Patent Publication Number: US-7596802-B2

Title: Method and system for providing connection handling

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is related to and claims the benefit of priority to: (i) U.S. Provisional Patent Application (Ser. No. 60/220,026), filed Jul. 21, 2000, entitled “Performance Enhancing Proxy,” and (ii) U.S. Provisional Patent Application (Ser. No. 60/225,630), filed Aug. 15, 2000, entitled “Performance Enhancing Proxy”; all of which are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is generally directed to a method and system for improving performance of a network, and more particularly, to a method and system which performs spoofing to improve network performance. 
     2. Description of the Background 
     The entrenchment of data networking into the routines of modern society, as evidenced by the prevalence of the Internet, particularly the World Wide Web, has placed ever-growing demands on service providers to continually improve network performance. To meet this challenge, service providers have invested heavily in upgrading their networks to increase system capacity (i.e., bandwidth). In many circumstances, such upgrades may not be feasible economically or the physical constraints of the communication system does not permit simply “upgrading.” Accordingly, service providers have also invested in developing techniques to optimize the performance of their networks. Because much of today&#39;s networks are either operating with or are required to interface with the Transmission Control Protocol/Internet Protocol (TCP/IP) suite, attention has been focused on optimizing TCP/IP based networking operations. 
     As the networking standard for the global Internet, TCP/IP has earned such acceptance among the industry because of its flexibility and rich heritage in the research community. 
     The transmission control protocol (TCP) is the dominant protocol in use today on the Internet. TCP is carried by the Internet protocol (IP) and is used in a variety of applications including reliable file transfer and Internet web page access applications. The four layers of the TCP/IP protocol suite are illustrated in  FIG. 39 . As illustrated, the link layer (or the network interface layer)  3710  includes device drivers in the operating system and any corresponding network interface cards. Together, the device driver and the interface cards handle hardware details of physically interfacing with any cable or whatever type of media is being used. The network layer (also called the Internet layer)  3712  handles the movement of packets around the network. Routing of packets, for example, takes place at the network layer  3712 . IP, Internet control message protocol (ICMP), and Internet group management protocol (IGMP) may provide the network layer in the TCP/IP protocol suite. The transport layer  3714  provides a flow of data between two hosts, for the application layer  3716  above. 
     In the TCP/IP protocol suite, there are at least two different transport protocols, TCP and a user datagram protocol (UDP). TCP, which provides a reliable flow of data between two hosts, is primarily concerned with dividing the data passed to it from the application layer  16  into appropriately sized chunks for the network layer  3712  below, acknowledging received packets, setting timeouts to make certain the other end acknowledges packets that are sent, and so on. Because this reliable flow of data is provided by the transport layer  3714 , the application layer  3716  can ignore these details. UDP, on the other hand, provides a much simpler service to the application layer  3716 . UDP just sends packets of data called datagrams from one host to another, but there is no guarantee that the datagrams reach the other end. Any desired reliability must be added by the application layer  3716 . 
     The application layer  3716  handles the details of the particular application. There are many common TCP/IP applications that almost every implementation provides. These include telnet for remote log-in, the file transfer protocol (FTP), the simple mail transfer protocol (SMTP) or electronic mail, the simple network management protocol (SNMP), the hypertext transfer protocol (HTTP), and many others. 
     As described above, TCP provides reliable, in-sequence delivery of data between two IP hosts. The IP hosts set up a TCP connection, using a conventional TCP three-way handshake and then transfer data using a window based protocol with the successfully received data acknowledged. 
     To understand where optimizations may be made, it is instructive to consider a typical TCP connection establishment. 
       FIG. 40  illustrates an example of the conventional TCP three-way handshake between IP hosts  3820  and  3822 . First, the IP host  3820  that wishes to initiate a transfer with IP host  3822 , sends a synchronize (SYN) signal to IP host  3822 . The IP host  3822  acknowledges the SYN signal from IP host  3820  by sending a SYN acknowledgement (ACK). The third step of the conventional TCP three-way handshake is the issuance of an ACK signal from the IP host  3820  to the IP host  3822 . IP host  3822  is now ready to receive the data from IP host  3820  (and vice versa). After all the data has been delivered, another handshake (similar to the handshake described to initiate the connection) is used to close the TCP connection. 
     TCP was designed to be very flexible and works over a wide variety of communication links, including both slow and fast links, high latency links, and links with low and high error rates. However, while TCP (and other high layer protocols) works with many different kinds of links, TCP performance, in particular, the throughput possible across the TCP connection, is affected by the characteristics of the link in which it is used. There are many link layer design considerations that should be taken into account when designing a link layer service that is intended to support Internet protocols. However, not all characteristics can be compensated for by choices in the link layer design. TCP has been designed to be very flexible with respect to the links which it traverses. Such flexibility is achieved at the cost of sub-optimal operation in a number of environments vis-à-vis a tailored protocol. The tailored protocol, which is usually proprietary in nature, may be more optimal, but greatly lacks flexibility in terms of networking environments and interoperability. 
     An alternative to a tailored protocol is the use of performance enhancing proxies (PEPs), to perform a general class of functions termed “TCP spoofing,” in order to improve TCP performance over impaired (i.e., high latency or high error rate) links. TCP spoofing involves an intermediate network device (the performance enhancing proxy (PEP)) intercepting and altering, through the addition and/or deletion of TCP segments, the behavior of the TCP connection in an attempt to improve its performance. 
     Conventional TCP spoofing implementations include the local acknowledgement of TCP data segments in order to get the TCP data sender to send additional data sooner than it would have sent if spoofing were not being performed, thus improving the throughput of the TCP connection. Generally, conventional TCP spoofing implementations have focused simply on increasing the throughput of TCP connections either by using larger windows over the link or by using compression to reduce the amount of data which needs to be sent, or both. 
     Many TCP PEP implementations are based on TCP ACK manipulation. These may include TCP ACK spacing where ACKs which are bunched together are spaced apart, local TCP ACKs, local TCP retransmissions, and TCP ACK filtering and reconstruction. Other PEP mechanisms include tunneling, compression, and priority-based multiplexing. 
     In addition network performance may be improved utilizing techniques such as connection establishment spoofing. 
     Based on the foregoing, there is a clear need for improved techniques for spoofing information. Therefore, an approach for improving network performance utilizing techniques such as spoofing is highly desirable. In particular, an approach for implementing spoofing rules within a PEP environment is highly desirable. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the above-stated need by providing a communication system with performance enhancing functionality. A spoofing apparatus communicates with a performance enhancing proxy (PEP) end point platform to configure the platform by utilizing profiles corresponding to the PEP end point platform. According to one aspect of the invention, a method for routing information in the communication system that includes a platform and a spoofing apparatus configured to perform a plurality of performance enhancing functions is provided. The method includes receiving information from the platform and receiving at least one of spoofing selection parameters and spoofing parameters, wherein the spoofing apparatus maintains a profile that contains at least one of spoofing selection and spoofing parameters and routing the information in accordance with the profile. 
     According to another aspect of the invention, a communication system includes a platform that is configured to provide performance enhancing functions. The platform includes a communication system including a platform configured to provide performance enhancing functions, the platform supplying information and at least one of spoofing selection and spoofing parameters and a spoofing apparatus communicating with the platform. The spoofing apparatus is configured to receive the information and the at least one of spoofing selection and spoofing parameters from the platform, wherein the spoofing apparatus has a profile which specifies at least one of spoofing selection and spoofing parameters, wherein the communication system is configured to route the information in accordance with the profile. 
     According to another aspect of the present invention, a spoofing apparatus for monitoring a communication system that includes a platform configured to perform a plurality of performance enhancing functions is disclosed. The apparatus includes means for receiving the information and the at least one of spoofing selection and spoofing parameters and means for maintaining a profile containing the at least one of the spoofing selection and spoofing parameters and means for routing the information in accordance with the profile. 
     In yet another aspect of the invention, a computer-readable medium carrying one or more sequences of one or more instructions for routing information in a communication system that includes a platform configured to perform a plurality of performance enhancing functions is disclosed. The computer-readable medium carries one or more sequences of one or more instructions, which, when executed by one or more processors, cause the one or more processors to perform the steps of receiving the information from the platform and receiving at least one of spoofing selection parameters and spoofing parameters, wherein the spoofing apparatus maintains a profile that contains the at least one of the spoofing selection and spoofing parameters and routing the information in accordance with the profile. 
     The method, communication system, spoofing apparatus, and computer-readable medium also are capable of compensating for maximum segment size mismatches. This compensation may be achieved by dynamically resizing the data segments which comprise the information which is being routed or may include disabling three-way handshake spoofing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a diagram of a communication system in which the performance enhancing proxy (PEP) of the present invention is implemented; 
         FIG. 2  is a diagram of a PEP end point platform environment, according to an embodiment of the present invention; 
         FIG. 3  is a diagram of a TCP Spoofing Kernel (TSK) utilized in the environment of  FIG. 2 ; 
         FIGS. 4A and 4B  are flow diagrams of the connection establishment with three-way handshake spoofing and without three-way handshake spoofing, respectively; 
         FIG. 5  is a diagram of a PEP packet flow between two PEP end points, according to an embodiment of the present invention; 
         FIG. 6  is a diagram of an IP (Internet Protocol) packet flow through a PEP end point, in accordance with an embodiment of the present invention; 
         FIG. 7  is a diagram of PEP end point profiles utilized in the platform of  FIG. 2 ; 
         FIG. 8  is a diagram of the interfaces of a PEP end point implemented as an IP gateway, according to an embodiment of the present invention; 
         FIG. 9  is a diagram of the interfaces of a PEP end point implemented as a Multimedia Relay, according to an embodiment of the present invention; 
         FIG. 10  is a diagram of the interfaces of a PEP end point implemented as a Multimedia VSAT (Very Small Aperture Terminal), according to an embodiment of the present invention; 
         FIG. 11  is a diagram of the interfaces of a PEP end point implemented in an earth station, according to an embodiment of the present invention; 
         FIG. 12  is a diagram of a TCP spoofing kernel message, according to an embodiment of the present invention; 
         FIG. 13  is a diagram of a TCP connection header, according to an embodiment of the present invention; 
         FIG. 14  is a diagram of TSK peers learning TSK backbone connection identifiers, according to an embodiment of the present invention; 
         FIG. 15  is a diagram illustrating the assignment of TCP connection identifiers, according to an embodiment of the present invention; 
         FIG. 16  is a diagram of TCB access via a TCB mapping table, according to an embodiment of the present invention; 
         FIG. 17  is a diagram illustrating CCB access via a CCB hash function, according to an embodiment of the present invention; 
         FIG. 18  is a diagram illustrating CCB access via a CCB mapping table, according to an embodiment of the present invention; 
         FIG. 19  is a diagram of the relationship between a CCB and a TCB, according to an embodiment of the present invention; 
         FIG. 20  is a diagram illustrating connection establishment, according to an embodiment of the present invention; 
         FIG. 21  is a diagram of the startup of the same TCP connection using the same backbone connection, according to an embodiment of the present invention; 
         FIG. 22  is a diagram of connection establishment with no local CCB, according to an embodiment of the present invention; 
         FIG. 23  is a diagram of connection establishment with no peer CCB, according to an embodiment of the present invention; 
         FIG. 24  is a diagram of simultaneous startup using the last CCB, according to an embodiment of the present invention; 
         FIG. 25  is a diagram of no response from a destination host, according to an embodiment of the present invention; 
         FIG. 26  is a diagram of no response from a source host, according to an embodiment of the present invention; 
         FIG. 27  is a diagram of spoofed data reception, according to an embodiment of the present invention; 
         FIG. 28  is a diagram of normal connection termination, according to an embodiment of the present invention; 
         FIG. 29  is a diagram of local host &lt;RST&gt; segment connection termination, according to an embodiment of the present invention; 
         FIG. 30  is a diagram of simultaneous normal connection termination, according to an embodiment of the present invention; 
         FIG. 31  is a diagram of simultaneous &lt;RST&gt; segment connection termination, according to an embodiment of the present invention; 
         FIG. 32  is a diagram of a premature connection restart, according to an embodiment of the present invention; 
         FIG. 33  is a diagram illustrating connection termination due to no response from a host, according to an embodiment of the present invention; 
         FIG. 34  is a diagram of the relationship between PEP End points, TCP spoofing selection profiles, and TCP spoofing parameter profiles, according to an embodiment of the present invention; 
         FIG. 35  is a diagram of selective TCP spoofing rules to TCP spoofing parameter profile mapping, according to an embodiment of the present invention; 
         FIG. 36  is a diagram illustrating dividing a segment into two or more smaller segments, according to an embodiment of the present invention; 
         FIG. 37  is a diagram illustrating reduction of the maximum segment size, according to an embodiment of the present invention; 
         FIG. 38  is a diagram of a computer system that can perform PEP functions, in accordance with an embodiment of the invention; 
         FIG. 39  is a diagram of the protocol layers of the TCP/IP protocol suite; and 
         FIG. 40  is a diagram of a conventional TCP 3-way handshake between IP hosts. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following description, for the purpose of explanation, specific details are set forth in order to provide a thorough understanding of the invention. However, it will be apparent that the invention may be practiced without these specific details. In some instances, well-known structures and devices are depicted in block diagram form in order to avoid unnecessarily obscuring the invention. 
     Although the present invention is discussed with respect to the Internet and the TCP/IP protocol suite, the present invention has applicability to other packet switched networks and equivalent protocols. 
       FIG. 1  illustrates an exemplary network  100  in which the performance enhancing proxy (PEP) of the present invention may be utilized. The network  100  in  FIG. 1  includes one or more hosts  110  connected to a network gateway  120  via TCP connections. The network gateway  120  is connected to another network gateway  140  via a backbone connection on a backbone link  130 . As seen in  FIG. 1 , the backbone link  130 , in an exemplary embodiment, is shown as a satellite link that is established over a satellite  101 ; however, it is recognized by one of ordinary skill in the art that other network connections may be implemented. For example, these network connections may be established over a wireless communications system, in general, (e.g., radio networks, cellular networks, etc.) or a terrestrial communications system. The network gateway  140  is further connected to a second group of hosts  150 , also via TCP connections. In the arrangement illustrated in  FIG. 1 , the network gateways  120 ,  140  facilitate communication between the groups of hosts  110 ,  150 . 
     The network gateways  120 ,  140  facilitate communication between the two groups of hosts  110 ,  150  by performing a number of performance enhancing functions. These network gateways  120 ,  140  may perform selective TCP spoofing, which allows flexible configuration of the particular TCP connections that are to be spoofed. Additionally, gateways  120 ,  140  employ a TCP three-way handshake, in which the TCP connections are terminated at each end of the backbone link  130 . Local data acknowledgements are utilized by the network gateways  120 ,  140 , thereby permitting the TCP windows to increase at local speeds. 
     The network gateways  120 ,  140  further multiplex multiple TCP connections across a single backbone connection; this capability reduces the amount of acknowledgement traffic associated with the data from multiple TCP connections, as a single backbone connection acknowledgement may be employed. The multiplexing function also provides support for high throughput TCP connections, wherein the backbone connection protocol is optimized for the particular backbone link that is used. The network gateways  120 ,  140  also support data compression over the backbone link  130  to reduce the amount of traffic to be sent, further leveraging the capabilities of the backbone connection. Further, the network gateways  120 ,  140  utilize data encryption in the data transmission across the backbone link  130  to protect data privacy, and provide prioritized access to backbone link  130  capacity on a per TCP connection basis. Each of the network gateways  120 ,  140  may select a particular path for the data associated with a connection to flow. The above capabilities of the network gateways  120 ,  140  are more fully described below. 
       FIG. 2  illustrates a performance enhancing proxy (PEP)  200  as implemented in a network gateway  120 ,  140 , according to one embodiment of the present invention. In this embodiment, the PEP  200  has a platform environment  210 , which includes the hardware and software operating system. The PEP  200  also includes local area network (LAN) interfaces  220  and wide area network (WAN) interfaces  230 . In the example in  FIG. 1 , the network gateway  120  may establish the TCP connections with the IP hosts  110 , via a local LAN interface  220  and may establish the backbone connection with the network gateway  140  via a WAN interface  230 . The PEP platform environment  210  may also include general functional modules: routing module  240 , buffer management module  250 , event management module  260 , and parameter management module  270 . As illustrated in  FIG. 2 , the network gateway also includes a TCP spoofing kernel (TSK)  280 , a backbone protocol kernel (BPK)  282 , a prioritization kernel (PK)  284 , and a path selection kernel (PSK)  286 . These four kernels essentially make up the functionality of the performance enhancing proxy  200 . 
     The platform environment  210  performs a number of functions. One such function is to shield the various PEP kernels  280 ,  282 ,  284 ,  286  from implementation specific constraints. That is, the platform environment  210  performs functions that the various PEP kernels  280 ,  282 ,  284 ,  286  cannot perform directly because the implementation of the function is platform specific. This arrangement has the advantageous effect of hiding platform specific details from the PEP kernels  280 ,  282 ,  284 ,  286 , making the PEP kernels  280 ,  282 ,  284 ,  286  more portable. An example of a platform specific function is the allocation of a buffer. In some platforms, buffers are created as they are needed, while in other platforms, buffers are created at start-up and organized into linked lists for later use. It is noted that platform specific functions are not limited to functions generic to all of the kernels  280 ,  282 ,  284 ,  286 . A function specific to a particular kernel, for example, the allocation of a control block for TCP spoofing, may also be implemented in the platform environment to hide platform specific details from the kernel. 
     In one exemplary embodiment, the platform environment  210  provides the task context in which the PEP kernels  280 ,  282 ,  284 ,  286  run. In another exemplary embodiment, all PEP kernels  280 ,  282 ,  284 ,  286  can run in the same task context for efficiency; however, this is not required. 
     Furthermore, the platform environment  210 , in an exemplary embodiment, provides an interface between the PEP functionality (embodied in kernels  280 ,  282 ,  284 ,  286 ) and the other functionality of the network gateway  120 ,  140 . The platform environment  210  may provide the interface between the PEP functionality and the routing function  240 , as seen in  FIG. 2 . It is noted that the platform specific functions illustrated in  FIG. 2  are examples and are not considered an exhaustive list. It is further noted that the PEP kernels shown touching each other ( 280 ,  282  and  284 ,  286 ) in  FIG. 2  may have a direct procedural interface to each other. Further, the kernels  280 ,  282 ,  284 ,  286  may include direct interfaces to improve performance, as opposed to routing everything through the platform environment  210  (as shown in  FIG. 2 ). 
     In addition to the PEP kernels  280 ,  282 ,  284 , and  286 , the PEP end point platform  210  may utilize a data compression kernel (CK)  290  and an encryption kernel (EK)  292 . These kernels  280 ,  282 ,  284 ,  286 ,  290 , and  292 , as described above, facilitate communication between the two groups of hosts  110 ,  150 , by performing a variety of performance enhancing functions, either singly or in combination. These performance enhancing functions include selective TCP spoofing, three-way handshake spoofing, local data acknowledgement, TCP connection to backbone connection multiplexing, data compression/encryption, prioritization, and path selection. 
     Selective TCP Spoofing is performed by the TSK  280  and includes a set of user configurable rules that are used to determine which TCP connections should be spoofed. Selective TCP spoofing improves performance by not tying up TCP spoofing-related resources, such as buffer space, control blocks, etc., for TCP connections for which the user has determined that spoofing is not beneficial or required and by supporting the use of tailored parameters for TCP connections that are spoofed. 
