Abstract:
An acceleration apparatus is adapted to operate in a direct mode and a proxy mode. In the direct mode, the acceleration apparatus decrypts data packets received from a client and forwards the decrypted data packets to a server using a communication session negotiated by the client and the server. In the proxy mode, the acceleration apparatus responds to the client on behalf of the server and forwards the decrypted data packets to the server using a communication session negotiated by the acceleration device and the server. The acceleration apparatus automatically switches from the direct mode to the proxy mode upon detection of a communication error associated with the communication session negotiated by the client and the server.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to improving the performance of secure communications between network-coupled devices, such as computers. In particular, to improving performance of secure communications using the Secure Sockets Layer (SSL) protocol between a client and a server communicating across an open source, global communications network such as the Internet. 
     2. Description of the Related Art 
     Many commercial and consumer networking applications require secure communications over a network. In particular, on the Internet, electronic commerce must be performed in a secure communications environment. Currently, the default standard for secure communications between a Web client and a Web server is the Secure Sockets Layer protocol or SSL, developed by Netscape Communications Corporation, Mountain View, Calif. 
     Virtually all online purchases and browser-based monetary transactions that occur on the Internet are secured by SSL. However, SSL is not just limited to securing e-commerce. Financial institutions implement SSL to secure the transmission of PIN numbers and other confidential account information. Insurance companies implement SSL to secure transmission of confidential policy information. Organizations who have established Business-to-Business (B2B) extranets implement SSL to secure transactions between the company and its partners, suppliers, and customers. Private organizations implement SSL in their intranets to confidentially transfer information to and from employees. 
     The process of SSL encryption and decryption is computationally intensive on the server and the client communicating via SSL. For the client, typically performing only one SSL communication session, this intensity is not a problem. However, for the server performing multiple sessions, SSL CPU overhead can be a significant problem. Many security-sensitive Web sites that have implemented SSL experience bottlenecks created by the managing and processing of SSL sessions. The end-result is that SSL degrades Web server performance considerably and Web transactions are slowed to a crawl. 
     In general, SSL is comprised of two protocols: the SSL Handshake protocol and the SSL Record protocol. An SSL transaction consists of two distinct parts: the key exchange, and the bulk data transfer. The SSL Handshake Protocol handles key exchange and the SSL Record Protocol handles the bulk data transfer. The key exchange begins with an exchange of messages called the SSL handshake. During the handshake, the server authenticates itself to the client using public-key encryption techniques. Then, the client and the server create a set of symmetric keys that they use during that session to encrypt and decrypt data and to detect if someone has tampered with the data. The SSL handshake also allows the client to authenticate itself to the server (as would be required for an on-line banking operation, for example). 
     Besides authenticating the server to the client, the SSL Handshake Protocol: allows the client and server to negotiate the cipher suite to be used; allows the client and the server to generate symmetric session keys; and establishes the encrypted SSL connection. Once the key exchange is complete, the client and the server use this session key to encrypt all communication between them. They perform this encryption with a symmetric key encryption algorithm, such as RC4 or DES. This is the function of the SSL Record Protocol. 
     Generally, the request for an SSL session comes from the client browser to the Web server. The Web server then sends the browser its digital certificate. The certificate contains information about the server, including the server&#39;s public key. Once the browser has the server&#39;s certificate, the browser verifies that certificate is valid and that a certificate authority listed in the client&#39;s list of trusted certificate authorities issued it. The browser also checks the certificates expiration date and the Web server domain name. Once a browser has determined that the server certificate is valid, the browser then generates a 48-byte master secret. This master secret is encrypted using server&#39;s public key, and is then sent to the Web server. Upon receiving the master secret from the browser, the Web server then decrypts this master secret using the server&#39;s private key. Now that both the browser and the Web server have the same master secret, they use this master secret to create keys for the encryption and MAC algorithms used in the bulk-data process of SSL. Since both participants used the same master key, they now have the same encryption and MAC key, and use the SSL encryption and authentication algorithms to create an encrypted tunnel through which data may pass securely. 
     An SSL session may include multiple secure connections; in addition, parties may have multiple simultaneous sessions. The session state includes the following elements: a session identifier (an arbitrary byte sequence chosen by the server to identify an active or resumable session state); a peer certificate (an X509.v3[X509] certificate of the peer); a compression method; a cipher spec (the bulk data encryption algorithm (such as null, DES, etc.) and a MAC algorithm (such as MD5 or SHA)); a master secret (a 48-byte secret shared between the client and server); an “is resumable” flag (indicating whether the session can be used to initiate new connections). The connection state includes the following elements: server and client random byte sequences that are chosen by the server and client for each connection; server write MAC secret used in MAC operations on data written by the server; client write MAC secret used in MAC operations on data written by the client; a server write key; a client write key; initialization vectors maintained for each key and initialized by the SSL handshake protocol; and sequence numbers maintained by ach party for transmitted and received messages for each connection. When a party sends or receives a change cipher spec message, the appropriate sequence number is set to zero. 
     When a number of Web clients are connecting to a particular Web site having a number of servers, each server will be required to handle a number of clients in the secure transaction environment. As a result, the processing overhead that is required by each server to perform to the secure sockets layer encryption and decryption is very high. If this were the only solution to providing secure communications protocols between the client and server, each transactional Web site would be required to provide an large number of servers to handle to the expected traffic. 
