Abstract:
Deciphering and verification of the checksum of enciphered and encapsulated UDP datagrams, particularly those which enclose a tunneling protocol such as L2TP, are achieved by the provision of a checksum verifier in parallel with a decipher block. Checksum logic creates a pseudo UDP header needed for checksum verification using fields that would occur at the start of the packet that encapsulates the UDP datagram. The first part of the packet to be deciphered is the UDP header; checksum logic can latch the checksum field into a local register. As the rest of the packet is deciphered the checksum verifier processes the data at the same time. Eventually the checksum logic will acquire a complete checksum which can be compared with the checksum that had been previously latched, so as to verify the checksum.

Description:
FIELD OF THE INVENTION 
     This invention relates to packet-switched communication networks. It more especially relates to the reception of datagrams conforming to the UDP (User Datagram protocol), encapsulated and enciphered within a packet conforming to another transport-layer protocol, such as IP (Internet Protocol) and more particularly to such datagrams which are carriers for a tunneling protocol such as L2TP (Layer 2 Tunneling Protocol). 
     BACKGROUND TO THE INVENTION 
     Modern communication practice has seen the development of virtual private networks (VPNs), which are useful for an organization which desires to provide a secure communication system within the organization but, owing for example to the geographical separation of parts of the organization, cannot conveniently employ a private local area network (LAN) separated from external networks by secure gateways or firewalls. One way in which a VPN can be organized is to employ datagrams which employ UDP as a transport protocol and a tunneling protocol such as L2TP, and to encapsulate datagrams using a enciphering protocol within packets that can be transported generally, i.e. packets conforming to an internetworking protocol (usually IP). The encapsulation may, where the overall transport protocol is IP, be an IPSEC (IP Security) protocol such as AH (Authentication Header) or ESP (Encapsulation Security Protocol). The former provides source authentication and data integrity but the latter provides, at the cost of greater complexity, confidentiality as well. In what follows it will be assumed that ESP is employed as an enciphering protocol but it will be understood that the invention extends to the decoding of UDP datagrams which are encapsulated by means of other enciphering protocols and which carry payloads via other tunneling protocols. 
     When a UDP datagram is prepared for transmission from a sender there is a computation of a checksum. A UDP checksum is computed by performing a 1&#39;s complement of the sum of all the 16-bit words in the entire UDP datagram and a pseudo-header (ignoring any overflow). The result is put into the checksum field of the UDP header. 
     The purpose of the checksum is to provide for error checking in the event that one of more of the links between source and destination does not provide error checking. If the datagram reaches the destination without error, the sum of the 16-bit words in the UDP datagram added to the checksum should provide in the absence of error a result consisting of all 1 s. If any bit in the result is a zero the datagram is in error and may be discarded. 
     When therefore a UDP datagram encapsulated in a packet reaches the end of a tunnel defined by the tunneling protocol, the receiver at or defining the end of the tunnel must first decipher the packet, using the relevant (secret) deciphering key. It is then necessary to verify the UDP checksum. 
     Any method of deciphering requires the use of memory. If shared memory is used for deciphering and the verification of the checksum, the whole packet (i.e. the datagram and its encapsulation) is read into memory and deciphered; then the checksum is verified. Such a process requires a lot of time (clock cycles) first to decipher the full packet and then to run through it again to verify the checksum. It limits the number of tunnels a system can terminate in a given time. Moreover the latency also increases. 
     If separate memories are used for deciphering and the verification of the checksum, the packet is decoded fully in one memory and then passed onto another memory wherein the checksum would be verified. Such a scheme requires extra memory, because each memory must be large enough to accommodate a packet of maximum size; again, with consecutive functions, there is an increase of the latency of tunnel termination. Such an increase is a serious disadvantage, especially if voice data is being tunneled through a VPN. 
     SUMMARY OF THE INVENTION 
     The object of the invention is therefore to improve the deciphering and verification of the checksum of enciphered UDP datagrams, particularly those which enclose a tunneling protocol such as L2TP. 
     This is achieved by the provision of a checksum verifier in parallel with the decipher block. Checksum logic creates a pseudo UDP header needed for checksum verification using fields that would occur at the start of the packet. The first part of the packet to be deciphered will always be the UDP header and checksum logic can latch the checksum field into a local register. As the rest of the packet is deciphered the checksum verifier processes the data at the same time. Eventually the checksum logic will acquire a complete checksum which can be compared with the checksum that had been previously latched, so as to verify the checksum. 