     In particular, the TSK  280  discriminates among the various TCP connections based on the applications using them. That is, TSK  280  discriminates among these TCP connections to determine which connection should be spoofed as well as the manner in which the connection is spoofed; e.g., whether to spoof the three-way handshake, the particular timeout parameters for the spoofed connections, etc. TCP spoofing is then performed only for those TCP connections that are associated with applications for which high throughput or reduced connection startup latency (or both) is required. As a result, the TSK  280  conserves TCP spoofing resources for only those TCP connections for which high throughput or reduced connection startup latency (or both) is required. Further, the TSK  280  increases the total number of TCP connections which can be active before running out of TCP spoofing resources, since any active TCP connections which do not require high throughput are not allocated resources. 
     One criterion for identifying TCP connections of applications for which TCP spoofing should and should not be performed is the TCP port number field contained in the TCP packets being sent. In general, unique port numbers are assigned to each type of application. Which TCP port numbers should and should not be spoofed can be stored in the TSK  280 . The TSK  280  is also re-configurable to allow a user or operator to reconfigure the TCP port numbers which should and should not be spoofed. The TSK  280  also permits a user or operator to control which TCP connections are to be spoofed based on other criteria. In general, a decision on whether to spoof a TCP connection may be based on any field within a TCP packet. The TSK  280  permits a user to specify which fields to examine and which values in these fields identify TCP connections that should or should not be spoofed. Another example of a potential use for this capability is for the user or operator to select the IP address of the TCP packet in order to control for which users TCP spoofing is performed. The TSK  280  also permits a user to look at multiple fields at the same time. As a result, the TSK  280  permits a user or operator to use multiple criteria for selecting TCP connections to spoof. For example, by selecting both the IP address and the TCP port number fields, the system operator can enable TCP spoofing for only specific applications from specific users. 
     The user configurable rules may include five exemplary criteria which can be specified by the user or operator in producing a selective TCP spoofing rule: Destination IP address; Source IP address; TCP port numbers (which may apply to both the TCP destination and source port numbers); TCP options; and IP differentiated services (DS) field. However, as indicated above, other fields within the TCP packet may be used. 
     As discussed above, in addition to supporting selective TCP spoofing rules for each of these criteria, AND OR combination operators can be used to link criteria together. For example, using the AND combination operator, a rule can be defined to disable TCP spoofing for FTP data received from a specific host. Also, the order in which the rules are specified may be significant. It is possible for a connection to match the criteria of multiple rules. Therefore, the TSK  280  can apply rules in the order specified by the operator, taking the action of the first rule that matches. A default rule may also be set which defines the action to be taken for TCP connections which do not match any of the defined rules. The set of rules selected by the operator may be defined in a selective TCP spoofing selection profile. 
     As an example, assuming sufficient buffer space has been allocated to spoof five TCP connections, if four low speed applications (i.e., applications which, by their nature, do not require high speed) bring up connections along with one high speed application, the high speed connection has access to only ⅕ of the available spoofing buffer space. Further, if five low speed connections are brought up before the high speed connection, the high speed connection cannot be spoofed at all. Using the TSK  280  selective spoofing mechanism, the low speed connections are not allocated any spoofing buffer space. Therefore, the high speed connection always has access to all of the buffer space, improving its performance with respect to an implementation without the selective TCP spoofing feature of the TSK  280 . 
     The TSK  280  also facilitates spoofing of the conventional three-way handshake. Three-Way Handshake Spoofing involves locally responding to a connection request to bring up a TCP connection in parallel with forwarding the connection requests across the backbone link  130  ( FIG. 1 ). This allows the originating IP host (for example,  110 ) to reach the point of being able to send the data it must send at local speeds, i.e. speeds that are independent of the latency of the backbone link  130 . Three-way Handshake Spoofing allows the data that the IP host  110  needs to send to be sent to the destination IP host  150  without waiting for the end-to-end establishment of the TCP connection. For backbone links  130  with high latency, this significantly reduces the time it takes to bring up the TCP connection and, more importantly, the overall time it takes to get a response (from an IP host  150 ) to the data the IP host  110  sends. 
     A specific example in which this technique is useful relates to an Internet web page access application. With three-way handshake spoofing, an IP host&#39;s request to retrieve a web page can be on its way to a web server without waiting for the end-to-end establishment of the TCP connection, thereby reducing the time it takes to download the web page. 
     With Local Data Acknowledgement, the TSK  280  in the network gateway  120  (for example) locally acknowledges data segments received from the IP host  110 . This allows the sending IP host  110  to send additional data immediately. More importantly, TCP uses received acknowledgements as signals for increasing the current TCP window size. As a result, local sending of the acknowledgements allows the sending IP host  110  to increase its TCP window at a much faster rate than supported by end to end TCP acknowledgements. The TSK  280  (the spoofer) takes on the responsibility for reliable delivery of the data which it has acknowledged. 
     In the BPK  282 , multiple TCP connections are multiplexed onto and carried by a single backbone connection. This improves system performance by allowing the data for multiple TCP connections to be acknowledged by a single backbone connection acknowledgement (ACK), significantly reducing the amount of acknowledgement traffic required to maintain high throughput across the backbone link  130 . In addition, the BPK  282  selects a backbone connection protocol that is optimized to provide high throughput for the particular link. Different backbone connection protocols can be used by the BPK  282  with different backbone links without changing the fundamental TCP spoofing implementation. The backbone connection protocol selected by the BPK  282  provides appropriate support for reliable, high speed delivery of data over the backbone link  130 , hiding the details of the impairments (for example high latency) of the link from the TCP spoofing implementation. 
     The multiplexing by the BPK  282  allows for the use of a backbone link protocol which is individually tailored for use with the particular link and provides a technique to leverage the performance of the backbone link protocol with much less dependency upon the individual performance of the TCP connections being spoofed than conventional methods. Further, the ability to tailor the backbone protocol for different backbone links makes the present invention applicable to many different systems. 
     The PEP  200  may optionally include a data compression kernel  290  for compressing TCP data and an encryption kernel  292  for encrypting TCP data. Data compression increases the amount of data that can be carried across the backbone connection. Different compression algorithms can be supported by the data compression kernel  290  and more than one type of compression can be supported at the same time. The data compression kernel  290  may optionally apply compression on a per TCP connection basis, before the TCP data of multiple TCP connections is multiplexed onto the backbone connection or on a per backbone connection basis, after the TCP data of multiple TCP connections has been multiplexed onto the backbone connection. Which option is used is dynamically determined based on user configured rules and the specific compression algorithms being utilized. Exemplary data compression algorithms are disclosed in U.S. Pat. Nos. 5,973,630, 5,955,976, the entire contents of which are hereby incorporated by reference. The encryption kernel  292  encrypts the TCP data for secure transmission across the backbone link  130 . Encryption may be performed by any conventional technique. It is also understood that the corresponding spoofer (in the example outlined above, the network gateway  140 ) includes appropriate kernels for decompression and decryption, both of which may be performed by any conventional technique. 
     The PK  284  provides prioritized access to the backbone link capacity. For example, the backbone connection can actually be divided into N (N&gt;1) different sub-connections, each having a different priority level. In one exemplary embodiment, four priority levels can be supported. The PK  284  uses user-defined rules to assign different priorities, and therefore different sub-connections of the backbone connection, to different TCP connections. It should be noted that PK  284  may also prioritize non-TCP traffic (e.g., UDP (User Datagram Protocol) traffic) before sending the traffic across the backbone link  130 . 
     The PK  284  also uses user-defined rules to control how much of the backbone link  130  capacity is available to each priority level. Exemplary criteria which can be used to determine priority include the following: Destination IP address; Source IP address; IP next protocol; TCP port numbers (which may apply to both the TCP destination and source port numbers); UDP port numbers (which may apply to both the UDP destination and source port numbers); and IP differentiated services (DS) field. The type of data in the TCP data packets may also be used as a criterion. For example, video data could be given highest priority. Mission critical data could also be given high priority. As with selective TCP spoofing, any field in the IP packet can be used by PK  284  to determine priority. However, it should be noted that under some scenarios the consequence of using such a field may cause different IP packets of the same flow (e.g., TCP connection) to be assigned different priorities; these scenarios should be avoided. 
     As mentioned above, in addition to supporting selective prioritization rules for each of these criteria, AND and OR combination operators can be used to link criteria together. For example, using the AND combination operator, a rule can be defined to assign a priority for SNMP data received from a specific host. Also, the order in which the rules are specified may be significant. It is possible for a connection to match the criteria of multiple rules. Therefore, the PK  284  can apply rules in the order specified by the operator, taking the action of the first rule that matches. A default rule may also be set which defines the action to be taken for IP packets which do not match any of the defined rules. The set of rules selected by the operator may be defined in a prioritization profile. 
     As regards the path selection functionality, the PSK  286  is responsible for determining which path an IP packet should take to reach its destination. The path selected by the PSK  286  can be determined by applying path selection rules. The PSK  286  also determines which IP packets should be forwarded using an alternate path and which IP packets should be dropped when one or more primary paths fail. Path selection parameters can also be configured using profiles. The path selection rules may be designed to provide flexibility with respect to assigning paths while making sure that all of the packets related to the same traffic flow (e.g., the same TCP connection) take the same path (although it is also possible to send segments of the same TCP connection via different paths, this segment “splitting” may have negative side effects). Exemplary criteria that can be used to select a path include the following: priority of the IP packet as set by the PK  284  (should be the most common criterion): Destination IP address; Source IP address; IP next protocol; TCP port numbers (which may apply to both the TCP destination and source port numbers); UDP port numbers (which may apply to both the UDP destination and source port numbers); and IP differentiated services (DS) field. Similar to selective TCP spoofing and prioritization, the PSK  284  may determine a path by using any field in the IP packet. 
     As with the prioritization criteria (rules) the AND and OR combination operators can be used to link criteria together. For example, using the AND combination operator, a rule can be defined to select a path for SNMP data received from a specific host. Also, the order in which the rules are specified may be significant. It is possible for a connection to match the criteria of multiple rules. Therefore, the PSK  286  can apply rules in the order specified by the operator, taking the action of the first rule that matches. A default rule may also be set which defines the action to be taken for IP packets which do not match any of the defined rules. The set of rules selected by the operator may be defined in a path selection profile. 
     By way of example, a path selection rule may select the path based on any of the following path information in which IP packets match the rule: a primary path, a secondary path, and a tertiary path. The primary path is specified in any path selection rule. The secondary path is used only when the primary path has failed. If no secondary path is specified, any IP packets that match the rule can be discarded when the primary path fails. The tertiary path is specified only if a secondary path is specified. The tertiary path is selected if both the primary and secondary paths have failed. If no tertiary path is specified, any IP packets that match the rule can be discarded when both the primary and secondary paths fail. Path selection may be generalized such that the path selection rule can select up to N paths where the Nth path is used only if the (N−1) the path fails. The example above where N=3 is merely illustrative, although N is typically a fairly small number. 
     By way of example, the operation of the system  100  is described as follows. First, a backbone connection is established between the PEPs  200  of two network gateways  120 ,  140  (i.e., the two spoofers), located at each end of the backbone link  130  for which TCP spoofing is desired. Whenever an IP host  110  initiates a TCP connection, the TSK  280  of the PEP  200  local to the IP host  110  checks its configured selective TCP spoofing rules. If the rules indicate that the connection should not be spoofed, the PEP  200  allows the TCP connection to flow end-to-end unspoofed. If the rules indicate that the connection should be spoofed, the spoofing PEP  200  locally responds to the IP host&#39;s TCP three-way handshake. In parallel, the spoofing PEP  200  sends a message across the backbone link  130  to its partner network gateway  140  asking it to initiate a TCP three-way handshake with the IP host  150  on its side of the backbone link  130 . Data is then exchanged between the IP host  110 ,  150  with the PEP  200  of the network gateway  120  locally acknowledging the received data and forwarding it across the backbone link  130  via the high speed backbone connection, compressing the data as appropriate based on the configured compression rules. The priority of the TCP connection is determined when the connection is established. The BPK  282  can multiplex the connection with other received connections over a single backbone connection, the PK  284  determines the priority of the connection and the PSK  286  determines the path the connection is to take. 
     The PEP  200 , as described above, advantageously improves network performance by allocating TCP spoofing-related resources, such as buffer space, control blocks, etc., only to TCP connections for which spoofing is beneficial; by spoofing the three-way handshake to decrease data response time; by reducing the number of ACKs which are transmitted by performing local acknowledgement and by acknowledging multiple TCP connections with a single ACK; by performing data compression to increase the amount of data that can be transmitted; by assigning priorities to different connections; and by defining multiple paths for connections to be made. 
       FIG. 3  shows an exemplary stack, which illustrates the relationship between the TCP stack and the PEP kernels  280 ,  282 ,  284 ,  286  of the present invention. The TSK  280  is primarily responsible for functions related to TCP spoofing. The TSK  280 , in an exemplary embodiment, includes two basic elements: a transport layer that encompasses a TCP stack  303  and an IP stack  305 ; and a TCP spoofing application  301 . The transport layer is responsible for interacting with the TCP stacks (e.g.,  303 ) of IP hosts  110  connected to a local LAN interface  220  of a PEP  210 . 
     The TSK  280  implements the TCP protocol, which includes the appropriate TCP state machines and terminates spoofed TCP connections. The TCP spoofing application  301  rests on top of the transport layer and act as the application that receives data from and sends data to the IP hosts  110  applications. Because of the layered architecture of the protocol, the TCP spoofing application  301  isolates the details of TCP spoofing from the transport layer, thereby allowing the transport layer to operate in a standard fashion. 
     As shown in  FIG. 3 , the TCP spoofing application  301  can also interface to the BPK  282  associated with the WAN interfaces  230 . The BPK  282  performs backbone protocol maintenance, implementing the protocol by which the network gateways  120 ,  140  (in  FIG. 1 ) communicate. The BPK  282  provides reliable delivery of data, uses a relatively small amount of acknowledgement traffic, and supports generic backbone use (i.e., use not specific to the TSK  280 ). An example of a protocol implemented by BPK  282  is the reliable data protocol (RDP). 
     The BPK  282  lies above the PK  284  and the PSK  286 , according to an exemplary embodiment. The PK  284  is responsible for determining the priority of IP packets and then allocating transmission opportunities based on priority. The PK  284  can also control access to buffer space by controlling the queue sizes associated with sending and receiving IP packets. The PSK  286  determines which path an IP packet should take to reach its destination. The path selected by the PSK  286  can be determined applying path selection rules. PSK  286  may also determine which IP packet should be forwarded using an alternate path and which packets should be dropped when one or more primary paths fail. It is noted that the above arrangement is merely exemplary; other arrangements would be evident to one skilled in the art. 
       FIGS. 4A and 4B  show flow diagrams of the establishment of a spoofed TCP connection utilizing three-way handshake spoofing and without three-way handshake spoofing, respectively. The TCP Spoofing Kernel  280  establishes a spoofed TCP connection when a TCP &lt;SYN&gt; segment is received from its local LAN or a Connection Request message from its TSK peer. It is noted that the three-way handshake spoofing may be disabled to support an end to end maximum segment size (MSS) exchange, which is more fully described below. For the purpose of explanation, the spoofed TCP connection establishment process is described with respect to a local host  400 , a local PEP end point  402 , a remote PEP end point  404 , and a remote host  406 . As mentioned previously, the TSK  280  within each of the PEP end points  402  and  404  provides the spoofing functionality. 
     In step  401 , the local host  400  transmits a TCP &lt;SYN&gt; segment to the local PEP end point  402  at a local LAN interface  220 . When a TCP segment is received from the local LAN interface  220 , the platform environment  402  determines whether there is already a connection control block (CCB) assigned to the TCP connection associated with the TCP segment. If there is no CCB, the environment  402  checks whether the TCP segment is a &lt;SYN&gt; segment that is being sent to a non-local destination. If so, the &lt;SYN&gt; segment represents an attempt to bring up a new (non-local) TCP connection, and the environment  402  passes the segment to the TCP Spoofing Kernel  280  to determine the TCP connection&#39;s disposition. When a TCP &lt;SYN&gt; segment is received from the local LAN interface  220  for a new TCP connection, the TCP Spoofing Kernel  280  first determines if the connection should be spoofed. If the connection should be spoofed, TSK  280  uses (in an exemplary embodiment) the priority indicated in the selected TCP spoofing parameter profile and the peer index (provided by the environment  210  with the TCP &lt;SYN&gt; segment) to construct the handle of the backbone connection which should be used to carry this spoofed TCP connection. In the exemplary embodiment, the peer index is used as the 14 high order bits of the handle and the priority is used as the two low order bits of the handle. The backbone connection handle is then used (via the TSK control block (TCB) mapping table) to find the TCB associated with the backbone connection. TSK  280  of PEP end point  402  then checks whether the backbone connection is up. If the backbone connection is up, TSK  280  determines whether the number of spoofed TCP connections that are already using the selected backbone connection is still currently below the TCP connection control block (CCB) resource limit. The CCB resource limit is the smaller of the local number of CCBs (provided as a parameter by the platform environment  210 ) and the peer number of CCBs (received in the latest TSK peer parameters (TPP) message from the TSK peer) available for this backbone connection. If the number of connections is still below the limit, TSK  280  of PEP end point  402  assigns a unique TCP connection identifier (e.g., a free CCB mapping table entry index) to the connection and calls the environment  210  to allocate a TCP connection control block for the connection. 
     TSK  280  of PEP end point  402  returns the TCP &lt;SYN&gt; segment back to the environment  210  to be forwarded unspoofed if any of the above checks fail. In other words, the following conditions result in the TCP connection being unspoofed. First, if the selective TCP spoofing rules indicate that the connection should not be spoofed. Also, there is no backbone connection for the priority at which the TCP connection should be spoofed (indicated by the absence of a TCB for the backbone connection). No spoofing is performed if the backbone connection is down. Additionally, if the number of spoofed TCP connections that are already using the backbone connection reaches or exceeds a predetermined threshold, then no spoofing is performed. Further, if there is no CCB mapping table entry available or there is no CCB available from the CCB free pool, then the TCP connection is forwarded unspoofed. For the case in which there is no backbone connection, TSK  280  of PEP end point  402  may also post an event to alert the operator that there is a mismatch between the configured TCP spoofing parameter profiles and the configured set of backbone connections. 
     Continuing with the example, if all of the above checks pass, TSK  280  of PEP end point  402  writes the backbone connection handle into the buffer holding the TCP &lt;SYN&gt; segment. It is noted that this is not done until a CCB is successfully allocated by the platform environment  402 , because the environment does not count the buffer unless a CCB is successfully allocated. TSK  280  then copies the parameters from the selected TCP spoofing parameter profile into the CCB. Consequently, relevant information (e.g., the maximum segment size that is advertised by the host (if smaller than the configured MSS), the initial sequence number, and etc.) is copied out of the TCP &lt;SYN&gt; segment and stored in the CCB. It is noted that the source and destination IP addresses and source and destination TCP port numbers will already have been placed into the CCB by the platform environment  402  when the CCB was allocated; the environment  402  uses this information to manage CCB hash function collisions. 
