Abstract:
A method for processing packets in a router includes specifying operations on packets as chains of processing elements. Each chain is uniquely associated with one interface/protocol pair, and each processing element performs at least one function on a packet. An incoming packet is received, and processed, first by a demultiplexor element which determines the protocol of the next higher level used by the packet. Then, the packet is processed by the elements of a decapsulation chain associated with the interface on which the packet was received, and by the elements of an encapsulation chain associated with the interface on which the packet is to be transmitted. The demultiplexor element or operation passes the packet on to a decapsulation chain associated with the protocol and with the incoming interface, depending on protocol information contained in the incoming packet. Decapsulation and encapsulation chains can be built dynamically, by inserting new and removing old elements as necessary as new protocols are developed and new features added. A chain walker walks through the chains, passing the processed packet to each element in a chain, until either the end of the chain is reached and processing is complete, or until the packet is dropped because no function can process it, or because a packet is processed by an outside process or by hardware, which may optionally stop the chain walk. A chain walk may be temporarily halted, or may be terminated. If temporarily halted, the chain walk can be resumed at any element in the chain, depending on the packet&#39;s requirements. A chain walk can also begin at any element in a chain. Alternate branches within the chain can be provided, where each branch can comprise zero or more elements. A classifier determines which branch to take, based on at least one characteristic of the packet being processed, and executes the elements of the selected branch.

Description:
RELATED APPLICATIONS 
     This application discloses subject matter that is related to subject matter disclosed in the following applications, assigned to Cisco Systems, Inc., the assignee of the present invention, and are incorporated herein by reference: 
     Kenneth Moberg, Claude A. Cartee, Greg Stovall, David Hampton, Kirk Lougheed, Pedro Marques, Timothy Kolar, Andrew Valencia and William May, PACKET PROCESSING USING ENCAPSULATION AND DECAPSULATION CHAINS, filed on Oct. 15, 1999, Ser. No. 09/419035. 
     Kenneth Moberg, Kristen Marie Robins, and William May, DISTRIBUTED PACKET PROCESSING USING ENCAPSULATION AND DECAPSULATION, filed on Oct. 15, 1999, Ser. No. 09/418723. 
    
    
     BACKGROUND OF THE INVENTION 
     A network is a communication system that allows a user on a computer to access resources and exchange messages with users on other computers . A network is typically a data communication system that links two or more computers and peripheral devices. It allows users to share resources on their own systems with other network users and to access information on centrally located systems or on systems that are located at remote offices. It may provide connections to the Internet or to the networks of other organizations. 
     A network typically includes a cable that attaches to a network interface card (NIC) in each of the devices within the network. 
     Users interact with network-enabled software applications to make a network request, such as to get a file or print on a network printer. An application may also communicate with the network software, and the network software may then interact with the network hardware to transmit information to other devices attached to the network. 
     A local area network (LAN) is a network located in a relatively small area, such as a department or building. A LAN typically includes a shared medium to which workstations attach and through which they communicate with one another by using broadcast methods. With broadcasting, any device on a LAN can transmit a message that all other devices on the LAN can listen to. The device to which the message is addressed actually processes the message. Data is typically packaged into frames for transmission on the LAN. 
     FIG. 1 is a block diagram illustrating a network connection between a user  2  and a particular web page  6 . This figure is an example, which may be consistent with any type of network, including a LAN, a wide area network (WAN), or a combination of networks, such as the Internet. 
     When a user  2  connects to a particular destination, such as a requested web page  6 , the connection from the user  2  to the web page  6  is typically routed through several routers  4 A- 4 D. Routers are internetworking devices, typically used to connect similar and heterogeneous network segments into Internetworks. For example, two LANs may be connected across a dial-up, integrated services digital network (ISDN), or a leased line via routers. 
     Routers are also found throughout the Internet. End users may connect to local Internet Service providers (ISPs), which are typically connected via routers to regional ISPs, which are in turn typically connected via routers to national ISPs. 
     To move packets from one network to another, packet processing software is required on each router. Generally, packets received from a network are processed and forwarded to another network, based on information contained in layers  2  and  3  of the ISO 7-layer model. 
     Layer  2  (L 2 ) of the ISO model is commonly referred to as the data-link layer. ethernet, HDLC and PPP are examples of commonly used L 2  protocols. ethernet is commonly used in LANs, while HDLC and PPP are commonly used in WANs. 
