INTERNET DRAFT IS-IS restart March 2003 Network Working Group M. Shand Internet Draft Les Ginsberg Expiration Date: September 2003 Cisco Systems March 2003 Restart signaling for IS-IS draft-ietf-isis-restart-03.txt Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026 [1]. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. 1. Abstract The IS-IS routing protocol (RFC 1142 [2], ISO/IEC 10589 [3]) is a link state intra-domain routing protocol. Normally, when an IS-IS router is restarted, the neighboring routers detect the restart event and cycle their adjacencies with the restarting router through the down state. This is necessary in order to invoke the protocol mechanisms to ensure correct synchronization of the LSP database. However, the cycling of the adjacency state causes the neighbors to regenerate their LSPs describing the adjacency concerned. This in turn causes temporary disruption of routes passing through the restarting router. In certain scenarios such temporary disruption of the routes is highly undesirable. Shand, Ginsberg Expires Sep 2003 [Page 1] INTERNET DRAFT IS-IS restart Mar 2003 This draft describes a mechanism for a restarting router to signal that it is restarting to its neighbors, and allow them to reestablish their adjacencies without cycling through the down state, while still correctly initiating database synchronization. When such a router is restarted, it is highly desirable that it does not recompute its own routes until it has achieved database synchronization with its neighbors. Recomputing its routes before synchronization is achieved will result in its own routes being temporarily incorrect. This draft additionally describes a mechanism for a restarting router to determine when it has achieved synchronization with its neighbors. This draft additionally describes a mechanism to optimize database synchronization and minimize transient routing disruption when a router starts. 2. Conventions used in this document The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC-2119 [4]. If the control and forwarding functions in a router can be maintained independently, it is possible for the forwarding function state to be maintained across a control function restart. This functionality is assumed when the terms "restart/restarting" are used in this document. The terms "start/starting" are used to refer to a router in which the control function has either been started for the first time or has been restarted but the forwarding functions have not been maintained in a prior state. The terms "(re)start/(re)starting" are used when the text is applicable to both a "starting" and a "restarting" router. 3. Overview There are two related problems with the existing specification of IS-IS with regard to synchronization of LSP databases when a router is restarted. Firstly, when a routing process restarts and an adjacency to a neighboring router is reinitialized the neighboring routing process does three things: 1. It reinitializes the adjacency and causes its own LSP(s) to be regenerated, thus triggering SPF runs throughout the area (or in the case of Level 2, throughout the domain). Shand, Ginsberg Expires Sep 2003 [Page 2] INTERNET DRAFT IS-IS restart Mar 2003 2. It sets SRMflags on its own LSP database on the adjacency concerned. 3. In the case of a Point-to-Point link it transmits a (set of) CSNP(s) over the adjacency. In the case of a restarting router process, the first of these is highly undesirable, but the second is essential in order to ensure synchronization of the LSP database. Secondly, whether or not the router is being restarted, it is desirable to be able to determine when the LSP databases of the neighboring routers have been synchronized (so that the overload bit can be cleared in the router's own LSP, for example). This document describes modifications to achieve this. It is assumed that the three-way handshake [5] is being used on Point-to-Point circuits. 4. Approach 4.1 Timers Three additional timers, T1, T2 and T3 are required to support the functionality defined in this document. An instance of T1 is maintained per interface, and indicates the time after which an unacknowledged (re)start attempt will be repeated. A typical value might be 3 seconds. An instance of T2 is maintained for each LSP database present in the system i.e. for a Level1/2 system, there will be an instance of T2 for Level 1 and an instance for Level 2. This is the maximum time that the system will wait for LSPDB synchronization. A typical value might be 60 seconds. A single instance of T3 is maintained for the entire system. It indicates the time after which the router will declare that it has failed to achieve database synchronization (by setting the overload bit in its own LSP). This is initialized to 65535 seconds, but is set to the minimum of the remaining times of received IIHs containing a restart TLV with RA set. 4.2 Restart TLV A new TLV is defined to be included in IIH PDUs. The presence of this TLV indicates that the sender supports the functionality defined in this document and it carries flags that are used to convey information during a (re)start. All IIHs transmitted by a router that supports this capability MUST include this TLV. Shand, Ginsberg Expires Sep 2003 [Page 3] INTERNET DRAFT IS-IS restart Mar 2003 Type 211 Length 3 Value (3 octets) Flags (1 octet) Bit 1 - Restart Request (RR) Bit 2 - Restart Acknowledgment (RA) Bit 3 û Suppress adjacency