     After allocating and setting up the CCB, the TCP Spoofing Kernel  280  of PEP end point  402  constructs a Connection Request (CR) message, per step  403 , and sends it to its TSK peer associated with the remote PEP end point  404 . The CR message basically contains all of the information extracted from the TCP spoofing parameter profile and the TCP &lt;SYN&gt; segment and stored in the local CCB, e.g., the source and destination IP addresses, the source and destination TCP port numbers, the MSS value, etc., with the exception of fields that have only local significance, such as the initial sequence number. (The IP addresses and TCP port numbers are placed into a TCP connection header.) In other words, the CR message contains all of the information that the peer TSK of PEP end point  404  requires to set up its own CCB. To complete the local connection establishment, the TCP Spoofing Kernel  280  of the local PEP end point  402  sends a TCP &lt;SYN,ACK&gt; segment to the local host  400  in response to the &lt;SYN&gt; segment received, per step  405 . TSK  280  of PEP end point  402  performs step  405  simultaneously with the step of sending the Connection Request message (i.e., step  403 ), if three-way handshake spoofing is enabled. Otherwise, TSK  280  of  402  waits for a Connection Established (CE) message from its TSK peer of the remote PEP end point  404  before sending the &lt;SYN,ACK&gt; segment. In an exemplary embodiment, TSK  280  of PEP end point  402  selects a random initial sequence number (as provided in IETF (Internet Engineering Task Force) RFC  793 , which is incorporated herein by reference in its entirety) to use for sending data. 
     If three-way handshake spoofing is disabled, the MSS value sent in the &lt;SYN,ACK&gt; segment is set equal to the MSS value received in the CE message. If three-way handshake spoofing is enabled, the MSS value is determined from the TCP spoofing parameter profile selected for the connection (and the configured path maximum transmission unit (MTU)). For this case, TSK  280  of PEP end point  402  then compares the MSS value received in the Connection Established message, when it arrives, to the value it sent to the local host in the TCP &lt;SYN,ACK&gt; segment. If the MSS value received in the CE message is smaller than the MSS value sent to the local host, a maximum segment size mismatch exists. (If an MSS mismatch exists, TSK may need to adjust the size of TCP data segments before sending them.) After sending the TCP &lt;SYN,ACK&gt; segment (step  405 ), TSK  280  of the local PEP end point  402  is ready to start accepting data from the local host  400 . In step  407 , the local host  400  transmits an &lt;ACK&gt; segment to the TSK  280  of PEP end point  402 ; thereafter, the local host forwards, as in step  409 , data to the TSK  280  of PEP end point  402  as well. When three-way handshake spoofing is being used, TSK  280  does not need to wait for the Connection Established message to arrive from its TSK peer before accepting and forwarding data. As seen in  FIG. 4A , in step  411 , TSK  280  of the local PEP end point  402  sends an &lt;ACK&gt; segment to the local host and simultaneously sends the TCP data (TD) from the local host  400  to the peer TSK of PEP end point  404  (per step  413 ) prior to receiving a CE message from the peer TSK of PEP end point  404 . 
     However, TSK  280  of PEP end point  402  does not accept data from its TSK peer of PEP end point  404  until after the CE message has been received. TSK  280  of PEP end point  402  does not forward any data received from its TSK peer of PEP end point  404  to the local host  400  until it has received the TCP &lt;ACK&gt; segment indicating that the local host has received the &lt;SYN,ACK&gt; segment (as in step  407 ). 
     When a Connection Request message is received from a peer TSK (step  403 ), the TCP Spoofing Kernel  280  allocates a CCB for the connection and then stores all of the relevant information from the CR message in the CCB. TSK  280  of PEP end point  404  then uses this information to generate a TCP &lt;SYN&gt; segment, as in step  415 , to send to the remote host  406 . The MSS in the &lt;SYN&gt; segment is set to the value received from the TSK peer of PEP end point  404 . When the remote host responds with a TCP &lt;SYN,ACK&gt; segment (step  417 ), TSK  280  of PEP end point  402  sends a Connection Established message to its TSK peer of the remote PEP end point  404  (step  419 ), including in the CE message the MSS that is sent by the local host in the &lt;SYN,ACK&gt; segment. TSK  280  of PEP end point  402  also responds, as in step  421 , with a TCP &lt;ACK&gt; segment to complete the local three-way handshake. The peer TSK of PEP end point  404  then forwards the data that is received from TSK  280  to the host, per step  423 . Concurrently, in step  425 , the remote host  406  sends data to the peer TSK of PEP end point  404 , which acknowledges receipt of the data by issuing an &lt;ACK&gt; segment to the remote PEP end point  404 , per step  427 . Simultaneously with the acknowledgement, the data is sent to TSK  280  of PEP end point  402  (step  429 ). 
     At this point, TSK  280  is ready to receive and forward data from either direction. TSK  280  forwards the data, as in step  431  to the local host, which, in turn, sends an &lt;ACK&gt; segment (step  433 ). If the data arrives from its TSK peer before a &lt;SYN,ACK&gt; segment response is received from the local host, the data is queued and then sent after the &lt;ACK&gt; segment is sent in response to the &lt;SYN,ACK&gt; segment (when it arrives). 
     Turning now to  FIG. 4B , a spoofed TCP connection is established with the three-way handshake spoofing disabled. Under this scenario, the local host  400  transmits a TCP &lt;SYN&gt; segment, as in step  451 , to the TSK  280  within the local PEP end point  402 . Unlike the TCP connection establishment of  FIG. 4A , the local PEP end point  402  does not respond to the a TCP &lt;SYN&gt; segment with a &lt;SYN,ACK&gt; segment, but merely forwards a CR message to the remote PEP end point  404  (step  453 ). Next, in step  455 , sends a TCP &lt;SYN&gt; segment to the remote host  406 . In response, the remote host  406  transmit a TCP &lt;SYN,ACK&gt; segment back to the remote PEP end point  404  (per step  457 ). Thereafter, the remote PEP end point  404 , as in step  459 , forwards a CE message to the local PEP end point  402 , which subsequently issues a &lt;SYN,ACK&gt; segment to the local host  400 , per step  461 . Simultaneous with step  459 , the remote PEP end point  404  issues an &lt;ACK&gt; segment to the remote host  406  (step  463 ). 
     Upon receiving the &lt;ACK&gt; segment, the remote host  406  may begin transmission of data, as in step  465 . Once the PEP end point  404  receives the data from the remote host  406 , the remote PEP end point  404  simultaneously transmits, as in step  467 , the TD message to the local PEP end point  402  and transmits an &lt;ACK&gt; segment to the remote host  406  to acknowledge receipt of the data (step  469 ). 
     Because the local host  400  has received a &lt;SYN,ACK&gt; segment from the local PEP end point  402 , the local host  400  acknowledges the message, per step  471 . Thereafter, the local host  400  transmits data to the local PEP end point  402 . In this example, before the local PEP end point  402  receives the data from the local host  400 , the local PEP end point  402  forwards the data that originated from the remote host  406  via the TD message (step  467 ) to the local host  400 , per step  475 . 
     In response to the data received (in step  473 ), the local PEP end point  402  issues an &lt;ACK&gt; segment, as in step  477 , and forwards the data in a TD message to the remote PEP end point  404 , per step  479 . The local host  400  responds to the received data of step  475  with an &lt;ACK&gt; segment to the local PEP end point  402  (step  481 ). The remote PEP end point  404  sends the data from the local host  400 , as in step  483 , upon receipt of the TD message. After receiving the data, the remote host  406  acknowledges receipt by sending an &lt;ACK&gt; segment back to the remote PEP end point  404 , per step  485 . 
       FIG. 5  shows the flow of packets with the PEP architecture, according to one embodiment of the present invention. As shown, a communication system  500  includes a hub site (or local) PEP end point  501  that has connectivity to a remote site PEP end point  503  via a backbone connection. By way of example, at the hub site (or local site) and at each remote site, PEP end points  501  and  503  handle IP packets. PEP end point  501  includes an internal IP packet routing module  501  a that receives local IP packets and exchanges these packets with a TSK  501   b  and a BPK  501   c . Similarly, the remote PEP end point  503  includes an internal IP packet routing module  503   a  that is in communication with a TSK  503   b  and a BPK  503   c . Except for the fact that the hub site PEP end point  501  may support many more backbone protocol connections than a remote site PEP end point  503 , hub and remote site PEP processing is symmetrical. 
     For local-to-WAN traffic (i.e., upstream direction), the PEP end point  501  receives IP packets from its local interface  220  ( FIG. 2 ). Non-TCP IP packets are forwarded (as appropriate) to the WAN interface  230  ( FIG. 2 ). TCP IP packets are internally forwarded to TSK  501   b . TCP segments which belong to connections that are not be spoofed are passed back by the spoofing kernel  501   b  to the routing module  501   a  to be forwarded unmodified to the WAN interface  230 . For spoofed TCP connections, the TCP spoofing kernel  501   a  locally terminates the TCP connection. TCP data that is received from a spoofed connection is passed from the spoofing kernel  501   a  to the backbone protocol kernel  501   c , and then multiplexed onto the appropriate backbone protocol connection. The backbone protocol kernel  501   c  ensures that the data is delivered across the WAN. 
     For WAN-to-local traffic (i.e., downstream direction), the remote PEP end point  503  receives IP packets from its WAN interface  230  ( FIG. 2 ). IP packets that are not addressed to the end point  503  are simply forwarded (as appropriate) to the local interface  220  ( FIG. 2 ). IP packets addressed to the end point  503 , which have a next protocol header type of “PBP” are forwarded to the backbone protocol kernel  503   c . The backbone protocol kernel  503   c  extracts the TCP data and forwards it to the TCP spoofing kernel  503   b  for transmission on the appropriate spoofed TCP connection. In addition to carrying TCP data, the backbone protocol connection is used by the TCP spoofing kernel  501   b  to send control information to its peer TCP spoofing kernel  503   b  in the remote PEP end point  503  to coordinate connection establishment and connection termination. 
     Prioritization may be applied at four points in the system  500  within routing  501   a  and TSK  501   b  of PEP end point  501 , and within routing  503   a , and TSK  503   b  of PEP end point  503 . In the upstream direction, priority rules are applied to the packets of individual TCP connections at the entry point to the TCP spoofing kernel  501   b . These rules allow a customer to control which spoofed applications have higher and lower priority access to spoofing resources. Upstream prioritization is also applied before forwarding packets to the WAN. This allows a customer to control the relative priority of spoofed TCP connections with respect to unspoofed TCP connections and non-TCP traffic (as well as to control the relative priority of these other types of traffic with respect to each other). On the downstream side, prioritization is used to control access to buffer space and other resources in the PEP end point  503 , generally and with respect to TCP spoofing. 
     At the hub (or local) site, the PEP end point  501  may be implemented in a network gateway (e.g. an IP Gateway), according to one embodiment of the present invention. At the remote site, the PEP end point  503  may be implemented in the remote site component, e.g. a satellite terminal such as a Multimedia Relay, a Multimedia VSAT or a Personal Earth Station (PES) Remote. 
     The architecture of system  500  provides a number of advantages. First, TCP spoofing may be accomplished in both upstream and downstream directions. Additionally, the system supports spoofing of TCP connection startup, and selective TCP spoofing with only connections that can benefit from spoofing actually spoofed. Further, system  500  enables prioritization among spoofed TCP connections for access to TCP spoofing resources (e.g., available bandwidth and buffer space). This prioritization is utilized for all types of traffic that compete for system resources. 
     With respect to the backbone connection, the system  500  is suitable for application to a satellite network as the WAN. That is, the backbone protocol is optimized for satellite use in that control block resource requirements are minimized, and efficient error recovery for dropped packets are provided. The system  500  also provides a feedback mechanism to support maximum buffer space resource efficiency. Further, system  500  provides reduced acknowledgement traffic by using a single backbone protocol ACK to acknowledge the data of multiple TCP connections. 
       FIG. 6  illustrates the flow of IP packets through a PEP end point, according to an embodiment of the present invention. When IP packets are received at the local LAN interface  220 , the PEP end point  210  determines (as shown by decision point A), whether the packets are destined for a host that is locally situated; if so, the IP packets are forwarded to the proper local LAN interface  220 . If the IP packets are destined for a remote host, then the PEP end point  210  decides, per decision point B, whether the traffic is a TCP segment. If the PEP end point  210  determines that in fact the packets are TCP segments, then the TSK  280  determines whether the TCP connection should be spoofed. However, if the PEP end point  210  determines that the packets are not TCP segments, then the BPK  282  processes the traffic, along with the PK  284  and the PSK  286  for eventual transmission out to the WAN. It should be noted that the BPK  282  does not process unspoofed IP packets; i.e., the packets flow directly to PK  284 . As seen in  FIG. 6 , traffic that is received from the WAN interface  230  is examined to determine whether the traffic is a proper PBP segment (decision point D) for the particular PEP end point  210 ; if the determination is in the affirmative, then the packets are sent to the BPK  282  and then the TSK  280 . 
     Routing support includes routing between the ports of the PEP End Point  210  ( FIG. 2 ), e.g., from one Multimedia VSAT LAN port to another. Architecturally, the functionalities of TCP spoofing, prioritization and path selection, fit between the IP routing functionality and the WAN. PEP functionality need not be applied to IP packets which are routed from local port to local port within the same PEP End Point  210 . TCP spoofing, prioritization and path selection are applied to IP packets received from a local PEP End Point interface that have been determined to be destined for another site by the routing function. 
       FIG. 7  shows the relationship between PEP End Points and PEP End Point profiles, in accordance with an embodiment of the present invention. PEP parameters are primarily configured via a set of profiles  701  and  703 , which are associated with one or more PEP end points  705 . In an exemplary embodiment, PEP parameters are configured on a per PEP End Point basis, such as whether TCP spoofing is globally enabled. These parameters are configured in the PEP End Point profiles  701  and  703 . It is noted that parameters that apply to specific PEP kernels may be configured via other types of profiles. Profiles  701  and  703  are a network management construct; internally, a PEP End Point  705  processes a set of parameters that are received via one or more files. 
     Whenever the PEP End Point  705  receives new parameters, the platform environment compares the new parameters to the existing parameters, figures out which of the PEP kernels are affected by the parameter changes, and then passes the new parameters to the affected kernels. In an exemplary embodiment, all parameters are installed dynamically. With the exception of parameters that are component specific (such as the IP addresses of a component), all parameters may be defined with default values. 
     As mentioned previously, the PEP end point  210  may be implemented in a number of different platforms, in accordance with the various embodiments of the present invention. These platforms may include an IP gateway, a Multimedia Relay, a Multimedia VSAT (Very Small Aperture Terminal), and a Personal Earth Station (PES) Remote, as shown in  FIGS. 8-11 , respectively. In general, as discussed in  FIG. 2 , the PEP end point  210  defines a local LAN interface  220  an interface through which the PEP End Point  210  connects to IP hosts located at the site. A WAN interface  230  is an interface through which the PEP End Point  210  connects to other sites. It is noted that a WAN interface  230  can physically be a LAN port.  FIGS. 8-11 , below, describe the specific LAN and WAN interfaces of the various specific PEP End Point platforms. The particular LAN and WAN interfaces that are employed depend on which remote site PEP End Points are being used, on the configuration of the hub and remote site PEP End Points and on any path selection rules which may be configured. 
       FIG. 8  shows the interfaces of the PEP end point implemented as an IP gateway, according to one embodiment of the present invention. By way of example, an IP Gateway  801  has a single local LAN interface, which is an enterprise interface  803 . The IP Gateway  803  employs two WAN interfaces  805  for sending and receiving IP packets to and from remote site PEP End Points: a backbone LAN interface and a wide area access (WAA) LAN interface. 
     The backbone LAN interface  805  is used to send IP packets to remote site PEP End Points via, for example, a Satellite Gateway (SGW) and a VSAT outroute. A VSAT outroute can be received directly by Multimedia Relays ( FIG. 9 ) and Multimedia VSATs ( FIG. 10 ) (and is the primary path used with these End Points); however, IP packets can be sent to a PES Remote ( FIG. 11 ) via a VSAT outroute. 
       FIG. 9  shows a Multimedia Relay implementation of a PEP end point, in accordance with an embodiment of the present invention. A Multimedia Relay has two or three local LAN interfaces  903 . A Multimedia Relay  901  has up to two WAN interfaces  905  for sending IP packets to hub site PEP End Points: one of its LAN interfaces and a PPP serial port interface, and four or five interfaces for receiving IP packets from hub site PEP End Points, a VSAT out route, all of its LAN interfaces, and a PPP serial port interface. It is noted that a PPP (Point-to-Point Protocol) serial port interface and a LAN interface are generally not be used at the same time. 
     A Multimedia Relay  901  supports the use of all of its LAN interfaces  903  at the same time for sending and receiving IP packets to and from hub site PEP End Points. Further, a Multimedia Relay  905  supports the use of a VADB (VPN Automatic Dial Backup) serial port interface for sending and receiving IP packets to and from the hub site PEP End Points. 
       FIG. 10  shows a Multimedia VSAT implementation of the PEP end point, according to one embodiment of the present invention. A Multimedia VSAT  1001 , in an exemplary embodiment, has two local LAN interfaces  1003 . Support for one or more local PPP serial port interfaces may be utilized. The Multimedia VSAT  1001  has two WAN interfaces  1005  for sending IP packets to hub site PEP End Points: a VSAT inroute and one of its LAN interfaces. The Multimedia VSAT  1001  thus has three interfaces for receiving IP packets from hub site PEP End Points, the VSAT outroute and both of its LAN interfaces  1003 . A Multimedia VSAT  1003  may support uses of both of its LAN interfaces  1003  at the same time for sending and receiving IP packets to and from hub site PEP End Points. The Multimedia VSAT  1003  further supports the use of a VADB serial port interface for sending and receiving IP packets to and from the hub site PEP End Points. 
       FIG. 11  shows a PES Remote implementation of a PEP end point, according to one embodiment of the present invention. A PES Remote  1101  may have a local LAN interface and/or several local IP (e.g. PPP, SLIP, etc.) serial port interfaces, collectively denoted as LAN interfaces  1103 . The particular LAN interfaces  1103  depend on the specific PES Remote platform. PES Remote  1101 , in an exemplary embodiment, has up to five WAN interfaces  1105  for sending IP packets to hub site PEP End Points, an ISBN inroute, a LAN interface, a VADB serial port interface, a Frame Relay serial port interface and an IP serial port interface, and up to five existing interfaces for receiving IP packets from hub site PEP End Points: an ISBN outroute, a LAN interface, a VADB serial port interface, a Frame Relay serial port interface, and an IP serial port interface. The physical Frame Relay serial port interface may be supporting multiple Permanent Virtual Circuits (PVCs); some of which are equivalent to local interfaces  1103  and some of which are WAN interfaces  1105 . 
     The TSK  280  is responsible for all of the functions related to TCP spoofing. The TSK  280  includes at least two basic parts, a TCP stack  303  and a TCP spoofing application  301  as illustrated in  FIG. 3 . The TCP stack  303  can be responsible for interacting with the TCP stacks of IP hosts connected to a PEP End Point&#39;s local LAN interface(s)  803 ,  903 ,  1003 ,  1103 . The TCP stack  303  can also implement the TCP protocol including the appropriate TCP state machines and terminates spoofed TCP connections. The TCP spoofing application  301  can sit on top of the TCP stack  303  and act as the application receiving data from and sending data to the IP host applications. The TCP spoofing application  301  can also hide the details of TCP spoofing from the TCP stack  303  as much as possible, allowing the TCP stack  303  to function as much like a standard TCP stack as possible. The TCP spoofing application  301  can also be responsible for interfacing to the BPK  282 . 
     TSK  280  parameters can be configured via profiles. Backbone connection parameters can be configured using connectivity profiles. TCP spoofing selection parameters and spoofing parameters can be defined in TCP spoofing selection and TCP spoofing parameters profiles, respectively. TCP spoofing selection profiles can define which TCP spoofing parameters profiles are being used. The other TSK  280  parameters and which TCP-spoofing selection profile is being used can be defined in PEP End Point profiles  701 ,  703 . Which PEP End Point profile  701 ,  703  is being used by a PEP End Point  705  can be configured as part of an individual PEP End Point&#39;s specific configuration. 
     Profiles can be a network management construct. TSK  280  may receive its parameters, except for parameters related to backbone connections, as a data structure passed to TSK  280  by the platform environment  210 . Backbone connection parameters can be passed to TSK  280  by the platform environment  210  on a per backbone connection basis. The platform environment  210 , in turn, can receive the parameters via files sent to it by a network manager. 
     The TSK  280  can receive parameters from the platform environment  210  at startup and whenever the platform environment  210  receives new parameters which include changes to TSK  280  related parameters. When TSK  280  receives new parameters, it can compare the new parameters to the existing parameters and then takes action to install the new parameters based on which parameters have changed. All parameters can be installed dynamically. In some cases the changes will only affect new TCP connections and not TCP connections already in the process of being spoofed. On the other hand, some parameter changes, such as the deletion of a backbone connection, might require that existing spoofed TCP connections be terminated. If TCP spoofing is disabled, TCP connections which are already in the process of being spoofed may be terminated because all the backbone connections will be closed when the platform environment  210  invokes TSK  280 &#39;s shut down procedure. 
     TSK peers can exchange messages in order to coordinate their spoofing functions.  FIG. 12  illustrates an exemplary format of a TSK message  1200 . Tables A and B describe exemplary message fields in  FIG. 12 . Other message formats may be used, if required for the particular application. For example, if a peer is being used in an environment where more backbone connections may exist than can be supported by a 16 bit connection identifier, a 32 bit connection identifier can be implemented instead. Table C lists exemplary reason codes associated with the various message types. Reason codes can be assigned to be unique across all message types to facilitate troubleshooting. 
     
