1) transport layer connection pooling, 2) transport layer connection multiplexing, 3) transport control protocol buffering, 4) compression, 5) caching, or other techniques. Appliance  200  may also provide load balancing of servers  106  to process requests from clients  102 , act as a proxy or access server to provide access to the one or more servers  106 , provide security and/or act as a firewall between a client  102  and a server  106 , provide Domain Name Service (DNS) resolution, provide one or more virtual servers or virtual internet protocol servers, and/or provide a secure virtual private network (VPN) connection from a client  102  to a server  106 , such as a secure socket layer (SSL) VPN connection and/or provide encryption and decryption operations. 
     Application delivery management system  190  may deliver computing environment  15  to a user (e.g., client  102 ), remote or otherwise, based on authentication and authorization policies applied by policy engine  195 . A remote user may obtain a computing environment and access to server stored applications and data files from any network-connected device (e.g., client  102 ). For example, appliance  200  may request an application and data file from server  106 . In response to the request, application delivery system  190  and/or server  106  may deliver the application and data file to client  102 , for example via an application stream to operate in computing environment  15  on client  102 , or via a remote-display protocol or otherwise via remote-based or server-based computing. In an embodiment, application delivery system  190  may be implemented as any portion of the Citrix Workspace Suite™ by Citrix Systems, Inc., such as Citrix Virtual Apps and Desktops (formerly XenApp® and XenDesktop®). 
     Policy engine  195  may control and manage the access to, and execution and delivery of, applications. For example, policy engine  195  may determine the one or more applications a user or client  102  may access and/or how the application should be delivered to the user or client  102 , such as a server-based computing, streaming or delivering the application locally to the client  120  for local execution. 
     For example, in operation, a client  102  may request execution of an application (e.g., application  16 ′) and application delivery system  190  of server  106  determines how to execute application  16 ′, for example based upon credentials received from client  102  and a user policy applied by policy engine  195  associated with the credentials. For example, application delivery system  190  may enable client  102  to receive application-output data generated by execution of the application on a server  106 , may enable client  102  to execute the application locally after receiving the application from server  106 , or may stream the application via network  104  to client  102 . For example, in some embodiments, the application may be a server-based or a remote-based application executed on server  106  on behalf of client  102 . Server  106  may display output to client  102  using a thin-client or remote-display protocol, such as the Independent Computing Architecture (ICA) protocol by Citrix Systems, Inc. of Fort Lauderdale, Fla. The application may be any application related to real-time data communications, such as applications for streaming graphics, streaming video and/or audio or other data, delivery of remote desktops or workspaces or hosted services or applications, for example infrastructure as a service (IaaS), desktop as a service (DaaS), workspace as a service (WaaS), software as a service (SaaS) or platform as a service (PaaS). 
     One or more of servers  106  may include a performance monitoring service or agent  197 . In some embodiments, a dedicated one or more servers  106  may be employed to perform performance monitoring. Performance monitoring may be performed using data collection, aggregation, analysis, management and reporting, for example by software, hardware or a combination thereof. Performance monitoring may include one or more agents for performing monitoring, measurement and data collection activities on clients  102  (e.g., client agent  120 ), servers  106  (e.g., agent  197 ) or an appliance  200  and/or  205  (agent not shown). In general, monitoring agents (e.g.,  120  and/or  197 ) execute transparently (e.g., in the background) to any application and/or user of the device. In some embodiments, monitoring agent  197  includes any of the product embodiments referred to as Citrix Analytics or Citrix Application Delivery Management by Citrix Systems, Inc. of Fort Lauderdale, Fla. 
     The monitoring agents  120  and  197  may monitor, measure, collect, and/or analyze data on a predetermined frequency, based upon an occurrence of given event(s), or in real time during operation of network environment  100 . The monitoring agents may monitor resource consumption and/or performance of hardware, software, and/or communications resources of clients  102 , networks  104 , appliances  200  and/or  205 , and/or servers  106 . For example, network connections such as a transport layer connection, network latency, bandwidth utilization, end-user response times, application usage and performance, session connections to an application, cache usage, memory usage, processor usage, storage usage, database transactions, client and/or server utilization, active users, duration of user activity, application crashes, errors, or hangs, the time required to log-in to an application, a server, or the application delivery system, and/or other performance conditions and metrics may be monitored. 
     The monitoring agents  120  and  197  may provide application performance management for application delivery system  190 . For example, based upon one or more monitored performance conditions or metrics, application delivery system  190  may be dynamically adjusted, for example periodically or in real-time, to optimize application delivery by servers  106  to clients  102  based upon network environment performance and conditions. 
     In described embodiments, clients  102 , servers  106 , and appliances  200  and  205  may be deployed as and/or executed on any type and form of computing device, such as any desktop computer, laptop computer, or mobile device capable of communication over at least one network and performing the operations described herein. For example, clients  102 , servers  106  and/or appliances  200  and  205  may each correspond to one computer, a plurality of computers, or a network of distributed computers such as computer  101  shown in  FIG.  1 C . 
     As shown in  FIG.  1 C , computer  101  may include one or more processors  103 , volatile memory  122  (e.g., RAM), non-volatile memory  128  (e.g., one or more hard disk drives (HDDs) or other magnetic or optical storage media, one or more solid state drives (SSDs) such as a flash drive or other solid state storage media, one or more hybrid magnetic and solid state drives, and/or one or more virtual storage volumes, such as a cloud storage, or a combination of such physical storage volumes and virtual storage volumes or arrays thereof), user interface (UI)  123 , one or more communications interfaces  118 , and communication bus  150 . User interface  123  may include graphical user interface (GUI)  124  (e.g., a touchscreen, a display, etc.) and one or more input/output (I/O) devices  126  (e.g., a mouse, a keyboard, etc.). Non-volatile memory  128  stores operating system  115 , one or more applications  116 , and data  117  such that, for example, computer instructions of operating system  115  and/or applications  116  are executed by processor(s)  103  out of volatile memory  122 . Data may be entered using an input device of GUI  124  or received from I/O device(s)  126 . Various elements of computer  101  may communicate via communication bus  150 . Computer  101  as shown in  FIG.  1 C  is shown merely as an example, as clients  102 , servers  106  and/or appliances  200  and  205  may be implemented by any computing or processing environment and with any type of machine or set of machines that may have suitable hardware and/or software capable of operating as described herein. 
     Processor(s)  103  may be implemented by one or more programmable processors executing one or more computer programs to perform the functions of the system. As used herein, the term “processor” describes an electronic circuit that performs a function, an operation, or a sequence of operations. The function, operation, or sequence of operations may be hard coded into the electronic circuit or soft coded by way of instructions held in a memory device. A “processor” may perform the function, operation, or sequence of operations using digital values or using analog signals. In some embodiments, the “processor” can be embodied in one or more application specific integrated circuits (ASICs), microprocessors, digital signal processors, microcontrollers, field programmable gate arrays (FPGAs), programmable logic arrays (PLAs), multi-core processors, or general-purpose computers with associated memory. The “processor” may be analog, digital or mixed-signal. In some embodiments, the “processor” may be one or more physical processors or one or more “virtual” (e.g., remotely located or “cloud”) processors. 
     Communications interfaces  118  may include one or more interfaces to enable computer  101  to access a computer network such as a LAN, a WAN, or the Internet through a variety of wired and/or wireless or cellular connections. 