advertisement(SA) Bits 4-8 û Reserved Remaining Time (2 octets) Remaining holding time (in seconds) (note: only required when RA bit is set) 4.2.1 Use of RR and RA bits The RR bit is used by a (re)starting router to signal to its neighbors that a (re)start is in progress, that an existing adjacency should be maintained even under circumstances when the normal operation of the adjacency state machine would require the adjacency to be reinitialized, and to request a set of CSNPs. The RA bit is sent by the neighbor of a (re)starting router to acknowledge the receipt of a restart TLV with the RR bit set. When the neighbor of a (re)starting router receives an IIH with the restart TLV having the RR bit set, if there exists on this interface an adjacency in state "Up" with the same System ID, and in the case of a LAN circuit, with the same source LAN address, then, irrespective of the other contents of the "Intermediate System Neighbors" option (LAN circuits), or the "Point-to-Point Adjacency State" option (Point-to-Point circuits): a) The state of the adjacency is not changed. It is an implementation choice whether or not the holding time of the adjacency is refreshed. Not refreshing the holding time preserves the intention of the original holding time. Refreshing it may allow a longer grace period for the completion of the (re)start process. Whichever option is chosen, the "remaining time" transmitted according to (b) below MUST reflect the actual time after which the adjacency will now expire. b) immediately (i.e. without waiting for any currently running timer interval to expire, but with a small random delay of a few 10s of milliseconds on LANs to avoid "storms"), transmit over the corresponding interface an IIH including the restart TLV with the RR bit clear and the RA bit set, having updated the "Point-to- Point Adjacency State" option to reflect any new values received from the (re)starting router. (This allows a restarting router to quickly acquire the correct information to place in its hellos.) The "Remaining Time" MUST be set to the current time (in seconds) before the holding timer on this adjacency is due to expire. This IIH SHOULD be transmitted before any LSPs or SNPs transmitted as a result of the receipt of the original IIH. Shand, Ginsberg Expires Sep 2003 [Page 4] INTERNET DRAFT IS-IS restart Mar 2003 c) if the corresponding interface is a Point-to-Point interface, or if the receiving router has the highest LnRouterPriority (with highest source MAC address breaking ties) among those routers whose IIHs contain the restart TLV, excluding the transmitting router (note the actual DIS is NOT changed by this process.), initiate the transmission over the corresponding interface of a complete set of CSNPs, and set SRMflags on the corresponding interface for all LSPs in the local LSP database. Otherwise (i.e. if there was no adjacency in the "UP" state to the system ID in question), process the IIH as normal by reinitializing the adjacency, and setting the RA bit in the returned IIH. 4.2.2 Use of SA bit The SA bit is used by a starting router to request that its neighbor suppress advertisement of the adjacency to the starting router in the neighbors LSPs. A router which is starting has no maintained forwarding function state. This may or may not be the first time the router has started. If this is not the first time the router has started, copies of LSPs generated by this router in its previous incarnation may exist in the LSP databases of other routers in the network. These copies are likely to appear "newer" than LSPs initially generated by the starting router due to the reinitialization of LSP fragment sequence numbers by the starting router. This may cause temporary blackholes to occur until the normal operation of the update process causes the starting router to regenerate and flood copies of its own LSPs with higher sequence numbers. The temporary blackholes can be avoided if the starting router's neighbors suppress advertising an adjacency to the starting router until the starting router has been able to propagate newer versions of LSPs generated by previous incarnations. When the neighbor of a starting router receives an IIH with the restart TLV having the SA bit set, if there exists on this interface an adjacency in state "Up" with the same System ID, and in the case of a LAN circuit, with the same source LAN address, then advertisement of the adjacency to the starting router in LSPs should be suppressed. Until an IIH with the SA bit clear has been received, the adjacency advertisement should continue to be suppressed. If the adjacency transitions to the UP state, the new adjacency should not be advertised until an IIH with the SA bit clear has been received. 