Abstract:
A protocol is provided for backing up ATM network devices should they fail. The protocol is implemented in a system running ATMARP and supporting IP over ATM. In the protocol, multiple ATM network devices are combined in a “standby group” and share a common IP address. When an active member of the standby group fails, one of the other members of the standby group takes over ATM responsibility for the functions of the failed device. An ATMARP Server determines which member of a standby group should handle IP packets destined for that group.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application is a continuation of U.S. patent application Ser. No. 09/281,621 filed Mar. 30, 1999 in the name of Prasad Miriyala, and entitled “FLEXIBLE SCHEDULING OF NETWORK DEVICES WITHIN REDUNDANT AGGREGATE CONFIGURATIONS,” which is incorporated herein by reference in its entirety and for all purposes. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to systems and methods for maintaining network functionality when a critical network device fails. More specifically, the invention relates to groups of devices using a procedure for backing up an active device should that device become functionally unavailable.  
         BACKGROUND OF THE INVENTION  
         [0003]    A computer network is a geographically distributed collection of interconnected communication links for transporting data between nodes, such as computers. By definition, a network is a group of computers and associated devices that are connected by communications facilities or links. Network connections can be of a permanent nature, such as cables, or can be of a temporary nature, such as connections made through telephone or other communication links. A plurality of computer networks may be further interconnected by intermediate nodes, or routers, to extend the effective “size” of the networks. A router is computer system that stores and forwards data packets from one local area network (LAN) or wide area network (WAN) to another. Routers see the network as network addresses and all the possible paths between them. They read the network address in a transmitted message and can make a decision on how to send it based on the most expedient route (traffic load, line costs, speed, bad lines, etc.). Routers typically communicate by exchanging discrete “packets” of data according to predefined protocols. In this context, a protocol comprises a set of rules defining how the nodes interact with each other.  
           [0004]    The Asynchronous Transfer Mode (ATM) protocol establishes point-to-point connections over a virtual connection oriented media. A properly configured network device within an ATM system will include an ATM interface and a mechanism for establishing and supporting virtual connections or circuits. The appropriate hardware and/or software for providing ATM interfaces is generally known in the field. In 1991, an entity known as the ATM Forum was founded to standardize ATM technology. A substantial body of information regarding deployment of ATM technology is available from the ATM Forum. Cites to many references pertaining to ATM technology are available through the ATM Forum&#39;s World Wide Web site at www.ATMForum.com. Specific references include McDysan et al., “ATM Theory and Application,” McGraw Hill, 1995; Minoi et al., “ATM &amp; Cell Relay Service for Corporate Environments,” McGraw Hill, 1994; and Prycker “Asynchronous Transfer Mode—Solution for Broadband ISDN, 2nd Edition, Ellis Horwood, 1993. Each of these references is incorporated herein by reference for all purposes.  
           [0005]    Point-to-point connections between ATM nodes are made by “virtual circuits” or “virtual connections.” A virtual connection may be a permanent virtual circuit (PVC) (e.g., a fixed line) or by a switched virtual circuit (SVC) (a temporary virtual connection). Since an SVC is temporary, it is established between two network devices of the ATM network  100  upon demand and then released after a predetermined time period. Note that the connection may include temporarily combining two PVCs and on either side of a switch capable of connecting the two PVCs.  
           [0006]    An ATM network device has an ATM address, such as a VPI address or a VCI address, which is required by a network device in order to establish the SVC with a second virtual device. The ATM address will sometimes be referred to herein as a Network Service Access Point (“NSAP”) Because PVCs dedicate network bandwidth to the two connected nodes (and preclude other nodes from using that bandwidth), PVC use is generally limited. Thus, to increase network applicability, many systems make use of SVCs. The process of establishing an SVC will be described broadly with respect to FIG. 1A.  
           [0007]    [0007]FIG. 1A describes the process of establishing a virtual connection between a network device  104  and a network device  108 . The network devices  104  and  108  have NSAP addresses of NSAP 1  and NSAP 2  respectively. To initiate the connection, the network device  104  constructs a ‘SETUP’ message  136  which indicates a desire to establish a connection with device  108 , and sends it to the NSAP 2  address. Message  136  may require multiple hops to reach device  108 . To simplify the discussion, only a single hop is depicted here. As the SETUP message propagates toward its destination, the network acknowledges receipt of the message with CALL PROCEEDING messages at each hop. As shown, in FIG. 1B, network device  108  replies with a CALL PROCEEDING message  138  if it is merely a hop on the path to the ultimate destination or a CONNECT message  140  if it is the ultimate destination. In this case where NSAP 2  is not the destination, another SETUP message is propagated to the next network device (not shown) along the path to the destination and a CALL PROCEEDING message returns from that device. This procedure continues until connection is made with the destination having NSAP 2 .  
           [0008]    As the setup message  138  propagates to the eventual destination IP address along a number of network devices and switches, a PNNI protocol may be found. The destination may specify a set of protocol parameters of message transmission and return them with the CONNECT message. For example, an AAL 5  platform with a 100 kBs bandwidth and a UBR service may be specified. If the network device  108  agrees to these parameters, the network device  108  will respond with a “CONNECT ACKNOWLEDGMENT” message  142 . In this case, an SVC is established between NSAP 1  and NSAP 2  as well as between any additional switches along the pathway from NSAP 2  to the destination. Once the virtual connections are established, data such as IP datagrams using ATM packets may be sent.  
           [0009]    ATM can be used to run IP in a procedure referred to as IP over ATM. See Laubach, M. and Halpern, J., “Classical IP and ARP over ATM”, RFC 2225 , April 1998 (http://www.ietf.cnri.reston.va.us/home.html), which is incorporated herein by reference for all purposes. In IP over ATM, each ATM host in a set of hosts is assigned its own IP address. The set of ATM hosts forms a logical IP subnet (“LIS”) which acts as a virtual LAN. All members of a LIS are directly connected to the ATM network and have the same IP network/subnet number and address mask. Hosts on the same LIS may exchange IP packets directly, but hosts on different ones are required to go through a router. A LIS may act as a bridge connecting existing LANs.  