     In described embodiments, a first computing device  101  may execute an application on behalf of a user of a client computing device (e.g., a client  102 ), may execute a virtual machine, which provides an execution session within which applications execute on behalf of a user or a client computing device (e.g., a client  102 ), such as a hosted desktop session, may execute a terminal services session to provide a hosted desktop environment, or may provide access to a computing environment including one or more of: one or more applications, one or more desktop applications, and one or more desktop sessions in which one or more applications may execute. 
     B. Appliance Architecture 
       FIG.  2    shows an example embodiment of appliance  200 . As described herein, appliance  200  may be implemented as a server, gateway, router, switch, bridge or other type of computing or network device. As shown in  FIG.  2   , an embodiment of appliance  200  may include a hardware layer  206  and a software layer  205  divided into a user space  202  and a kernel space  204 . Hardware layer  206  provides the hardware elements upon which programs and services within kernel space  204  and user space  202  are executed and allow programs and services within kernel space  204  and user space  202  to communicate data both internally and externally with respect to appliance  200 . As shown in  FIG.  2   , hardware layer  206  may include one or more processing units  262  for executing software programs and services, memory  264  for storing software and data, network ports  266  for transmitting and receiving data over a network, and encryption processor  260  for encrypting and decrypting data such as in relation to Secure Socket Layer (SSL) or Transport Layer Security (TLS) processing of data transmitted and received over the network. 
     An operating system of appliance  200  allocates, manages, or otherwise segregates the available system memory into kernel space  204  and user space  202 . Kernel space  204  is reserved for running kernel  230 , including any device drivers, kernel extensions or other kernel related software. As known to those skilled in the art, kernel  230  is the core of the operating system, and provides access, control, and management of resources and hardware-related elements of application  104 . Kernel space  204  may also include a number of network services or processes working in conjunction with cache manager  232 . 
     Appliance  200  may include one or more network stacks  267 , such as a TCP/IP based stack, for communicating with client(s)  102 , server(s)  106 , network(s)  104 , and/or other appliances  200  or  205 . For example, appliance  200  may establish and/or terminate one or more transport layer connections between clients  102  and servers  106 . Each network stack  267  may include a buffer  243  for queuing one or more network packets for transmission by appliance  200 . 
     Kernel space  204  may include cache manager  232 , packet engine  240 , encryption engine  234 , policy engine  236  and compression engine  238 . In other words, one or more of processes  232 ,  240 ,  234 ,  236  and  238  run in the core address space of the operating system of appliance  200 , which may reduce the number of data transactions to and from the memory and/or context switches between kernel mode and user mode, for example since data obtained in kernel mode may not need to be passed or copied to a user process, thread or user level data structure. 
     Cache manager  232  may duplicate original data stored elsewhere or data previously computed, generated or transmitted to reducing the access time of the data. In some embodiments, the cache memory may be a data object in memory  264  of appliance  200 , or may be a physical memory having a faster access time than memory  264 . 
     Policy engine  236  may include a statistical engine or other configuration mechanism to allow a user to identify, specify, define or configure a caching policy and access, control and management of objects, data or content being cached by appliance  200 , and define or configure security, network traffic, network access, compression or other functions performed by appliance  200 . 
     Encryption engine  234  may process any security related protocol, such as SSL or TLS. For example, encryption engine  234  may encrypt and decrypt network packets, or any portion thereof, communicated via appliance  200 , may setup or establish SSL, TLS or other secure connections, for example between client  102 , server  106 , and/or other appliances  200  or  205 . In some embodiments, encryption engine  234  may use a tunneling protocol to provide a VPN between a client  102  and a server  106 . In some embodiments, encryption engine  234  is in communication with encryption processor  260 . Compression engine  238  compresses network packets bi-directionally between clients  102  and servers  106  and/or between one or more appliances  200 . 
     Packet engine  240  may manage kernel-level processing of packets received and transmitted by appliance  200  via network stacks  267  to send and receive network packets via network ports  266 . Packet engine  240  may operate in conjunction with encryption engine  234 , cache manager  232 , policy engine  236  and compression engine  238 , for example to perform encryption/decryption, traffic management such as request-level content switching and request-level cache redirection, and compression and decompression of data. 
     User space  202  is a memory area or portion of the operating system used by user mode applications or programs otherwise running in user mode. A user mode application may not access kernel space  204  directly and uses service calls in order to access kernel services. User space  202  may include graphical user interface (GUI)  210 , a command line interface (CLI)  212 , shell services  214 , health monitor  216 , and daemon services  218 . GUI  210  and CLI  212  enable a system administrator or other user to interact with and control the operation of appliance  200 , such as via the operating system of appliance  200 . Shell services  214  include the programs, services, tasks, processes or executable instructions to support interaction with appliance  200  by a user via the GUI  210  and/or CLI  212 . 
     Health monitor  216  monitors, checks, reports and ensures that network systems are functioning properly and that users are receiving requested content over a network, for example by monitoring activity of appliance  200 . In some embodiments, health monitor  216  intercepts and inspects any network traffic passed via appliance  200 . For example, health monitor  216  may interface with one or more of encryption engine  234 , cache manager  232 , policy engine  236 , compression engine  238 , packet engine  240 , daemon services  218 , and shell services  214  to determine a state, status, operating condition, or health of any portion of the appliance  200 . Further, health monitor  216  may determine if a program, process, service or task is active and currently running, check status, error or history logs provided by any program, process, service or task to determine any condition, status or error with any portion of appliance  200 . Additionally, health monitor  216  may measure and monitor the performance of any application, program, process, service, task or thread executing on appliance  200 . 
     Daemon services  218  are programs that run continuously or in the background and handle periodic service requests received by appliance  200 . In some embodiments, a daemon service may forward the requests to other programs or processes, such as another daemon service  218  as appropriate. 
     As described herein, appliance  200  may relieve servers  106  of much of the processing load caused by repeatedly opening and closing transport layer 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 (e.g., “connection pooling”). To perform connection pooling, appliance  200  may translate or multiplex communications by modifying sequence numbers and acknowledgment numbers at the transport layer protocol level (e.g., “connection multiplexing”). Appliance  200  may also provide switching or load balancing for communications between the client  102  and server  106 . 
     As described herein, each client  102  may include client agent  120  for establishing and exchanging communications with appliance  200  and/or server  106  via a network  104 . Client  102  may have installed and/or execute one or more applications that are in communication with network  104 . Client agent  120  may intercept network communications from a network stack used by the one or more applications. For example, client agent  120  may intercept a network communication at any point in a network stack and redirect the network communication to a destination desired, managed or controlled by client agent  120 , for example to intercept and redirect a transport layer connection to an IP address and port controlled or managed by client agent  120 . Thus, client agent  120  may transparently intercept 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 layers. Client agent  120  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. 
     In some embodiments, client agent  120  is implemented as an Independent Computing Architecture (ICA) client developed by Citrix Systems, Inc. of Fort Lauderdale, Fla. Client agent  120  may perform acceleration, streaming, monitoring, and/or other operations. For example, client agent  120  may accelerate streaming an application from a server  106  to a client  102 . Client agent  120  may also perform end-point detection/scanning and collect end-point information about client  102  for appliance  200  and/or server  106 . Appliance  200  and/or server  106  may use the collected information to determine and provide access, authentication and authorization control of the client&#39;s connection to network  104 . For example, client agent  120  may identify and determine one or more client-side attributes, such as: the operating system and/or a version of an operating system, a service pack of the operating system, a running service, a running process, a file, presence or versions of various applications of the client, such as antivirus, firewall, security, and/or other software. 