       
         
           
               
             
               
                 TABLE A 
               
             
            
               
                   
               
               
                 Exemplary TSK Message Field Descriptions 
               
            
           
           
               
               
               
            
               
                 Field 
                 Size 
                 Description 
               
               
                   
               
               
                 Message 
                 1 Byte 
                 Indicates the type of TSK message. Exemplary 
               
               
                 Type 
                   
                 message types are defined in Table B. Any 
               
               
                 1202 
                   
                 message type value not defined in Table B can 
               
               
                   
                   
                 be reserved for future use. 
               
               
                 Version 
                 2 Bits 
                 Indicates the current version of the TSK 
               
               
                 1204 
                   
                 “protocol”. 
               
               
                 Flags 1206 
                 2 Bits 
                 The flags field includes at least two flag bits: 
               
               
                   
                   
                 The first (MSB) flag bit can hold the PUSH 
               
               
                   
                   
                 flag. The second (LSB) flag bit can hold the 
               
               
                   
                   
                 optional TCP connection header flag. A value 
               
               
                   
                   
                 of 1 can indicate that the header is present. 
               
               
                 Sequence 
                 4 bits 
                 A unique sequence number can be tracked for 
               
               
                 Number 
                   
                 each TCP connection being spoofed with the 
               
               
                 1208 
                   
                 sequence number incremented by 1 each time a 
               
               
                   
                   
                 TSK message is sent. Since the PEP Backbone 
               
               
                   
                   
                 Protocol should guarantee in order delivery of 
               
               
                   
                   
                 messages, TSK may choose to use this field as 
               
               
                   
                   
                 additional flags. 
               
               
                 Destination 
                 2 Bytes 
                 Connection identifiers. 
               
               
                 Connection 
               
               
                 ID 1210 
               
               
                 Source 
                 2 Bytes 
                 Connection identifiers. 
               
               
                 Connection 
               
               
                 ID 1212 
               
               
                 Optional 
                 12 Bytes 
                 The TCP connection header is described below. 
               
               
                 TCP 
                   
                 Table C provides examples of types of TSK 
               
               
                 Connection 
                   
                 messages that included a TCP connection 
               
               
                 Header 1214 
                   
                 header (and when). 
               
               
                 Parameters 
                 N Bytes 
                 For TCP Data messages, this field can contain 
               
               
                 and/or Data 
                   
                 TCP data. For Urgent Data messages, this field 
               
               
                 1216 
                   
                 can contain a TCP Urgent Pointer followed by 
               
               
                   
                   
                 TCP data. For all other types of messages, this 
               
               
                   
                   
                 field can contain parameters which are specific 
               
               
                   
                   
                 to the Message Type. 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE B 
               
             
            
               
                   
               
               
                 Exemplary TCP Connection Related TSK 
               
            
           
           
               
               
               
               
            
               
                 Message 
                   
                 TCP Connection 
                   
               
               
                 Type 
                   
                 Header 1201 
               
               
                 1202 
                 Value 
                 Included When: 
                 Description 
               
               
                   
               
               
                 TCP Data 
                 0 
                 DST ID = 0xFFFF 
                 A TD message can carry the 
               
               
                 (TD) 
                   
                 and 
                 data from a TCP connection 
               
               
                   
                   
                 Header Fits 
                 data segment. 
               
               
                 Urgent Data 
                 1 
                 DST ID = 0xFFFF 
                 A UD message can carry the 
               
               
                 (UD) 
                   
                 and 
                 data from a TCP connection 
               
               
                   
                   
                 Header Fits 
                 &lt;URG&gt; segment. A UD 
               
               
                   
                   
                   
                 message can include an extra 
               
               
                   
                   
                   
                 two byte header in front of the 
               
               
                   
                   
                   
                 data to carry the TCP Urgent 
               
               
                   
                   
                   
                 Pointer field. 
               
               
                 Connection 
                 2 
                 Always 
                 A CR message can be sent 
               
               
                 Request 
                   
                   
                 when a TCP &lt;SYN&gt; segment 
               
               
                 (CR) 
                   
                   
                 is received to trigger the 
               
               
                   
                   
                   
                 sending of a TCP &lt;SYN&gt; 
               
               
                   
                   
                   
                 segment by the TSK peer. 
               
               
                 Connection 
                 3 
                 DST ID = 0xFFFF 
                 A CE message can be sent 
               
               
                 Established 
                   
                 (or Always) 
                 when a TCP &lt;SYN, ACK&gt; 
               
               
                 (CE) 
                   
                   
                 segment is received to 
               
               
                   
                   
                   
                 indicate successful 
               
               
                   
                   
                   
                 establishment of the TCP 
               
               
                   
                   
                   
                 connection. 
               
               
                 Connection 
                 4 
                 Always 
                 A CT message can be sent to 
               
               
                 Terminated 
                   
                   
                 terminate a TCP connection. 
               
               
                   
                   
                   
                 A CT message may include a 
               
               
                   
                   
                   
                 reason code which indicates 
               
               
                   
                   
                   
                 the reason for the termination. 
               
               
                   
                   
                   
                 Exemplary termination reason 
               
               
                   
                   
                   
                 codes are listed in Table C. 
               
               
                 No 
                 5 
                 Always 
                 An NR message can be sent 
               
               
                 Resources 
                   
                   
                 to refuse a TCP connection 
               
               
                 (NR) 
                   
                   
                 due to a lack of resources for 
               
               
                   
                   
                   
                 spoofing the connection. An 
               
               
                   
                   
                   
                 NR message can include a 
               
               
                   
                   
                   
                 reason code which indicates 
               
               
                   
                   
                   
                 which resource is unavailable. 
               
               
                   
                   
                   
                 Exemplary no resource 
               
               
                   
                   
                   
                 reason codes are listed in 
               
               
                   
                   
                   
                 Table C. 
               
               
                 Termination 
                 6 
                 DST ID = 0xFFFF 
                 A TP message can be sent to 
               
               
                 Pending 
                   
                 (or Always) 
                 indicate that a TCP &lt;FIN&gt; 
               
               
                 (TP) 
                   
                   
                 segment has been received 
               
               
                   
                   
                   
                 and termination of the TCP 
               
               
                   
                   
                   
                 connection is pending. 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE C 
               
             
            
               
                   
               
               
                 Exemplary Termination Reason Codes 
               
            
           
           
               
               
            
               
                 Message 
                   
               
               
                 Type(s) 
                 Description 
               
               
                   
               
               
                 CT 
                 Connection terminated due to a &lt;RST&gt; segment received 
               
               
                   
                 from the local host. 
               
               
                 CT 
                 Connection terminated due to no response from local host. 
               
               
                 CT 
                 Connection terminated due to the detection of a simultaneous 
               
               
                   
                 startup condition using different backbone connections. 
               
               
                 CT 
                 Connection refused because the previous incarnation of the 
               
               
                   
                 same connection has not yet terminated. 
               
               
                 NR 
                 Connection refused because no CID (i.e., CCB mapping table 
               
               
                   
                 entry) for the connection is available. 
               
               
                   
               
            
           
         
       
     
       FIG. 13  illustrates an exemplary format of a TCP connection header  1201 . Table D describes exemplary fields of the TCP connection header  1201 . A TCP connection header  1201  may contain the IP addresses  1302 ,  1304  and TCP port numbers  1306 ,  1308  which uniquely identify a TCP connection. TCP connection headers  1201  can be included in a TSK message when the TCP connection identifier used as the destination connection identifier field in the TSK message is set equal to 0xFFFF. A TCP connection header  1201  need not be used in a Data or Urgent Data message unless the data segment size is at least 12 bytes (the size of an exemplary header) smaller than the selected maximum segment size being used for the TCP connection. This can ensure that TSK  280  does not accidentally generate a TSK message which is larger than can be handled by the path being used by the backbone connection to the TSK peer. TCP connection headers  1201  can also be included in TSK control messages to help ensure that a control message is mapped to the correct TCP connection. This may not be necessary for some types of TSK control messages but including the header can simplify control message processing and may also help when debugging problems. 
     There are at least two types of connection identifiers used by the TSK  280 . A TSK peer connection identifier (TID) can be assigned to each TSK backbone connection. A TCP connection identifier (CID) can be assigned to each spoofed TCP connection. Exemplary TIDs and CIDs are described below. 
     
       
         
           
               
             
               
                 TABLE D 
               
             
            
               
                   
               
               
                 Exemplary TCP Connection Header Field Descriptions 
               
            
           
           
               
               
               
            
               
                 Field 
                 Size 
                 Description 
               
               
                   
               
               
                 Destination 
                 4 Bytes 
                 The destination IP address of the TCP connection, 
               
               
                 IP Address 
                   
                 i.e. the IP address of the host not local to the 
               
               
                 1302 
                   
                 sender of the TSK message. 
               
               
                 Source IP 
                 4 Bytes 
                 The source IP address of the TCP connection, i.e. 
               
               
                 Address 
                   
                 the IP address of the host local to the sender of 
               
               
                 1304 
                   
                 the TSK message. 
               
               
                 Destination 
                 2 Bytes 
                 The destination TCP port number of the TCP con- 
               
               
                 TCP Port 
                   
                 nection, i.e. the source TCP port number of the 
               
               
                 1306 
                   
                 host not local to the sender of the TSK message. 
               
               
                 Source TCP 
                 2 Bytes 
                 The source TCP port number of the TCP con- 
               
               
                 Port 1308 
                   
                 nection, i.e. the source TCP port number of the 
               
               
                   
                   
                 host local to the sender of the TSK message. 
               