           [0010]    To move IP packets along a route from source to destination, in a conventional non-ATM IP network, an Address Resolution Protocol (“ARP”) is used. See Plummer, D., “An Ethernet Address Resolution Protocol—or—Converting Network Protocol Addresses to 48.bit Ethernet Addresses for Transmission on Ethernet Hardware,” STD 37, RFC 826, November 1982 (which is incorporated herein by reference). In such protocol, a network device holding a packet to be delivered asks its peers which one of them is responsible for handling packets having the IP destination address of the packet. The device makes this inquiry via an “ARP packet.” The correct device replies via the Address Resolution Protocol with its hardware address. The device holding the packet then encapsulates that packet with a header indicating the hardware address of the responding device and sends the packet to it.  
           [0011]    In SVC cases, devices must learn the ATM addresses of their peers in order to forward IP packets to them. The ARP protocol immediately suggests itself for this purpose. ARP, as currently implemented and described in RFC 826, requires a broadcast medium (e.g., Ethernet) on which to transmit the ARP request. ATM, which is a point-to-point protocol, cannot support ARP as described in RFC 826.  
           [0012]    One suitable method for transmitting IP datagrams over an ATM network where the destination lower level address is unknown uses ATM Address Resolution Protocol (“ATMARP”) as described in RFC 2225. ATMARP determines the lower level address of the next network device along a suitable path when given the destination IP address. An ATMARP system is typically comprised of an ATMARP Server and numerous ARP Clients who require assistance in transmitting to a destination IP address.  
           [0013]    The ATMARP Server is typically responsible for determining the ATM address of the next network device along a suitable path when given the destination IP address. An ATMARP Client having a packet with a destination  1 P address needs to determine which of its ATM peers should serve as the next hop. It determines this by sending an ATMARP request to the ATMARP server, which resolves the request and returns the ATM address of the ATMARP Client serving as the next hop.  
           [0014]    [0014]FIG. 1B illustrates the components of an ATM network  100  capable of running IP over ATM. The ATM network  100  includes an ATMARP Server  102 . The server  102  is responsible for facilitating associations between ATMARP Clients  104 ,  106  and  108 . Physically, the server  102  as well as the clients  104 ,  106  and  108  may be any conventional network device including routers or bridges. They may also be conventional hosts configured to run ATM.  
           [0015]    Because individual ATM network devices are incapable of broadcasting an ARP message, the ATMARP Server  102  acts in conjunction with the network devices to facilitate transmission to a destination IP address. For example, the ARP Server  102  may return the appropriate ATM address (NSAP) when given an IP destination request by the ARP Client  104 . For this purpose, the server  102  and the network device  104  are shown to have a virtual connection  112 .  
           [0016]    As an example of ATMARP, consider an IP message from a node handled by device  104  to a node handled by device  108 . Device  104  has the message with its associated destination IP address but does not know which of its ATM peers should act as the next hop. To identify this device, network device  104  (IP address  1 ) sends an ARP request over the virtual connection  112  to the ATMARP Server  102  requesting the ATM address of the device handling transmissions to the destination IP address (IP address  3 ). The server  102  determines that IP address  3  corresponds to NSAP address  3  (device  108 ) and then responds, along the connection  112 , with an NSAP address (NSAP 3 ) corresponding to network device  108  (IP address  3 ). The NSAP address corresponding to network device  108  provided by the server  102  allows the network device  104  to set up a virtual connection  114  with the network device  108  and thus send the data packet. Typically, the ATMARP Server  102  is capable of providing an ATM address for each network device it is connected to.  
           [0017]    [0017]FIG. 2 illustrates a problem that can arise using the ATMARP protocol on an ATM network such as the ATM network  100  illustrated in FIG. 1B. ATMARP Clients  104 ,  106  and  108  are distinguishable by NSAP addresses NSAP 1 , NSAP 2  and NSAP 3  respectively. ATMARP Clients  104  and  108  are coupled to external networks (networks beyond ATM network  100 ) such as Internet  206  and a private local network  210 . In the illustrated environment, the ATMARP Client  104  may be a gateway router leading to the Internet  206 , which includes an entity  206  connected to the Internet  206 . The ATMARP Client  108  connects with the local network  210 , which includes various network nodes such as an arbitrary entity  212 .  
           [0018]    When a data packet is to be sent from entity  208  to the arbitrary entity  212 , a series of steps is taken in order to establish the required network connections. First, the packet from entity  208  must proceed through the relevant connections in the Internet  206  to reach network device  104 . At this point, the packet must proceed through the ATM network  100  to reach the ATMARP Client  108 . If the virtual connection does not exist, then the corresponding low level NSAP address is required to establish a virtual connection  114 . First, the Client  104  may check an internal cache (corresponding to a list of ATMARP entries that may have been stored) to find the low level NSAP address. If it is not in the internal cache, the ATMARP Client  104  relays an ATMARP request specifying the IP address of device  108  to the server  102 .  
           [0019]    At this point, the ATMARP Server  102  checks whether there is an existing NSAP address for the destination IP address in a cache which stores existing external responsibilities of the ARP Clients it is responsible for. The ATMARP Server responds with the NSAP address for ATMARP Client  108 . After the ARP Client  104  receives the ARP response corresponding to the ARP Client  108  NSAP address, the ARP Client  104  proceeds to establish a virtual circuit  114  with the ARP Client  108  in the manner described in FIG. 1A.  
           [0020]    Suppose that ARP Client  108  malfunctions, breaks down or temporarily shuts down for service, and thus the virtual connection  114  cannot be made. The data package is thus incapable of reaching its destination and communication with nodes on network  210  via ATM network  100  is impossible. If ARP Client  108  is the sole link for handling access to local network  206 , local network  210  is essentially shut off from all external communication. This inability to communicate will persist until the faulty network device is corrected. As there may be hundreds of network devices relying on the ATM link through device  108 , this inability to communicate through a single non-functioning network device seriously compromises the effectiveness of ATM switching systems.  