     Layer  3  (L 3 ) is commonly referred to as the network layer. Internet Protocol (IP) is the most commonly used L 3  protocol. ARP, SAP and SNAP are IP-related protocols that, although not strictly in layer  2 , are considered to be layer  2  protocols for purposes herein. 
     Layer  4  (L 4 ) is commonly referred to as the transport layer. TCP is the most commonly used L 4  protocol. 
     Router software designers use the term encapsulation to refer to the layering of protocols. A packet containing TCP, IP and ethernet protocols is said to have an encapsulation of TCP over IP over ethernet. A packet containing TCP, IP and HDLC protocols has an encapsulation of TCP over IP over HDLC. 
     The proliferation of features and protocols has greatly complicated the software involved in processing packets. Because of this, the software responsible for each layer must examine each packet to determine what the next step is in the handling of the packet. 
     For example, when a packet is received on an ethernet interface, the ethernet protocol handling software must examine the header, and based on values in the header, determine what to do with the packet. Data contained within a packet, for example, a type or length field in the protocol header, contains information about the next layer of the packet. 
     A traditional ethernet processing algorithm for this might be: 
     If type=IP and IP is configured for this interface 
     Pass packet to IP code 
     If type=ARP and ARP is configured for this interface 
     Pass packet to ARP code 
     If type=RARP and RARP is configured for this interface 
     Pass packet to RARP code 
     If type=IPV 6  and IPV 6  is configured for this interface 
     Pass packet to IPV 6  code 
     . . . 
     Each time a new protocol is added, the ethernet code must be modified to handle the new protocol. This may be necessary for each layer every time new features or protocols are added. Router software has grown to the point where maintenance has become very difficult. Each change can cause wide variations in performance. A new method is thus needed to allow growth in features and protocols without affecting the stability of existing software. 
     SUMMARY OF THE INVENTION 
     The present invention solves the above problems by logically separating the various packet operations into chain elements. These elements are then dynamically chained together as needed, at runtime, to form encapsulation and decapsulation chains. 
     The chains can also be dynamically rebuilt upon a change of configuration. Such changes typically come from a customer command line interface, however, in some cases there may be features that are configured after a protocol negotiation. 
     With the employment of chains, developers of new features do not need to change existing code to introduce the new features. Instead, new chain elements can be developed and inserted into existing chains as necessary. 
     As a result of the improved modularity, performance is improved for complex operations. 
     Accordingly, a method for processing packets in a router includes specifying packet processing function elements and linking them together to form chains. Each chain is associated with a unique interface/protocol pair. As used herein, a protocol may be a router-level protocol such as IP, or it may be a bridge-level protocol. Furthermore, the term protocol is meant to encompass the concept of tunneling, where a packet which has already been encapsulated using one protocol is further encapsulated using the same or a different protocol at the same level. In such a case, one or more virtual interfaces may be encountered by a packet which encapsulate and decapsulate the packet according to various protocols. An interface, then, is simply the target of a packet, and may be implemented in hardware, software, or both. 
     Each processing element within a chain performs at least one function on a packet. An incoming packet is received, and processed, first by a demultiplexor (demux) element which determines the protocol of the next higher level used by the packet. Then, the packet is processed by the elements of a decapsulation (decaps) chain associated with the interface on which the packet was received, and by the elements of an encapsulation (encaps) chain associated with the interface on which the packet is to be transmitted. 
     In one embodiment, the demultiplexor element or operation passes the packet on to a decapsulation chain associated with the protocol and with the incoming interface, depending on protocol information contained in the incoming packet. The protocol information may be contained, for example, in a type or length field in a header. 
     Decapsulation and encapsulation chains can be built dynamically, by inserting new and removing old elements as necessary as new protocols are developed and new features added. 
     In addition to decapsulation, the decapsulation chain can include functions including, but not limited to, decryption, decompression and filtering. Similarly, an encapsulation chain can include, in addition to an encapsulation operation, encryption, compression, flow control and filtering functions, as well as other function. 
     A chain walker walks through the chains, passing the processed packet to each element in a chain, until either the end of the chain is reached and processing is complete, or until the packet is dropped because no function can process it, or because a packet is processed by an outside process or by hardware, which may optionally stop the chain walk. 