               
                   
               
            
           
         
       
     
       FIG. 14  illustrates the learning of TSK backbone connection identifiers by a TSK peer as described below. 
     A TSK  280  which has more than one configured TSK peer can use a unique, non-zero local TSK peer connection identifier (TID) to each of its TSK peers. The TID can be assigned by the platform environment  210  in order to allow it correspond to the environment&#39;s identifier for the peer. The value assigned can be used as an index into a table of TSK peer control block pointers. The local TID  1402 ,  1404  can be used by TSK  280  as the source connection ID value in messages which are not associated with any particular TCP connection. 
     Since there may be a one to one mapping between TIDs and backbone connections (by definition), a backbone connection&#39;s handle, assigned by the platform environment  210 , can simply be used as the backbone connection&#39;s TID. The TID is used as an index into a table of TSK backbone connection control block (TCB) pointers, the TCB mapping table  1406 . The local TID  1402 ,  1404  is used by TSK as the source connection ID value in messages which are not associated with any particular TCP connection, e.g., TSK Peer Parameter (TPP) messages. TSK  280  learns the TID being used for the backbone connection by its TSK peer when it receives a TPP message from its peer and uses this value as the destination connection ID value in messages sent across the backbone connection which are not associated with any particular TCP connection. (Learning the TID being used by a TSK peer is not a critical requirement for communication. Learning the peer TID is only done as a performance optimization to allow easy mapping of messages to TCBs and for east of debugging. Note that 0xFFFF should not be used as a TID because 0xFFFF is sent as the destination connection ID when TSK  280  has not yet learned its TSK peer&#39;s local TID. 
     When a TSK  280  needs to forward a message for a TCP connection prior to learning the TCP CID assigned to the connection by its TSK peer, the TSK  280  can set the destination connection ID field in the TSK message to 0xFFFF (exemplary). (Thus, 0xFFFF is not a valid TCP CID.) And, if doing so does not cause the size of the message to exceed the TCP connection&#39;s selected maximum segment size, TSK  280  also can include a TCP connection header  1201 . Note that this does not necessarily apply only to Connection Request messages. If the three-way handshake is being spoofed, TSK  280  may need to forward data messages to its TSK peer prior to receiving the Connection Established message. And, if an error occurs, TSK  280  may need to send a Connection Terminated (CT) message to its TSK peer to abort a connection. 
     When TSK  280  receives a TSK TCP connection related message with a destination connection ID of 0xFFFF, TSK  280  can use the TCP connection header  1201 , if present, and the source connection ID  1212  in the message, combined with the information regarding which backbone connection the TSK message was received from (i.e. the handle passed to TSK  280  by the BPK  282  with the message), to find the appropriate CCB for the connection. The information in the TCP connection header  1201  can be used to find the CCB using a hash function. When there is no TCP connection header  1201 , the source connection ID  1212  can be used along with TSK&#39;s active hash list for the backbone connection to find the CCB. If there is no CCB and the TSK message is a CR message, a CCB can be allocated. If the message is not a CR message and there is no CCB, the message can be discarded. These methods for looking up a message&#39;s CCB can be less efficient than using the local TCP CID. But, these methods may only need to be used for a few messages at the start of a connection.  FIG. 15  illustrates the assignment of TCP connection identifiers. 
     At startup, the platform environment  210  calls the TSK  280  to open each backbone connection that it needs to each TSK peer. A separate call may be made for each backbone connection. The platform environment  210  informs the TCP Spoofing Kernel  280  when the backbone connection becomes active. TSK  280  should never close a backbone connection unless explicitly requested to do so by the platform environment  210 . 
     Every time that TSK  280  is informed that a backbone connection has transition from DOWN to UP, TSK  280  sends a TSK Peer Parameters message to its TSK peer. A TPP message is used to send resource availability information to the PEP End Point peer. The following information may be sent in a TPP message:
     The amount of buffer space available for spoofing in the WAN to LAN direction for this backbone connection;   The local number of TCP connection control blocks available for spoofing for this backbone connection.   

     These values are provided by the platform environment  210  when the backbone connection is opened (and stored in the backbone connection&#39;s TCB). Until a PEP End Point  705  has received at least one TPP message from its peer for a given backbone connection, no spoofed TCP connections will be able to use the connection. 
     If the amount of WAN to LAN buffer space or the number of CCBs available for spoofing on a backbone connection changes, the platform environment  210  will inform TSK  280  of the changes. Whenever TSK  280  receives an indication that one or both of these parameters has changed, the new values for the parameters are stored in the backbone connection&#39;s TCB and a new TPP message is sent to the TSK peer. 
     The TSK  280  uses at least two types of control blocks. TSK backbone connection control blocks are used to store information related to backbone connections established to TSK peers. TCP connection control blocks are used to store information with respect to TCP connections which are being spoofed by TSK  280 . 
     TSK  280  can support some number of backbone connections to TSK peers, determined by the particular PEP End Point platform software build. In general, this number is equal to the number of backbone connections that the PEP End Point platform as a whole supports. Backbone connections may be used for things other than TCP spoofing and, therefore, TSK  280  can support fewer backbone connections than are supported by the PEP End Point  705  as a whole. At startup, the platform environment  210  calls TSK  280  to add backbone connections to the TCP Spoofing Kernel&#39;s configuration. For each backbone connection, the platform environment  210  provides the handle that it will use for the connection, derived from the PEP End Point peer&#39;s peer index and the priority of the connection. After startup, the platform environment  210  may call TSK  280  to add, change the parameters of, or delete a backbone connection. 
     When the platform environment  210  calls TSK  280  to open (add) a backbone connection, the environment  210  provides a TCB for the backbone connection. The environment  210  allocates the TCB to allow for platform specific memory management of the TCBs. For example, an IP Gateway  801  can be designed to support up to 16,000 remote site PEP End Point peers (since an IP Gateway can currently support up to 16,000 remote IP subnets) and 64,000 backbone connections. Therefore, up to 64,000 TCBs may be required. On the other hand, a Multimedia Relay, Multimedia VSAT or PES Remote is likely to only have a few PEP End Point peers and, thus, only a few TCBs. Therefore, the IP Gateway implementation of TCB management is likely to be more complex than the Multimedia Relay, Multimedia VSAT or PES Remote implementation of TCB management. 
     TCBs are provided to the TSK  280  by the platform environment  210  when backbone connections are opened. TCBs are returned by TSK  280  to the platform environment  210  when backbone connections are closed. As indicated elsewhere, the allocation and deallocation of TCBs is done by the platform environment  210  in order to allow the use of an allocation strategy (e.g., dynamic versus static) appropriate for the particular platform.) A TCB mapping table, created and maintained by TSK  280 , is used to access allocated TCBs. The size of the mapping table (and the number of TCBs required) is determined by the software build of the PEP End Point  705 . The TSK backbone connection handle provided by the platform environment  210  is used as the index into the mapping table with the indexed table entry pointing to the TCB. This is illustrated in  FIG. 16 . The handle  1602  is passed by the environment  210  to TSK  280  when the backbone connection is referenced (either directly or by the way of a TCP connection&#39;s CCB). The handle is also passed to TSK  280  by the BPK  282  whenever a TSK message is received from the handle&#39;s backbone connection. The handle is also used as the TSK backbone connection identifier (TID) used as the source connection ID value in TSK messages sent to the TSK peer. 
     A TCB  1606  is used to store the configuration information passed to the TSK  280  by the platform environment  210  about the backbone connection. It also includes the connection&#39;s current state (UP or DOWN) and a pointer to the head and tail of the linked list of CCBs belonging to TCP connections which are currently using the backbone connection. Access to the list of CCBs may be required in order to find the TCP connections which are affected when backbone connections fail or are deleted. 
     Connection control blocks  1608  can be used to store information related to specific TCP connections. CCBs  1608  can be managed by the platform environment  210  because many details of their management are platform specific. The platform environment  210  can provide mechanisms for allocating and deallocating CCBs and a function for mapping a received TCP segment to its corresponding CCB. When a TCP segment is passed to the TSK  280 , the platform environment  210  can pass a pointer to the appropriate CCB  1608  to TSK  280  along with the TCP segment. A NULL pointer can be passed if there is no CCB  1608  currently associated with the particular TCP connection. The mapping of received TSK messages to CCBs, however, can be done by TSK itself. 
     TSK  280  can support some number of CCBs  1608 , determined by configuration and/or by compilation as appropriate for the particular PEP End Point  705  platform and software build. In order for a TCP connection to be spoofed, a CCB  1608  should be available in both TSK peers. Ideally, the number of CCBs  1608  will be large to ensure that all TCP connections which the operator desires to be spoofed can be spoofed. In practice, the memory constraints of some of PEP End Point  503  platforms may limit the number of CCBs  1608  such that occasionally a TCP connection cannot be spoofed because no CCB  1608  is available. When a TCP connection which should be spoofed cannot be spoofed because of a lack of CCB  1608 , an appropriate statistic is incremented and the TCP connection is carried unspoofed. TSK peers exchange information on the number of CCBs  1608  available for spoofed TCP connections using a particular backbone connection at startup (and whenever parameters change or the backbone connection restarts) via TSK Peer Parameters messages. The smaller value of the two TSK peers is then used as the limiting value for that backbone connection. Both TSKs  280  track the number of CCBs  1608  currently allocated (per backbone connection). If a new TCP connection is detected but the current number of CCBs  1608  allocated (for this backbone connection) is at the “negotiated” limit, the TSK  280  treats the connection as if no CCB  1608  is available (even if one is). 
     Because of propagation delay or because the PEP End Point is sharing its pool of CCBs  1608  among all of its peers, it is possible for a CCB  1608  to be available when a TCP &lt;SYN&gt; segment is received by a TSK  280  but for a corresponding CCB  1608  to not be available at the TSK peer. The handling of this error scenario is described below. 
     Unlike TCBs  1606  which can be accessed via the TCB mapping table  1604  both for TCP segments received from the local LAN and for TSK messages received from a backbone connection, CCBs  1608  may require different mechanisms for being accessed via TCP segments versus TSK messages. Exemplary mechanisms are described below. 
     CCBs  1608  which are not currently associated with any TCP connection can be stored by the platform environment  210  in a CCB free pool. Free CCBs can be stored using various platform dependent methods. A first method is a pool of memory from which CCBs are created using a malloc function or equivalent. With this method, the number of free CCBs  1608  can be tracked numerically or via the amount of buffer space set aside for use in creating CCBs  1608 . CCBs  1608  can be returned to the free pool using a free function or equivalent. A second method is by means of a FIFO queue. With this method, all of the CCBs  1608  are created at platform startup and then chained together using their next CCB pointers. The CCB next CCB pointer is described below. A CCB  1608  can be allocated by removing it from the head of the FIFO queue and a CCB  1608  can be freed by placing it at the end of the FIFO queue. 
     A CCB  1608  which is associated with a TCP connection can be considered active. Active CCBs  1608  are referenced in various ways. For mapping TSK messages received from its TSK peer to CCBs  1608 , TSK  280  can use a CCB mapping table. The CCB mapping table can also be used by TSK  280  in a round robin fashion to access CCBs  1608  to check for TCP connection timeouts. For mapping TCP segments received from the local host to CCBs  1608 , a CCB hash function  1702  can also be used to find CCB pointers. The CCB hash function  1702  can also be used, in some cases, to find the CCBs for received TSK messages when the CCB mapping table cannot be used. 
     For being accessed when a TCP segment is received from the local LAN, a hash function  1702  can be used. The hash function  1702  produces an index  1704  into a CCB hash table  1706 . The CCB hash table  1706  points to a doubly linked list of CCBs  1708  which match the hash value. Each CCB  1608  can include a next CCB pointer field which is used by the platform environment  210  to implement the linked list.  FIG. 17  illustrates CCB access via the CCB hash function  1702 . The maintenance of the CCB pointers  1710  used by the hash function  1702  may be the responsibility of the platform environment  210 . The platform environment  210  can simply pass a pointer to the appropriate CCB to the TSK  280  along with a TCP segment it passes to TSK  280 . The environment  210  can also provide a function call interface which TSK  280  can call to use the hash function itself. This interface can be used by TSK  280  to find a CCB  1608  using the information in the TCP connection header  1201  of a received TSK message  1200 . 
     The fact that the platform environment  210  is responsible for managing the CCB hash table  1706  means that the platform environment  210  should have access to the some of the fields in the CCB  1608 . To keep the platform environment  210  from needing to know the complete format of the CCB  16087 , the fields in the CCB  1608  which are accessible to the platform environment  210  can be placed at the front of the CCB  1608 . The platform environment  210  may then be responsible for maintaining the following exemplary CCB fields:
     the next and previous CCB pointer;   the IP addresses and TCP port numbers which uniquely identify the TCP connection; and   the backbone connection handle used to map to the TCB of the backbone connection being used to carry this spoofed connection (i.e. the TID of the peer).   

     In general, the IP addresses and TCP port numbers of received TCP segments can be used as input into the CCB hash function  1702 . However, the hash function  1702  used can be platform specific. For example, because it will be supporting a large number of TCP connections to different remote sites, the IP Gateway  801  hash function should give emphasis to the subnet portion of the IP addresses. However, the subnet portion of the IP addresses can be the same for all of the TCP connections associated with a particular remote site. Therefore, a remote site platform environment  210  should give more emphasis to the host part of the IP addresses. 
     CCBs  1608  can be allocated and deallocated by the TSK  280  via function calls to the platform environment  210 . A CCB mapping table  1802 , created and maintained by TSK  280 , can be used to access CCBs  1608  for purposes of timer processing and when TSK messages are received from the BPK  282 . A mapping table  1802  can be used to support the TSK peers. The size of the mapping table  1802  and the number of CCBs  1608  required can be determined by the software build of the PEP End Point  705 . In a given PEP End Point  705 , the number of entries in the mapping table  1802  and the number of CCBs  1608  available may be the same since TSK  180  should not use a CCB  1608  which it cannot access via the mapping table  1802  and TSK  280  does not need mapping table entries into which it cannot place a CCB  1608 . Each entry in the mapping table  1802  has at least two fields:
     a CCB pointer; and   a next entry index.   

     The next index can be used to implement linked lists of CCBs. At least two types of linked lists can be maintained using the next entry index:
     a free entry list  1804 ; and   active CCB lists  1806 .   

     The free entry list can store the list of free mapping table entries. TSK  280  can maintain a pointer to the front and rear of the list and uses these pointers to implement a free entry FIFO queue. When a new CCB  1608  is allocated, an entry from the free entry list  1804  can also be allocated. TSK  280  uses the index of the mapping table entry as the TCP connection&#39;s local TCP CID. When a CCB  1608  is deallocated, the CCB&#39;s mapping table  1802  entry can be returned to the free list  1804 . 
     Active CCB lists  1806  can be used to chain together the CCBs  1608  of TCP connections which are currently active. The CCBs  1608  of all of the TCP connections which are sharing a particular backbone connection can be linked together. The indices for the first and last entries of a backbone connection&#39;s CCB linked list can be stored with the backbone connection state in the TCB associated with the backbone connection. The active CCB lists  1806  can be implemented as doubly linked lists in order to make it easier to remove entries from the middle of the list. However, in the interests of conserving space in the CCB mapping table  1802  and keeping the list maintenance software simpler, singly linked lists  1806  may be used. Active CCB lists can be used for at least two purposes:
     to find all of the CCBs  1608  affected by the failure or deletion of a backbone connection. When a backbone connection fails or is deleted, all of the TCP connections using the backbone connection can be terminated; and   to find the appropriate CCB  1608  when a TSK message is received with a destination TCP CID value of 0xFFFF but without a TCP connection header.   