           [0021]    In view of the foregoing, a technique for protecting against failure of a network device in an LIS running ATMARP would be highly beneficial.  
         SUMMARY OF THE INVENTION  
         [0022]    The present invention provides systems and methods for backing up ATM network devices should they fail. The invention may be conveniently implemented in a system running ATMARP and supporting IP over ATM. In the invention, multiple ATM network devices are combined in a “standby group” and share a common IP address. When an active member of the standby group fails, one of the other members of the standby group takes over ATM responsibility for the functions of the failed device. In the context of ATMARP, an ATMARP Server determines which member of a standby group should handle IP packets destined for that group.  
           [0023]    The present invention relates in accordance with one embodiment to a method of providing a network service using a standby group of ATM network devices within an ATM network. Each ATM network device within the standby group has its own ATM address and shares a non-ATM network address with other members of the standby group. The method includes determining that a first member of the standby group of network devices is not available to provide the network service. The method also includes identifying a second member of the standby group of network devices to provide the network service.  
           [0024]    The present invention relates in accordance with another embodiment to a method, for a single network device, of providing a network service using a standby group of ATM network devices within an ATM network. Each ATM network device within the standby group has its own ATM address and shares a non-ATM network address with other members of the standby group. The method includes sending a notification identifying the first network device by its ATM address and shared non-ATM network address. The method also includes receiving one or more packets destined for the shared non-ATM network address.  
           [0025]    The present invention relates in accordance with another embodiment to a server for use in an ATM network including a plurality of network devices. The server includes one or more processors and at least one interface for establishing a connection between the server and a network device of the plurality of network devices. The server also includes a collection of entries, wherein each entry corresponds to a network device. The entry includes the corresponding network device&#39;s ATM address, a shared non-ATM address used by the corresponding network device and one or more other network devices, and a value used in determining whether the network device corresponding to the entry is currently acting as the device having the non-ATM address.  
           [0026]    The present invention relates in accordance with another embodiment to a network device for use in an ATM network having a plurality of network devices and a server. The network device includes one or more processors. The network device also includes at least one interface for establishing a connection between the network device and a second network device. The network device further includes an ATM address and additionally includes a non-ATM address shared by at least one other network device of the ATM network.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]    The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which:  
         [0028]    [0028]FIG. 1A describes the process of establishing a virtual connection between two network devices in a simple ATM network.  
         [0029]    [0029]FIG. 1B illustrates the components of a simple ATM network that supports ATMARP.  
         [0030]    [0030]FIG. 2 illustrates a problem that can arise if one of the ATM nodes fails in the network shown in FIG. 1B.  
         [0031]    [0031]FIG. 3 illustrates an ATM system including standby groups in accordance with one embodiment of the present invention.  
         [0032]    [0032]FIG. 4 illustrates two standby groups of ATMARP Clients, each capable of servicing a separate group of nodes and each having a unique IP address.  
         [0033]    [0033]FIG. 5 illustrates a sub-interface structure of an ARP Client suitable for supporting multiple IP addresses in accordance with one embodiment of the present invention.  
         [0034]    [0034]FIG. 6A illustrates a cache or table in which the ATMARP Server maintains a table of information including information for each ARP Client in the ATM system in accordance with one embodiment of the present invention.  
         [0035]    [0035]FIG. 6B illustrates the cache of FIG. 6A at a later time after one of the server&#39;s ATMARP Clients&#39; has failed.  
         [0036]    [0036]FIG. 7 illustrates a method in which the ATMARP Server uses a standby ATMARP Client upon non-response or unavailability of a primary ATMARP Client within a standby group.  
         [0037]    [0037]FIG. 8 illustrates an extension of the method of FIG. 7 subsequent to the return of the primary ATMARP Client.  
         [0038]    [0038]FIG. 9 illustrates the format of an exemplary KEEP ALIVE Message, which may be sent within a given time-out period from an ARP Client to the ATMARP Server.  
         [0039]    [0039]FIG. 10A is a block diagram of a hardware system or apparatus that may be employed to implement an ATMARP Server in accordance with an embodiment of this invention.  
         [0040]    [0040]FIG. 10B is a block diagram of a hardware system or apparatus that may be employed to implement an ATMARP Client or Server in accordance with one embodiment of this invention. 
     
    
     DETAILED DESCRIPTION  
       [0041]    In the following detailed description of the present invention, numerous specific embodiments are set forth in order to provide a thorough understanding of the invention. However, as will be apparent to those skilled in the art, the present invention may be practiced without these specific details or by using alternate elements or processes. In other instances well known processes, procedures, components, and circuits have not been described in detail so as not to unnecessarily obscure aspects of the present invention.  
         [0042]    The present invention provides a standby group of ATMARP clients, which together reduce the likelihood of a network transmission failure due to the unavailability of a single network device. The ATMARP Clients may run IP over ATM and may share an IP address. When one device running IP breaks down, the effectiveness of the ATM network (or network portion such as a LIS) running IP is not compromised. The grouping of network devices may provide one or more redundant ATM devices for an IP address. The entire standby group may be viewed as a virtual ATMARP Client within the network. Typically, the groups are organized such that a primary network device assumes the functions of the shared IP address and the remaining network devices in the group are in a standby state ready to take over the functions in the event that primary network device is unavailable. The device currently handling the responsibilities of the shared IP address will be referred to herein as a “primary” or “active” network device.  
         [0043]    In a specific embodiment, the present invention is implemented by configuring all the ATMARP Clients with a shared IP address (usually in addition to one or more other IP addresses) and distinct priority levels in order to derive an implementation order. In such a design, the available network device having the highest priority for the group is responsible for the responsibilities of the primary network device. Such responsibilities include, for example, routing IP packets to and from a local network or network segment. Such segment need not run ATM. Alternatively, the IP address may represent a LAN entity on an ATM network (or LIS) emulating a LAN. An ATMARP Server keeps track of the network devices, their corresponding addresses and required group information. Thus, upon failure of the primary network device for a particular group, the server may readily direct a client to an alternate network device.  
