Abstract:
A transmission control protocol (TCP) is used for data communications, yet in order to provide security against denial of service attacks and the like, a connection negotiation phase is required before the TCP handshake. Without a successful connection negotiation, a TCP handshake is unable to complete thereby preventing connection.

Description:
CROSS REFERENCE TO PRIOR APPLICATION 
     This is a U.S. national phase application under 35 U.S.C. §371 of International Patent Application No. PCT/GB2003/005345, filed Dec. 8, 2003, and claims benefit of Great Britain Patent Application No. 0228713.4, filed Dec. 9, 2002. The International Application was published in English on Jun. 24, 2004 as WO 2004/054200 A2 under PCT Article 21(2). 
     FIELD OF THE INVENTION 
     The present invention relates to a system and method for securing data communications over a network and is particularly relevant to networks using the Transmission Control Protocol (TCP). 
     BACKGROUND TO THE INVENTION 
     In data communications, Internet Protocol (IP) networks have become pervasive. In particular, the majority of public data communication networks, particularly the Internet, use IP. Given the availability of cheap, high speed, access to public data communications networks using DSL connections or similar, many organisations wish to use these networks to provide interconnectivity between trusted areas or devices. The trusted devices may be located, for example at branch offices or homes. Trusted areas would include networks within corporate offices. 
     However, by connecting trusted areas and devices to public networks, they become open to attack and abuse. This means that organisations are forced to take defensive measures against attack. 
     The Transmission Control Protocol is the most common, reliable and popular protocol used in IP networks to control connection establishment and transfer. In addition, TCP/IP is the protocol most Microsoft Windows® systems use to access networks. Unfortunately, it is well known that existing session establishment for TCP over IP networks is inherently insecure and prone to exploitation by Active and Intrusion attacks. 
     It is easy for an attacker on an IP network to determine those TCP/IP services present using simple techniques and/or software. Once the TCP/IP services are detected, it is often easy to exploit and/or attack them. For example, denial of service attacks are possible. 
     Denial of service (DoS) attacks cost businesses millions of dollars each year and are now a serious threat to any system or network connected to a public network. These costs are related to system downtime, lost revenues and the labour involved in identifying and reacting to such attacks. Essentially, a DoS attack disrupts or completely denies services to legitimate users, networks, systems or other resources. The intent of such attacks is usually malicious and often takes little skill or resources because the requisite tools are readily available. 
     In the case of TCP/IP, attacks normally focus on the way systems handle handshaking and connection initiation.  FIG. 1  is a schematic diagram of a data communications system using the normal 3-way handshaking used in TCP to process connection requests. 
     A SYN packet is sent from a specific port on a source  10  to the same port at a destination  20 . Upon receipt of the SYN packet, the destination  20  then sends an SYN/ACK packet to the source  10 . Upon receipt of the SYN/ACK packet, the source  10  then sends an ACK packet to the destination  20  and the connection is then considered established (also referred to as open). Data can then be communicated between the source  10  and destination  20 . 
     The packets discussed above have a predetermined format, as is defined in RFC0793 available from ftp://ftp.rfc-editor.org/in-notes/rfc793.txt, and is incorporated herein by reference. 
     One such DoS attack is referred to as an SYN flood attack. While the standard 3-way handshake works well most of the time, most systems have only a finite number of resources available for setting up connections and potential. While most systems can sustain hundreds of concurrent connections to a specific port, it may only take a dozen or so potential connection requests to exhaust all resources allocated to setting up connections. It is this weakness that attackers use to disable a system. 
     When a SYN flood attack is initiated, attackers send a SYN packet from a source to a destination, as is normal in the handshaking procedure. However, the attackers commonly spoof the source address, selecting an address that does not exist. When the destination tries to send the SYN/ACK packet to the spoofed address, it receives no response. Typically, the destination system places each pending connection request in a connection queue to await the ACK packet. In the case of a SYN flood attack, the ACK packet never arrives. The resources allocated for the spoofed request will only be released when a timer associated with the connection queue expires. In standard configurations of systems, timer settings vary from 75 seconds up to as much as 23 minutes or more and the size of the connection queue is often very small. Thus, attackers only need to send a small number of SYN packets to completely disable a specific port. The system under attack will never be able to clear the queue before receiving new SYN requests. 