     A chain walk may be temporarily halted, or may be terminated. If temporarily halted, the chain walk can be resumed at any element in the chain, depending on the packet&#39;s requirements. A chain walk can also begin at any element in a chain. 
     A packet may be intended for the router, in which case processing ends after the decapsulation chain terminates. In addition, the router itself can generate packets. The destination or outgoing interface is determined, and the packet is processed by executing processing elements within a selected encapsulated chain associated with the outgoing interface. Finally, the packet is transmitted from the outgoing interface. 
     Preferably, an embodiment uses a chain walker which, upon receipt of an incoming packet in an incoming interface, executes the demux chain associated with the incoming interface, a decapsulation chain associated with the incoming interface, and an encapsulation chain associated with an identified outgoing interface. After an element is executed from chain walker, the element returns a reference to the next element in the chain. The chain walker then executes the referenced next element. On the other hand, if the element returns a null reference, the chain walker stops. 
     In addition, a return code can notify the chain walker that the packet should be dropped. 
     One embodiment implements the chains and chain walker using an interface descriptor block (IDB) associated with each interface. An IDB includes a reference or pointer to a demux element associated with the associated interface, a reference to a decapsulation chain array, and a reference to an encapsulation chain array. Each encapsulation or decapsulation array holds references or pointers to chains of encapsulation or decapsulation processing elements, respectively. 
     In a further embodiment, alternate branches within the chain are provided. Each branch can comprise zero or more elements. Preferably, a classifier determines which branch to take, based on at least one characteristic of the packet being processed, and executes the elements of the selected branch. 
     Packet characteristics on which the classifier may depend in selecting a branch, include, but are not limited to protocol type, e.g., IP, TCP, UDP, HTML, ARP, FTP, etc., characteristics such as broadcast, multicast, unicast, etc., TCP or UDP port number, source and/or destination address, media type such as voice, data, or video. 
     Alternatively, where a branch comprises a first element which normally performs some function such as, for example, encapsulation, the function can be performed in a second element which occurs in the chain prior to the first element, and the first element will be skipped. The first element can communicate to the second element how to perform the function. 
     Preferably, elements of a selected branch are executed sequentially by passing the packet to each element and transferring execution control to the element. Each element processes the packet and returns an indication, to the chain walker, of the next element in the branch to be executed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
     FIG. 1 is a block diagram of an example of a network connection between a user and a web page. 
     FIG. 2 is a block diagram illustrating a simple network connecting a user on an ethernet LAN to a remote server on a different ethernet LAN, with a serial WAN using HDLC connecting the two LANs. 
     FIG. 3 is a schematic diagram illustrating decapsulation and encapsulation of a message as performed by a router. 
     FIG. 4 is a schematic diagram illustrating an embodiment of the present invention including a chain walker. 
     FIG. 4A is a schematic diagram illustrating an embodiment of the present invention having a virtual interface. 
     FIG. 4B is a schematic diagram of an embodiment of the present invention with different chains than those of the previous figures. 
     FIG. 4C is a schematic diagram of an alternative embodiment of the present invention illustrating a non-sequential chain. 
     FIG. 4D is a schematic diagram of an alternative embodiment of the present invention illustrating another non-sequential chain. 
     FIG. 5 is a schematic diagram of the data structures used to implement decapsulation and encapsulation chains of the present invention. 
     FIG. 6 is a flowchart illustrating an embodiment of the chain walker. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2 illustrates a simple network  10  to which several systems  14  are connected. In particular, the network  10  connects a user  14 A on one ethernet LAN  12 A with a remote server  14 B on a different ethernet LAN  12 B, with a serial WAN  16  using HDLC to connect the two LANs  12 A,  12 B. Two routers  15 A and  15 B provide the interfaces between the two types of networks. 
     The ethernet LANs  12 A,  12 B use ethernet protocol, while the serial WAN  16  uses HDLC protocol. For a TCP packet to reach the server  14 B, the routers  15 A,  15 B must change the encapsulation from ethernet to HDLC, and then back to ethernet. These encapsulations are associated with and configured for each interface. 
     Local area network  12 A uses, for example, the protocol shown in block  18 A. Here, HTTP protocol is used by the application. TCP and IP protocols as well as ethernet protocol are used throughout the local area network. The same protocols are used in local area network  12 B as shown in block  18 B. 