         [0044]    Note that the invention is illustrated by an ATMARP protocol implemented in an IP over ATM network. The invention need not be so limited. Generally any ATM network running a network layer or other higher level protocol may profit from this invention. Thus, the network addresses resolved by a server, as described herein, need not be IP addresses in some embodiments. Further, the client-server protocol need not be an ARP protocol. As long as the server is providing some service to ATM clients and those clients can profit from a standby group, the invention may be applicable. In a preferred embodiment, the invention employs many of the processes described in RFC 2225 and subsequent descriptions of ATMARP protocols.  
         [0045]    [0045]FIG. 3 illustrates an ATM network group  300  (e.g., an LIS) in accordance with one embodiment of the present invention. The ATM system  300  is capable of implementing the ATMARP protocol and includes an ATMARP Server  302 , which is responsible for implementing ARP protocol together with ARP Clients  304 ,  306 ,  308 ,  310  and  312 . A standby group is defined as a set of ATMARP Clients sharing a common non-ATM or lower level address (e.g., they share an IP address). For example, a group may be responsible for routing traffic to and from an external network such as a WAN, LAN, or the Internet. In FIG. 3, a Group  1  includes ARP Clients  306 ,  308  and  310  and assumes the responsibilities of an IP address denoted “IP- 1 ” herein. More specifically, standby group  1  is responsible for communication to and from an external network  316 . Network group  300  also includes a Group  2  that includes ARP Clients  304  and  306  and assumes the responsibilities of an IP address denoted “IP- 2 ” herein. Group  2  is responsible for communication to and from an external network  318 . Note that the IP routing protocols used by the ATM devices in group  300  may be configured to direct packets addressed to external network  316  through a device having the network address IP- 1  and to direct packets addressed to external network  318  through a device having the network address IP- 2 .  
         [0046]    To specify an active device within a standby group, each ARP Client within that group may be given a value distinguishing it from the other ARP Clients in the group. For example, the value may be a priority configured relative to the other ARP Clients within a group. In this case, the priority indicates the order in which the individual ARP Clients within the group are implemented to service the IP address corresponding to the group. For Group  1 , ARP Clients  310 ,  308  and  306  are assigned priority values of 100, 90 and 80 respectively. With this priority designation, the available ARP Client having the highest priority would be the primary ARP Client and would be responsible for servicing the shared IP address (IP- 1 ) associated with Group  1 . In the event that the highest priority ARP Client cannot function properly or is unavailable to service the IP address, the ARP Client corresponding to the next highest priority will service the IP address. In other words, upon functional failure of the ARP Client  310  to transmit data within the ATM network  300  or across a connection  320  to external network  316 , the ATMARP Server  302  may designate ARP Client  308  to handle traffic (via a connection  322 ) to and from external network  316 .  
         [0047]    Similar to Group  1 , the clients within Group  2  also provide redundant virtual connection capabilities to the external network  318 . In this case, ARP Client  304  normally acts as the primary ARP Client for Group  2  since it has the highest active priority. The ARP Client  306  acts as a standby in the event of failure of the ARP Client  304  to allow an alternative communication path leading to the external network  318 .  
         [0048]    It is important to note that a single ARP Client may be responsible for multiple IP addresses or network connections. For example, the ARP Client  306  maintains standby status with respect to Group  1 , as well as a standby status with respect to Group  2 . It must possess the shared IP addresses for both of these standby groups (i.e., IP- 1  and IP- 2 ). It may possess other IP addresses (not illustrated) that it uses in other roles. Thus, it is entirely possible that an individual ARP Client may be responsible for routing traffic simultaneously over multiple IP addresses. Further, at any given instance in time, a single ATM device may be the active device in two or more standby groups. For example, in the event that the ARP Clients  310  and  308  are shut down for servicing, coupled with failure of the ARP Client  304 , ARP Client  306  will be dually responsible for servicing connections to and from external networks  316  and  318 .  
         [0049]    As two ARP Clients within a group may have the same IP address, distinguishing the two is thus the responsibility of the ATMARP Server  302  using the relevant lower level address system. For example, in addition to the IP address designations, ARP Clients  310 ,  308  and  306  may have NSAP address designations of NSAP- 1 , NSAP- 2  and NSAP- 3 , respectively.  
         [0050]    The ATMARP Clients may be any type of network device configured to handle ATM traffic. Common examples include routers, switches, cable modem termination systems, and the like. Note that the invention is not limited to devices devoted entirely to routing or otherwise controlling network traffic. The invention may also apply to work stations, personal computers, laptop computers, and other such devices that can run ATM and another network protocol such as IP.  
         [0051]    Specific ATM-capable hardware includes the  7000  series of routers and the LightStream® line of ATM switches available from Cisco Systems, Inc. of San Jose Calif. In addition, add-on ATM adapters are available from Cisco Systems and other companies providing networking equipment.  
         [0052]    The ATMARP Server  302  may generally be any server that provides ATMARP information for a collection of ATMARP Clients, which look to it for such configuration information. Preferably, the ATMARP information is arranged such that the relevant information is stored in non-volatile memory. One way of achieving this result is through a cache or list of entries for the ATMARP Clients. The cache may include such standard ATMARP information as the NSAP addresses and associated IP addresses of the individual ATMARP Clients. It may, in addition, include respective priorities of the individual ARP Clients. An arrangement of ATMARP entries suitable for implementation of the present invention will be described with reference to FIG. 6A.  
         [0053]    An ATMARP Server  302  suitable for implementing the present invention may include a central processing unit (CPU)  314 , memory  319 , and one or more ATM interfaces (not shown). When acting under the control of appropriate software or firmware, the CPU  314  is responsible for such router tasks as routing table computations and network management. It may also be responsible for issuing ARP Client communications, applying configuration data, etc. It preferably accomplishes all these functions under the control of software including an operating system (e.g., the Internetwork Operating System (IOS®) of Cisco Systems, Inc.) and any appropriate applications software. Memory  319  stores configuration information for server  302  and may also store the list of ATMARP entries described above.  