     Various mechanisms have been employed to counter such attacks, such as implementing firewalls or other ways of blocking some or all ports. However, if a TCP/IP system is to communicate successfully with another at least one port must be left open and this in itself creates a vulnerability when the connection is over a public network. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, there is provided a communication system arranged to communicate under the Transmission Control Protocol (TCP), the system being arranged to not accept a TCP connection request unless a connection has already been negotiated. 
     Preferably the connection is negotiated by receipt at the communication system of a connection request message. 
     Preferably the request message comprises a datagram. 
     Preferably, the datagram includes data on the connection requested and/or information on the source. 
     Preferably, negotiation includes evaluation of the request message. 
     Preferably the evaluation includes processing the data on the connection requested and/or the information on the source. Most preferably, the evaluation includes authenticating the source. 
     The evaluation may include satisfactory authentication of the source and/or negotiation of an encryption key. 
     According to another embodiment of the present invention, there is provided a further communication system arranged to communicate using TCP with the communication system of any of the preceding claims, the further communication system being arranged to negotiate a connection with the communication system prior to transmitting a TCP connection request. 
     Dependant on the network infrastructure in place the received datagram may be responded to with a further datagram, this is desirable when the intervening network is not able to allow the responding end to successfully send a TCP SYN. The effect of this further datagram is to cause the initiating party to send a TCP SYN in response to the responding datagram. In some circumstances the ‘requesting’ datagram may result in a ‘responding’ datagram which is not a TCP SYN. 
     The or each communication system may comprise a computer network communication protocol stack, computer system or network communications device such as a router, bridge, gateway, firewall or switch. 
     According to another embodiment of the present invention, there is provided a data communications method for communicating using the Transmission Control Protocol (TCP) comprising: requiring a connection negotiation with a source system to be completed prior to acceptance of TCP communication packets from the source system. 
     According to a further embodiment of the present invention, there is provided a data communications connection method for the Transmission Control Protocol (TCP) comprising the steps of: 
     prior to the establishment of a TCP/IP connection the initiating party sending a connection request message to a receiving party; 
     receiving the connection request message at the receiving party; 
     opening a TCP connection at the receiving party for the initiating party, and, communicating between the initiating and receiving parties using TCP communications. 
     The present invention seeks to provide a data communications method and system in which the transmission control protocol (TCP) is used for data communications. In order to provide security against denial of service attacks and the like, a connection negotiation phase is required before the TCP handshake. Without a successful connection negotiation, a TCP handshake is unable to complete thereby preventing connection. 
     The present invention seeks to provide a solution for protecting against these attacks whilst at the same time providing a scalable and flexible method for exchanging data securely over Public IP networks using TCP connections. In particular the use of specific datagrams for session establishment provides for the end-point devices, initiator and receiver, to be invisible to attackers on the Public IP infrastructure. TCP is chosen as a reliable data carrier over IP networks. 
     The present invention can be implemented in a manner that is transparent in operation to users and can be implemented in any system or device using any version of the TCP stack. The present invention would also be applicable to other TCP like protocol stacks. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Examples of the present invention will now be described in detail, by way of example only, with reference to the accompanying drawings in which: 
         FIG. 1  is a schematic diagram of a data communications system using the normal 3-way handshaking used in TCP to process connection requests; 
         FIG. 2  is a schematic diagram of a data communications system according to a first embodiment of the present invention; 
         FIG. 3  is a schematic diagram of a data communications system according to a second embodiment of the present invention; 
         FIG. 4  is a schematic diagram of a third embodiment of the present invention; 
         FIG. 5  illustrates a datagram format suitable for use in the embodiments of  FIGS. 2 ,  3  and  4 ; and, 
         FIG. 6  is a flow diagram of a data communication method according to an embodiment of the present invention. 