     The wide area network  16 , however, uses HDLC protocol instead of ethernet, as shown in block  18 C, because this protocol is better suited to wide area networks. Therefore, it is necessary for the routers  15 A,  15 B to translate packets from the ethernet Protocol to HDLC protocol and back to ethernet protocol. 
     FIG. 3 illustrates basic decapsulation and encapsulation as performed by router  15 A of FIG.  2 . An incoming packet  20 A contains a message or other data  22  which is encapsulated with an HTTP header  24  which in turn is encapsulated with a TCP header  26 . This in turn is encapsulated with an IP header  28  and the packet is finally encapsulated in an ethernet frame, including an ethernet header  30 . 
     The router  15 A reformats and readdresses the packet by stripping off the ethernet header  30 , or decapsulating the frame, and reading the EP header  28  to obtain a destination address. The packet now appears as shown at  20 B. 
     Next the router prepends the packet with an HDLC header  32  as shown at  20 C. The router has thus encapsulated the message in a HDLC packet format. 
     In practice, the decapsulation and encapsulation processes can be more complex, including compression, encryption and other functions. The present invention breaks these functions into individual elements, and dynamically chains the elements together at runtime, as needed, using a linked chain structure in one embodiment. For example, a chain can be dynamically built at the start of a session. Each session can then have its own chain. 
     A chain walker processes a packet by walking through a chain, passing the packet to each element in the chain, until either processing is completed, the chain is halted, or the process is dropped. Each element also has the ability to “punt” the packet to an external software or hardware function, and the punted packet may be returned anywhere in the chain, or may not be returned at all. 
     FIG. 4 illustrates an embodiment of the present invention. The router shown  15  represents either router  15 A or  15 B of FIG.  2 . While a typical router may have multiple interfaces, only two  51 ,  53  are shown in FIG.  4 . 
     When a packet is received by the router  15 , it is passed to the chain walker  55 . The chain walker  55  in turn retrieves a pointer to a de-multiplexor, or demux, element. Because in this example, interface  51  is an ethernet interface, the demux element  57  performs an ethernet decapsulation operation. For other types of interfaces, other decapsulation functions would be used in the demux chain. 
     The demux operation or element  57  determines the protocol of the next higher level (L 3 ) used by the incoming packet, and passes the packet to an appropriate decapsulation chain. Although it will be understood that there can be many more, four possible decapsulation chains are shown: an ARP decapsulation chain comprising a single element arp_decaps  59 ; a SAP decapsulation chain comprising a single element sap_decaps  61 ; a SNAP decapsulation chain comprising a single element snap_decaps  63 ; and a multi-element IP decapsulation chain  64 . 
     Of course, other decapsulation chains can exist for processing other protocols as well. For illustrative purposes, assume the packet uses internet protocol (IP). In this case, the demux element Ether_decaps  57  returns a pointer to the first element  65  of the IP decapsulation chain  64  to the chain walker  55 , as indicated by dotted line  86 . Logically, decrypt  65  follows ether_decaps  57  as indicated by solid arrow  85 . 
     Decrypt  65  is the first element in the IP decapsulation chain  64 . In this example, the decapsulation chain  64  is shown to comprise a decryption element  65 , a decompression element  67 , and an ip_switch element  69 . In an actual application, some or all of these elements may be absent, or additional elements may be in the chain. 
     In actuality, for IP packets that are switched, there is no decapsulation, although the term “decapsulation chain” is retained. The ip_switch element  69  determines if the packet is to be switched or punted for further processing. For A TCP packet that is destined for the router itself, the actual decapsulation is performed in the IP process after the packet has been punted. Similarly, the arp_decaps element  59  simply punts the packet to the ARP process. 
     As indicated by the dashed lines  79 , the process of chain walking can be implemented by having each chain element return to the chain walker  55  a pointer to the next chain element. The chain walker then calls the next element in the chain. 
     After decapsulation, the router must prepare the packet for further transmission by encapsulating it in the desired protocol, assuming the packet is not intended for the router itself. Dashed line  88  separates the decapsulation chain  64  from the encapsulation chain  74 . The packet is passed from the ip_switch element  69  to an encapsulation chain  74  associated with the intended output HDLC interface  53 , beginning with the ip_rewrite element  70 . Ip_rewrite  70  performs functions such as updating (decrementing) the time-to-live count in an IP packet&#39;s header, and recalculating the check sum for the IP header. 