     For the latter case, TSK  280  can walk down the active CCB list  1806  of the backbone connection from which the TSK message was received looking for a CCB  1608  with a peer CID equal to the source connection ID in the TSK message. 
     A CCB  1608  can be removed from its active CCB list  1806  when the CCB is deallocated. 
       FIG. 18  illustrates the use of the CCB mapping table  1802 . 
     A CCB  1608  can be allocated when a new TCP connection is detected which needs to be spoofed. TSK  280  allocates a free entry from the CCB mapping table  1802  and then calls the platform environment  210  to allocate the CCB  1608 , providing the IP addresses and TCP port numbers which uniquely identify the connection. The platform environment  210  can allocate a CCB  1608  from the free CCB pool and can use the provided IP addresses and port numbers to determine the correct hash table entry for the CCB  1608 . The CCB pointer can then be added to the hash table  1706  (chained to the end of any existing CCBs already mapped to this hash table entry in the event of a hash table collision). Finally, before passing the CCB  1608  back to TSK  280 , the platform environment  210  can fill in the CCB&#39;s TCB index value. When TSK  280  receives the CCB  1608 , it uses the TCB index in the CCB  1608  to find the TCB  1606 . The CCB  1608  is then linked into the active CCB list  1806  for the backbone connection associated with the TCP connection&#39;s priority. When allocating a CCB  1608  for a new TCP connection detected from the local LAN, before actually placing the CCB  1608  into the CCB mapping array, TSK  280  first checks to make sure that the backbone connection is up. If the backbone connection is down, the connection cannot be spoofed and the CCB  1608  for the connection is returned to the platform environment  210 . 
     When a CCB  1608  is deallocated, it can be dequeued from its active CCB list  1806 , its CCB mapping table entry can be returned to the free entry list  1804  and the CCB  1608  can be returned to the platform environment  210 . The environment  210 , in turn, can remove the CCB  1608  from the CCB hash table  1706  and return the CCB  1608  to the free CCB pool. 
     The total number of CCBs  1608  available in a PEP End Point platform  705  is configurable. The value may actually be specified in terms of the number of CCBs  1608  available per PEP End Point  705  peer, as part of a PEP End Point profile  701 ,  703 . However, each PEP End Point  705  platform software build will have some maximum number of CCBs  1608  it can support. If the operator configures the number of CCBs  1608  to be larger than the number supported by the software build, the smaller number will be used and an event may be posted to alert the operator that this has occurred. However, in a PEP End Point  501 , where the CCB pool is shared among all of the peers, the operator may intentionally configure the per peer CCB limit such that multiplying the limit by the number of peers would require more CCBs  1608  than actually exist to improve performance by statistically sharing the CCBs  1608 . 
     Having the number of CCBs  1608  in a PEP End Point  705  be configurable allows the operator to control the point at which TCP connections stop being spoofed. The total number of TCP connections being carried by the system can reach a point where the total amount of bandwidth divided by the number of TCP connections actively using it is less than the throughput possible for each TCP connection without TCP spoofing. Therefore, the operator may want to set the number of CCBs  1608  such that spoofing only occurs when performance will be improved. However, TCP spoofing performance improvement is not limited to just high data throughput. TCP spoofing includes spoofing the TCP three-way handshake, as discussed above regarding  FIG. 4A . Depending on the applications being used, the operator may decide that spoofing the three-way handshake is useful even when throughput is limited by the presence of a large number of TCP connections. In addition, for spoofed TCP connections, when resources (e.g., buffer space) are low, flow control can be applied to spoofed TCP connections (by shrinking the TCP windows being advertised by the PEP End Point  705 ). This is not possible for unspoofed TCP connections. 
     In addition to the total number of PEP End Point CCBs  1608 , the operator can also configure the percentage of the available CCBs  1608  which can be used with the backbone connection for each priority. This allows the operator to reserve CCBs  1608  for use by higher priority TCP connections. 
     When a TCP segment is received from the local LAN, the platform environment  210  can use the CCB hash function  1702  to find the CCB  1608  associated with the TCP connection and passes a pointer to this CCB  1608  to the TSK  280  along with the TCP segment. An index into the TCB mapping array stored in the CCB  1608  can then used by TSK  280  when it wants to reference the TCB  1606  associated with the backbone connection being used to spoof the TCP connection. For a TCP segment received from the local LAN, TSK  280  shouldn&#39;t need to access the TCB  1606  first to find the connection&#39;s CCB  1608 . 
     When a TSK message is received from the BPK  282  by TSK  280 , TSK  280  can extract the destination TCP CID from the TSK message. If the TCP CID is not 0xFFFF, it can represent the CCB mapping table index for the CCB associated with the TCP connection of the TSK message. If the TCP CID is 0xFFFF, TSK  280  should determine if a new TCP CID is required (because the TSK message may be a Connection Request message), if the message belongs to an existing TCP connection for which the TSK peer has not yet received the TCP CID or if the message should be discarded because neither of the previous two conditions apply. TSK  280  can first check the message to see if there is a TCP connection header  1201  included with the message. If a TCP connection header  1201  is included, TSK  280  can use the information in the TCP connection header  1201  as input into the hash function  1702  to find the CCB  1608 . If no TCP connection header  1201  is included in the message, TSK  280  can search the list of active CCBs  1608  currently associated with the backbone connection from which the message was received, searching for a match with the source TCP CID in the TSK message. BPK  282  can pass to TSK  280  the handle to find the appropriate TCB  1606  when it passes the TSK message to TSK.  FIG. 19  illustrates the relationship between a CCB  1608  and TCB  1606 . 
     The priority of a spoofed TCP connection can be determined for the connection at the time the connection&#39;s CCB, is allocated. TSK  280  only uses the priority to determine the appropriate backbone connection for carrying a spoofed TCP connection. After making this determination, TSK  280  need never references a TCP connection&#39;s priority again. 
     For a CCB  1608  being allocated because of the reception of a Connection Request (CR) message  1506 , the priority is set equal to the priority of the backbone connection on which the CR message  1506  was received, i.e., the backbone connection from which the CR message  1506  was received is used as the backbone connection for the spoofed TCP connection. For a CCB  1608  which is about to be allocated due to the reception of a TCP &lt;SYN&gt; segment, the priority is the priority indicated in the selected TCP spoofing parameter profile. However, prior to actually using a CCB  1608  allocated for the connection, TSK  280  checks to make sure that a backbone connection to the appropriate TSK peer is currently up at the priority level indicated in the TCP spoofing parameter profile. If the desired backbone connection is not up (or does not exist), the CCB  1608  is not allocated and the TCP connection is not spoofed. The priority of a TCP connection which is not spoofed may be determined by prioritization rules implemented in the PK  284 . 
     The following describes the handling of spoofed TCP connections. 
     The TCP Spoofing Kernel  280  can establish a spoofed TCP connection when it receives a TCP &lt;SYN&gt; segment from its local LAN or it receives a Connection Request message from its TSK peer.  FIGS. 4A and 4B  illustrate spoofed TCP connection establishment with and without three-way handshake spoofing. Three-way handshake spoofing may be disabled to support an end to end MSS exchange. 
     When a TCP segment is received from the local LAN, the platform environment  210  checks to see if there is already a CCB  1608  assigned to the TCP connection associated with the TCP segment. If there is no CCB  1608 , the environment checks to see if the TCP segment is a &lt;SYN&gt; segment being sent to a non-local destination. If so, the &lt;SYN&gt; segment represents an attempt to bring up a new (non-local) TCP connection and the environment passes the segment to the TSK  280  to determine the TCP connection&#39;s disposition. 
     When a TCP &lt;SYN&gt; segment is received from the local LAN for a new TCP connection, the TSK  280  first must determine if the connection should be spoofed. If the connection should be spoofed, TSK  280  can uses the priority indicated in the selected TCP spoofing parameter profile and the peer index (provided by the environment with the TCP &lt;SYN&gt; segment) to construct the handle of the backbone connection which should be used to carry this spoofed TCP connection. The backbone connection handle is then used (via the TCB mapping table) to find the TCB associated with the backbone connection. TSK  280  then checks to see if the backbone connection is up. If the backbone connection is up, TSK  280  checks to see if the number of spoofed TCP connections already using the selected backbone connection is still currently below the CCB resource limit. The CCB resource limit is the smaller of the local number of CCBs (provided as a parameter by the platform environment  210 ) and the peer number of CCBs (received in the latest TPP message from the TSK peer) available for this backbone connection. If the number of connections is still below the limit, TSK  280  assigns a unique TCP connection identifier (e.g., a free CCB mapping table entry index) to the connection and calls the environment to allocate a TCP connection control block for the connection. 
     TSK  280  will return the TCP &lt;SYN&gt; segment back to the environment  210  to be forwarded unspoofed in any of the above checks failed. In other words, if:
     The selective TCP spoofing rules indicate that the connection should not be spoofed;   There is no backbone connection for the priority at which the TCP connection should be spoofed (indicated by the absence of a TCB for the backbone connection);   There is a backbone connection but the backbone connection is down;   The number of spoofed TCP connections already using this backbone connection is at (or above) the limit; or   There is no CCB mapping table  1802  entry available or there is no CCB  1608  available from the CCB free pool, then the TCP connection is forwarded unspoofed.   