         [0054]    Server  302  may be a conventional computer or workstation outfitted with one or more appropriate ATM interfaces or it may be router or other network device. Further structural details of a workstation embodiment are presented in FIG. 10A and further structural details of a network device embodiment are presented in FIG. 10B.  
         [0055]    The aggregation of individual ARP Clients into standby groups may be undertaken by configuring appropriate entries in the ATMARP server  302  and by configuring the appropriate IP addresses on the ATMARP Clients of the standby group. As mentioned, a logical representation of ATMARP server entries is presented in FIG. 6A and described below. Individual ATMARP Clients may be configured with standby or shared IP addresses in the same manner that they are configured with any IP address. In one embodiment, the Dynamic Host Configuration Protocol (DHCP) is employed. See RFC 2131, which is incorporated herein by reference for all purposes.  
         [0056]    Various criteria may be employed to determine membership in standby groups and priority designation of individual ATMARP Clients making up the standby groups. For example, the anticipated traffic to and from external network  318  may be used in determine which clients and how many clients should be used in a standby group. If the external network will have a high volume of traffic, the standby group may include multiple high capacity ATMARP clients. Other criteria influencing the likelihood of failure of any device within the standby group, the service schedule of the devices, etc.  
         [0057]    Algorithms may be applied in any manner as to aid in allocating of the ARP Clients within the ATMARP system  300 . As the number of ARP Clients within the ATMARP system  300  may number in the hundreds, ARP Client configurations within the ATMARP network  300  based on software algorithms becomes more valuable as the ATMARP network  300  complexity grows.  
         [0058]    In many applications it is common for an ATMARP Client to be responsible for a high volume of traffic—at least temporarily. To facilitate the efficient transfer of data over the potentially overloaded ATMARP Client, load sharing among individual ARP Clients within the ATMARP system  300  may be implemented. For example, if the ATMARP Client  310  servicing external network  316  is periodically responsible for handling an excessive amount of data which may compromise the transmission of data, the system (or an administrator) may designate an alternate ATMARP Client such as ARP Client  308  to facilitate efficient data transmission. Together, Clients  310  and  308  serve as a standby group for the purpose of load sharing.  
         [0059]    Alternately, load sharing may be implemented between groups of the present invention. For example, for the case in which the External Entities  316  and  318  respectively represent two excessively large networks such as a large company&#39;s WAN or the Internet, the ATMARP system  300  will be responsible for the large Volume of data being transmitted between the two entities. For the illustrative case in which twenty ARP Clients are included in Group  1  which are normally responsible for Internet communication, and three ARP Clients are included in Group  2  which are normally responsible for the WAN External Entity, load sharing may be implemented between Group  1  and Group  2 .  
         [0060]    As mentioned, virtual connections in an ATM network may be classified as either a permanent virtual connection (PVC) or a switched virtual connection (SVC). A PVC is a virtual connection that does not change over time and is analogous to a leased line. Alternately, an SVC is a temporary virtual connection. Thus, SVCs are continually established within the network  300  and maintained as necessary for a suitably finite period of time. More specifically, the SVCs may be flexibly dissolved at a predetermined time in order to ease overhead within the ATM system. For example, a SVC may be specified to have a duration of five seconds. Further, for SVCs that are continually used, the duration may be much longer, or reset upon use, in order to reduce SVC establishment overhead within the ATM system.  
         [0061]    Note that SVCs fall into various service categories, of different costs, including CBR (Constant Bit Rate), VBR (Variable Bit Rate), ABR (Available Bit Rate), and UBR (Unspecified Bit Rate). The type of service chosen typically depends upon the type of data being transferred. For example, audio or video transmission is conventionally sent using CBR or real-time VBR, while background file transfer is usually made using UBR.  
         [0062]    [0062]FIG. 4 illustrates an ATMARP LIS  400  capable of serving two separate groups each corresponding to a unique IP address. For example, Group  1  includes ARP Clients  404  and  406  and is responsible for servicing an IP address of 20.1.1.1. Group  2  includes ARP Clients  408  and  406  and is responsible for servicing an IP address of 10.1.1.1. A set of External Entities  410  and  412  employ the ATM services Group  1  and Group  2  respectively. For example, the External Entities  410  and  412  may represent two large WAN networks for a very large company. Or they may represent large computers that require ATM services. In the case of transmission between the External Entities  410  and  412  where the ARP Client  404  and the ARP Client  408  both go down or are unable to carry out transmission, service to and from both External Entities  410  and  412  must be solely serviced by the ARP Client  406 .  
         [0063]    The number of IP interfaces for a specific ATMARP Client will depend on the particular router (or other network device) used. In order for such device to serve in a standby group and carry out other dedicated functions outside the group, it must present multiple IP addresses. In one embodiment, separate interfaces within a given network device are used for a standby IP address and a second IP address. Some network devices, such as a Cisco  3810  router, provided by Cisco Technology, Inc. of San Jose, Calif., have only a single interface. One manner in which to implement the present invention is to divide an interface into multiple sub-interfaces, each having a separate IP address.  
         [0064]    [0064]FIG. 5 illustrates an ATMARP Client having a sub-interface system in accordance with one embodiment of the invention. For an ARP Client  502  with one interface  506 , the interface  506  may be divided into sub-interfaces  508 ,  510  and  512  wherein each sub-interface presents an individual IP address. Each sub-interface is capable of providing the transmission capabilities required for a normal IP interface. The IP address of each sub-interface is mapped to an NSAP address, which may be repeated between sub-interfaces of the Client  502 . For example, the IP addresses for sub-interfaces  508  and  510  correspond to NSAP 6  while the IP address for sub-interface  512  corresponds to NSAP 8 . It is important to note that while an IP address is shared among the members of a standby group within the ATM system  300 , the lower level NSAP addresses (ATM addresses in this case) normally are not shared within a group. Note that a single device may reside in two or more standby groups, and therefore one of its NSAP addresses may appear in two distinct standby groups.  