     C. Systems and Methods for Virtualizing an Application Delivery Controller 
     Referring now to  FIG.  3   , a block diagram of a virtualized environment  300  is shown. As shown, a computing device  302  in virtualized environment  300  includes a virtualization layer  303 , a hypervisor layer  304 , and a hardware layer  307 . Hypervisor layer  304  includes one or more hypervisors (or virtualization managers)  301  that allocates and manages access to a number of physical resources in hardware layer  307  (e.g., physical processor(s)  321  and physical disk(s)  328 ) by at least one virtual machine (VM) (e.g., one of VMs  306 ) executing in virtualization layer  303 . Each VM  306  may include allocated virtual resources such as virtual processors  332  and/or virtual disks  342 , as well as virtual resources such as virtual memory and virtual network interfaces. In some embodiments, at least one of VMs  306  may include a control operating system (e.g.,  305 ) in communication with hypervisor  301  and used to execute applications for managing and configuring other VMs (e.g., guest operating systems  310 ) on device  302 . 
     In general, hypervisor(s)  301  may provide virtual resources to an operating system of VMs  306  in any manner that simulates the operating system having access to a physical device. Thus, hypervisor(s)  301  may be used to emulate virtual hardware, partition physical hardware, virtualize physical hardware, and execute virtual machines that provide access to computing environments. In an illustrative embodiment, hypervisor(s)  301  may be implemented as a Citrix Hypervisor by Citrix Systems, Inc. of Fort Lauderdale, Fla. In an illustrative embodiment, device  302  executing a hypervisor that creates a virtual machine platform on which guest operating systems may execute is referred to as a host server.  302   
     Hypervisor  301  may create one or more VMs  306  in which an operating system (e.g., control operating system  305  and/or guest operating system  310 ) executes. For example, the hypervisor  301  loads a virtual machine image to create VMs  306  to execute an operating system. Hypervisor  301  may present VMs  306  with an abstraction of hardware layer  307 , and/or may control how physical capabilities of hardware layer  307  are presented to VMs  306 . For example, hypervisor(s)  301  may manage a pool of resources distributed across multiple physical computing devices. 
     In some embodiments, one of VMs  306  (e.g., the VM executing control operating system  305 ) may manage and configure other of VMs  306 , for example by managing the execution and/or termination of a VM and/or managing allocation of virtual resources to a VM. In various embodiments, VMs may communicate with hypervisor(s)  301  and/or other VMs via, for example, one or more Application Programming Interfaces (APIs), shared memory, and/or other techniques. 
     In general, VMs  306  may provide a user of device  302  with access to resources within virtualized computing environment  300 , for example, one or more programs, applications, documents, files, desktop and/or computing environments, or other resources. In some embodiments, VMs  306  may be implemented as fully virtualized VMs that are not aware that they are virtual machines (e.g., a Hardware Virtual Machine or HVM). In other embodiments, the VM may be aware that it is a virtual machine, and/or the VM may be implemented as a paravirtualized (PV) VM. 
     Although shown in  FIG.  3    as including a single virtualized device  302 , virtualized environment  300  may include a plurality of networked devices in a system in which at least one physical host executes a virtual machine. A device on which a VM executes may be referred to as a physical host and/or a host machine. For example, appliance  200  may be additionally or alternatively implemented in a virtualized environment  300  on any computing device, such as a client  102 , server  106  or appliance  200 . Virtual appliances may provide functionality for availability, performance, health monitoring, caching and compression, connection multiplexing and pooling and/or security processing (e.g., firewall, VPN, encryption/decryption, etc.), similarly as described in regard to appliance  200 . 
     In some embodiments, a server may execute multiple virtual machines  306 , for example on various cores of a multi-core processing system and/or various processors of a multiple processor device. For example, although generally shown herein as “processors” (e.g., in  FIGS.  1 C,  2  and  3   ), one or more of the processors may be implemented as either single- or multi-core processors to provide a multi-threaded, parallel architecture and/or multi-core architecture. Each processor and/or core may have or use memory that is allocated or assigned for private or local use that is only accessible by that processor/core, and/or may have or use memory that is public or shared and accessible by multiple processors/cores. Such architectures may allow work, task, load or network traffic distribution across one or more processors and/or one or more cores (e.g., by functional parallelism, data parallelism, flow-based data parallelism, etc.). 
     Further, instead of (or in addition to) the functionality of the cores being implemented in the form of a physical processor/core, such functionality may be implemented in a virtualized environment (e.g.,  300 ) on a client  102 , server  106  or appliance  200 , such that the functionality may be implemented across multiple devices, such as a cluster of computing devices, a server farm or network of computing devices, etc. The various processors/cores may interface or communicate with each other using a variety of interface techniques, such as core to core messaging, shared memory, kernel APIs, etc. 
     In embodiments employing multiple processors and/or multiple processor cores, described embodiments may distribute data packets among cores or processors, for example to balance the flows across the cores. For example, packet distribution may be based upon determinations of functions performed by each core, source and destination addresses, and/or whether: a load on the associated core is above a predetermined threshold; the load on the associated core is below a predetermined threshold; the load on the associated core is less than the load on the other cores; or any other metric that can be used to determine where to forward data packets based in part on the amount of load on a processor. 
     For example, data packets may be distributed among cores or processes using receive-side scaling (RSS) in order to process packets using multiple processors/cores in a network. RSS generally allows packet processing to be balanced across multiple processors/cores while maintaining in-order delivery of the packets. In some embodiments, RSS may use a hashing scheme to determine a core or processor for processing a packet. 
     The RSS may generate hashes from any type and form of input, such as a sequence of values. This sequence of values can include any portion of the network packet, such as any header, field or payload of network packet, and include any tuples of information associated with a network packet or data flow, such as addresses and ports. The hash result or any portion thereof may be used to identify a processor, core, engine, etc., for distributing a network packet, for example via a hash table, indirection table, or other mapping technique. 
     D. Systems and Methods for Providing a Distributed Cluster Architecture 
     Although shown in  FIGS.  1 A and  1 B  as being single appliances, appliances  200  may be implemented as one or more distributed or clustered appliances. Individual computing devices or appliances may be referred to as nodes of the cluster. A centralized management system may perform load balancing, distribution, configuration, or other tasks to allow the nodes to operate in conjunction as a single computing system. Such a cluster may be viewed as a single virtual appliance or computing device.  FIG.  4    shows a block diagram of an illustrative computing device cluster or appliance cluster  400 . A plurality of appliances  200  or other computing devices (e.g., nodes) may be joined into a single cluster  400 . Cluster  400  may operate as an application server, network storage server, backup service, or any other type of computing device to perform many of the functions of appliances  200  and/or  205 . 
     In some embodiments, each appliance  200  of cluster  400  may be implemented as a multi-processor and/or multi-core appliance, as described herein. Such embodiments may employ a two-tier distribution system, with one appliance if the cluster distributing packets to nodes of the cluster, and each node distributing packets for processing to processors/cores of the node. In many embodiments, one or more of appliances  200  of cluster  400  may be physically grouped or geographically proximate to one another, such as a group of blade servers or rack mount devices in a given chassis, rack, and/or data center. In some embodiments, one or more of appliances  200  of cluster  400  may be geographically distributed, with appliances  200  not physically or geographically co-located. In such embodiments, geographically remote appliances may be joined by a dedicated network connection and/or VPN. In geographically distributed embodiments, load balancing may also account for communications latency between geographically remote appliances. 