         FIG. 7  is a flow diagram of a data communication method according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2  is a schematic diagram of a data communications system according to a first embodiment of the present invention. 
     A first communication system  100  is connected to an insecure network  110  and communicates using TCP. However, the first communication system  100  is configured to not acknowledge, reply or otherwise give away its existence to new incoming TCP traffic. As described with reference to  FIG. 1 , in a standard TCP communication system if a connection request is received in the form of a SYN TCP packet, the communication system would give away its existence by replying. In the present embodiment, upon receipt of such a SYN packet (or indeed any other unexpected packet type), the first communication system  100  does nothing. Preferably, the first communication system  100  discards such packets. Thus, potential attackers are not able to implement DoS attacks and the like. 
     In order to establish a connection with the first communication system  100 , a second communication system  120  must send a connection request message  130  of a predetermined format before sending the standard TCP SYN packet  140  to initiate the handshake. Preferably, a predetermined delay is applied by the second communication system  120  before sending the SYN packet  140  in order for the connection request message to be received and processed at the first communication system  100 . 
     Upon receipt of the connection request message  130 , the first communication system  100  examines it for validity and, if the request message  130  is found to be valid then the first communication system opens a TCP/IP connection to the second communication system  120 . Upon receipt of the TCP SYN packet  140  from the second communication system, the standard TCP handshake continues, as is illustrated with reference to  FIG. 1 . 
       FIG. 3  is a schematic diagram of a data communications system according to a second embodiment of the present invention. 
     The second embodiment operates in a similar manner to the first embodiment, as discussed with reference to  FIG. 1 . However, instead of it being reliant on the second communication system  120  to follow the connection request message  130  with a TCP SYN packet  140 , the first communication system  100  instead instigates the handshake by sending the SYN packet  140  upon receipt of a valid connection request message  130 . 
       FIG. 4  is a schematic diagram of a third embodiment of the present invention. 
     In this embodiment, the first communication system  100  is configured to require more than a valid connection request message  130  of a predetermined format to permit a connection. The connection request message  130  is preferably used by the second communication system  120  to transmit data content that is examined by the first communication system  100  to authenticate the first communication system  120 . Authentication may, for example be dependent on the first communication system  120  providing one of a number of approved identities, network addresses, passwords or other authentication data. Alternatively, or in addition, authentication may include the first communication system sending a response message  150  to the address of the second communication system  120 , which the second communication system  120  must reply to in a further response message  160 . In this example, the response message may include a code that must be included in the response message  160  of the second communication system  120  in order for the network address of the second communication system  120  to be confirmed. 
     In addition, or as an alternative, the authentication may require successful negotiation of an encryption key between the first communication system  100  and the second communication system  120 . 
     Depending on the configuration it may require only one or a predetermined number of the authentication requirements be failed by the second communication system  120  for the connection request to be discarded by the first communication system  100 . 
       FIG. 5  is a schematic diagram of a fourth embodiment of the present invention. 
     In order to protect trusted networks  200 ,  210  from the untrusted network  110 , communication systems as discussed above with reference to  FIGS. 2 to 4  are placed at the boundary of the trusted networks  200 ,  210 . The communication systems in the embodiments discussed with reference to  FIGS. 2 to 4  have been illustrated as Client or server computer terminals, such as a Windows PCs. However, as the present invention could be implemented as an addition to a TCP protocol stack, it could be included within any TCP enabled device that is directly connected to the untrusted network  110 . 
     In the embodiment of  FIG. 5 , the present invention is implemented in a gateway device  220 ,  230  connecting the two trusted network  200 ,  210  to the untrusted network. Client computers  240 ,  250  and server computers  260 ,  270  are connected to the respective gateway device  220 ,  230  via their respective trusted network  200 ,  210 . 