     Further features of the invention will be described with reference to the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of a packet employing L2TP sent over UDP encapsulated by IPSEC for sending over IP; 
         FIG. 2  illustrates a security block according to the invention; 
         FIG. 3  illustrates a checksum verifier; 
         FIG. 4  illustrates a pseudo-header; and 
         FIG. 5  is a flow diagram which illustrates the method of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  of the drawings illustrates the structure of a packet which employs a tunneling protocol (specifically, L2TP) and which is to be sent over a transport protocol in a virtual private network. Such a packet is in this example intended for sending over a public network, i.e. the Internet and therefore the packet commences with an IP header  11  and an IPSec header  12 . The virtual private network employs UDP as a transport protocol and L2TP (layer 2 transmission protocol) as a tunneling protocol. This usage accounts for the UDP header  13  and the L2TP header  14 . The UDP header comprises a UDP source port number  131 , a UDP destination port number  132 , a length field  133  and a checksum  134 . The datagram represented by the UDP header, the L2TP header and a payload  15  is encapsulated by means of an IPSEC protocol and in this particular example by means of the Encapsulation Security Payload (ESP) protocol. This protocol provides authentication, data integrity and confidentiality, specifically by enciphering between an ESP header and an IPSEC authentication trailer. Currently IPSEC authentication, for both the ESP protocol in the example and the AH (Authentication Header) protocol, uses an HMAC (Hashed Message Authentication Code) which relies on a shared secret key rather than public keys; however, the type of key is not relevant to the present invention. 
     The transmission protocol header, in this example the IP header  11 , includes an identification of the transport protocol which the packet employs. This is done for an IPv4 packet by setting the ‘higher-level protocol’ field (the 10 th  byte of the IP header) to a number which conventionally identifies the transport protocol, i.e. ‘50’ to denote the ESP protocol. For an IPv6 packet, the same value (50) would be set into the ‘Next header’ field (the seventh byte) to denote the ESP protocol. The IPSec ESP header  12  conventionally consists of a 32-bit field called the SPI (Security Parameter Index) field and a 32-bit Sequence Number field. The SPI field in combination with the network destination address and the security protocol uniquely identifies a Security Association (SA) for the datagram. The sequence number is initially set to zero at the establishment of a Security Association and is employed to inhibit intrusion (for example by ‘man-in-the-middle’ attacks). 
     The ESP trailer field  16  includes the protocol number that identifies the transport protocol (in this case UDP) of the encapsulated datagram. 
     The UDP header, the tunneling protocol header, the payload and the encapsulation protocol trailer (in this case the ESP trailer) are enciphered. The enciphering includes the protocol number so that an intruder should not be able to determine that UDP is the transport protocol. 
     The authentication trailer  17  terminating the packet is a variable-length field containing a signed message digest, computed for example by some suitable algorithm, such as MD5 (Message Digest 5) or SHA (Secure Hash Algorithm). 
     When a packet of the kind shown in  FIG. 1  reaches its destination in the virtual private network, i.e. it reaches the end of the L2TP/IPSEC tunnel, the receiver has to decode the packet, using whatever decryption key has been agreed for the VPN over the IPSEC protocol, and to verify the UDP checksum. Unlike some other protocols, UDP requires that the whole datagram (rather than just the header) be processed for the checksum to be verified. 
     Checksum verification of a UDP datagram at a receiver customarily requires the following steps:
         (a) add a pseudo-header to the UDP user datagram. This pseudo-header is defined by the UDP protocol and conventionally includes the IP source address, the IP destination address and transport protocol from the IP header and the length field from the UDP header;   (b) add padding if needed to make data 16-bit aligned   (c) divide the total bits into 16-bit sections   (d) add all 16-bit sections using one&#39;s complement arithmetic   (e) complement the result   (f) if the result matches the checksum in the UDP header, the checksum is verified. If so, the pseudo-header and any added padding are discarded and the packet is accepted, for example for further forwarding or other processing. If not, the packet should be discarded.       

     As has been previously explained it is possible to employ separate memories for decoding and the examination of the UDP checksum. A packet is received at an input (for example a port of a network unit) and passed to a cipher memory coupled to a cipher block. The deciphered packet is loaded into the checksum memory while a checksum block determines the checksum; the packet; the packet, on the assumption that the checksum is correct, is passed on for processing and/or forwarding by the unit. 
     An initial time interval is occupied by the deciphering of the whole packet by the cipher block. Only when the deciphering is complete is packet transferred to the checksum memory and processing is complete at the end of a second interval. The next packet can be deciphered in the cipher memory block during the second interval but the process inevitably introduces latency and requires double the memory space. 