     In some embodiments, cluster  400  may be considered a virtual appliance, grouped via common configuration, management, and purpose, rather than as a physical group. For example, an appliance cluster may comprise a plurality of virtual machines or processes executed by one or more servers. 
     As shown in  FIG.  4   , appliance cluster  400  may be coupled to a first network  104 ( 1 ) via client data plane  402 , for example to transfer data between clients  102  and appliance cluster  400 . Client data plane  402  may be implemented a switch, hub, router, or other similar network device internal or external to cluster  400  to distribute traffic across the nodes of cluster  400 . For example, traffic distribution may be performed based on equal-cost multipath (ECMP) routing with next hops configured with appliances or nodes of the cluster, open-shortest path first (OSPF), stateless hash-based traffic distribution, link aggregation (LAG) protocols, or any other type and form of flow distribution, load balancing, and routing. 
     Appliance cluster  400  may be coupled to a second network  104 ( 2 ) via server data plane  404 . Similarly to client data plane  402 , server data plane  404  may be implemented as a switch, hub, router, or other network device that may be internal or external to cluster  400 . In some embodiments, client data plane  402  and server data plane  404  may be merged or combined into a single device. 
     In some embodiments, each appliance  200  of cluster  400  may be connected via an internal communication network or back plane  406 . Back plane  406  may enable inter-node or inter-appliance control and configuration messages, for inter-node forwarding of traffic, and/or for communicating configuration and control traffic from an administrator or user to cluster  400 . In some embodiments, back plane  406  may be a physical network, a VPN or tunnel, or a combination thereof. 
     E. Systems and Methods for Cluster-Aware Multipath Transmission Control Protocol (MPTCP) Session Load Balancing 
     Referring now to  FIG.  5   , depicted is a system  500  for cluster-aware multipath transmission control protocol (MPTCP) session load balancing, according to one or more embodiments. The system  500  is shown to include a processing system  502  intermediary to a client  102  and a server  106 . The processing system  502  may include various clusters of processors  504 . For example, the processing system  502  may include a first cluster of processors  504 ( 1 ),  504 ( 2 ) (also referred to herein as a first cluster or a first tier) intermediary to the client  102  and server  106 , and a second cluster of processors  504 ( 3 )- 504 (N) (also referred to herein as a second cluster or a second tier) intermediary to the first cluster and the server  106 . As described in greater detail below, a processor of the first cluster may receive a first request from the client  102  to establish a first connection between the client  102  and the server  106 . The processor of the first cluster may select a processor of the second cluster to forward the first request. The processor of the first cluster may forward the first request to the selected processor of the second cluster. The selected processor of the second cluster may receive the request and establish the first connection between the client  102  and the server  106 . Another processor of the first cluster may receive a second request to establish a multipath connection between the client  102  and the server  106 . The processor of the first cluster may identify the selected processor of the second cluster to which to forward the second request responsive to determining that the second request is to establish a multipath connection. The processor may forward the second request to the selected processor. The selected processor may receive the second request, and establish a multipath connection that includes the first connection and the second connection used as paths of the multipath connection. 
     The systems and methods of the present solution may be implemented in any type or form of device, including clients, servers or appliances described above with reference to  FIG.  1 A - FIG.  4   . For instance, and as described in greater detail below, the processors  504 ( 1 )- 504 (N) may be incorporated or otherwise implemented as processing engines in an appliance similar to the appliances  200  described above with reference to  FIG.  2   - FIG.  4   . The clients  102  may be similar in some respects to the clients  102  described above with respect to  FIG.  1 A - FIG.  1 B . The clients  102  may request access to a domain (e.g., a website, application, service, etc.) corresponding to a server  106 , which may be similar in some respects to the server  106  described above with respect to  FIG.  1 A - FIG.  1 B . In some implementations, the clients  102 , servers  106 , and/or processors  504  may include or incorporate components and devices similar in some aspects to those described above with reference to  FIG.  1 C , such as a memory and/or one or more processors operatively coupled to the memory. The present systems and methods may be implemented in any embodiments or aspects of the appliances or devices described herein. 
     The clients  102  may be the same as or similar to the clients  102  described above with respect to  FIG.  1 A - FIG.  1 B . The clients  102  may be personal computers, laptops, desktops, tablets, mobile devices, etc. The clients  102  may be configured to access services, websites, webpages, applications, etc. corresponding to a domain hosted on various servers  106 . The clients  102  may be configured to access the domain by generating requests for a processor  504  or processing engine of the processing system  502 . The clients  102  may be configured to generate the requests when a user selects a service, launches a service, provides a uniform resource locator (URL) address to a browser, etc. The request may include, for instance, the URL address for the domain, a domain name, etc. The clients  102  may be configured to transmit, send, or otherwise provide the requests to a processor  504  of the first cluster for establishing a connection with the server  106 . In some embodiments, the clients  102  may be configured to generate additional requests for establishing secondary connections with the server  106 . For example, the clients  102  may be configured to generate requests to establish a multipath connection between the client  102  and server  106 . The multipath connection may include at least a primary and secondary connection between the client  102  and server  106 . The clients  102  may be configured to generate request to establish a multipath connection to, for example, increase persistency of connections (i.e., by providing multiple connections to use as a fallback in the event of one connection having an interruption), to aggregate bandwidth of different networks (i.e., aggregate bandwidth of the primary and secondary connections), to expedite delivery of content from the server  106  to the client  102  by using each of the connections, to ensure that client  102  data is secure by sending data via a more secure connection from the primary and secondary connections, and so forth. As described in greater detail below, the processors  504  of the processing system  502  may be configured to select a processor  504  of the second cluster which is to receive the connection requests from the client  102  (i.e., according to one or more load balancing algorithms), and forward each of the connection requests originating from the client  102  as part of the same session to the same processor  504 . 
     Referring now to  FIG.  5    and  FIG.  6   , the system  500  is shown to include a processing system  502  including a plurality of processors  504 . Specifically,  FIG.  6    shows the processing system  502 , according to an illustrative embodiment. The processing system  502  may include a first cluster of processors  504  forming a first tier of the processing system  502 , and a second cluster of processors  504  forming a second tier of the processing system  502 . In some embodiments, processors  504  of the processing system  502  may be assigned to a respective cluster. For example, an administrator or other user may provide each of the processors  504  with a configuration file which assigns the processors  504  to a particular cluster (i.e., at initialization, at installation, at enrollment, etc.). The first cluster of processors  504  may receive each of the requests received from clients  102 . As such, the first cluster of processors  504  may be assigned a first (or front-end) network location (i.e., a location along a network path between the client  102  and the server  106 ) in the configuration file. The first cluster of processors  504  may be communicably coupled to the second cluster of processors  504  such that processors  504  of the first cluster may exchange data with processors  504  of the second cluster. As described in greater detail below, the processors  504  of the first cluster may select a processor  504  of the second cluster to which to forward requests from the client to establish connections with the server  106 . The second cluster of processors  504  may be configured to establish connections between the clients  102  and the server  106  based on requests received from the clients  102  via the processors  504  of the first cluster. As such, the second cluster of processors  504  may be assigned a second (or back-end) network location in the configuration file. 