         [0065]    For ATM interface  506 , each sub-interface has at least one IP address corresponding to a unique transmission responsibility. For example, a sub-interface  508  (denoted ATM 0.1) may specify an IP address of 10.1.1.1 and be responsible for routing all video data to and from an External Entity using classical IP. A sub interface  510  (denoted ATM 0.2) may have an IP address of 20.1.1.1 and may be responsible for LAN emulation on an ATM network. A third interface  512  (denoted ATM 0.3) may be represented by an IP address of 76.32.1.9 and may be responsible for video and voice transmission.  
         [0066]    One method of implementing sub-interfaces  508 ,  520  and  512  for a single hardware interface  506  is using software. The structure may be stored in any suitable non-volatile memory  514  located within the ATMARP Client  502 .  
         [0067]    [0067]FIG. 6A illustrates a collection of entries  600 , arranged in table format, in which the ATMARP Server  302  maintains information for each ATMARP Client in the ATM system  300 . The collection  600  may be stored or cached in the non-volatile memory of the ATMARP Server  302 . The collection of entries  600  includes an entry for each unique interface in an ARP Client within the system  300 . Thus, the entries  602 ,  610 ,  612 ,  614 ,  616 ,  618 ,  620  and  622  each represent a unique interface corresponding to a unique combination of an IP address and an NSAP address within the ATMARP network  300 . Each entry in table  600  includes an IP address component  604 , an NSAP address component  606  and a priority component  608 . Each of these components may be provided in a field having the necessary number of bytes for unambiguously designating its respective address or priority.  
         [0068]    Note that the table includes eight separate NSAP addresses, which may correspond to eight separate ARPATM Clients. A first standby group, specified by entries  602 ,  610 , and  612 , shares the IP address denoted by “IP-1” and includes devices having ATM addresses given by NSAP 1 , NSAP 2 , and NSAP 3 . Within this group, the device denoted by NSAP 1  is the active device because it has a higher priority than any of its peers in the group. A second standby group, specified by entries  614 ,  616 , and  618 , shares an IP address denoted by “IP-2” and includes devices having ATM addresses denoted by NSAP 2 , NSAP 4 , and NSAP 5 . The device denoted by NSAP 5  is the active device in this group because it has the highest priority. Note that the device specified as NSAP 2  participates in both standby groups. Entries  620  and  622  in table  600  specify ATMARP Clients that are not participating in any standby groups. This is evident by the fact that they have IP addresses that do not appear in any other entries in table  600 . Note also that they do have associated priorities.  
         [0069]    [0069]FIG. 6B illustrates the table  600  of FIG. 6A at a subsequent time. During this subsequent time, one of the individual ARP Clients has become unavailable within the ATM network  300 . In this example, the ARP Client  310  corresponding the NSAP address of NSAP 1 , which is currently the primary ARP Client for Group  1  (which is responsible for servicing IP- 1 ), may be shut down for service. The ATMARP Server  302  may thus adjust the table  600  to reflect the non-availability of the ARP Client  310 . As illustrated in FIG. 6B, the entry  602  corresponding to the ARP Client  310  is removed from the table  600  until the ATMARP Client  310  again becomes available. Correspondingly, the next highest priority ATMARP Client within Group  1  may be designated as the primary ARP Client for IP- 1  (ATMARP Client  2 ). Similarly, upon the notice of availability of ARP Client  310 , the table  600  may add the entry  602  and thereby reinstate the ARP Client as the primary ARP Client for Group  1 . Thus, the table  600  may facilitate the implementation of alternate network devices in order to avoid single network device dependency on order to maintain communication with the external network  316 .  
         [0070]    The table  600  is one suitable example of how the ATMARP Server  302  may maintain priority scheduling of the ATM system. The server may use any suitable table, chart or data management system capable of managing the ATMARP Clients flexibly according to their redundant availability. For example, rather than deleting entries that have become unavailable, the system may provide an additional component such as an “availability flag” for each entry in table  600 . The flag may be, for example, a one-bit tag added to or within the entry that signals the availability of an ATMARP Client. Obviously, alternate methods of configuring and manipulating the information required in scheduling the network devices within an ATM network can be easily implemented, as one skilled in the art would appreciate.  
         [0071]    In order to maintain the table  600  within the ARP system  300 , each individual ARP Client may be responsible for informing the ATMARP Server  302  of its availability. For example, a message may be sent on a periodic basis, which informs the ATMARP Server  302  of the functional status of an individual ARP Client. A “KEEP ALIVE Message” is a periodic message sent by an ARP Client that signals the availability of the ATMARP. Client when processed by the ATMARP Server  302 . If a KEEP ALIVE Message is not received for an individual ATMARP Client within a specified time period, then the negligent ARP Client may be considered unavailable and may be removed from the cache  600 .  
         [0072]    The temporally based signals may be sent at flexibly predetermined time periods in order to reduce system traffic or increase system maintenance resolution. Additionally, other manners of ARP Client health reporting may be administered. For example, ARP Clients may be probed by the ATMARP Server  302  sequentially according to their location within the table  600 .  
         [0073]    [0073]FIG. 7 illustrates a method  700  in which an ATMARP Server  702  schedules a standby ATMARP Client  708  upon non-response or unavailability of a primary ATMARP Client  706  client in accordance with one embodiment of the present invention. An ATMARP Client  704  having IP- 1  address and NSAP 1  address desires to transmit data with an ATMARP Client having an IP address of IP- 2 . This IP address is shared by ATMARP Client  706  (ATM address NSAP 2 ) and ATMARP Client  708  (ATM address NSAP 3 ). Together clients  706  and  708  form a standby group  1  using an IP- 2 . Group  1  may also include other ARP Clients configured with the IP- 2  address. ATMARP Client  706  acts as the primary ATMARP Client within Group  1  while the ARP Client  708  has the second highest priority within the group  708  and is currently on standby.  