     In this example, if client computer  240  wishes to connect to server computer  270 , the client computer  240  sends a TCP SYN packet to the server computer  270  in accordance with the standard TCP handshake. However, when the packet reaches the gateway device  220 , it is intercepted by the gateway device  220  and held in a queue. A connection request message is then sent from gateway device  220  to gateway device  230  and is processed in the same manner as has been discussed with reference to  FIGS. 2 to 4 . Once the connection has been granted, the TCP SYN packet is passed from the queue, over the untrusted network  110  to the server computer  270  (via gateway device  230  and network  210 ). Obviously if the embodiment of  FIG. 3  were being used, the TCP stack at the client computer  240  and server computer  270  would have to be changed to cope with the SYN request from the client computer  240  being dropped in place of one from the server computer  240 . 
     The gateway device  220  may be a router, bridge, switch, firewall or other network device or system. In addition, one communication device may be a gateway device as discussed above whilst another may be in the form of software on a computer system. 
     In some network topologies (such as those using gateways above), the intervening network device will change the end point address for the connection. In these circumstances it is necessary to either have datagrams exchanged in both directions prior to the establishment of a TCP connection, or for the originating party to send a datagram and to follow this directly with an opening TCP packet (ie. a SYN packet). The receiving gateway may respond with its own address and include additional information such as ECHO options into the responding SYN packet. 
     Receiving devices would await incoming datagrams requesting connections these could contain a payload which offers authentication of the requesting device. The responding device can process these requests as system resources allow, although this may be accelerated by the use of specific hardware devices. Once the responding device evaluates a requesting datagram as acceptable it will attempt to open a TCP connection to the initiating device. 
       FIG. 6  illustrates a request message format suitable for use in the embodiments of  FIGS. 2 ,  3 ,  4  and  5 . 
     The format of the request message will be dependent on the addressing protocol used by the network  110 . In the case of an IP network, an IP datagram could be used and this is illustrated in  FIG. 6 . 
     Irrespective of the addressing protocol, the request message  300  will at least contain a destination address  310  and a payload  320 . The destination address  310  is needed for the datagram to be delivered by the network  110  to its destination whilst the payload  320  is needed in order that the request message  300  can be differentiated from other traffic the destination may receive. 
     Preferably, the request message  300  also includes an address  330  of the source sending the request. It may also include one or more fields  340  indicating the type and/or position of data in the payload  320 , such as request, authentication information, encryption information. Other fields such as checksums, time-to-live, length and version identifiers may also be included. 
     Preferably, the request message is a datagram, that is, a self-contained packet of information that is sent through a network with a minimum protocol overhead. It is preferred that the connection request message is not a TCP packet to avoid confusion and also to make implementation simpler. However, implementations can be envisaged, including the use of a standard TCP packet format for the request message. 
       FIG. 7  is a flow diagram of a data communication method according to an embodiment of the present invention. 
     In step  400 , Prior to the establishment of a TCP/IP connection the initiating party will send a datagram to the receiving device requesting a connection be initiated by the receiving party to the initiating party. 
     The datagram may optionally contain data content which can be examined by the receiving party to authenticate the initiating party. 
     Following receipt of the datagram in step  410 , the receiving party will open a TCP connection in step  420  to the initiating party, and optionally, in step  430  negotiate a payload encryption key. It would be during the opening of the TCP connection and encryption key establishment for the end devices to authenticate each other. 
     Once opened successfully both entities can use the TCP/IP connection to communicate in step  440 . The option exists to incorporate TCP payload encryption at this point to ensure the data being transferred is secure and will remain confidential. 
     Where authentication and encryption have been discussed, it will be apparent that many known authentication and cryptographic techniques could be used to establish the encryption key and detailed party authentication. Furthermore, although the emphasis of the present invention is on securing TCP based communication systems on publicly accessible data networks, it will be apparent that the systems and methods disclosed are equally applicable to non-public data networks. 
     Although the description has referred to Internet Protocol (IP) networks and using TCP over IP networks at various points, it will be apparent that the present invention is applicable for use over other network types as long as they support TCP.