     The first tier of the processing system  502  may be intermediary to the client  102  and the second tier of the processing system  502 , and the second tier of the processing system  502  may be intermediary to the first tier of the processing system  502  and the server  106 . In some embodiments, the processors  504  may be incorporated or embodied on an appliance or device  602  (such as an appliance  200  described above with reference to  FIG.  2    and  FIG.  4   ). In some embodiment, the processors  504  may each be incorporated in or embodied on a separate device  602 . For example, and as shown in  FIG.  6   , some devices  602 ( 1 ),  602 ( 3 ) may include a single processor  504 . In some embodiments, an appliance or device  602  may include multiple processors  504 . For instance, an appliance or device  602  may be or include a multi-core processing system which includes multiple processors  502 . As shown in  FIG.  6   , devices  602 ( 2 ),  602 ( 4 ) may each include multiple processors  504 ( 2 ),  504 ( 3 ),  504 ( 5 ),  505 ( 6 ). 
     The devices  602  are shown to include a data structure  506 . The data structure  506  may include, for example, memory. The memory may include volatile memory (e.g., RAM), non-volatile memory (e.g., one or more hard disk drives (HDDs) or other magnetic or optical storage media, one or more solid state drives (SSDs) such as a flash drive or other solid state storage media, one or more hybrid magnetic and solid state drives, and/or one or more virtual storage volumes, such as a cloud storage, or a combination of such physical storage volumes and virtual storage volumes or arrays thereof), or other data structure. In some embodiments, the data structure  506  may be shared, distributed, or otherwise accessible across processors  504 . For example, and in some embodiments, the data structure  506  may include a distributed hash table. The distributed hash table may be accessible by each of the processors  504  within a cluster of processors  504 . For example, and in some embodiments, the distributed hash table may be accessible by the processors  504 ( 1 )- 504 ( 3 ) of the first cluster shown in  FIG.  6   . As described in greater detail below, the processors  504 ( 1 )- 504 ( 3 ) may be configured to store data in the distributed hash table (or other data structure) relating to connections between clients  102  and the server  106  established by processors  504  of the second cluster. Such data may be used by the processors  504 ( 1 )- 504 ( 3 ) to forward requests for multipath connections from a client  102  to the same processor  504  of the second cluster which established the primary connection for the client  102 , as described in greater detail below. 
     Referring now to  FIG.  7 A  together with  FIG.  5   - FIG.  6   , depicted is a flow diagram showing an example method  700  of establishing a primary connection, according to an illustrative embodiment. As a brief overview, at step  702 , a client  102  may be configured to generate a request for a connection. At step  704 , a processor  504  of the first cluster may be configured to receive the request for the connection from the client  102  and select a processor of the second cluster. At step  706 , the processor  504  of the first cluster may be configured to forward the request to the selected processor of the second cluster. At step  708 , the processor  504  of the first cluster may be configured to create a persistency session and store a token in the data structure  506 . At step  710 , the processor  504  of the first cluster may be configured to create a session replica and forward the token to other processors of the first cluster. 
     At step  702 , a client  102  may be configured to generate a request for a connection. The request may be a request to establish the connection between the client  102  and a server  106 . In some instances, the request may be a request to establish a primary connection (or primary subflow) of a multipath connection between the client  102  and the server  106 . In some embodiments, the client  102  may be configured to generate the request responsive to a user launching an application hosted on the server  106 , a user providing an address (such as a uniform resource locator (URL)) for a resource (such as a webpage, document, or other data) hosted on the server  106 , and so forth. The client  102  may be configured to generate the request for transmission to a device  602  intermediary to the client  102  and the server  106 . In some embodiments, the request may include a synchronize (SYN) packet from the client  102 . The SYN packet may be a packet which includes a source port for the server  106 , a sequence or identifier from the client  102 , and data relating to the connection requested by the client  102 . In some embodiments, the client  102  may be configured to send the request to the device  602  (i.e., to an address or port for the device  602 ). For example, and in some embodiments, the device  602  may be managed by an entity which also manages the server  106  and/or the resource hosted on the server  106 . For instance, the device  602  may be a server front-end device. As such, the address of the device  602  may be used for sending requests to establish the connection with the server  106 . In other words, the device  602  configured to automatically receive requests which are routed to the server  106  as part of being a server front-end device. As such, when requests to connect to the server  106  are sent by clients  102 , such requests may be received (or intercepted) at the device  602 . As an example, when a client  102  requests access to an application hosted on the server  106 , the client  102  may generate a DNS request or query for a DNS server. The DNS server may respond with an IP address and port of the device(s)  602 . 
     At step  704 , a processor  504  of the first cluster may be configured to receive the request for the connection from the client  102  and select a processor of the second cluster. In some embodiments, the processor  504  of the first cluster may be embodied on or otherwise a component of one of the devices  602  of the first cluster. For example, the processor  504  may be a processor of the device  602 ( 1 ) or device  602 ( 2 ). The processor  504  may be configured to receive the request at an address of the device  602 . The processor  504  may be configured to select a processor  502  of the second cluster to which to forward the request from the client  102 . The processor  504  may be configured to select the processor  502  of the second cluster from a plurality of processors  504  of the second cluster. 
     In some embodiments, the processor  504  may be configured to identify or select the processor  504  of the second cluster according to (i.e., using or based on) one or more load balancing algorithms. For example, the processor  504  may be configured to identify or select the processor  504  of the second cluster according to a round robin load balancing algorithm. The round robin load balancing algorithm may be weighted (i.e., according to a load on the processors  504  of the second cluster). The processor  504  may be configured to access data corresponding to a current load on the processors  504  of the second cluster. In some implementations, the processor  504  may be configured to query the data structure  506  to determine a load on each of the processors  504  of the second cluster. As described in greater detail below, the processor  504  of the first cluster may be configured to populate the data structure  506  with data corresponding to connections maintained by the processors  504  of the second cluster. The processor  504  of the first cluster may determine a current load on each of the processors  504  of the second cluster by querying the data structure  506  to determine a current load on each of the processors  504  of the second cluster. In some embodiments, the processor  504  may determine the current load on each of the processors  504  of the second cluster by querying each of the processors  504  of the second cluster to determine a current load on each of the processors  504 . The processor  504  of the first cluster may be configured to identify or select the processor  504  of the second cluster according to a round robin algorithm which is weighted based on a load on the processors  504  of the second cluster. In some embodiments, the processor  504  of the first cluster may be configured to identify or select the processor  504  of the second cluster according to a least loaded load balancing algorithm. For example, the processor  504  may be configured to identify the processor  504  of the second cluster based on which of the processors  504  of the second cluster has fewest loads or connections. The processor  504  of the first cluster may determine the current load on the processors of the cluster  504  based on data from the data structure  506  and/or based on load data received from the processors  504  as described above. The processor  504  of the first cluster may identify or select the processor  504  of the second cluster having the least load (i.e., the fewest connections, processing the fewest packets, etc.). 
     At step  706 , the processor  504  of the first cluster may be configured to forward the request to the selected processor  506  of the second cluster. In some embodiments, the processor  504  may be configured to send, transmit, provide, or otherwise forward the request (i.e., the SYN packet) to the selected processor  504  of the second cluster. The processor  504  of the first cluster may be configured to provide the request to an address of the selected processor  504 . The selected processor  504  may be configured to respond to the request with an acknowledgement (i.e., a SYN-ACK packet). The acknowledgement may be or include an acknowledgement of the request and indicating that the selected processor  504  has established a connection (i.e., the primary or first connection) responsive to receiving the request. The selected processor  504  may be configured to provide, generate, or otherwise establish the connection between the client  102  and the server  106  responsive to sending the acknowledgement. For example, once the processor  504  of the first cluster receives the acknowledgement from the selected processor  504  of the second cluster, the processor  504  of the first cluster may be configured to transmit, forward, or otherwise provide the acknowledgement to the client  102 . The client  102  may be configured to use the primary or first connection for transmitting, receiving, or otherwise exchanging data between the client  102  and the server  106 . 