     It is also possible to employ a shared memory. It is acted on in consecutive intervals first by a cipher block and then by a checksum block. Although this scheme uses half the memory space as the previous scheme, it is obviously much slower because the memory cannot be used for the next packet until the processing of the current packet has been completed. Furthermore, the technique is unnecessarily time consuming, because it is necessary first to decode the full packet for IPSEC and then to run through the packet again after the decode to verify the checksum. The technique limits the number of tunnels a system can terminate per second. 
       FIG. 2  illustrates generally one embodiment of the invention.  FIG. 3  illustrates a checksum verifier block that forms part of the embodiment more generally shown in  FIG. 2 .  FIG. 4  illustrates a pseudo-header employed in the technique to be described and  FIG. 5  is a flow diagram. 
     As is shown in  FIG. 2 , a packet is received by way of a Fifo  21  and written into a memory block  22  (Stage  51 ,  FIG. 5 ). Coupled to the memory is a decipher block  23 , which progressively deciphers the packet and returns the deciphered packet back to the memory  22 . In parallel with the return path from the decipher block  23  to the memory  22  is a checksum block  24 . A controller  25  controls the reading of data to and from the memory and also provides an enable signal to the decipher block at a particular stage in a received packet. 
     The checksum verifier, as shown in  FIG. 3 , receives data at an input  30  and couples the data to a checksum logic circuit  31  (coupler to registers  32 ) and to a ones-complement adder  33 . The adder  33  is coupled to a checksum register  34  which can signal a pass/fail register  35 . 
     The system allows, as will now be described, deciphering and checksum verification to proceed in parallel. 
     Before the technique is described, it is relevant to mention that algorithms, such as DES-CBC (Data Encryption Standard—Cipher Block Chaining) used for enciphering in the present context are block ciphers which operate on a segment of data at a time. Typically the segment size is 64, 128 or 256 bits. This allows progressive deciphering a block at a time. Further, it will be understood that it is desirable to provide pre-filtering to ensure that only IPSEC packets are sent to the deciphering logic. This is readily achievable because the enciphered part of the packet occurs after the IPSEC header and the IPSEC header will indicate the type of the IPSEC packet. 
     The packet is passed from the memory into the IPSEC logic (the decipher block  23 ) in segments one at a time (the size depending on the cipher used). Since the first part of the packet (the IP header and the IPSec ESP header are not encrypted, the controller  25  will inhibit the decipher block  23  from deciphering until the correct point in the packet is reached. From the IPSec header the type of IPSec packet can be determined and accordingly how many bytes into the packet the ciphered section starts. 
     The checksum logic  31  creates the pseudo UDP header needed for checksum verification using some of the fields in the outer IP Header that would be at the top of the packet. To do this it will latch into registers  32  the IPSA, IPDA and IP protocol fields obtained form the IP header (stage  53 ,  FIG. 5 ) and wait for the deciphered UDP length field. The ‘reserved’ field is set to zero. The pseudo UDP header can be added at any time, conveniently at the end of the packet, to the packet data to complete the computation of the checksum. 
     The first part of the packet to be decoded by the decipher block  23  in this mode will always be the UDP header. As the decoded data comes out, the checksum logic  31  will complete the pseudo-header (stages  55  and  57 ,  FIG. 5 ), create the checksum field and latch it into a checksum register  34 . This field is zeroed in the packet (so that the checksum is set to 16×0) and then the data is passed through the checksum verifier. 
     As the rest of the packet is deciphered (stage  56 ,  FIG. 5 ) the checksum verifier will process the data simultaneously. 
     When it comes to the ESP trailer which can include some of the data as well as padding, a pad length and next header fields, the checksum logic will have to work out if, and so, what part of it is actual packet data. This may be done by subtracting from the length field in the IP header the length of the IPSec header; the result is the length of the packet. A simple computation then gives the number of bytes required to align the packet to 16 bits. 
     The checksum logic should now have a complete checksum in the checksum register  34 . This is then compared to the checksum that was previously latched and a pass/fail registered in register  35 . The pass/fail value is provided when the decipher is complete to allow forwarding or to cause discard of the packet accordingly. 
     If the UDP checksum passes, all the tunnel/IPSEC headers and trailers are stripped off the packet and the raw packet is forwarded. 
     The advantages of this proposal are that as fast as the packet is decoded, the checksum is also being verified, thus overcoming the throughput and/or latency problems with the prior art and avoiding extra memory. For example the main performance indicator for VPNs is the number of tunnels that can be terminated per second. Latency is becoming increasingly important too with the use of VPNs for voice and delay sensitive traffic. 
     This proposal is mainly applicable to L2TP/IPSEC a widely used tunneling protocol but can also be used in any tunneling protocol which uses UDP and security e.g. enciphered IP over UDP used to traverse NAT.