4.3 Adjacency (re)acquisition Adjacency (re)acquisition is the first step in (re)initialization. Both restarting and starting routers will make use of the RR bit in the restart TLV, though at different stages of the (re)start procedure. Shand, Ginsberg Expires Sep 2003 [Page 5] INTERNET DRAFT IS-IS restart Mar 2003 4.3.1 Adjacency reacquisition during restart The restarting router explicitly notifies its neighbor that the adjacency is being reacquired, and hence that it should not reinitialize the adjacency. This is achieved by setting the RR bit in the restart TLV. When the neighbor of a restarting router receives an IIH with the restart TLV having the RR bit set, if there exists on this interface an adjacency in state "Up" with the same System ID, and in the case of a LAN circuit, with the same source LAN address, then the procedures described in 4.2.1 are followed. A router that does not support the restart capability will ignore the restart TLV and reinitialize the adjacency as normal, returning an IIH without the restart TLV. On restarting, a router initializes the timer T3, starts timer T2 for each LSPDB and for each interface (and in the case of a LAN circuit, for each level) starts a timer T1 and transmits an IIH containing the restart TLV with the RR bit set. On a Point-to-Point circuit the "Point-to-Point Adjacency State" SHOULD be set to "Init", because the receipt of the acknowledging IIH (with RA set) MUST cause the adjacency to enter "Up" state immediately. On a LAN circuit the LAN-ID assigned to the circuit SHOULD be the same as that used prior to the restart. In particular, for any circuits for which the restarting router was previously DIS, the use of a different LAN-ID would necessitate the generation of a new set of pseudonode LSPs, and corresponding changes in all the LSPs referencing them from other routers on the LAN. By preserving the LAN-ID across the restart, this churn can be prevented. To enable a restarting router to learn the LAN-ID used prior to restart, the LAN-ID specified in an IIH w RR set MUST be ignored. Transmission of "normal" IIHs is inhibited until the conditions described below are met (in order to avoid causing an unnecessary adjacency reinitialization). On expiry of the timer T1, it is restarted and the IIH is retransmitted as above. On receipt of an IIH by the restarting router, a local adjacency is established as usual, and if the IIH contains a restart TLV with the RA bit set, the receipt of the acknowledgement over that interface is noted. T3 is set to the minimum of its current value and the value of the "Remaining Time" field in the received IIH. Receipt of an IIH not containing the restart TLV is also treated as an acknowledgement, since it indicates that the neighbor is not restart capable. In this case the neighbor will have reinitialized the adjacency as normal, which in the case of a Point-to-Point link Shand, Ginsberg Expires Sep 2003 [Page 6] INTERNET DRAFT IS-IS restart Mar 2003 will guarantee that SRMflags have been set on its database, thus ensuring eventual LSPDB synchronization. In the case of a LAN interface, the usual operation of the update process will also ensure that synchronization is eventually achieved. However, since no CSNP is guaranteed to be received over this interface, T1 is cancelled immediately without waiting for a CSNP. Synchronization may therefore be deemed complete even though there are some LSPs which are held(only) by this neighbor (see section 4.4). In the case of a Point-to-Point circuit, the "LocalCircuitID" and "Extended Local Circuit ID" information contained in the IIH can be used immediately to generate an IIH containing the correct 3-way handshake information. The presence of "Neighbor System ID" or "Neighbor Extended Local Circuit ID" information which does not match the values currently in use by the local system is ignored (since the IIH may have been transmitted before the neighbor had received the new values from the restarting router), but the adjacency remains in the initializing state until the correct information is received. In the case of a LAN circuit the information in the Intermediate Systems Neighbors option is recorded and used for the generation of subsequent IIHs as normal. When BOTH a complete set of CSNP(s) (for each active level, in the case of a pt-pt circuit) and an acknowledgement have been received over the interface, the timer T1 is cancelled. Once T3 has expired or been cancelled, subsequent IIHs are transmitted according to the normal algorithms, but including the restart TLV with both RR and RA clear. If a LAN contains a mixture of systems, only some of which support the new algorithm, database synchronization is still guaranteed, but the "old" systems will have reinitialized their adjacencies. If an interface is active, but does not have any neighboring router reachable over that interface the timer T1 would never be cancelled, and according to clause 4.4.1.1 the SPF would never be run. Therefore timer T1 is cancelled after some pre-determined number of expirations (which MAY be 1). (By this time any existing adjacency on a remote system would probably have expired anyway.) A router which supports restart SHOULD ensure that the holding time of any IIHs it transmits is greater than the expected time to complete a restart. However, where this is impracticable or undesirable a router MAY transmit one or more normal IIHs (containing a restart TLV with RR and RA clear) after the initial RR/RA exchange, but before synchronization has been achieved, in order to extend the holding time of the neighbors adjacencies beyond that indicated in the remaining time field of the neighbors IIH with the RA bit set. Shand, Ginsberg Expires Sep 2003 [Page 7] INTERNET DRAFT IS-IS restart Mar 2003 4.3.2 Adjacency acquisition during start The starting router wants to ensure that in the event a neighboring router has an adjacency to the starting router in the UP state (from a previous incarnation of the starting router) that this adjacency is reinitialized. The starting router also wants neighboring routers to suppress advertisement of an adjacency to the starting router until LSP database