     At step  708 , the processor  504  of the first cluster may be configured to create, generate, or otherwise establish a persistency session and store a token in the data structure  506 . In some embodiments, the processor  504  may be configured to establish the persistency session between the processor  504  and the selected processor  504  of the second cluster. The processor  504  of the first cluster may establish the persistency session responsive to forwarding, sending, transmitting, or otherwise providing the request to the selected processor and receiving the acknowledgement. The acknowledgement may include a key which is used for generating a token or other identifier. The selected processor  504  may be configured to generate the key based on the request. In some embodiments, the selected processor  504  may be configured to generate the key using data specific to the client  102  and the request, including a key of the client  102 , a client port or IP address, an IP or port of the server  106  for the request. The selected processor  504  may be configured to generate the key using or based on a security key maintained by the selected processor  504  (such as a recycled key which may be changed at various intervals). In some embodiments, the selected processor  504  may be configured to generate the key using a hash function of a combination of, for example, the client key, the client IP address, the server IP address, and/or the recycled key. The key may be or include a data string which is used by the client  102  and the processor  504  of the first cluster to derive a token which is unique to the session between the client  102  and the server  106 . The key may be unique to the selected processor  504 . In some embodiments, the key may be unique to the selected processor and the request from the client  102 . As described in greater detail below, the client  102 , upon receiving the key from the SYN-ACK, may generate the token and include the token in subsequent requests/packets/etc. from the client  102  to the server  106  (i.e., requests for content from the server  102 , requests for additional or secondary connections between the client  102  and the server  106 , etc.). 
     The processor  504  of the first cluster may receive the acknowledgement from the selected processor including the key. The processor  504  of the first cluster may be configured to extract the key from the acknowledgement from the selected processor  504 . The processor  504  of the first cluster may be configured to generate, derive, or otherwise determine the token or identifier using the key. In some embodiments, the processor  504  may determine the token by computing or determining a cryptographic hash of the key. The cryptographic hash may be dependent on the authentication algorithm selected or negotiated by the client  102  and the server  106 , the client  102  and the processor  504  of the first cluster, the client  102  and the processor  504  of the second cluster, and so forth. The processor  504  may be configured to store the token or identifier in the data structure  506  in association with an identifier of the selected processor  504 . In some embodiments, the processor  504  may be configured to store the token in a hash table of the data structure  506  in association with an identifier of the selected processor  504 . For example, the processor  504  may be configured to compute a hash of the token or identifier and store the computed hash in the hash table of the data structure. The processor  504  may be configured to store the computed hash indefinitely, for a predetermined duration (i.e., until expiration of the key or the token), for a duration of the connection between the client  102  and the server  106  (i.e., until a processor  504  of the first cluster receives a request to terminate the connection), and so forth. 
     At step  708 , the processor  504  of the first cluster may be configured to create a session replica and forward the token to other processors of the first cluster. The processor  504  may be configured to generate, determine, derive, or otherwise create the session replica by generating one or more additional or replica tokens. The replica tokens may be or include a duplicate token which is similar to the token described above and derived from the key received in the acknowledgement from the processor of the first cluster. The processor  504  may be configured to create the session replica to include the replica token(s) and an identifier for the selected processor  504  of the second cluster. The session replica may be used by the processors  504  of the first cluster to determine or identify the selected processor  504  of the second cluster which is associated with the replica token(s). Additionally, the session replica may provide persistence of the multipath connection by ensuring that each of the processors  504  of the first cluster include data corresponding to the primary connection in the event that one of the processors  504  are removed from the first cluster, go offline, crash, restart, etc. At step  710 , the processor  504  may be configured to forward, transmit, send, or otherwise provide the replica token to other processors  504  of the first cluster. In some embodiments, the processor  504  may be configured to update the hash table of the data structure  506 , which may be accessible by other processors  504  of the first cluster. In this regard, the hash table may be or include a distributed hash table which is accessible by each of the processors  504  of the first cluster. 
     As one example use case of the method  700  and with reference to  FIG.  5    and FIG.  7 A, the client  102  may generate a request (i.e., a SYN packet) to establish a primary connection between the client  102  and the server  106 . The client  102  may transmit the request to the server  106 , which may be received by the processor  504 ( 2 ) of the first cluster (shown in solid). The processor  504 ( 2 ) may select the processor  504 ( 3 ) of the second cluster according to a load balancing algorithm based on a current load of the processors  504  of the second cluster. The processor  504 ( 2 ) may forward the request from the client  102  to the selected processor  504 ( 3 ) of the second cluster (shown in solid). The processor  504 ( 3 ) may establish the connection between the client  102  and the server  106  (shown in solid), and respond to the request with an acknowledgement (i.e., a SYN-ACK packet) which includes a key for deriving or generating a token for the primary connection. The processor  504 ( 3 ) may send the acknowledgement to the client  102  via the processor  504 ( 2 ). The processor  504 ( 2 ) may receive the acknowledgement from the processor  504 ( 3 ). The processor  504 ( 2 ) may extract the key and derive the token. The processor  504 ( 2 ) may store the token for the primary connection in the data structure  506 ( 2 ). The processor  504 ( 2 ) may forward the acknowledgement back to the client  102 . The client  102  may derive the token using the key received in the acknowledgement. The client  102  may include the token in subsequent requests/data/packets/etc. sent from the client  102  to the server  106  via the processors  504 . 
     Referring now to  FIG.  7 B  together with  FIG.  5   - FIG.  6   , depicted is a flow diagram showing an example method  712  of establishing a secondary connection, according to an illustrative embodiment. As a brief overview, at step  714 , the client  102  may be configured to generate a request for a secondary connection. At step  716 , a processor  504  of the first cluster may be configured to receive the request for the secondary connection from the client  102 . At step  718 , the processor  504  of the first cluster may be configured to perform a local session lookup in the local data structure  506  of the processor  504 . If the processor  504  of the first cluster does not identify a destination processor  504  of the second cluster from the lookup in the local data structure  506 , at step  720 , the processor  504  of the first cluster may perform a remote session lookup in data structures  506  of other processors  504  of the first cluster to identify a destination processor  504 . On the other hand, if the processor  504  of the first cluster identifies a destination processor  504  in the local data structure, the processor  504  of the first cluster forwards the request from the client  102  to the destination processor  504  of the second cluster. 
     At step  714 , the client  102  may be configured to generate a request for a secondary connection. Step  714  may be similar in some aspects to step  702  described above. In this example, the request for the secondary connection may include a token. The client  102  may derive the token using the key received from the processor  504  of the second cluster as described above. The client  102  may be configured to include, incorporate, or otherwise provide the token with the request for the secondary connection. The client  102  may be configured to transmit, send, or otherwise provide the request including the token to a processor  504  of the first cluster. 
     At step  716 , a processor  504  of the first cluster may be configured to receive the request for the secondary connection from the client  102 . In some instances, the client  102  may be configured to provide the request to the processor  504  of the first cluster which received the first request (i.e., the request to establish the primary connection). In some instances, the client  102  may be configured to provide the request to a different processor  504 . In other words, the processor  504  which receives the request for the secondary connection from the client  102  at step  716  may or may not be the same processor  504  which receives the request for the primary connection from the client  102  at step  704  of  FIG.  7 A . The processor  504  which receives the request for the secondary connection may parse the request to extract the token included in the request. 