         [0074]    To make connection with the device acting on behalf of IP address IP- 2 , ARP Client  704  initially sends an ATMARP request ( 736 ) to the ATMARP Server  702  requesting the NSAP address of the device configured as IP- 2 . In accordance with conventional ATMARP protocol, the ATMARP Server  702  performs address resolution ( 737 ) to identify the NSAP address of the active device corresponding to destination IP address of the request (IP- 2 ). Note that ATMARP Server  702  may update its entry for ATMARP Client  1  with information about that client contained in the ARP request.  
         [0075]    Thereafter, ATMARP Server  702  then sends an ATMARP reply ( 738 ) containing an NSAP address (NSAP 2 ) to the ATMARP Client  704 , thereby enabling the ARP Client  704  to establish a virtual connection with the ARP Client  706 . Upon receipt of the NSAP 2  address from the ATMARP Server  702 , the ARP Client  704  configures a setup message ( 739 ) for the ARP Client  706 . In a manner analogous to that described with reference to FIG. 1A, a virtual connection is established ( 742 ) between the ARP Client  704  and the ARP Client  706 .  
         [0076]    Subsequent to the establishment of the virtual connection  740 , the ARP Client  706  goes down ( 744 ). At the next subsequent time-out period, the ARP Client  706  will not broadcast a KEEP ALIVE Message ( 746 ). As described above, notice of the unavailability of the ARP Client  706  is established within the ATMARP Server  702  due to the lack of a KEEP ALIVE Message. The ATMARP Server  702  then removes the ARP Client  706  as the primary ARP Client for Group  1  and promotes the standby ARP Client  708  to service the IP- 2  address.  
         [0077]    At a subsequent time, the ARP Client  704  is required to deliver another data package to a device having an IP address of IP- 2 . Similar to the case above, after determining that an NSAP address is not available in its local cache, the ARP Client  704  sends an ATMARP request ( 750 ) to the ATMARP Server  702 . After address resolution ( 752 ), the ATMARP Server  702  then responds ( 754 ) with the NSAP 3  address corresponding to the ATMARP Client  708 . The ARP Client  704  then addresses the NSAP address received from the ATMARP Server  702  ( 756 ) and prepares a setup message ( 760 ) to establish a virtual connection  758  with the ATMARP Client  708 .  
         [0078]    [0078]FIG. 8 illustrates a continuation of the method  700  subsequent to a return  802  of the ATMARP Client  706 . Initially, client  706  constructs a KEEP ALIVE message at  803 . Thus, the return  802  may be signaled by the ATMARP Client  706  sending its KEEP ALIVE message ( 804 ) to the ATMARP Server  702 . Upon receipt of the KEEP ALIVE Message, the ATMARP Server  702  will update the cache ( 806 ) indicating that the ARP Client  706  is active and thus reinstate the ARP Client  706  as the primary ARP Client for IP- 2 .  
         [0079]    [0079]FIG. 9 illustrates an exemplary KEEP ALIVE Message  900  that may be sent at a given time-out period from the ATMARP Client  706  to the ATMARP Server  702 . The KEEP ALIVE Message  900  contains the necessary information for identifying the ATMARP Client  706  and reporting the ARP Client&#39;s status. An ATM header portion  902  provides information that references the KEEP ALIVE Message  900  to the ATM system  700  using suitable protocol. An IP address portion  904  allows referencing of the ARP Client to a specific IP address. An NSAP address portion  906  provides the lower level address designation within the ATMARP system  700 . In addition, the KEEP ALIVE Message  900  may also include a priority section  908  which designates the priority level of the respective NSAP address within aggregate group.  
         [0080]    Although the KEEP ALIVE Message  900  has been described with reference to uniquely identifying the status of a single ATMARP Client, the ATMARP Client  706  may also identify the status of all the sub-interfaces for an ATMARP Client. In the case where the KEEP ALIVE Message  900  is sent within a single ATM cell, an appropriately sized cell may be used. For example, a 52-byte cell may be used in which five bytes may be used for the ATM cell header  902 , leaving 47 bytes for the remaining three or more sections. Obviously, the size of the cell may be varied to accommodate the needs of the system.  
         [0081]    Note that an ATMARP request typically contains the NSAP address and IP address of the sending client. This information may also be used by the ATMARP Server to note that client is still alive. It may also be used to update an entry in its cache. See RFC 2225. Preferably, the KEEP ALIVE messages are send using SVCs having UBR service. This limits the network bandwidth dedicated to notifying the server of client availability.  
         [0082]    Generally, standby methods of this invention can be implemented on software and/or hardware. For example, they can be implemented in an operating system kernel, in a separate user process, in a library package bound into network applications, on a specially constructed machine, or on a network interface card. In a preferred embodiment of this invention, standby group technology is partially implemented in server software for accessing and reporting the ATM address of a currently active network device. It is also partially implemented in client code on a network device. Both components may be implemented in an operating system or in an application running on an operating system.  
         [0083]    [0083]FIG. 10A illustrates a typical computer system that may be used to run server software in accordance with an embodiment of the present invention. The computer system  1000  includes any number of processors  1002  (also referred to as central processing units, or CPUs) that are coupled to storage devices including primary storage  1006  (typically a random access memory, or “RAM”), primary storage  1004  (typically a read only memory, or “ROM”). As is well known in the art, primary storage  1004  acts to transfer data and instructions uni-directionally to the CPU and primary storage  1006  is used typically to transfer data and instructions in a bi-directional manner. Both of these primary storage devices may include any suitable type of the computer-readable media described above. A mass storage device  1008  is also coupled bi-directionally to CPU  1002  and provides additional data storage capacity and may include any of the computer-readable media described above. The mass storage device  1008  may be used to store programs, data and the like and is typically a secondary storage medium such as a hard disk that is slower than primary storage. It will be appreciated that the information retained within the mass storage device  1008 , may, in appropriate cases, be incorporated in standard fashion as part of primary storage  1006  as virtual memory. A specific mass storage device such as a CD-ROM  1014  may also pass data uni-directionally to the CPU.  