     The packet is then passed to the compression element  71 , the encryption element  73  and finally to an HDLC encapsulation element  77 . The encapsulation element  77  then passes the packet to the queue  81  for the intended output interface  53 . A scheduler  83  removes the packet from the queue  81  and transmits via the HDLC interface  53 . 
     Packets do not need to be switched in the de-encapsulation chain walk. Rather, they can be handed to an outside process for further processing and then passed back to the start of the encapsulation chain. This is done for ‘ping’ packets that are destined for the router, for example. A chain walk can be stopped, started or resumed from any point on the chain. This allows elements such as an encryption element to pass the packet to an outside process, or piece of hardware, to perform the actual encryption. 
     The processed packet may then be passed back to continue the chain walk. This mechanism is called ‘punt’ and ‘puntback’. The punting element hands the packet to the outside process or hardware. If the punting element wants the packet to resume the chain walk at the next element in the chain, it stores the chain next node pointer in the packet. The puntback routine starts the chain walk with the node previously stored in the packet. 
     For example, as indicated by lines  87 , an element such as encryption element  73  can pass the packet to an encryption process  75  which is external to the chain. This is known as “punting” the process packet. The packet may be returned to the punting element, encryption element  73  in this example, after processing. 
     Alternatively, the chain walk could resume processing with any element in the chain. 
     ARP, SAP and SNAP are various protocols which fall under the IP protocol family. As shown with respect to the ARP, SAP and SNAP decapsulation elements  59 ,  61  and  63  respectively, packets may be processed by additional processes  60 , 62 ,  66  external to the chains. Packets processed by these functions  60 ,  62 ,  66  may then be queued directly at queue  81  without passing through an encapsulation chain. 
     Of course, while only IP, ARP, SAP and SNAP decapsulation chains are shown, additional protocols can also be supported for this interface  51 . Each protocol would have its own decapsulation chain. 
     Note that although the incoming and outgoing interfaces are typically different interfaces, this is not always true. For example, Ping and ARP are protocols where the incoming and outgoing interface is typically the same interface. 
     FIG. 4A illustrates an embodiment of the present invention having a virtual interface  90 , which has been inserted at the interface  88  (FIG. 4) between the incoming and outgoing chains. 
     Lines  88 A and  88 B represent the beginning and ending edges of the virtual interface  90 , respectively. Message packets pass from the ip_switch element  69  to the ip_tunnel encaps element  91  within the virtual interface  90 . The ip_tunnel encaps element  91  encapsulates the IP packet into another IP packet, i.e., it implements IP over IP, and passes the tunneled packet to an ip_switch element  69 A, which may or may not be the same as ip_switch  69  in the incoming chain. The encapsulated IP packet is then passed out of the virtual interface  90  across line  88 B to the encapsulation chain, specifically to the ip_rewrite element  70 . 
     FIG. 4B illustrates chains which are more practical than those of FIG. 4, which shows many elements for illustrative purposes. 
     FIG. 4B shows two interfaces: Serial  1   51 B and Serial  2   53 B. The decaps chain encompasses a decryption element  65 B, a decompress element  67 B, a point to point protocol (PPP) decapsulation element  92 , and an ip_switch element  69 B. The encapsulation chain comprises an ip_rewrite element  70 B, HDLC encapsulation element  77 B, and queue element  81 B, followed by a schedule element  83 B which passes the packet to the output serial port Serail  2   53 B. 
     FIG. 4C is a schematic diagram of an alternative embodiment of the present invention illustrating a non-sequential chain. In this example, the incoming interface is Ethernet  1   51 C and the outgoing interface is Serial  1   53 C. The decapsulation chain comprises an ethernet decapsulation module  57 C followed by ip_switch  69 C. 
     Ip_switch  69 C passes the packet to the encapsulation chain through the interface  88 , to ip_rewrite  70 C which, among other tasks, updates the time-to-live counter and updates the header checksum accordingly. The updated packet is then passed from ip_rewrite to a PPP encapsulation element  94 . 
     The classifier element  95  then examines the packet, for example, for packet type or some other characteristic. For example, packets may be voice or data, as shown in FIG.  4 C. In the case of voice, the packet is passed along path  96 A. Voice data is compressed in this example by compression element  71 C- 1 . 