     At step  718 , the processor  504  of the first cluster may be configured to perform a local session lookup in the local data structure  506  of the processor  504 . In some embodiments, the processor  504  may perform the local session lookup in the local data structure  506  using the token from the request. If the processor  504  of the first cluster does not identify a destination processor  504  of the second cluster from the lookup in the local data structure  506 , at step  720 , the processor  504  of the first cluster may perform a remote session lookup in data structures  506  of other processors  504  of the first cluster to identify a destination processor  504 . In these or other embodiments, the processor  504  may configured determine whether the token corresponds to an existing primary connection between the client  102  and the server  106  based on the token matching a token in one of the data structures  706  stored by one of the processors  504  of the first cluster. 
     The processor  504  may be configured to perform a lookup function using the token from the request in the local or remote data structures  506  to determine whether the token matches a token in the local or remote data structures  506 . The processor  504  may be configured to determine or identify a destination processor  504  of the second cluster responsive to the token matching a token in the local or remote data structure  506 . As described above, the tokens may be stored in the local and/or remote data structure  506  in association with an identifier of a processor  504  of the second cluster. The processor  504  may extract the identifier of the destination processor  504  from the local and/or remote data structure which is associated with the matching token. 
     If the processor  504  of the first cluster identifies a destination processor  504  in the local or remote data structure from the local or remote session lookup, at step  722 , the processor  504  of the first cluster forwards the request from the client  102  to the destination processor  504  of the second cluster. The destination processor  504  of the second cluster may receive the request from the processor  504  of the first cluster. The destination processor  504  may be configured to establish the secondary connection and respond to the client  102  with an acknowledgement including a key for the secondary connection. The destination processor  504  may transmit, send, or otherwise provide the acknowledgement of the secondary request to the client via the processor  504  of the first cluster. The processor  504  of the first cluster may generate a second token based on the key for the secondary connection, and store the token in the data structure as described above. The method  712  shown in  FIG.  7 B  may be performed any number of times to establish any number of secondary connections between the client  102  and the server  106 . 
     Continuing the use case of the method  700  and in reference to  FIG.  5    and  FIG.  7 B , the client  102  may generate a request (i.e., a SYN packet) to establish a secondary connection between the client  102  and the server  106 . The client  102  may transmit the request to the server  106  including a token derived by the client  102  based on the key received in the acknowledgement of the primary connection. A processor  504 ( 1 ) of the first cluster may receive the request to establish the secondary connection (shown in dot-dash). The processor  504 ( 1 ) may extract the token from the request received from the client  102 . The processor  504 ( 1 ) may perform a lookup in the local data structure  506 ( 1 ) using the token from the request to identify a destination processor. In this example, since the processor  504 ( 1 ) did not receive the first request (i.e., the request to establish the primary connection), the processor  504 ( 1 ) may not identify a matching token in the data structure  506 ( 1 ). The processor  504 ( 1 ) may then perform a remote session lookup in the data structure  506 ( 2 ) of other processors  504 ( 2 ) of the first cluster using the token. In this example, since the processor  504 ( 2 ) received the first request, the data structure  506 ( 2 ) may include a token which matches the token received in the request for the secondary connection. The processor  504 ( 1 ) may retrieve, from the data structure  506 ( 2 ), the identifier which was stored in association with the matching token. The processor  504 ( 1 ) may identify the destination processor  504 ( 3 ) using the identifier from the data structure  506 ( 2 ). 
     The processor  504 ( 1 ) may forward the request from the client  102  to the destination processor  504 ( 3 ) (shown in dot-dash). The processor  504 ( 3 ) may receive the request from the processor  504 ( 1 ), and establish the connection (shown in dot-dash) between the client  102  and the server  106 . The processor  504 ( 3 ) may respond to the request with an acknowledgement (i.e., a SYN-ACK packet) which includes a key for deriving or generating a token for the secondary connection. The processor  504 ( 3 ) may send the acknowledgement to the client  102  via the processor  504 ( 1 ). The processor  504 ( 1 ) may receive the acknowledgement from the processor  504 ( 3 ). The processor  504 ( 1 ) may extract the key and derive the token. The processor  504 ( 1 ) may store the token for the secondary connection in the data structure  506 ( 1 ). The processor  504 ( 1 ) may forward the acknowledgement back to the client  102 . The client  102  may derive the token using the key received in the acknowledgement. 
     Referring to  FIG.  8 A , depicted is a flow diagram showing a method  800  for establishing a multipath connection, according to an illustrative embodiment. The method  800  may be implemented or performed by one or more of the processors of the first cluster described above with reference to  FIG.  5   - FIG.  6   . As a brief overview, at step  802 , a processor forward a first request to establish a first connection with a server to a second processor. At step  804 , a processor receives a second request to establish a multipath connection. At step  808 , the processor forwards the second request to the second processor. 
     At step  802 , a processor forward a first request to establish a first connection (or subflow) with a server to a second processor. In some embodiments, a first processor of a first cluster may forward a first request from the client to establish a first connection with the server to a second processor of the second cluster. The first connection may be a primary connection or subflow of a multipath connection. In some embodiments, the processor of the first cluster may receive a first request from a client. The processor of the first cluster may receive the first request responsive to the client launching an application hosted on the server. The processor of the first cluster may receive the first request responsive to the client accessing a domain or resource hosted on the server. The client may send the first request to an address of the server. The processor may receive the request. The processor may be arranged intermediary to the client and the server. The processor(s) of the first cluster may share the address with the server such that requests for the server are routed to the processor(s) of the first cluster. The first processor may intercept or receive the first request. 
     The first processor may select the second processor of the second cluster from a plurality of processors of the second cluster. In some embodiments, the first processor may receive load data for each of the processors of the second cluster. In some embodiments, the first processor may receive the load data from the processors of the second cluster. In some embodiments, the first processor may retrieve or otherwise access the load data from a data structure maintained or otherwise accessible by the first processor. The first processor may select the second processor from the plurality of second cluster based on the load of the second processor. In some embodiments, the first processor may select the second processor based on a load balancing algorithm maintained or otherwise used by the first processor. The load balancing algorithm may be or include a weighted or straight round robin, a least loaded processor algorithm, and so forth. The first processor may forward the request from the client to the second processor responsive to selecting the second processor. 
     In some embodiments, the first processor may receive a response to the first request from the second processor. The first processor may receive a response from the second processor responsive to the second processor establishing the primary connection. In some embodiments, the response may be an acknowledgement of establishing the primary connection. The response may include a key for deriving a token. The token may be unique to the connection between the client and the server. The second processor may generate the response and including the key responsive to establishing the primary connection. The second processor may transmit the response including the key to the client via the first processor. The first processor may receive the response. The first processor may extract or otherwise identify the key from the response from the second processor. The first processor may generate a token based on the key from the response to the request. The first processor may store the token in a data structure maintained by the first processor and accessible by the processors of the first cluster. For example, the data structure may include a table including a plurality of tokens for a plurality of clients between clients and processors of the second cluster. The first processor may add an entry to the table to include the token. In some embodiments, the first processor may store the token in association with an identifier of the second processor. In some embodiments, the first processor may add an entry to tables maintained by other processors of the first cluster. For example, the first processor may transmit the token to other processor(s) of the first cluster for storage in a table maintained by the other processors of the first cluster. 