     Accordingly, a solution has been developed to provide an acceleration device as a built-in expansion card in the server or as a separate stand-alone device on the network. The accelerator provides SSL encryption and offloads the processing task of encryption and decryption for the client using SSL from the server. A general representation of this solution is shown in  FIG. 1 . 
       FIG. 1  shows a Web client  100  coupled to the Internet  50  that may be coupled via a router  75  to an SSL accelerator device  250 . The SSL accelerator device  250  is coupled to a plurality of Web servers  300 . Generally, a secure SSL session with encrypted traffic is first established between SSL accelerator  120  and the Web client. Communication between the SSL accelerator  250  and the Web servers  300  occurs as clear text traffic. Hence, a secure network must connect the Web servers  300  and the SSL accelerator  250 . 
     Commercial SSL acceleration devices include Rainbow&#39;s CryptoSwiftâeCommerce accelerator and F5&#39;s BIG IP e-Commerce Controller. Typically, commercially available SSL acceleration devices operate as shown in  FIG. 2A  and  FIG. 2B . In  FIG. 2A , the SSL accelerator is coupled between the Web client  100  and the Web server  300 . Communication between the SSL accelerator and the Web client occurs through a secure TCP protocol such as HTTPS. Communication between the SSL accelerator and the Web server occurs through clear HTTP/TCP protocol. 
       FIG. 2B  illustrates how SSL functions in the Open Systems Interconnect (OSI) Reference Model and in typical accelerators. The web client transmits data to the accelerator  250  in an encrypted form to the secure port  443  of the accelerator. In the client, the application layer protocol hands unencrypted data to the session layer; SSL encrypts the data and hands it down through the layers to the network IP layer, and on to the physical layers (now shown). Normally, a server will receive the encrypted data and when the server receives the data at the other end, it passes it up through the layers to the session layer where SSL decrypts it and hands it off to the application layer (HTTP). The same happens in the typical SSL accelerator within the accelerator, where the data is handed to the application layer, processed, then returned down the stack from the HTTP layer to the IP layer for transmission to port  80  (in the clear) on the server coupled to the SSL accelerator. Once at the server, the data returns up the stack for processing in the application layer. Since the client and the SSL device have gone through the key negotiation handshake, the symmetric key used by SSL is the same at both ends. 
     In essence, the HTTP packet must travel through the TCP stack four times, creating a latency and CPU overhead and requiring full TCP stack support in the accelerator. This also requires a great deal of random access memory, usually around 8-10 kB per TCP session, for retransmission support. This type of architecture also has scalability and fault tolerance problems because all of the TCP and SSL state databases are concentrated on one SSL accelerator device. 
     The device of the present invention overcomes these limitations by providing a packet based decryption mechanism and intercepting secure packets between a Internet coupled Web server and Internet coupled Web client. 
     SUMMARY OF THE INVENTION 
     In one aspect, the invention comprises an accelerator coupled between a client computer and a server computer, both of which are coupled to the Internet. The accelerator intercepts packet based communications between the client and the server, such as TCP/IP packet communications, and encrypts or decrypts data carried in the packets to reduce the workload of servers communicating in encrypted formats with a number of concurrent clients. In one advantageous implementation, the invention is utilized in a routing device positioned to conduct communications traffic between the client and the server. The invention finds particular usefulness in accelerating the secure sockets layer (SSL) protocol utilized in Internet commerce applications. 
     In a further embodiment, the invention comprises a communications encryption and decryption device, coupled to a first computer and a second computer via a network. The device may include a communications bus; a network interface coupled to the communications bus; a storage memory coupled to the bus; at least one processor coupled to the communications bus; a proxy module; and a packet-level encryption and decryption module. 
     In a still further embodiment, the invention comprises a method for encrypting and decrypting packet data in a network traveling between a client and a server. The method comprises the steps of: intercepting a packet communicated between the client and the server on an unsecured network; decrypting data in the packet; forwarding data in the packet to the server as clear text over a secure network; intercepting a packet communicated between the server and a client on a secure network; encrypting the data in the packet; and forwarding the encrypted data in the packet to the client over an unsecured network. 
     The present invention can be accomplished using hardware, software, or a combination of both hardware and software. The software used for the present invention is stored on one or more processor readable storage media including hard disk drives, CD-ROMs, DVDs, optical disks, floppy disks, tape drives, RAM, ROM or other suitable storage devices. In alternative embodiments, some or all of the software can be replaced by dedicated hardware including custom integrated circuits, gate arrays, FPGAs, PLDs, and special purpose computers. 
     These and other objects and advantages of the present invention will appear more clearly from the following description in which the preferred embodiment of the invention has been set forth in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described with respect to the particular embodiments thereof. Other objects, features, and advantages of the invention will become apparent with reference to the specification and drawings in which: 
         FIG. 1  is a block diagram illustrating the arrow usage of a SSL accelerator in accordance with the prior art. 
         FIG. 2A  is a block diagram illustrating the protocol connections scheme between a client, SSL accelerator, and Web server. 
         FIG. 2B  is a block diagram illustrating the computational exercise of SSL accelerator accordance with the prior art. 
         FIG. 3  is a block diagram illustrating the computational exercise of an SSL accelerator prepared in accordance with the present invention. 
         FIG. 4  is block diagram illustrating the initial TCP/IP connection between a client and a server. 
         FIG. 5  is a block diagram illustrating the sequence of communications in a first embodiment of the present invention between a client, an SSL accelerator device implementing a direct mode in accordance with the present invention, and a Web server. 
         FIG. 6  is a block diagram illustrating the sequence of communications and a second embodiment of the present invention between a client, an SSL accelerator device implementing a load balancing mode in accordance with the present invention, and a Web server. 