     In the case of data, the packet is passed along path  96 B. In this example, the packet&#39;s data is compressed by compression element  71 C- 2 , which may use a different compression scheme than the voice compression element  71 C- 1 . The packet is then encrypted by encryption element  73 C. Packets from either path are then passed to queue element  81 C, and finally to the scheduler element  83 C which then outputs the packet through serial interface  53 C. 
     Of course, packets characteristics are not limited to just data and voice. These are simply offered as examples. Packet characteristics on which the classifier  95  may base a decision, include but are not limited to, for example, protocol type, e.g., IP, TCP, UDP, HTML, ARP, FTP, etc., characteristics such as broadcast, multicast, unicast, etc., TCP or UDP port number, source and/or destination address, media type such as voice, data, or video. 
     As with all other elements in the chain, the classifier  95  passes, to the chain walker  55 , a pointer to the next element. However, in one embodiment, the classifier  95  uses internal knowledge plus its analysis of the packet type to decide along which chain path  96 A,  96 B to send the packet. Thus, the chain is not sequential, in that there are multiple branches a chain may take. 
     FIG. 4D is a schematic diagram of another chain which can be implemented by a non-sequential embodiment. FIG. 4D shows just an encapsulation chain. A packet is received, from the incoming interface, or alternatively, from a virtual interface, by the ip_rewrite element  70 B. In the event ip_rewrite  70 B already knows how to encapsulate the packet, it performs the encapsulation, and there is no need to execute the HDLC encapsulation element  77 B. The encapsulation element  77 B is thus skipped by having the packet take the path  97 . In this case ip_rewrite  70 B passes back a pointer to the next element to be executed through path  97 A, the next element being the queue element  81 B. 
     If ip_rewrite  70 B does not know how to encapsulate the packet, it will process the packet normally and return a pointer to HDLC encapsulation element  77 B to the chain walker  55 . 
     Finally, the packet is passed from queue element  81 B to the scheduler  83 B and transmitted out through the Serial  2  port  53 B. 
     The advantage of this embodiment is that it cuts out the extra processing required to do an extra chain walk. Often, ip_rewrite  70 B will know how to encapsulate a packet based on destination. This information may be returned, for example, by the HDLC encapsulation element  77 B after the first encapsulation of the packet. Ip_rewrite  70 B can then store this information and use it for later packets with the same destination or within the session. 
     There are other reasons for having non-sequential chains. For example, different types of encryption could be applied to packets depending on destination, type or some other attribute. Furthermore, ip_rewrite can observe the congestion on the network and for example, skip right to the scheduler if there is no congestion. In very fast Ethernet, for example, a router needs to get packets in and out fast. By allowing the chain elements to be skipped or executed in a non-sequential way, packets are routed through the router expediently. 
     Another example where non-sequential chains is useful is, for example, in implementing an access list or other filter. Different chain elements can be executed depending on the destination and/or depending on access rights. 
     The element which causes the non-sequential skip, such as the classifier  95  of FIG. 4C, can have internal data which allows it to determine, when a chain is built, what the jump targets are. Determination may be based on data stored by or for the classifier  94 , and based on the incoming packet itself. 
     More generally, different feature sets may be applied to packets depending on their source address, destination IP address, types or other attributes. The classifier element  95  can, for example, maintain this information in an internal data base and return, to the chain walker  55 , a pointer to the applicable element. 
     FIG. 5 illustrates the data structures used to implement sequential decapsulation and encapsulation chains, as well as the sequential portions of non-sequential chains. First, a separate interface descriptor block (IDB)  100  is defined for each interface. Each IDB  100  includes several fields including a demux field  100 A, a decaps array pointer  100 B, an encaps array  100 C, queuing information  100 D and scheduling information  100 E. 
     The demux pointer  100 A points to a demux element  102 . The demux element  102  is typically the only element in the demux chain. It is responsible for removing the outer encapsulation, e.g., ethernet or HDLC, of an incoming packet, and for examining the packet to determine which protocol the packet uses. 
     The decaps array pointer  100 B of the IDB  100  points to a decaps array  104 . The decaps array  104  holds pointers  104 A- 104 D to the first elements of various decapsulation chains associated with the various protocols. For example, the first entry  104 A in the decaps array  104  is a pointer to the first element  106  of an IP decapsulation chain  103 . 