     At step  804 , a processor receives a second request to establish a multipath connection. In some embodiments, a third processor of the first cluster of processors may receive a second request to establish a multipath connection between the client and the server. In other words, the processor which receives the second request at step  804  may be different from the processor which receives the first request at step  802 . In some embodiments, the third processor may receive the second request responsive to the client requesting a multipath connection to the server. In some embodiments, the third processor may determine that the second request is to establish a multipath connection responsive to the second request including the token. For example, the client may generate the token using the key received in the acknowledgement from the second processor. The third processor may determine that the second request is to establish a multipath connection based on the second request including the token (i.e., indicating that the client currently has a primary connection or subflow which has already been established). 
     In some embodiments, the third processor may identify the second processor to which the third processor is to forward the second request. The third processor may identify the second processor using the token received in the second request. For example, the third processor may parse the second request to extract the token received from the client. The third processor may query the data structure of the third processor (and/or the data structure of other processor(s) of the first cluster) to determine whether the token received from the request matches any tokens from the data structure(s). In some embodiments, the data structure maintained by the first and/or third processor(s) may include a table (such as a distributed hash table). The third processor may query the table using the token received in the request to determine whether the token matches any tokens included in the table. Where the data structure includes a token which matches the token received from the client in the second request, the third processor may identify the corresponding processor (i.e., the second processor) to which to forward the second request. As described above, the tokens may be stored in the data structure in association with an identifier of a processor of the second cluster. When the third processor identifies the matching token from the data structure, the third processor may extract or otherwise identify the identifier of the processor which is associated with the matching token in the data structure. 
     At step  808 , the processor forwards the second request to the second processor. In some embodiments, the third processor forwards the second request to the second processor to establish the multipath connection that includes the first connection and a second connection used as paths of the multipath connection. In some embodiments, the third processor forwards the second request to the second processor responsive to determining that the second request is to establish a multipath connection. In some embodiments, the third processor may determine that the second request is to establish a multipath connection based on the second request including the token. In some embodiments, the third processor may determine that the second request is to establish a multipath connection based on the token included in the second request matching a token included in the data structure of the first and/or the third processor. The third processor may forward the second request to an address of the second processor. In some embodiments, the address of the second processor may be or include the identifier stored in the data structure. In some embodiments, the third processor may identify the address of the second processor using the identifier (i.e., by performing a lookup or query in an address table using the identifier received or retrieved from the data structure). As described in greater detail below with reference to  FIG.  8 B , the second processor may receive the first and second requests (i.e., from the first and third processors) and establish the multipath connection between the client and the server. 
     Referring now to  FIG.  8 B , depicted is a flow diagram showing a method  808  for establishing a multipath connection, according to an illustrative embodiment. The method  808  may be implemented or performed by one or more of the processors of the second cluster described above with reference to  FIG.  5   - FIG.  6   . As a brief overview, at step  810 , a first processor receives a request to establish a first connection with a server from a second processor. At step  812 , the first processor receives a request to establish a second connection from a third processor. At step  814 , the first processor establishes a multipath connection. 
     At step  810 , a first processor receives a request to establish a first connection with a server from a second processor. In some embodiments, the first processor may receive a first request to establish a first connection between the client and a server from a second processor of the cluster of processors that is intermediary to the first processor and a client. As such, the first processor may be intermediary to a server and the cluster of processors which includes the second processor. In this embodiment, the first processor may be a processor of the second cluster of processors, and the second processor may be a processor of the first cluster of processors. For example, the first processor described herein with reference to  FIG.  8 B  is referred to as the second processor above in  FIG.  8 A . On the other hand, the second (and third) processor described herein with reference to  FIG.  8 B  is referred to as the first and third processor above in reference to  FIG.  8 A . 
     In some embodiments, the first processor may receive the first request responsive to the second processor selecting the first processor from a plurality of processors of the second cluster. The second processor may select the first processor according to a load of the first processor relative to a load of other processors of the second cluster. The first processor may receive the first request originating from the client. The first request may include a SYN packet. The first processor may establish the first connection (i.e., primary connection or subflow) responsive to receiving the SYN packet included in the first request. The first processor may generate response or acknowledgement of the first request (i.e., a SYN-ACK packet). The first processor may generate the response responsive to receiving the first request. The first processor may generate the response responsive to establishing the first connection. The first processor may generate a key for including in the response. The first processor may generate the key based on the first request. For example, the processor may use data from the SYN packet to generate the key. In some embodiments, the first processor may generate the key using data specific to the client and the request, including a key of the client, a client port or IP address, an IP or port of the server for the request. The first processor may generate the key using or based on a security key maintained by the selected processor (such as a recycled key which may be changed at various intervals). In some embodiments, the selected processor may generate the key using a hash function of a combination of, for example, the client key, the client IP address, the server IP address, and/or the recycled key. The first processor may incorporate the generated key into the response which is sent to the second processor and forwarded to the client. As described above, both the client and the second processor may derive, determine, or otherwise generate a token based on or using the key. The client may generate the token based on the key and include the token in subsequent requests/packets/data/etc. sent from the client to the server (i.e., via the processor(s) of the first and second cluster). The second processor may generate the token for storage in a data structure of the second processor. In some embodiments, the second processor may share the token with other processors of the first cluster. For example, the second processor may store the token in a table of a data structure which is accessible by other processors of the first cluster. As another example, the second processor may transmit the token to other processors of the first cluster for storage in a table of a data structure maintained by the other processors. 
     At step  812 , the first processor receives a request to establish a second connection from a third processor. In some embodiments, the first processor may receive a second request to establish a second connection between the client and the server from a third processor of the cluster of processors (i.e., the first cluster of processors including the second processor which sent the first request to the first processor). The first processor may receive the second request responsive to the third processor identifying the first processor. The third processor may identify the first processor using a token received in the second request. For example, and as described above, the client may generate the token using the key received in the response from the first processor. The client may include the token in the second request. The third processor may receive the second request from the client, and perform a lookup in the data structure to identify the first processor as described above with reference to  FIG.  8 A . The third processor may transmit the second request to the first processor including the token. 
     The first processor may receive the second request including the token. The first processor may determine that the second request is to establish a multipath connection based on the second request including the token. The first processor may determine that the second request is to establish a multipath connection based on the token corresponding to (i.e., being generated from) the key previously generated by the first processor. The first processor may determine or identify the primary connection maintained by the first processor using the token. For example, the key may include data relating to (i.e., identifying) the primary connection. Since the token is generated by the client using the key, the token may correspondingly include the data (or may be used to derive the data) which identifies the primary connection. 
     At step  814 , the first processor establishes a multipath connection. In some embodiments, the first processor may establish a multipath connection between the client and the server. The multipath connection may include the first connection (i.e., the primary connection or subflow) and the secondary connection (or subflow). The first and second connections may be used as paths of the multipath connection. In some embodiments, certain traffic may be routed between the client and the server via the first and second connection. For example, secure traffic may be routed via the primary connection (which may be a more secure connection) and other traffic may be routed via the secondary connection. As another example, the primary and secondary connections may be used to provide redundancy and persistence of the multipath connection. For instance, the secondary connection may be used as a fallback connection in the event of a disruption, interruption, or disconnection of the primary connection. 
     Various elements, which are described herein in the context of one or more embodiments, may be provided separately or in any suitable sub-combination. For example, the processes described herein may be implemented in hardware, software, or a combination thereof. Further, the processes described herein are not limited to the specific embodiments described. For example, the processes described herein are not limited to the specific processing order described herein and, rather, process blocks may be re-ordered, combined, removed, or performed in parallel or in serial, as necessary, to achieve the results set forth herein. 
     It will be further understood that various changes in the details, materials, and arrangements of the parts that have been described and illustrated herein may be made by those skilled in the art without departing from the scope of the following claims.