         FIG. 7  is a block diagram illustrating the sequence of communications and a second embodiment of the present invention between a client, an SSL accelerator device implementing a full TCP/IP and SSL proxy mode in accordance with the present invention, and a Web server. 
         FIG. 8  is a block diagram illustrating SSL multisegmentation. 
         FIGS. 9   a  and  9   b  are block diagrams illustrating the various modes of implementing the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention provides a unique system and method for implementing SSL acceleration, and indeed any encryption or decryption methodology, to offload to the computational overhead required with the methodology from a server or client. The invention is particularly suited to offloading encryption and decryption tasks from a server which is normally required to handle a multitude of concurrent sessions. The system may include an SSL acceleration device, which operates to intercept secure communications between, for example, a Web based Internet client such as a Web browser operating on a personal computer, and a Web server. The SSL acceleration device will intercept communications directed to the server and act as a proxy in various embodiments of the invention. In a first embodiment, the SSL acceleration device acts as a complete proxy, substituting itself for the server and both the TCP/IP handshaking sequence and the SSL encryption and decryption sequence. In a second embodiment, the SSL acceleration device passes through the TCP/IP handshaking sequence and performs only SSL proxy encryption and decryption. In yet another embodiment, a layer-7 switching interface is utilized between the server and the client in the accelerator device. In additional embodiments, both a full TCP/IP proxy mode and a pass through mode are used interchangeably. 
       FIG. 3  shows how the system of the present invention differs in general from that of the prior art, and illustrates the manner in which the SSL encryption and decryption proxy is implemented. Typically, when a Web client wishes to send data via a secure protocol to an SSL enabled Web server, it will do so by communicating via a secure port  443 . As shown in  FIG. 3 , in accordance with the present invention, the SSL accelerator will intercept data destined for port  443  of the web server and, rather than the transmitting packets up and down the TCP/IP stack as shown in  FIG. 2B , will perform the SSL encryption and decryption at the packet level before forwarding the packet on to its destination. The accelerator will thus decode the packet data and forward a clear text (HTTP) packet the HTTP port  80  of the Web server  300 . A number of operational modes of encryption and decryption, including a direct or pass-though mode, a load balancing mode, and a full proxy mode, are supported and the manner in which the system of the invention performs these tasks is hereinafter described. 
     It should be recognized that the system of the present invention may include a hardware device which may comprise a server add-in card, a network coupled device specifically constructed to perform the functions described herein, or a network coupled device having the capability of providing a plurality of functions, such as, for example, routing functions on network communications. In one embodiment, a dedicated device coupled to a network and suitable for performing the operations described herein will include network interface hardware, random access memory and a microprocessor. In an alternative embodiment, a hardware device may include a plurality of processors each with a dedicated memory or sharing a common memory, with one or more of the processors dedicated to one or more specific tasks, such as performing the SSL encryption and decryption needed to implement the present invention. One such device which is optimal for performing the method of the present invention is described in co-pending patent application Ser. No. 09/900,481 [NEXSI-01020USO entitled MULTI-PROCESSOR SYSTEM, inventors Gruner, et al., filed Jul. 6, 2001. It will be recognized that any number of hardware configurations are available to implement the system and method of the present invention. 
       FIG. 4  illustrates the typical TCI/IP handshake sequence. The “threeway handshake” is the procedure used to establish a TCP/IP connection. This procedure normally is initiated by one TCP device (in  FIG. 3 , the client) and responded to by another TCP device (in  FIG. 3 , the server). The procedure also works if two TCP simultaneously initiate the procedure. 
     The simplest TCP/IP three-way handshake begins by the client sending a SYN segment indicating that it will use sequence numbers starting with some sequence number, for example sequence number  100 . Subsequently, the server sends a SYN and an ACK, which acknowledges the SYN it received from the client. Note that the acknowledgment field indicates the server is now expecting to hear sequence  101 , acknowledging the SYN which occupied sequence  100 . The client responds with an empty segment containing an ACK for the server&#39;s SYN; the client may now send some data. 
     In the various embodiments of the present invention, the SSL accelerator system intercepts all communication intended for the server from the client and vice versa, in order to implement SSL when required. 
     The general system and method of the present invention will be described with respect to  FIGS. 5-7 . Various modes of the invention are illustrated. It should be understood that the methods illustrated in  FIGS. 5-7  are performed using instructions sets to direct performance of the aforementioned hardware, and that one objective of implementing the system is to minimize hardware requirements. 
       FIG. 5  illustrates a direct, cut through processing method. Packets from client to server are addressed from the client to the server and from server to client, with the intermediary, SSL device being transparent to both. In the embodiment shown therein, the SSL accelerator allows the client and server to negotiate the TCP/IP session directly, making only minor changes to the TCP/IP headers passing through the accelerator device, and tracking session data in a data structure in memory to enable SSL session handling to occur. As described herein, this mode is referred to herein as the “direct, cut-through” mode, since the client and server “think” they are communicating directly with each other, and the SSL accelerator is essentially transparent. 
       FIG. 6  illustrates a cut though, load balancing approach where the SSL device acts as a proxy for one or more servers, and the client recognizes the device as the server (i.e. packets from the client are addressed to the device, and the device handles passing of communications to the server via a secure network in an unencrypted format.) In this embodiment, TCP packets are re-addressed to the appropriate client or server by altering the address of the packet before forwarding. The SSL device acts as an SSL proxy for the server and may implement a load balancing function, appearing to all clients as a single server, while in reality directing traffic to a multitude of servers as illustrated in  FIG. 1 . 
       FIG. 7  illustrates a full proxy mode, wherein the SSL device acts as a proxy for one or more servers, and handles both the SSL and TCP communications for the server. 