     For the case where there is no backbone connection, TSK  280  can also post an event to alert the operator that there is a mismatch between the configured TCP spoofing parameter profiles and the configured set of backbone connections. 
     If all of the above checks pass, TSK  280  writes the backbone connection handle into the buffer holding the TCP &lt;SYN&gt; segment  401 . This is not done until a CCB  1608  is successfully allocated by the platform environment  210  because the environment  210  does not count the buffer unless a CCB is successfully allocated.) TSK  280  then copies the parameters from the selected TCP spoofing parameter profile into the CCB. Then relevant information (the maximum segment size advertised by the host (if smaller than the configured MSS), the initial sequence number, etc.) is copied out of the TCP &lt;SYN&gt; segment  401  and stored in the CCB  1608 . The source and destination IP addresses and source and destination TCP port numbers will already have been placed into the CCB by the platform environment  210  when the CCB was allocated. The environment  210  needs this information to manage CCB hash function collisions. 
     After allocating and setting up the CCB  1608 , the TSK  280  constructs a Connection Request message  403  and sends it to its TSK peer. The CR message  403  basically contains all of the information extracted from the TCP spoofing parameter profile and the TCP &lt;SYN&gt; segment  401  and stored in the local CCB  1608 , e.g., the source and destination IP addresses, the source and destination TCP port numbers, the MSS value, etc., with the exception of fields, such as the initial sequence number, which have only local significance. The IP addresses and TCP port numbers are placed into a TCP connection header  1201 . In other words, the CR message  403  contains all of the information that the peer TSK will need to set up its own CCB  1608 . 
       FIG. 20  illustrates completion of the local connection establishment.  FIG. 20  is identical to  FIGS. 4A and 4B  but repeated here for clarity and ease on understanding. TSK  280  needs to send a TCP &lt;SYN,ACK&gt; segment  405  in response to the &lt;SYN&gt; segment  401  received. TSK  280  can do this at the same time it sends the CR message  403 , if three-way handshake spoofing is enabled. Otherwise, it may wait for a CE message  459  from its TSK peer  404  before sending the &lt;SYN,ACK&gt; segment  405 . TSK  280  picks a random initial sequence number (following the guidelines provided in RFC  793 , the entire contents of which are hereby incorporated by reference) to use for sending data. If three-way handshake spoofing is disabled, the MSS value sent in the &lt;SYN,ACK&gt; segment  461  is set equal to the MSS value received in the CE message  459 . However, if the MSS value is larger than the configured MSS value the configured MSS value will be sent instead. If three-way handshake spoofing is enabled, the MSS value is determined from the TCP spoofing parameter profile selected for the connection. For this case, TSK  280  must then compare the MSS value received in the CE message  419 , when it arrives, to the value it sent to the local host in the TCP &lt;SYN,ACK&gt; segment  405 . If the MSS value received in the CE message  419  is smaller than the MSS value sent to the local host, a maximum segment size mismatch exists. MSS mismatch handling is described below. 
     After sending the TCP &lt;SYN,ACK&gt; segment  405 , TSK  280  is ready to start accepting data from the local host. When three-way handshake spoofing is being used, TSK  280  does not need to wait for the CE message  419  to arrive from its TSK peer  404  before accepting and forwarding data. Doing so would defeat the purpose of spoofing the three-way handshake. However, TSK  280  will not accept data from its TSK peer  404  until after the CE message  419  has been received. And, TSK  280  will not forward any data received from its TSK peer  404  to the local host  400  until it has received the TCP &lt;ACK&gt; segment  407  indicating that the local host  400  has received the &lt;SYN,ACK&gt; segment  405 . 
     When a CR message  403  is received from a peer TSK  280 , TSK  280  can allocate a CCB for the connection and then store all of the relevant information from the CR message  403  in the CCB  1608 . Handling of the case where no CCB is available is described below. TSK  280  then can use this information to generate a TCP &lt;SYN&gt; segment  415  to send to the local host. The MSS in the &lt;SYN&gt; segment  415  can be set to the value received from the TSK peer. When the local host responds with a TCP &lt;SYN,ACK&gt; segment  417 , TSK  280  can send a CE message  419  to its TSK peer  402 , including in the CE message  419  the MSS sent by the local host in the &lt;SYN,ACK&gt; segment  417 . TSK  280  can also respond with a TCP &lt;ACK&gt; segment  421  to complete the local three-way handshake. At this point, TSK  280  is ready to receive and forward data from either direction. If data arrives from its TSK peer before a &lt;SYN,ACK&gt; segment response is received from the local host, the data can be queued and then sent after the &lt;ACK&gt; segment is sent in response to the &lt;SYN,ACK&gt; segment (when it arrives). 
     There are many TCP connection establishment error scenarios which can be handled by TSK  280 . The following describes some exemplary scenarios. 
     A TCP connection can be uniquely identified by the combination of its associated destination and source IP addresses and destination and source TCP port numbers. It is possible for two hosts to attempt to bring up the same TCP connection at the same time with the TCP &lt;SYN&gt; segment from each host passing each other. With TCP spoofing in the present invention, this may result in two CR messages  403  passing each other. To handle this situation, when TSK  280  receives a CR message  403 , prior to allocating a CCB  1608  for the TCP connection, it can first check to see if there is already a CCB  1608  allocated for the TCP connection (by using the CCB hash function  1702  on the IP addresses and TCP port numbers included in the TCP connection header  1201  of the CR message  403 ). If there already is a CCB  1608  allocated, then TSK  280  can treat the CR message  403  as if it was a CE message  419 , extracting its TSK peer&#39;s TCP CID from the CR message  403 .  FIG. 21A  illustrates the startup of the same TCP connection from each host. 
     Each TSK peer should be able to allocate a CCB  1608  for each TCP connection in order for the connection to be spoofed. When a TCP &lt;SYN&gt; segment  401  is received for a new connection and no CCB  1608  is available (or the number of CCBs allocated for this TSK peer has reached its limit), the TCP &lt;SYN&gt; segment  401  can be forwarded unspoofed. This is illustrated in  FIG. 22 . However, due to propagation delay (or the potential overbooking of CCBs if they are shared among TSK peers), it is possible for a CCB  1608  to be available when a TCP &lt;SYN&gt; segment  401  is received but for a CCB  1608  to not be available at the TSK peer. 
     When a CR  403  is received for a new connection and no CCB  1608  is available, TSK  280  can respond to the CR message  403  with a No Resource (NR) message  439  (with a reason code indicating “no CCB available”). Any subsequent data messages received from the TSK peer corresponding to this TCP connection can simply be discarded. 
     When TSK  280  receives a NR message  439  with a reason code of “no CCB available” in response to a CR message  403 , TSK  280  can set the current state for the TCP connection to “unspoofed” and starts the connection&#39;s “unspoofed” timer. A purpose of the “unspoofed” state is to allow TSK  280  to remember that it was unable to spoof this connection and, thus, allow the connection to come up unspoofed on a reattempt by the local host  400 . While in the “unspoofed” state, if TSK  280  receives any non-&lt;SYN&gt; segment for the TCP connection before it receives a &lt;SYN&gt; segment  401 , TSK  280  can discard the non-&lt;SYN&gt; segment and respond with a &lt;RST&gt; segment  437 . If TSK  280  receives a TCP &lt;SYN&gt; segment  401  while in the “unspoofed” state, TCP can forward the &lt;SYN&gt; segment  401  unspoofed and starts waiting for a non-&lt;SYN&gt; segment (in the meantime, forwarding any additional &lt;SYN&gt; segments  401  received unspoofed). When TSK  280  receives a non-&lt;SYN&gt; segment after having received a &lt;SYN&gt; segment  401 , TSK  280  can assume that the connection must have been successfully established unspoofed and, therefore, deallocates the connection&#39;s CCB  1608  (after forwarding the received segment unspoofed). In any case, when the connection&#39;s “unspoofed” timer expires, the connection&#39;s CCB can be deallocated.  FIG. 23  illustrates connection establishment scenarios when no CCBs are available at the TSK peer. 
     Because of propagation delay, it is possible for the last available CCB  1608  to be allocated by each TSK peer to different TCP connections if a host on each side of the network sends a TCP &lt;SYN&gt; segment  401  at the same time. This is illustrated in  FIG. 24 . This situation can result in each connection being forwarded unspoofed even though there is a CCB  1608  at each end of the backbone connection which could be used to spoof one of the connections. However, since this scenario will be rare, the available CCB  1608  will simply be used to spoof the next TCP connection. 
     When a TSK  280  receives a CR message  403  and sends a TCP &lt;SYN&gt; segment  401  to a local host, it can start a local response timer. If the timer expires before a TCP &lt;SYN,ACK&gt; segment response  405  is received from the host, TSK  280  can resend the &lt;SYN&gt; segment  401  and restart the timer. TSK  280  will retransmit the &lt;SYN&gt; segment  401  N times, where the retry count, N, is either a compile time constant or an operator configurable parameter. If TSK  280  does not receive a &lt;SYN,ACK&gt; segment  405  response after N retries, it can conclude that the host is not reachable. It can send a TCP &lt;RST&gt; segment  437  to the local host in case the problem only exists for traffic from the local host and, thus, the local host did actually receive the &lt;SYN&gt; segment)  401 , send a CT message  435  to its TSK peer (with a reason code indicating “no response from local host”) and then close the connection, discarding any data segments which were received from its TSK peer and queued for later delivery and deallocating the connection&#39;s CCB  1608 . 
     The behavior of a TSK  280  when it receives a CT message  435  can depend on whether or not the kernel has already locally established the TCP connection (i.e. by sending the TCP &lt;SYN,ACK&gt; segment). If TSK  280  has not locally established the TCP connection, TSK  280  can simply close the connection and deallocate the connection&#39;s CCB. Nothing need be sent to the local host. If TSK  280  has locally established the TCP connection, TSK  280  can send a TCP &lt;RST&gt; segment  437  to the local host and then closes the connection. 
     The no response from destination host error scenario is illustrated in  FIG. 25 . 
     When TSK  280  receives a TCP &lt;SYN&gt; segment  401  and sends a TCP &lt;SYN,ACK&gt; segment  405  in response, it can start a local response timer. If the timer expires before a TCP &lt;ACK&gt; segment  405  response is received from the host to complete the three-way handshake, TSK  280  can resend the &lt;SYN,ACK&gt; segment  405  and restart the timer. TSK  280  can retransmit the &lt;SYN,ACK&gt; segment  405  N times, where the retry count, N, is either a compile time constant or an operator configurable parameter. If TSK  280  does not receive an &lt;ACK&gt; segment  407  response after N retries, it can conclude that the host is not reachable. It can send a TCP &lt;RST&gt; segment  437  to the local host just in case the problem only exists for traffic from the local host and, thus, the local host did actually receive the &lt;SYN,ACK&gt; segment  405 , send a CT message  435  to its TSK peer (with a reason code indicating “no response from local host”) and close the connection, discarding any data segments which were received from its TSK peer (after the CE message  419  was received from the TSK peer) and queued for later delivery before deallocating the connection&#39;s CCB  1608 . 
     When the TSK peer receives the CT message  435 , it can send a TCP &lt;RST&gt; segment  437  to the local host and closes the connection. The no response from source host error scenario is illustrated in  FIG. 26 . 
     After a TCP connection has been established between a local host and the TSK  280 , the host and TSK  280  can send data to each other. When a data segment is received, the platform environment  210  will pass a pointer to the CCB  1608  to TSK  280  along with the received TCP segment. The existence of a CCB  1608  for the segment&#39;s TCP connection (for any type of TCP segment other than a &lt;SYN&gt; segment  401 ) can be used as the indication as to whether or not the connection is being spoofed. The presence of a CCB  1608  does not guarantee that the TCP connection is being spoofed (as indicated in the discussion above re the “unspoofed” state). The absence of a CCB  1608  can indicate that the TCP connection is not being spoofed. 
     With respect to sending data to the host (and recovering from any dropped data segments), TSK  280  should follow all of the relevant Internet Engineering Task Force (IETF) standards which relate to TCP, including the standards which govern slow start and congestion avoidance. With respect to receiving data from the host, TCP can advertise a receive window  441  to the host and locally acknowledges data received from the host. Acknowledged data can be forwarded by TSK  280  to its TSK peer. This is illustrated in  FIG. 27 . 
     The receive window  441  advertised by TSK  280  to the local host in any given TCP &lt;ACK&gt; segment  405  can be the minimum of the windows determined from several calculations. These windows can be determined by the receive window size configured in the TCP connection&#39;s selected TCP spoofing profile the previous window size advertised and the amount of buffer space available for TCP spoofing. 
     A host will normally terminate a TCP connection by sending a TCP &lt;FIN&gt; segment  443 . A &lt;FIN&gt; segment  443  indicates that the host has no more data to send but does not terminate the connection until the other host also sends a &lt;FIN&gt; segment  443  indicating that it also has no more data to send. In some cases, a host may terminate a connection by sending a TCP &lt;RST&gt; segment  437 . A &lt;RST&gt; segment  437  immediately terminates a TCP connection. In general, a TCP connection will only be terminated using a &lt;RST&gt; segment  437  when the application intentionally wants to interrupt the flow of TCP data or the application is sure that there is no more data which needs to be transferred and wants to terminate the TCP connection faster than a &lt;FIN&gt; segment  443  exchange allows. TSK  280  handling of these connection termination messages is described below. 
     When TSK  280  receives a TCP &lt;FIN&gt; segment  443 , it can enter a local FIN wait state, send a TCP &lt;ACK&gt; segment  407  to acknowledge reception of the &lt;FIN&gt; segment  443  and send a Termination Pending (TP)  445  message to its TSK peer. After receiving the &lt;FIN&gt; segment  443 , TSK  280  can discard any TCP data received on the TCP connection from the local host. However, TSK  280  can continue to accept and forward data it receives from its TSK peer until it receives a TP message  445  from the TSK peer. When TSK  280  receives a TP message  445  from its TSK peer, it can enter a local FIN pending state. TSK  280  can then continue to send any data segments which have not been transmitted and acknowledged. After all data has been sent and acknowledged, TSK  280  can then send a TCP &lt;FIN&gt; segment  443  to the local host. If there is no data remaining when the TP message is received, the &lt;FIN&gt; segment  443  can be sent immediately. When the local host acknowledges the &lt;FIN&gt; segment  443 , TSK  280  can start the connection&#39;s “time-wait” timer and enters the “time-wait” state. 
     When TSK  280  receives a TP message  445  from its TSK peer while it is in the data transfer state, it can enter a peer FIN wait state. TSK  280  can then continue to send any data segments which have not been transmitted and acknowledged. After all data has been sent and acknowledged, TSK  280  can send a TCP &lt;FIN&gt; segment  443  to the local host and enter a peer FIN pending state. If there is no data remaining when the TP message is received, the &lt;FIN&gt; segment  443  can be sent immediately and TSK  280  can go to the peer FIN pending state. TSK  280  can continue to accept, acknowledge and forward data received from its local host until the local host sends a TCP &lt;FIN&gt; segment  443 . When the local host sends the TCP &lt;FIN&gt; segment  443  while TSK  280  is in the peer FIN pending state, TSK  280  can send a TCP &lt;ACK&gt; segment  441  to acknowledge reception of the &lt;FIN&gt; segment  443  and sends a TP message  445  to its TSK peer. TSK  280  then can start its “time-wait” timer and enter the “time-wait” state.  FIG. 28  illustrates spoofed TCP connection &lt;FIN&gt; segment  443  handling. 
     It is possible for the final segment, e.g. the &lt;RST&gt; segment  437  or the &lt;ACK&gt; segment  433  sent in response to a &lt;FIN&gt; segment  443 , sent to close a connection to be lost. The purpose of the “time-wait” state is to hold on to the CCB  1608  long enough to resend the final segment if any new segments for the same connection are received from the local host. For TCP in general, the “time-wait” state also provides some protection against the arrival of stale segments which have been significantly delayed transiting the network. This problem should be rare but could occur in the presence of temporary routing loops. 
     When the “time-wait” state is entered, the segment sent at the time the state was entered is (logically) queued. This segment can be resent in response to any segments (other than &lt;SYN&gt; segments  401 ) received from the local host for the connection. If no segment was sent at the time the “time-wait” state was entered, a TCP &lt;RST&gt; segment  437  can be sent in response to all non-&lt;SYN&gt; segments received from the local host for the connection. If a TCP &lt;SYN&gt; segment  401  is received, TSK  280  can process the &lt;SYN&gt; segment  401  in the same manner it does for a &lt;SYN&gt; segment  401  received in the “closed” state, with the exception that TSK  280  can reuse the existing CCB  1608  rather than allocate a new CCB  1608 . Note that the “closed” state is a logical state, i.e. it represents the state when there is no CCB  1608  allocated for a TCP connection. A TCP connection&#39;s state variable is not actually set to “closed”. TSK  280  cannot assume that the CCB  1608  (and, hence, CID) being used by its TSK peer has not already been deallocated. Even if the two TSK peers are using the same value for the “time-wait” timeout, propagation delay makes it likely that the two TSK peers do not start their “time-wait” timers at the same time. 
     Whenever the “time-wait” state is entered, the “time-wait” timer is started. The “time-wait” timeout is configured as part of a TCP connection&#39;s selected TCP spoofing parameter profile. When the “time-wait” timer expires, the CCB  1608  can be moved from its backbone connection&#39;s active CCB list to the CCB free pool. 
     When TSK  280  receives a TCP &lt;RST&gt; segment  437  from a host, it can terminate the TCP connection being spoofed. The buffers from any data segments not acknowledged or not transmitted can be freed, a CT message  435  is sent to the TSK peer (with a reason code indicating “&lt;RST&gt; segment  437  received from local host”), the connection can be closed and the connection&#39;s CCB  1608  is deallocated. 
     When the TSK peer receives the CT message  435 , it can free up the buffers from any data segments not acknowledged or not transmitted and sends a TCP &lt;RST&gt; segment  437  to its local host. TSK  280  then can close the connection and deallocates the connection&#39;s CCB  1608 .  FIG. 29  illustrates spoofed TCP connection &lt;RST&gt; segment  437  handling. 
     There are many TCP connection termination error scenarios which can be handled by the TSK  280 . There are also many error scenarios which can lead to the termination of TCP connections. The following describes some of these exemplary scenarios. 
     It is possible for both of the hosts involved in a TCP connection to decide to terminate the connection. If this occurs, the TCP &lt;FIN&gt; segments  443  from each host may pass each other. With TCP spoofing in the present invention, this can result in TP messages  445  passing each other. This does not cause any problems because the TSK peers cannot tell that the TP messages  445  passed each other. Each TSK peer sees the normal case of receiving a TP message  445  after having sent a TP message  445 .  FIG. 30  illustrates a simultaneous normal connection termination. 
     It is also possible for both of the hosts involved in a TCP connection to decide to terminate the connection with one (or both) of the hosts sending a TCP &lt;RST&gt; segment  437  rather than a &lt;FIN&gt; segment  443 . With TCP spoofing in the present invention, this can result in a TP message  445  passing a CT message  435  (or two CT messages  435  passing each other). This also does not cause any problems. The TSK  280  receiving the &lt;RST&gt; segment  437  can close the connection when it sends the CT message  435  and, thus, can simply discard the TP  445  (or CT  435 ) message when it is received. A TSK  280  receiving a &lt;FIN&gt; segment  443  can send a &lt;RST&gt; segment  437  to its local host and close the connection when it receives the CT message  435 .  FIG. 31  illustrates exemplary scenarios involving termination with TCP &lt;RST&gt; segments  437 . 
     After TSK  280  has both sent and received a TCP &lt;FIN&gt; segment  443 , it can enter the “time-wait” state. However, because it is possible for the delivery of data to be delayed to a local host, it is possible for one of the TSK peers to enter the “time-wait” state while the other TSK peer is still trying to deliver data. In an extreme case, it is possible for one TSK&#39;s “time-wait” timer to expire for the TCP connection, freeing the TCP connection&#39;s CCB  1608 , while its TSK peer is still trying to deliver data. This can be avoided by requiring a final handshake between the TSK peer&#39;s prior to exiting the “time-wait” state. But, the extra overhead for this handshake is not desirable, especially given that this scenario should be rare. This still would not prevent the local host which has also both sent and received a &lt;FIN&gt; segment  443  from exiting its “time-wait” state. 
     Not including a final handshake can introduce some additional error scenarios which can be handled by TSK  280 . One scenario is that a CCB  1608  becomes available at one TSK peer without being available at the other. This could lead to an unsuccessful attempt to spoof a new TCP connection. However, this case can already be handled for other reasons by TSK  280 . 
     A slightly more complicated scenario arises if the local host tries to restart the exact same TCP connection which existed before. In this case, the TSK peer which was not able to terminate the connection promptly could receive a CR message  403  for an existing connection while it is still delivering the data from the previous incarnation of the connection. TSK  280  can recognize that this has occurred because it uses the CCB hash function  1702  on the information provided in the TCP connection header  1201  of the CR message  403  to search for an existing CCB  1608 . This check can prevent TSK  280  from trying to bring up a second instance of the same TCP connection. When TSK  280  receives a CR message  403  prior to successful termination of the previous incarnation of a connection, it can reject the CR message  403  by sending a CT message  435  to its TSK peer (with a reason code indicating “previous incarnation of this connection still pending”). The TSK peer then can handle the reception of a CT message  435  in response to a CR message  403  in the same manner that it does for other reasons for receiving a CT message  435 . TSK  280  can continue to reject any new attempts to restart the same connection until it either successfully delivers the data or it tears down the connection due to an unrecoverable error (e.g. it reaches the maximum retry count while trying to retransmit a data segment).  FIG. 32  illustrates one of many exemplary versions of the premature connection restart scenario. 
     TSK  280  resources (e.g. CCBs) are generally very valuable because they are limited. Thus, it is desirable for TSK  280  to be able to detect when a host has died in order to free up the resources being used by the TCP connections associated with the host. Therefore, when the TSK  280  sends a TCP segment to its local host which requires a response, it can start a response timer to wait for the response. If no response is received before the timer expires, TSK  280  retransmits the TCP segment for which it is waiting for a response. TSK  280  can retransmit the TCP segment N times, where the retry count, N, is either a compile time constant or an operator configurable parameter. If TSK  280  does not receive an appropriate response after N retries, it can conclude that the host is not reachable. It will then send a TCP &lt;RST&gt; command  437  to the local host and a CT message  435  (with a reason code indicating “no response from local host”) to its TSK peer. TSK  280  then can free up the buffers of any data segments not acknowledged or not transmitted and closes the connection, deallocating the connection&#39;s CCB  1608 . 
     When the TSK peer receives the CT message  435 , it can send a TCP &lt;RST&gt; segment  437  to its local host, if appropriate, free up the buffers from any data segments not acknowledged or not transmitted and close the connection, deallocating the connection&#39;s CCB  1608 . 
       FIG. 33  illustrates no response from local host handling for spoofed TCP connections in the data transfer state.  FIGS. 25 and 26 , discussed above, illustrate other no response from host error scenarios. 
     The response timer can provide a mechanism for detecting that a host has died when there is data (or some other message) outstanding. However, if a TCP connection is idle, i.e. if there is currently no data being transferred on the connection, the response timer may not be running. The host having died could even be the reason why there is no data being carried. To detect when hosts associated with idle connections die, the TSK  280  can run a keepalive timer whenever a connection is idle. When the keepalive timer expires, TSK  280  can send a keepalive message to the local host. The length of the keepalive timer is configured as part of a connection&#39;s selected TCP spoofing parameter profile and can range from minutes to hours. The sending of keepalive messages can also be completely disabled. After sending a TCP keepalive message, the TSK  280  can start its response timer and follows the procedure to detect if the local host is still alive. 
     A TCP keepalive message can be a zero length TCP data segment, sent with a sequence number equal to the last byte already sent and acknowledged. When the host receives this data segment, it can discard it (because it represents data already received and acknowledged). However, the host can respond to this data segment with an &lt;ACK&gt; segment just in case the data segment was resent because its last &lt;ACK&gt; segment was lost. TCP keepalive messages are described in RFC  1122 , the entire contents of which is hereby incorporated by reference. 
     Backbone connections may fail due to link failures between two PEP End Points  705  or because of a failure of a PEP End Point  705  itself. If at any time the BPK  282  indicates (via the environment  210 ) that a backbone connection has failed, TSK  280  can terminate all of the TCP connections which are associated with the failed backbone connection. For each such TCP connection, TSK  280  can send a TCP &lt;RST&gt; segment  437  to its local host, free up the buffers from any data segments not acknowledged or not transmitted and close the connection, deallocating the connection&#39;s CCB  1608 . TSK  280  can also terminate the TCP connections which are associated with a backbone connection which it needs to close for some reason (e.g. because the operator deleted it). 
     TSK  280  can be provided periodically, by the platform environment  210 , with a background processing opportunity. One of the functions performed by TSK  280  during its background processing opportunity can be to check for TCP connection timeouts. In one exemplary embodiment, whenever TSK  280  starts a timer, it can set the expiration time of the timer to be equal to the timeout value plus the current system time. To check to see if a timer has expired, TSK  280  can compare the expiration time of the timer to the current system time provided by the platform environment  210 . System time may be represented in terms of timer ticks. The platform environment  210  converts timeout parameters provided (in units of deciseconds) by the network manager into system tick counts prior to passing the parameters to TSK  280 . 
     TSK  280  can make use of the CCB mapping table  1802  to find CCBs  1608  which need timer processing. TSK  280 , in a round robin fashion, can walk through the mapping table  1802  (as an array) looking for valid CCB pointers. When a valid CCB pointer is found, TSK  280  can check the CCB  1608  to see if any timeouts have occurred and, if so, processes the timeouts. To keep from holding onto control of the CPU for too long a period of time, TSK  280  can limit the number of CCBs  1608  it checks and the number of timeouts it processes during each background call. The specific limits may be different for each PEP End Point platform. 
     The PEP End Point platform environment  210  passes an IP packet received from the local LAN interface  803 ,  903 ,  1003 ,  1103  which has a protocol type of TCP to TSK  280  if TCP spoofing is globally enabled and the TCP segment meets one or both of the following conditions:
     A TCP connection control block exists for the TCP connection to which the TCP segment belongs;   The TCP segment is a &lt;SYN&gt; segment  401 , destined to a non-local destination. (Local TCP connections, for example, TCP connections between hosts located local to the LAN ports of a VSAT, and TCP connections which are not associated with a known non-local destination subnet are not forwarded to TSK  280 .)   

     A TCP segment which does not match either condition is either forwarded locally or of forwarded unspoofed (as appropriate) by the platform environment  210 . TSK  280  applies selection criteria to TCP &lt;SYN&gt; segments  401  to determine whether or not a connection should be spoofed. For all TCP segments, if a connection control block exists for the connection then the connection has already had selection criteria applied. This includes &lt;SYN&gt; segments  401 . TCP &lt;SYN&gt; segments  401  generally mark the initiation of a new connection but, from the point of view of the TCP state machine, a &lt;SYN&gt; segment  401  can be received in the middle of an existing connection to resynchronize or restart the connection. TCP &lt;SYN&gt; segments  401  which fail the selection criteria are returned back to the platform environment  210  to be forwarded unspoofed. The subsequent TCP segments received for these TCP connections will then be received with no CCB  1608  present and, thus, will also be forwarded unspoofed. 
     There are at least five exemplary criteria which can be specified by the operator in a selective TCP spoofing rule. The first exemplary criterion is destination IP address. Selective TCP spoofing can be performed based on destination IP addresses. A mask can be associated with each IP address to support multiple addresses matching a single rule. For example, a mask of 0.0.0.255 with an address of 0.0.0.1 could be used to select any IP address of the form x.x.x.1 and a mask of 255.255.255.0 with an address of 10.1.1.0 could be used to select all IP addresses in the 10.1.1.0 subnet. A mask of 0.0.0.0 represents a “don&#39;t care” value for the IP address field, i.e., a mask of 0.0.0.0 matches all IP addresses. 
     A second exemplary criterion is source IP address. Selective TCP spoofing can be performed based on source IP addresses. As with destination IP addresses, a mask can be associated with each IP address to support multiple addresses matching a single rule. 
     A third exemplary criterion is TCP port number. Selective TCP spoofing can be performed based on TCP port numbers. TCP port numbers can identify the type of application being carried by a TCP connection. Currently assigned TCP port numbers can be tracked at the following location:
 
http://www.isi.edu/in-notes/iana/assignments/port-numbers.
 
     Port number rules can apply to both the TCP destination and source port numbers, i.e. a TCP port number rule can apply if either the destination or source port number matches. A value of 0 is used as the “don&#39;t care” value for the TCP port number fields, i.e., a port number value of 0 in a rule matches all TCP port numbers. 
     A fourth exemplary criterion is TCP options. Selective TCP spoofing can be performed based on the TCP options which are present. 
     A fifth exemplary criterion is IP DS field. Selective TCP spoofing can be performed based on the differentiated services (DS) field in the IP header. A bit mask is used in conjunction with a configured DS field value in order to specify meaningful bits. A mask of 0 represents a “don&#39;t care” value for the DS field, i.e., a mask of 0 matches all DS field values. The use of the IP header DS field is described in RFCs  2474  and  2475 , the entire contents of which are hereby incorporated by reference. 
     In addition to supporting selective TCP spoofing rules for each of these criteria, AND and OR combination operators can also be supported to link the criteria together. For example, using the AND combination operator, a rule can be defined to disable TCP spoofing for FTP data (TCP port number  20 ) received from a specific host. Also, the order in which rules are specified can be significant. It is possible for a connection to match the criteria of multiple rules. Therefore, the TSK  280  can apply the rules in the order specified by the operator, taking the action of the first rule which matches. A default rule can also exist which defines the action to be taken for TCP connections which do not match any of the defined rules. 
     After checking the selective TCP spoofing rules to see if a TCP connection should be spoofed, before actually attempting to spoof the connection, a couple of additional checks can be made:
     Is there a CCB  1608  available?   Is the appropriate backbone protocol connection to the destination PEP End Point up?   If there is no CCB  1608  available or the backbone protocol connection to the PEP End Point  705  associated with the destination IP address of this connection is down, spoofing is probably not possible.   