     In one embodiment, each chain element has the structure shown at  106 . A next field  106 A points to the next element in the chain. A previous field  106 B points to the previous element in the chain, and is 0 or some other null indicator for the first element. Furthermore, each chain element  106  contains a data processing element pointer  106 C and a control processing element pointer  106 D which point to the actual processing elements. Control data  106 E may also be contained in the chain element  106 . 
     A second chain element  107  is shown. The chain may contain a plurality of elements. Each element returns a pointer to the next element to be executed to the chain walker. 
     Similarly, the encaps array pointer  100 C in the IDB  100  points to an encaps array  108 . Each entry in the encaps array  108  points to an encapsulation element for a specific protocol. For example, the first entry  108 A is a pointer to an ethernet encapsulation element. The next entries  108 B- 108 D contain pointers to encapsulation chain elements for other protocols, such as HDLC. 
     FIG. 6 is a flow chart  120  illustrating an embodiment of the present invention. 
     First, at step  122 , a packet is received on an interface. Then, at step  124 , an IDB is retrieved which is associated with that interface. At step  126 , a pointer to the demux chain is retrieved from the IDB, and passed to the chain walker. 
     The chain walker  55  receives the packet and the pointer from the IDB and sets a return code to 0 at step  130 . At step  132  the chain walker executes the current chain element pointed to by the pointer received from the IDB. A typical chain element is shown at  134 . Each element  134  on the chain processes the packet, and can return the next element to the chain walker or set the return code to 0 to stop the chain walk. The return code is set to a value less than 0 if, for some reason, the element  134  cannot process the packet, in which case the packet is to be dropped. Otherwise, the chain element  134  returns to the chain walker a pointer to the next chain element. 
     As described earlier in the description of FIG. 4, a chain element  134  may punt or pass the packet to an external routine or to hardware  138  for further processing. The external routine or hardware  138  processes the packet, and may return control to the chain element, as shown. Alternatively the external routine or hardware  138  may pass the packet to another chain or may halt the process. 
     A code value is also used to notify the chain walker as to whether it should continue. If the packet was passed to a higher layer for further processing, the code is set to a value that tells the chain walker to do nothing more. If the packet is to be dropped, the code is set to a value that tells the chain walker to drop the packet. 
     When the packet is returned to the chain walker, in step  140  the chain walker determines whether the return code is 0. If not, either the packet is dropped at step  142  or the chain walker stops. If the return code is 0, then at step  144 , the chain walker determines whether a chain element has been returned. If so the chain walker sets the current element to the next element at step  146  and returns to step  132  to execute the next chain element. This loop is repeated until either the packet is dropped or until there are no more chain elements. 
     During initialization, the router&#39;s hardware control software determines which network interfaces exist on the router. An IDB  100  (FIG. 5) is created for each interface. The demux, decapsulation element and the encapsulation element fields are initially empty. 
     A default demux element is associated with each interface based on the interface media type, e.g., ethernet, HDLC, etc. Configuration information is determined for each interface, for example, by reading it from a file or from a programmable memory. Default queuing and scheduling elements are then added to the chain. Finally, decapsulation and encapsulation elements are added to each interface according to the protocols available as determined by the configurations. 
     Once the chain for an interface is built, the interface is ready for packet processing. When a packet is received by an interface it is handed to the chain walker, as described with respect to FIG.  4 . The chain walker has no knowledge of protocols. For an encapsulation of IP over ethernet, the chain might include only an IP decapsulation element, followed by an encapsulation chain comprising ethernet encapsulation, queuing and scheduling elements. 
     Chains can have elements added or deleted based on configuration. Each element in a chain has no specific knowledge of the other elements on the chain. The chain walker has no knowledge of the number of elements or types of protocols on a chain. Each element or node on the chain determines the whether or not the chain walk continues based on its processing of the packet. 
     The above chain could thus be modified by inserting elements into the chains without changing the ethernet, IP, queuing or scheduling elements. For example, an ethernet address filter could be inserted before the IP decapsulation element, or a new compression element could be inserted at the beginning of the encapsulation chain. 
     For packets entering the router from an interface, the chain walker typically starts with the demux element. While there can be any number of encapsulation and de-encapsulation elements chained together on an interface, there is typically only one demux element for each interface. For packets that are sourced on the router, the chain walk starts with the first encapsulation element. 
     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.