     While  FIGS. 5-7  illustrate a single process of communication, it will be understood that multiple sessions similar to those illustrated in  FIGS. 5-7  may be occurring on a single SSL accelerator device. Moreover, it should be understood that various embodiments may likewise occur on a single device. 
     In the embodiment shown in  FIG. 5 , SSL accelerator device  250  intercepts communications between the client  100  and server  300 . Device  250  passes on the TCP/IP negotiation communications between the client  100  and the server  300 . 
       FIG. 5  illustrates a client device  100 , having an IP address of 1.1.1.1, attempting to establish an SSL session with server  300 , having an IP address of 3.3.3.3. The SSL accelerator device (SSLAD)  250  having an exemplary IP address of 2.2.2.2 will intercept traffic between client  100  and server  300  according to routing tables present on the Internet in accordance with well-known techniques. 
     Initially, the client  100  sends a SYN packet to TCP port  443  of server  300  (at step  202 ). The SYN packet will define, for example, an MSS of some length, for example, 1460 bytes. (As should be generally understood, the MSS is the maximum segment size and is a configurable TCP parameter allowing an understanding between the communicating devices of the maximum number of bytes of data allowed in a data packet; the default is 576 bytes.) The SSL Accelerator device  250  will intercept (at step  204 ) the SYN packet transmitted by client  100  (at step  202 ). The SSL Accelerator may also perform other functions on packet to enable the SSL acceleration device to continue to perform its SSL proxy functions. For example, the SSL accelerator may reduce the initially defined MSS value in the communication sequence between the client and server in order to accommodate headers and extensions utilized in the system of the present invention in the packet. MSS reduction takes place by, responding to the initial SYN packet from the client  100  with a setting in the options field of the TCP/IP header. For example, in the method of the present invention, if Server  300  uses the same MSS value as the client communicating with the SSL accelerator, the server will output data equal to the MSS value in each packet, but the SSL accelerator will require space for SSL overhead in returning an encrypted packet to the client. Hence, the SSL may reduce the SSL-Server MSS value to leave room for header information back to the server. An exemplary value would be for the MSS to equal the Client&#39;s MSS less the SSL Overhead, but other modifications or schemes may be used in accordance with the present invention. 
     Next, the SSL accelerator will forward the client&#39;s initial SYN packet on to the server  300  at step  206  as clear text on port  80 . Server  300  will respond to the TCP SYN packet at step  208  with its own SYN and ACK packet addressed to the client  100 . The SSL accelerator device will then respond from port  443  with SYN packet at step  210 , and acknowledgement packet ACK which verifies the MSS. The client will then respond with an ACK on port  443  (at step  212 ) that is forwarded on to server  300  at step  214  and the TCP session is now established. 
     Client  100  will then begin an SSL session at  220  by starting the SSL handshake with the SSL accelerator device. In accordance with the invention shown in  FIG. 5 , the SSL accelerator device  250  responds to the client with all appropriate handshake responses  230 ,  235 . 
     As is well known in the art, it is typically the responsibility of the SSL handshake protocol to coordinate the states of the client and server, thereby allowing the protocol state machines of each to operate consistently, despite the fact that the state may not be exactly parallel. Logically the state is represented twice, once as the current operating state, and (during the handshake protocol) again as the pending state. Additionally, separate read and write states are maintained. When the client or server receives a change cipher spec message, it copies the pending read state into the current read state. When the client or server sends a change cipher spec message, it copies the pending write state into the current write state. When the handshake negotiation is complete, the client and server exchange change cipher spec messages), and then communicate using the newly agreed-upon cipher spec. 
     In the system of the present invention, the SSL device takes over the role typically occupied by the server in the handshake protocol. 
     The SSL handshake occurring at step  235 ,  230  may occur as follows. The client  200  sends a client hello message to which the SSL accelerator  250  must respond with a server hello message, or a fatal error will occur and the connection will fail. The client hello and server hello are used to establish security enhancement capabilities between client and server. The client hello and server hello establish the following attributes: protocol version, session ID, cipher suite, and compression method. Additionally, two random values are generated and exchanged: ClientHello.random and ServerHello.random. 
     Following the hello messages, the SSL Accelerator  250  will send the certificate of server  300 , if it is to be authenticated. Additionally, a server key exchange message may be sent, if it is required (e.g. if their server has no certificate, or if its certificate is for signing only). If the server is authenticated, it may request a certificate from the client, if that is appropriate to the cipher suite selected. 
     Next the SSL accelerator will send the server hello done message, indicating that the hello-message phase of the handshake is complete. The server will then wait for a client response. 
     If the SSL accelerator has sent a certificate request message, the client must send either the certificate message or a no certificate alert. The client key exchange message is now sent, and the content of that message will depend on the public key algorithm selected between the client hello and the server hello. If the client has sent a certificate with signing ability, a digitally signed certificate verify message is sent to explicitly verify the certificate. 
     At this point, the client sends a change cipher spec message, and the client copies the pending Cipher Spec into the current Cipher Spec. The client then immediately sends the finished message under the new algorithms, keys, and secrets. In response, the SSL accelerator will send its own change cipher spec message, transfer the pending to the current Cipher Spec, and send its Finished message under the new Cipher Spec. At this point, the handshake is complete and the client and SSL accelerator may begin to exchange application layer data. 
     During the handshaking sequence, the SSL accelerator will update the TCP/SSL database and associate the SSL sequence numbers with the TCP sequence numbers for the session. Hence, each session will include a plurality of TCP-SSL sequence number pairs, with the number of pairs per session being variable based on a set number or time. These pairs can then be used for rollback recovery in the event that TCP or SSL packets are lost. The database storing these pairs is typically stored in the memory of the apparatus. 