     Selective TCP spoofing rules can be defined by the operator in a selective TCP spoofing selection profile  3402 . PEP End Points  705  can then be defined to use a particular TCP spoofing selection profile  3402 . All of the PEP End Points  705  in a network can be configured to use the same profile. Or, different profiles may be used by different subsets of PEP End Points  705 . There is no requirement that the same TCP spoofing selection profile be used by the two PEP End Points  705  at the ends of a backbone connection. Not requiring that the same selective TCP spoofing rules be used by each PEP End Point  705  allows the operator to add one more dimension to the rules. The added dimension can be allowing selective TCP spoofing to be based on which end of the backbone link the connection originates from. For example, an FTP TCP connection originating from a remote site can be treated differently than an FTP TCP connection originating from the hub site. This capability can be useful but should be used with care to avoid unexpected side effects regarding what does and does not get spoofed. 
     Selective TCP spoofing rules can be used to select an appropriate TCP spoofing selection profile  3042 . A TCP spoofing parameter profile  3404  can indicate whether connections which are mapped to the profile should be spoofed and, if so, defines the spoofing parameters which should be applied to the connections which match the rule. TCP spoofing parameter profile  3404  parameters include (but are not necessarily limited to):
     Three-way handshake spoofing—This parameter indicates whether or not three-way handshake spoofing is enabled or disabled;   Connection priority—This parameter indicates the priority which should be used for connections which are selected to this profile.   Priority disposition—This parameter indicates the appropriate handling, discard or forward unspoofed, of TCP connections which map to this TCP spoofing profile if a backbone connection at the indicated connection priority is not currently available;   TCP protocol related parameters—These parameters are related to TCP protocol operation and include the maximum MSS which should be used, keepalive, response and time-wait timeouts, etc.   

     As indicated above, a TCP spoofing parameter profile  3404  can also indicate whether or not TCP spoofing is enabled. However, since all of the rest of the TCP spoofing parameter profile parameters may be meaningless when TCP spoofing is disabled, only one TCP spoofing parameter profile  3404  with TCP spoofing disabled is needed. Rather than require that such a profile be created, an “unspoofed” profile can always exist for selection by a selective TCP spoofing rule. This approach can eliminate the need for an explicit TCP spoofing enabled or disabled parameter in the TCP spoofing parameter profile. TCP spoofing can always be enabled in every TCP spoofing parameter profile except the “unspoofed” profile. 
     A TCP spoofing selection profile  3042  may include a default rule which the operator can configure to select the TCP spoofing parameter profile  3404  which should be used for any TCP connections which do not match any of the configured selective TCP spoofing rules. The default value may select the “unspoofed” TCP spoofing parameter profile if TCP connections which do not match any of the selective TCP spoofing rules should not be spoofed. 
       FIG. 34  illustrates an exemplary relationship between PEP End Points  905 , TCP spoofing selection profiles  3402  and TCP spoofing parameter profiles  3402 . TCP spoofing parameter profiles  3402  can be referenced by name by the operator when they are configured but the names can be converted into index numbers for use by TSK  280 . An index number of  255  may be used to indicate the “unspoofed” TCP spoofing parameter profile  3502 . This is illustrated in exemplary  FIG. 35 . 
     When TSK  280  spoofs the three way handshake by responding to a TCP &lt;SYN&gt; segment  401  without waiting for a response to be received from the distant host, it can send an operator configured maximum segment size to the host in an MSS option in the TCP &lt;SYN,ACK&gt; segment  403 . Later, after its TSK peer completes its local three-way handshake, TSK  280  can receive the maximum segment size value sent by the distant host to the TSK peer. If the distant host indicates an MSS smaller than the MSS sent to the local host by TSK (for example, TSK sent an MSS option of  1460  while the distant host sent an MSS option of  536 ), a maximum segment size mismatch exists. 
     A maximum segment size mismatch can lead to TSK  280  receiving a TCP data segment from the backbone connection which is larger than the maximum segment size advertised by its local host. TSK  280  could just forward it anyway. And, in many cases, this would work because the host&#39;s buffer size is actually equal to the MTU. However, in some cases, the larger segment size would overflow the host&#39;s buffer causing the segment to be discarded. Each retransmission of the segment would be similarly discarded. 
     To avoid this problem, TSK  280  includes a capability to resize data segments it receives from the its TSK peer before sending them to the local host. This capability is described below. However, resizing segments requires CPU and may add complexity to the TSK  280 . TSK  280  can depend upon operator configuration to prevent the problem. The operator configures the MSS to be used for a connection in a TCP spoofing parameter profile and uses selection rules to map the connection to the profile. 
     While knowing which hosts to map to which MSS values seems somewhat labor intensive (appearing to require the operator to determine this value for all of the hosts of the network), in practice, it is not that difficult. First, most TCP implementations (with the number increasing over time) support an MSS which is based on the MTU of the paths being used by the connection with the default value being the MSS (1460 bytes) supported by the Ethernet MTU (1500 bytes). In addition, when a host initiates a TCP connection, it sends an MSS option in its TCP &lt;SYN&gt; segment  401 . Thus, TSK  280  learns the MSS of the initiating host when it attempts to bring up a connection. It is only the responding host&#39;s MSS which needs to be “guessed” by TSK  280  in order to spoof the TCP three-way handshake. A TSK peer which receives a CR message  403  can use the MSS value in the CR message to initiate the connection on its side of the link. In general, since most VSAT-based intranet TCP connections are initiated from client site applications (e.g. web browsers), this means that it is only the server side hosts for which the operator needs to know the MSS values supported. And, since the server site hosts tend to be servers, the server sites hosts are likely to support the maximum (for Ethernet) MSS value of 1460 bytes. 
     However, even though configuring the MSS should not be difficult, spoofing in the present invention includes an additional mechanism which can be used in cases where the operator cannot be sure of the MSS. This mechanism is the ability to disable three-way handshake spoofing, allowing the MSS of a connection to be determined end to end. The operator can disable three-way handshake spoofing via a parameter in the TCP spoofing parameter profile  3404 . By default, three-way handshake spoofing will be enabled to gain the performance benefits it provides. 
     As indicated above, spoofing the three-way handshake can potentially lead to MSS mismatch if the PEP End Point  705  advertises a larger MSS in the TCP &lt;SYN,ACK&gt; segment  405  it sends than the distant host ultimately advertises in its &lt;SYN,ACK&gt; segment  405 . 
     To dynamically recover from an MSS mismatch, if TSK  280  receives a TCP data segment from the backbone connection which is larger than the maximum segment size that it can send to the local host on the connection, TSK  280  could break the segment into several smaller segments before forwarding them to the local host. This could be done at the TCP layer, not at the IP layer, i.e., this is not done via IP packet fragmentation. 
     Breaking a large segment into two or more smaller segments is illustrated in  FIG. 36 . With three-way handshake spoofing enabled, TSK  280  will respond to a TCP &lt;SYN&gt; segment  401  with a TCP &lt;SYN,ACK&gt; segment  461  which indicates an MSS value equal to the value configured in the selected TCP spoofing parameter profile  3404 . If the remote host  406  responds to the TCP &lt;SYN&gt; segment  415  sent by the remote PEP End Point  404  with a TCP &lt;SYN,ACK&gt; segment  467  which has an MSS less than that sent by the local PEP End Point  402  to the local host  400 , the local host  400  may send a TCP data segment  463  which is larger than the MSS sent by the remote host  406 . When this occurs, the TSK  280  in the remote PEP End Point  404  must break the large TD message  465  into multiple smaller TCP data segments  469 ,  471  before forwarding them to the remote host  406 . 
     Breaking larger segments into small enough segments guarantees that the segments will be accepted by the local host. But, breaking larger segments into smaller segments does have a CPU penalty associated with it for the PEP End Point  705 . The extent of the CPU penalty for a particular PEP End Point platform can be dependent on the buffering strategy used with the platform. For example, if a chain of small physical buffers is used, breaking a large segment into smaller segments mainly involves breaking the buffer chain in appropriate places and can be done with no data copies. However if a single large physical buffer is used, breaking a larger segment into smaller segments would involve data copies. 
     While maximum segment size mismatches incur a CPU penalty, sending larger segments over the backbone connection improves bandwidth efficiency by reducing header overhead. In some cases, it may be more desirable to use bandwidth efficiently at the expense of the use of CPU in the PEP End Points  705 . To this end, if segment resizing is implemented, the operator may intentionally configure the MSS to be used for a TCP connection to be larger than the MSS actually supported by the host. 
     If an MSS mismatch occurs, if configured to do so by the operator, the TCP Spoofing Kernel  280  could attempt to get the sending end host to reduce the size of the MSS it is using to send data across the wide area network. A mechanism which could be used to do this is based on the Path MTU (PMTU) discovery algorithm described in RFC  1191 , the entire contents of which are incorporated by reference. PMTU discovery works by setting the “Don&#39;t Fragment” bit in the IP header of all of the IP packets being sent, including TCP data segments. Then, if a packet is received by a router which is too big for the MTU of the next hop of the path, the router sends an ICMP “Datagram Too Big” message. The ICMP “Datagram Too Big” message also includes an indication of the next hop MTU. This allows the host to break the original TCP data segments into smaller segments and resend them. 
     To address a maximum segment size mismatch, TSK  280  could attempt to invoke the PMTU discovery behavior of the end host. If TSK  280  receives a CE message  419  from its TSK peer with an MSS less than the MSS it has already locally sent to the end host, TSK  280  would generate an ICMP “Datagram Too Big” message to the host indicating an MTU value which corresponds to the smaller MSS. Hopefully, the host would react by starting to send smaller TCP data segments. If it does not, the TSK  280  peer could be forced to continuously break larger segments into smaller segments. But, this is no different than if no ICMP message was sent. Note that even when the ICMP “Datagram. Too Big” message does work, the end host may have already sent some large TCP data segments which would have been locally acknowledged by TSK  280 . These large data segments would need to be broken into smaller segments before being transmitted to the destination host. But, the CPU to do this would only need to be expended for the first few data segments. 
     Reducing the maximum segment size being used by the host is illustrated in  FIG. 37 . When the local PEP End Point  402  receives the CE message  473  which indicates that a smaller MSS is being used by the remote host  406  than was indicated to the local host  400 , TSK  280  can send an ICMP “Datagram Too Big” message  475  indicating that the MTU of the network is only equal the MSS sent by the remote host  406  plus the size the IP and TCP headers (40 bytes). The local host  400  can then start sending TCP data segments  477  which match the MSS being used by the remote host  406 . 
     As indicated above, attempting to reduce the maximum segment size being sent by a host would be controlled by operator configuration. By default, MSS reduction would be enabled. But the operator could disable it for specific connections by means of the TCP spoofing parameter profiles selected for the connections. The operator might choose to turn off the MSS reduction ICMP messages either because of some unforeseen side effect of their use with particular hosts or because of a decision to expend the CPU in the PEP End Points  705  for breaking large segments into smaller segments in order to gain the wide area bandwidth efficiency associated with using larger packets over the WAN. 
     If, for some reason, a host ignores the MSS value sent to it by TSK and sends a segment larger than the configured MSS to TSK, TSK can discard the segment. This may eventually cause the TCP connection to fail, requiring operator intervention to correct. But, not discarding such segments could cause the backbone connection to fail and a backbone connection failure will affect many TCP connections instead of just one. 
     When TSK  280  needs to determine a window size to advertise in a TCP segment, it starts by calling the platform environment  210  to get the current LAN to WAN buffer space availability for the backbone connection associated with the spoofed TCP connection. TSK  280  then divides this number by the number of TCP connections which are currently using the backbone connection. (TSK keeps track of the number of TCP connections using a backbone connection in the backbone connection&#39;s TCB, incrementing the count whenever a CCB is allocated and decrementing the count whenever a CCB is deallocated.) TSK  280  then converts this value from buffers into bytes by multiplying the number of buffers by the MSS being used by the local host to send TCP segments TSK  280 . This value represents the potential window size which can be advertised. However, TSK  280  must make two additional checks before using this value. First, the potential value is compared to the window size limit. If the potential value is larger than the window size limit, the window size limit is advertised instead. If the potential value is smaller than the window size limit, TSK  280  then checks to see if advertising the potential value would shrink the window to a value smaller than previously advertised (i.e., would move the right edge of the rotating window to the left). A TCP receiver is not supposed to shrink its window. Therefore, if the potential value would shrink the window, TSK  280  instead advertises the smallest possible window which does not shrink the previously advertised window (i.e., the value which represents keeping the right edge of the window in the same place). 
       FIG. 38  illustrates a computer system  3601  upon which an embodiment according to the present invention may be implemented. Such a computer system  3601  may be configured as a server to execute code that performs the PEP functions of the PEP end point  210  as earlier discussed. Computer system  3601  includes a bus  3603  or other communication mechanism for communicating information, and a processor  3605  coupled with bus  3603  for processing the information. Computer system  3601  also includes a main memory  3607 , such as a random access memory (RAM) or other dynamic storage device, coupled to bus  3603  for storing information and instructions to be executed by processor  3605 . In addition, main memory  3607  may be used for storing temporary variables or other intermediate information during execution of instructions to be executed by processor  3605 . Main memory  3607  may also be used to share PEP control blocks and buffers used to store packets. Computer system  3601  further includes a read only memory (ROM)  3609  or other static storage device coupled to bus  3603  for storing static information and instructions for processor  3605 . A storage device  3611 , such as a magnetic disk or optical disk, is provided and coupled to bus  3603  for storing information and instructions. 
     Computer system  3601  may be coupled via bus  3603  to a display  3613 , such as a cathode ray tube (CRT), for displaying information to a computer user. An input device  3615 , including alphanumeric and other keys, is coupled to bus  3603  for communicating information and command selections to processor  3605 . Another type of user input device is cursor control  3617 , such as a mouse, a trackball, or cursor direction keys for communicating direction information and command selections to processor  3605  and for controlling cursor movement on display  3613 . 
     Embodiments are related to the use of computer system  3601  to perform the PEP functions of the PEP end point  210 . According to one embodiment, this automatic update approach is provided by computer system  3601  in response to processor  3605  executing one or more sequences of one or more instructions contained in main memory  3607 . Such instructions may be read into main memory  3607  from another computer-readable medium, such as storage device  3611 . Execution of the sequences of instructions contained in main memory  3607  causes processor  3605  to perform the process steps described herein. One or more processors in a multi-processing arrangement may also be employed to execute the sequences of instructions contained in main memory  3607 . In alternative embodiments, hard-wired circuitry may be used in place of or in combination with software instructions. Thus, embodiments are not limited to any specific combination of hardware circuitry and software. 
     The term “computer-readable medium” as used herein refers to any medium that participates in providing instructions to processor  3605  for execution the PEP functions of the PEP end point  210 . Such a medium may take many forms, including but not limited to, non-volatile media, volatile media, and transmission media. Non-volatile media includes, for example, optical or magnetic disks, such as storage device  3611 . Volatile media includes dynamic memory, such as main memory  3607 . Transmission media includes coaxial cables, copper wire and fiber optics, including the wires that comprise bus  3603 . Transmission media can also take the form of acoustic or light waves, such as those generated during radio wave and infrared data communications. 
     Common forms of computer-readable media include, for example, a floppy disk, a flexible disk, hard disk, magnetic tape, or any other magnetic medium, a CD-ROM, any other optical medium, punch cards, paper tape, any other physical medium with patterns of holes, a RAM, a PROM, and EPROM, a FLASH-EPROM, any other memory chip or cartridge, a carrier wave as described hereinafter, or any other medium from which a computer can read. 
     Various forms of computer readable media may be involved in carrying one or more sequences of one or more instructions to processor  3605  for execution. For example, the instructions may initially be carried on a magnetic disk of a remote computer. The remote computer can load the instructions relating to execution of the PEP functions of the PEP end point  210  into its dynamic memory and send the instructions over a telephone line using a modem. A modem local to computer system  3601  can receive the data on the telephone line and use an infrared transmitter to convert the data to an infrared signal. An infrared detector coupled to bus  3603  can receive the data carried in the infrared signal and place the data on bus  3603 . Bus  3603  carries the data to main memory  3607 , from which processor  3605  retrieves and executes the instructions. The instructions received by main memory  3607  may optionally be stored on storage device  3611  either before or after execution by processor  3605 . 
     Computer system x01 also includes one ore more communication interfaces  3619  coupled to bus  3603 . Communication interfaces  3619  provide a two-way data communication coupling to network links  3621  and  3622  which are connected to a local area network (LAN)  3623  and a wide area network  3624 , respectively. A WAN  3624 , according to one embodiment of the present invention, may be a satellite network. For example, communication interface  3619  may be a network interface card to attach to any packet switched LAN. As another example, communication interface  3619  may be an asymmetrical digital subscriber line (ADSL) card, an integrated services digital network (ISDN) card, a cable modem, or a modem to provide a data communication connection to a corresponding type of telephone line. Wireless links may also be implemented. In any such implementation, communication interface  3619  sends and receives electrical, electromagnetic or optical signals that carry digital data streams representing various types of information. 
     Network link  3621  typically provides data communication through one or more networks to other data devices. For example, network link  3621  may provide a connection through local area network  3623  to a host computer  3625  or to data equipment operated by an Internet Service Provider (ISP)  3627 . ISP  3627  in turn provides data communication services through the Internet  505 . In addition, LAN  3623  is linked to an intranet x29. The intranet x29, LAN x23 and Internet  505  all use electrical, electromagnetic or optical signals that carry digital data streams. The signals through the various networks and the signals on network link  3621  and through communication interface  3619 , which carry the digital data to and from computer system  3601 , are exemplary forms of carrier waves transporting the information. 
     Computer system  3601  can send messages and receive data, including program code, through the network(s), network link  3621  and communication interface  3619 . In the Internet example, a server x31 might transmit a requested code for an application program through Internet  505 , ISP  3627 , LAN  3623  and communication interface  3619 . 
     The received code may be executed by processor  3605  as it is received, and/or stored in storage device  3611 , or other non-volatile storage for later execution. In this manner, computer system  3601  may obtain application code in the form of a carrier wave. 
     Computer system  3601  can transmit notifications and receive data, including program code, through the network(s), network link  3621  and communication interface  3619 . 
     The techniques described herein provide several advantages over prior approaches to improving network performance, particularly in a packet switched network such as the Internet. A local PEP end point and a remote PEP end point communicate to optimize the exchange of data through a TCP spoofing functionality. Ease of configuration of the end points is provided through the use of profiles. 
     Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.