         [0084]    CPU  1002  is also coupled to an interface  1010  that includes one or more input/output devices such as such as video monitors, track balls, mice, keyboards, microphones, touch-sensitive displays, transducer card readers, magnetic or paper tape readers, tablets, styluses, voice or handwriting recognizers, or other well-known input devices such as, of course, other computers. Finally, CPU  1002  is coupled to an ATM network using a network connection as shown generally at  1012 . With such a network connection, it is contemplated that the CPU might receive information from the network, or might output information to the network in the course of performing the above-described method steps. The above-described devices and materials will be familiar to those of skill in the computer hardware and software arts.  
         [0085]    A network device that is configured in accordance with this invention (as an ATMARP Client or an ATMARP Server) typically includes multiple network interfaces including frame relay and ISDN interfaces, for example. Specific examples of such network devices include routers and switches. For example, the standby systems of this invention may be specially configured routers such as specially configured router models  1600 ,  2500 ,  2600 ,  3600 ,  4500 ,  4700 ,  7200 ,  7500 , and  12000  available from Cisco Systems, Inc. of San Jose, Calif. A general architecture for some of these machines will appear from the description given below. The invention may be at least partially implemented on a card (e.g., an interface card) for a network device or a general-purpose computing device.  
         [0086]    Referring now to FIG. 10B, a router  1011  suitable for implementing the present invention includes a master central processing unit (CPU)  1062 , interfaces  1068 , and a bus  1015  (e.g., a PCI bus). When acting under the control of appropriate software or firmware, the CPU  1062  is responsible for such router tasks as routing table computations and network management. It may also be responsible for constructing or processing ATMARP requests, identifying the appropriate ATM address of an IP device, etc. It preferably accomplishes all these functions under the control of software including an operating system (e.g., the Internetwork Operating System (IOS®) of Cisco Systems, Inc.) and any appropriate applications software. CPU  1062  may include one or more processors  1063  such as a processor from the Motorola family of microprocessors or the MIPS family of microprocessors. In an alternative embodiment, processor  1063  is specially designed hardware for controlling the operations of router  1011 . In a preferred embodiment, a memory  1061  (such as non-volatile RAM and/or ROM) also forms part of CPU  1062 . However, there are many different ways in which memory could be coupled to the system.  
         [0087]    The interfaces  1068  are typically provided as interface cards (sometimes referred to as “line cards”). Generally, they control the sending and receiving of data packets over the network and sometimes support other peripherals used with the router  1011 . Among the interfaces that may be provided are ATM interfaces, frame relay interfaces, cable interfaces, DSL interfaces, token ring interfaces, and the like. In addition, various very high-speed interfaces may be provided such as fast Ethernet interfaces, Gigabit Ethernet interfaces, ATM interfaces, HSSI interfaces, POS interfaces, FDDI interfaces and the like. Generally, these interfaces may include ports appropriate for communication with the appropriate media. In some cases, they may also include an independent processor and, in some instances, volatile RAM. The independent processors may control such communications intensive tasks as packet switching, media control and management. By providing separate processors for the communications intensive tasks, these interfaces allow the master microprocessor  1062  to efficiently perform routing computations, network diagnostics, security functions, etc.  
         [0088]    Although the system shown in FIG. 10B is one preferred router of the present invention, it is by no means the only router architecture on which the present invention can be implemented. For example, an architecture having a single processor that handles communications as well as routing computations, etc. is often used. Further, other types of interfaces and media could also be used with the router.  
         [0089]    As indicated both server machines and client machines (e.g., routers) may employ one or more memories or memory modules configured to store program instructions for the general-purpose network operations and configuration operations described herein. The program instructions may control the operation of an operating system and/or one or more applications, for example. The memory or memories may also be configured to store relevant state information, data structures, etc., such as the ATMARP entries containing IP addresses, ATM addresses, and priorities described herein.  
         [0090]    Because such information and program instructions may be employed to implement the systems/methods described herein, the present invention relates to machine readable media that include program instructions, state information, etc. for performing various operations described herein. Examples of machine-readable media include, but are not limited to, magnetic media such as hard disks, floppy disks, and magnetic tape; optical media such as CD-ROM disks; magneto-optical media such as floptical disks; and hardware devices that are specially configured to store and perform program instructions, such as read-only memory devices (ROM) and random access memory (RAM). The invention may also be embodied in a carrier wave travelling over an appropriate medium such as airwaves, optical lines, electric lines, etc. Examples of program instructions include both machine code, such as produced by a compiler, and files containing higher level code that may be executed by the computer using an interpreter.  
         [0091]    Although only a few embodiments of the present invention have been described in detail, it should be understood that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Particularly, the standby ATMARP Clients may be chosen based on the efficiency in which they route packages to an end destination as opposed to the critical nature of the associated entity or the number of IP addresses the ATMARP Client is responsible for.  
         [0092]    Alternately, as the application of the ATMARP system  300  may vary over time, flexible control of the aggregate groups of ARP Clients and priority levels therein may be performed. For example, in the case where the ATM system original begins with  50  ATMARP Clients and then grows over time to  100  ARP Clients, the ATMARP Server  302  may reallocate the composition of each aggregate group or individual ATMARP Client. For example, the ARP Clients may be added or removed from an aggregate group or may have their priority within a group altered. Alternatively, if an ATMARP Client responsible as a primary ATMARP Client for one IP address and a stand-by ATMARP Client for another IP address continually becomes responsible as a primary of client for both IP addresses, the ATMARP Server  302  is capable of readjusting the aggregate groups in order alleviate the burden upon a single ARP Client.  
         [0093]    Additionally, the allocation of ATMARP Clients within groups may be designated with respect to reserved bandwidths, data transmitting schemes or data classes such as video or simple email data. Although the foregoing invention has been described in some detail for purposes of clarity of understanding, it will be apparent that certain changes and modifications may be practiced within the scope of the appended claims.