     As shown at reference number  265 , client  100  will now begin sending encrypted application data to the SSL accelerator device  250 . The client will send a request on port  443 . In the client&#39;s request, the source IP will be mapped to the client&#39;s IP, the destination IP will be mapped to the virtual IP of the SSL accelerator device, the source port will be mapped to the client&#39;s port and the destination port will be  443 . This request will include the sequence number and acknowledgement (SEQ/ACK). 
     The accelerator device will process the data at step  270  on the packet level and forward it to the server as clear text. When encrypted application data is received by SSL acceleration device  250  at step  270 , the data in the packet is decrypted and the SSL record extracted, and the TCP/SSL database record is updated by storing the TCP sequential number, the SSL sequential pair, the initialization vector and expected ACK. 
     The SSL accelerator  250  includes a TCP/SSL session database to track all communication sessions occurring through it. Each session will have one or more records associated with it, with each record comprising an association of the TCP session sequence and the SSL sequence. Hence, on receiving the initial SYN from client  100  at step  202 , the SSL accelerator will create a database entry for the particular session, associating the TCP-SSL sequence number pairs. The data may be considered as a table, with each row in the table representing one entry in a given session. Hence, for each session, a typical record might include up to about 8-16 records, which include a TCP sequence number, SSL session number, an initialization vector (for DES and 3DES) and an expected ACK. 
     During decryption, the device may utilize portions of its memory to buffer segments as necessary for decryption. The number and size of the buffers will depend on the cipher scheme used and the configuration of the packets, as well as whether the packets contain application data spanning multiple packets, referred to herein as multi-segment packets (and illustrated with respect to  FIG. 8 ). The SSL device can allocate SSL buffers as necessary for TCP segments. If, for example, application data having a length of 3000 bytes is transmitted via TCP segments having a length of 100 bytes, the device can, copy TCP segment  1  to a first SSL buffer, and start a timer, wait for packet  2  and when received, copy it to an SSL buffer and restart the timer, and finally when packet  3  is received, the SSL accelerator will copy it, decrypt all application data, authenticate it and forward the data on in the clear. (An alternative, bufferless approach is described below). 
     Decrypted packets are then forwarded in clear text to server  300  at port  80 . The SSL accelerator device will forward the data decrypted to port  80  of server with the client IP mapped to the source IP, the virtual IP as the destination IP, the client port as the source port, and port  80  as the destination port. The SSL accelerator device will also send a SEQ/ACK to the server  300 . 
     The server  300  receives packet at step  275  and processes the packet as required, and returns the packet in the clear to SSL accelerator device  250 . The server will respond with a SEQ 1 /ACK 1  acknowledging the data and if necessary, sending data of its own with the destination IP as the client IP, the source IP as the virtual IP, the destination Port as the clients port, and a source port of  80 . 
     Upon receiving the clear packet at step  280 , the accelerator device will extract the ACK, look to the database to compare the ACK with all expected server ACKs less than or equal to the received ACK, and save the TCP sequential number and SSL sequential pair. The SSL accelerator device will then encrypt the data for sPort  443 , assigning the virtual IP of the SSL accelerator as the source IP, the client IP as the destination IP, the destination port as the client port, the source port as port  443 , along with the appropriate SEQ/ACK, and return the information to client&#39;s HTTP  443  port at step  372 . 
     Client  100  will then receive and decrypt the packet at  282 , and send and ACK back to the server at  284 . This ACK is received by the SSL accelerator device at step  285 , compare with all expected client ACKS, clear all entries which have expected ACKs less than or equal to this received ACK, and forward the ACK on to server  300 . 
     This process continues as long as the client and server require. Upon completion of the transmission, the SSL accelerator will send a closed notify alert to the client, and the client will respond to close notify alert. 
       FIG. 6  shows an alternative method of the present invention wherein the SSL device may be utilized for load balancing amongst a number of servers. In the embodiment of  FIG. 5 , the packet destination addresses and source addresses were not modified. In the embodiment of  FIG. 6 , the SSL accelerator assumes the identity of the server, and handles and distributes sessions to a multitude of servers by altering the source and destination addresses of packets in a manner similar to that utilized in Network Address Translation (NAT). While this example is illustrated with respect to a single session, it should be understood that a multitude of similar TCP/SSL sessions may be simultaneously occurring with a multitude of servers. Routing tables associated with the SSL sessions may be utilized by the SSL accelerator device to track the routing of the sessions to individual servers in accordance with well-known techniques. 
     In a manner similar to the embodiment shown in  FIG. 5 , the client  100  sends a handshaking packet SYN packet to TCP port  443  of SSL accelerator  250  rather than directly to server  300  (at step  202   a ) The SSL Accelerator device  250  will receive (at step  204   a ) the SYN packet transmitted by client  100  and may perform functions on packet to enable the SSL acceleration device to continue to perform its SSL proxy functions. 
     The SSL accelerator will forward the client&#39;s initial SYN packet on to the server  300  at step  206  as clear text on port  80 . The SSL accelerator SYN packet to server will identify the source IP as the SSLAD  250  IP, the source port as the client&#39;s port, the destination IP as the virtual IP assigned by the SSL accelerator device, and the destination port as port  80 . Server  300  will respond to the TCP SYN packet at step  208   a  with its own SYN and ACK packet addressed to the client  100 . Upon receipt of the SYN/ACK packet from server  300 , the SSL acceleration device will change the state of the SSL-TCP database by examining the database for expected ACKS from the server, and once found, will clear the entry for the expected ACK and any previous ACKS in the table. The SSL accelerator device will then respond from port  443  to the client with SYN packet at step  210   a  and the client will then respond with an ACK on port  443  (at step  212 ) that is forwarded on to server  300  at  214   a  and the TCP session is now established. 
     It should be noted that the SSL device may implement a load balancing selection algorithm in accordance with any of a number of techniques to select one or more servers  300 ,  301 ,  302 , etc. to provide an even resource load amongst any number of servers communicating with the intermediary device. 
     The client  100  will then begin an SSL session at  220   a  by starting the SSL handshake with the SSL accelerator device  250 . In the embodiment shown in  FIG. 6 , the SSL accelerator device  250  responds to the client with all appropriate handshake responses  230 ,  235  and uses its own IP as the source. 
     A typical handshake occurring at step  235 ,  230  may occur as set forth above with respect to  FIG. 5 , except that the client is communicating directly with the SSL accelerator device (e.g. the destination IP from the client is that of the SSL accelerator). 
     As shown at reference number  265   a , client  100  will now begin sending encrypted application data to the SSL accelerator device  250 . 
     When encrypted application data is received by SSL acceleration device  250  at step  270 , the data in the packet is decrypted and the SSL record extracted, and the TCP/SSL database record is updated by storing the TCP sequential number, the SSL sequential pair, the initialization vector and expected ACK. The packet is then forwarded in clear text to server  300  at port  80 . The SSL accelerator device will forward the data decrypted to port  80  of server utilizing the client IP as the source IP, the SSL virtual IP as the destination IP, the client port as the source port, and port  80  as the destination port. 
     The server  300  receives packet at step  275 , processes the packet as required, and returns the packet in clear to SSL accelerator device  250 . Upon receiving the packet at step  280 , the accelerator device will extract the ACK, compare the ACK with all expected server ACKs less than or equal to the received ACK, save the TCP sequential number and SSL sequential pair, encrypt the packet and forward the encrypted packet to client  100 . The SSL accelerator device will then encrypt the data for sPort  443 , assigning as the source IP as the virtual IP, the destination IP as the client IP, the destination port as the client port, the source port as port  443 , along with the appropriate SEQ/ACK, and return the information to client&#39;s HTTP port  443 . Upon completion of the transmission, the SSL accelerator will send a closed notify alert and the client will respond to close notify alert. 
     Client  100  will then receive and decrypt the packet at  282 , and send and ACK back to the server at  284 . This ACK is received by the SSL accelerator device, compared with all expected client ACKS, clear all entries which have expected ACKs less than or equal to this received ACK, and update the sequential number pair. This ACK is then forwarded on to server  300 . 
       FIG. 7  shows yet another embodiment of the present invention wherein the SSL accelerator performs a full proxy for both the TCP/IP negotiation process as well as the SSL encryption process. As shown in  FIG. 7 , a SYN packet destined for server will be received and responded to by the SSL acceleration device  250 . The SSL acceleration device, at step  207 , performs all functions performed by the server and set forth in steps  206 ,  208  and  216  in  FIGS. 5 and 6 . Later, at step  236 , the SSL acceleration device  250  will negotiate its own TCP/IP session with server  300  to forward decrypted information to the server  300  in the clear. 
     Client  100  sends a SYN packet TCP port  443  of server  300 . The SYN packet will define, for example, an MSS of 1460 bytes. The SSL accelerator device will respond from port  443  with SYN packet V, and acknowledgement packet ACK which verify MSS=1460. The Client will then respond with an ACK on port  443 . 
     On receipt of the ACK packet at step  210 , the TCP session is established and the TCP state is set to “established”. The client  100  will then begin an SSL session at  220   b  by starting the SSL handshake with the SSL accelerator device  250 . In the embodiment shown in  FIG. 7 , the SSL accelerator device  250  responds to the client with all appropriate handshake responses  230 ,  235  and uses its own IP as the source. 
     A typical handshake occurring at step  235 ,  230  may occur as set forth above with respect to  FIG. 7 , except that the client is communicating directly with the SSL accelerator device. It should be understood that the SSL Encryption in this embodiment is essentially the same as the embodiment of  FIG. 6 . 
     Concurrently, at step  236 , the SSL accelerator device will negotiate with server  300 , to establish a clear text session with server  300 . This is accomplished by the SSL accelerator device sending a TCP/80 SYN packet to server identifying the source IP (sIP) as the client  200  IP (cIP), the source port (sPort) as the client&#39;s port (cPort), the destination IP (dIP) as the virtual IP (vIP) assigned by the SSL accelerator device, and the destination port (dPort) as port  80   
     The server responds ( 238 ) with a SYN packet and ACK packet, which will draw ACK from the SSL accelerator  250 . The SSL accelerator  250  is now positioned to receive SSL encrypted data from the client  100  and forward it as clear text to server  300 . 
     Once the SSL and TCP sessions are established, the client can send SSL encrypted data to the accelerator  250 . The SSL session is terminated on the accelerator  250  and decrypted SSL data is copied to the server&#39;s TCP session at step  270   c . Likewise, after clear data is forwarded to the server and responded to (at step  275 ), clear data is received by the SSL accelerator at step  280 , copied to the client&#39;s SSL session and returned in encrypted form to the client at step  280 . The server&#39;s TCP session within the SSL device  250  is terminated on SSL device  250 . An ACK is sent when SSL data returned to client  100  is received. 
     In yet another alternative embodiment of the invention, a further enhancement implemented in the SSL acceleration device is that of a web switching or layer 7 protocol interface. Devices incorporating content or “layer 7” switching are well known in the art. Content or layer 7 switching may be implemented any SSL acceleration device and communicate directly with the Web server  300 . 
     In this embodiment, the SSL accelerator device SSL layer will negotiate with the layer 7 switching implementation on the SSL device, to establish a clear TCP session on Port  80  to the server  300 . The SSL accelerator device will send a TCP/80 SYN packet to the layer 7 switching which identifies the source IP as the client  200  IP, the source port as the client&#39;s port, the destination IP as the virtual IP assigned by the SSL accelerator device, and the destination port as port  80   
     The switching layer responds with a SYN packet and ACK packet which will draw the acknowledgement ACK from the SSL accelerator device. The SSL accelerator device  250  is now positioned to receive SSL encrypted data from the client  100  and forward it as clear text to server  300 . SSL accelerator device will then send the finished code to the client  100  to indicate that the SSL protocol is ready. 
     The SSL accelerator device will decrypt the encrypted data at the packet level by extracting data from the TCP packet sent by client  100 . and will forward the data decrypted to port  80  of the switching layer utilizing the client IP as the source IP, the SSL virtual IP as the destination IP, the client port as the source port, and port  80  as the destination port. The SSL accelerator device will also send SEQ/ACK to the Web switching layer. The switching layer will forward, the decrypted data to TCP port  80  identifying the client IP as the source IP port, the switching port as the source port, the destination IP as real server  300  IP address, and the destination port as port  80 . 
     The switching layer will then translate the destination IP address to be source IP address, source IP address to the client IP address, the destination IP address to the real server IP address, and the source port to the real switching port. The destination port will be  80  and the HTTP/ 80  request will be forwarded to server  300 . The server will respond the HTTP  80  response indicating that the destination IP is the client IP, the source IP is the real server IP address, the destination port is the Web switching port, the source port is port  80  and the appropriate SEQ/ACK. 
     Switching layer will forward the HTTP  80  response to the SSL accelerator device substituting for IP the virtual IP assigned to the server  300  by the SSL accelerator device, substituting for the data for the client port, and the source port equals 80 with the appropriate SEQ/ACK. 
     Once received by SSL accelerator device, the SSL accelerator device will encrypt the a data for port  443 , assign the source IP as the virtual IP, the destination IP as the client  100  IP, the destination port as the client port, the source port as port  443 , along with the appropriate SEQ/ACK and return the information to client&#39;s HTTP  443  port. Upon completion of the transmission, the SSL accelerator will send a closed notify alert and the client will respond to close notify alert. 
     It should be further recognized that the system of the present invention can implement hybrid of the foregoing schemes.  FIG. 9   a  shows an overview of the various modes which may be implemented by the SSL device. As shown therein, using cut-through communication, both a direct mode (one to one communication between client and server) and a load balancing (address redirection) schemes may be utilized. In a full proxy mode, the SSL device performs both TCP and SSL functions, with this mode being optionally utilized for load balancing.  FIG. 9   b  shows a further feature of the device, allowing for mode switching: the system can begin a full TCP proxy mode session (in accordance with the description of  FIG. 6 ) and switch to cut through/direct modes depending on the circumstances of the data transfer. Full proxy TCP mode has the advantage that all cases of transmission are supported. However, this embodiment requires more buffer memory than TCP cut through mode shown in  FIG. 5 . 
     In the cut through modes, certain types of packet transmissions can cause problems. For example, when the SSL record transverses more than one TCP segment or when the client window is very small, (for example, on the order of 200-300 bytes) and many small TCP segments are received. 
     The switching mode shown in  FIG. 9   b  can therefore allow the TCP proxy mode for SSL and TCP session setup, and then cut through mode for normal data, with a roll back to the proxy TCP mode for problem cases. 
     There are numerous types of communications problems which may occur at various stages of data transfer between the SSL Accelerator, the client and the server. Some examples of these problems, and how the SSL device handles them, are set forth below. However, it will be understood that the number and type of errors which are possible in this sequence, and their attendant solutions, are too numerous to detail here. 
     One type of problem is lost packets. Most lost packet cases can be recovered through use of the data structure mentioned above. As the data structure maintains the TCP sequence number, SSL sequence number, expected ACK and DES&#39;s Initialization vector, the SSL Accelerator device can roll back the SSL number to the previous TCP number received. 
     A different problem occurs not packets are lost, but when there is an SSL segmentation problem. Segmentation problems may occur when, for example, 1 SSL record spans over 3 TCP segments, i.e.: where SSL length=3000, and the TCP packet&#39;s length=1000. This segmentation issue is illustrated in  FIG. 8 . In this case, the Accelerator device cannot decrypt and authenticate the packet, since the MAC algorithm data will not arrive for another two segments. 
     If, in the method of the invention, the accelerator uses a memory buffer, (as described above with respect to  FIG. 5 ) the Accelerator can allocate an SSL buffer for 3000 bytes, copy TCP segment  1  to the SSL buffer, and start a timer. When packet SSL/TCP packet  2  is received, it will be copied to an SSL buffer and the timer restarted. Then when packet  3  is received, the SSL accelerator will copy it, decrypt it, allocate 3 TCP, segments, and copy HTTP data into it. This may then be forwarded on in the clear. 
     An alternative embodiment of the present invention utilizes a bufferless or small buffer approach to handle the multisegment problem. In the bufferless approach, individual segments of multisegment SSL records are decrypted, but not authenticated prior to being sent to the server. Upon receipt of the last segment in the series (packet  3  in the above example), the data will be authenticated, however, individual segments are not. This greatly reduces the hardware requirements of the device be requiring little or no buffer memory allocated to multi segment SSL packets. For non-block ciphers, such as RC2 and RC4, this decryption can be performed on the fly. However, for block ciphers such as 3DES/DES, some buffering must occur. This is due to the fact that data for these ciphers must be combined from blocks. In these cases, only part of the data is decrypted and the rest is moved to the next segment. Hence, if there are more than two segments, and the encryption cipher is DES, with 8 byte blocks, the SSL device will buffer up to 7 bytes with additional 7 bytes sequentially moved until the last segment, with the last segment always having enough room to accommodate the data without breaking the server&#39;s MSS. In an exemplary design, the operational modes are configurable by a user so that the sacrifice of whether to potentially compromise security by not authenticating each packet is the user&#39;s choice. Nevertheless, because for block ciphers it is impossible to know the padding length before decryption is finished and the padding length is used to start calculating authentication, then authentication of the data in the multi-segment SSL data does occur upon receipt of the last segment—and the receipt of the MAC algorithm data and one is required to store all decrypted data into a buffer. If, however, the data cannot be authenticated at that time, the SSL device will send a reset to the server and an ALERT to the client, indicating a problem with the session has occurred and notifying the user. For block ciphers, the system does some buffering, but this minimal buffering will reduce latency. 
     Another issue may occur when a “small” window problem occurs. Normally, communications between the Sever to Client occur as shown in Table 1: 
     
       
         
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Client 
                 SSL Accelerator 
                 Server 
               
               
                   
               
             
             
               
                   
                   
                 ← TCP80 1 = 0 
               
               
                   
                 encrypt 
               
               
                   
                 ← SSL TCP443 
               
               
                   
                 1 = 0 
               
               
                   
                   
                 ← TCP80 2 = 1000 
               
               
                   
                 Encrypt 
               
               
                   
                 ← SSL TCP443 
               
               
                   
                 2 = 1000 
               
               
                   
                   
                 ← TCP80 3 = 2000 
               
               
                   
                 Encrypt 
               
               
                   
                 ← SSL TCP443 
               
               
                   
                 3 = 2000 
               
               
                 TCP443 ACK = 3000 → 
               
               
                   
                 TCP80 ACK3000 → 
               
               
                   
               
             
          
         
       
     
     The small window problem may occur when, for example, the ServerMSS=1000, but Client understands an MSS=900. In this situation, if the client sends an ACK W=3000, the SSL accelerator will understand it is going to receive 3, 1000 byte segments. This problem is illustrated in Table 3. In Table 3, the server&#39;s packet length is, for example, 100 bytes. So instead of receiving 3, 1000 byte segments, the SSL accelerator will receive 30, 100 byte segments from the server. Once the SSL accelerator adds the SSL overhead, which in this example is 100 bytes, the packet size to be returned to the client doubles for each packet from the server: 
     
       
         
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Client 
                 SSL Accelerator 
                 Server 
               
               
                   
               
             
             
               
                 Ack W = 3000 --&gt; 
                   
                   
               
               
                   
                 Ack W = 2700 
               
               
                   
                 (SSL expecting 3 
               
               
                   
                 1000 Segments) 
               
               
                   
                   
                 &lt;-- TCP 1 = 0, 
               
               
                   
                   
                 I = 100 
               
               
                   
                 Encrypt 
               
               
                   
                 &lt;-- SSL TCP 1 = 0, 
               
               
                   
                 I = 200 
               
               
                   
                   
                 &lt;-- TCP 2 = 100, 
               
               
                   
                   
                 I = 100 
               
               
                   
                 Encrypt 
               
               
                   
                 &lt;-- SSL TCP 2 = 200, 
               
               
                   
                 I = 200 
               
               
                   
                   
                 &lt;-- TCP 3 = 200, 
               
               
                   
                   
                 I = 100 
               
               
                   
                 Encrypt 
               
               
                   
                 &lt;-- SSL TCP 3 = 400, 
               
               
                   
                 1 = 200 
               
               
                   
                 * 
               
               
                   
                 * 
               
               
                   
                 * 
               
               
                   
                   
                 &lt;-- TCP 14 = 1400, 
               
               
                   
                   
                 I = 100 
               
               
                   
                 Encrypt 
               
               
                   
                 &lt;-- SSL TCP 4 = 2800, 
               
               
                   
                 I = 200 
               
               
                   
                   
                 &lt;-- TCP 15 = 1500, 
               
               
                   
                   
                 I = 100 
               
               
                   
                 Encrypt 
               
               
                   
                 &lt;-- SSL TCP 5 = 3000, 
               
               
                   
                 I = 200 
               
               
                   
                   
                 &lt;-- TCP 16 = 1600, 
               
               
                   
                   
                 I = 100 
               
               
                   
               
             
          
         
       
     
     The SSL accelerator cannot send TCP packet  16  because client&#39;s window is full already (with 15, 200 byte packets). 
     In this case, the SSL accelerator will buffer the Server&#39;s responses, starting from this point so that when a next TCP ACK=3000 is received from the client, the SSL accelerator will take the server response (packet  16 ) from the buffer, encrypt it and return it to the client. 
     If one of the foregoing problems occurs when the SSL accelerator is in a mode which does not support that particular type of communication, the SSL accelerator may switch modes to enable that type of communication to be handled. 
     The foregoing detailed description of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. The described embodiments were chosen in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.