synchronization is achieved. This is achieved by sending IIHs with the RR bit clear and the SA bit set in the restart TLV. The RR bit remains clear and the SA bit remains set in subsequent transmissions of IIHs until the adjacency has reached the UP state and the initial T1 timer interval (see below) has expired. Receipt of an IIH with RR bit clear will result in the neighboring router utilizing normal operation of the adjacency state machine. This will ensure that any old adjacency on the neighboring router will be reinitialized. On receipt of an IIH with SA bit set the behavior described in 4.2.2 is followed. On starting, a router initializes the timer T3, and starts timer T2 for each LSPDB. For each interface (and in the case of a LAN circuit, for each level), when an adjacency reaches the UP state, the starting router starts a timer T1 and transmits an IIH containing the restart TLV with the RR bit clear and SA bit set. On expiry of the timer T1, it is restarted and the IIH is retransmitted with both RR and SA bits set(only the RR bit has changed state from earlier IIHs). On receipt of an IIH with RR bit set(regardless of whether SA is set or not) the behavior described in 4.2.1 is followed. When an IIH is received by the starting router and the IIH contains a restart TLV with the RA bit set, the receipt of the acknowledgement over that interface is noted. T3 is set to the minimum of its current value and the value of the "Remaining Time" field in the received IIH. Receipt of an IIH not containing the restart TLV is also treated as an acknowledgement, since it indicates that the neighbor is not restart capable. In this case the neighbor will have reinitialized the adjacency as normal, which in the case of a Point-to-Point link will guarantee that SRMflags have been set on its database, thus ensuring eventual LSPDB synchronization. In the case of a LAN interface, the usual operation of the update process will also ensure that synchronization is eventually achieved. However, since no CSNP is guaranteed to be received over this interface, T1 is cancelled immediately without waiting for a CSNP. Synchronization Shand, Ginsberg Expires Sep 2003 [Page 8] INTERNET DRAFT IS-IS restart Mar 2003 may therefore be deemed complete even though there are some LSPs which are held(only) by this neighbor (see section 4.4). When BOTH a complete set of CSNP(s) (for each active level, in the case of a pt-pt circuit) and an acknowledgement have been received over the interface, the timer T1 is cancelled. Subsequent IIHs sent by the starting router have the RR and RA bits clear and the SA bit set in the restart TLV. Once T3 has expired or been cancelled, subsequent IIHs are transmitted according to the normal algorithms, but including the restart TLV with RR, RA, and SA bits clear. Timer T1 is cancelled after some pre-determined number of expirations (which MAY be 1). During the period when T1 is active, according to the rules defined in 4.3 the neighbor of the starting router may choose not to update the holding time for an adjacency because the RR bit is set in the received IIH. To prevent holding time expiration a starting router MAY transmit one or more IIHs containing a restart TLV with RR and RA bits clear and SA bit set after the initial RR/RA exchange. When T2 is cancelled or expires transmission of "normal" IIHs (with RR,RA, and SA bits clear) will begin. 4.3.3 Multiple levels A router which is operating as both a Level 1 and a Level 2 router on a particular interface MUST perform the above operations for each level. On a LAN interface, it MUST send and receive both Level 1 and Level 2 IIHs and perform the CSNP synchronizations independently for each level. On a pt-pt interface, only a single IIH (indicating support for both levels) is required, but it MUST perform the CSNP synchronizations independently for each level. 4.4 Database synchronization When a router is started or restarted it can expect to receive a (set of) CSNP(s) over each interface. The arrival of the CSNP(s) is now guaranteed, since an IIH with RR bit set will be retransmitted until the CSNP(s) are correctly received. The CSNPs describe the set of LSPs that are currently held by each neighbor. Synchronization will be complete when all these LSPs have been received. Shand, Ginsberg Expires Sep 2003 [Page 9] INTERNET DRAFT IS-IS restart Mar 2003 When (re)starting, a router starts the timer T3 and an instance of timer T2 for each LSPDB as described in 4.3.1 or 4.3.2. In addition to normal processing of the CSNPs, the set of LSPIDs contained in the first complete set of CSNP(s) received over each interface is recorded, together with their remaining lifetime. If there are multiple interfaces on the (re)starting router, the recorded set of LSPIDs is the union of those received over each interface. LSPs with a remaining lifetime of zero are NOT so recorded. As LSPs are received (by the normal operation of the update process) over any interface, the corresponding LSPID entry is removed (it is also removed if the LSP had arrived before the CSNP containing the reference). When an LSPID has been held in the list for its indicated remaining lifetime, it is removed from the list. When the list of LSPIDs is empty and T1 has been cancelled for all the interfaces that have an adjacency at this level, the timer T2 is cancelled. At this point the local database is guaranteed to contain all the LSP(s) (either the same sequence number, or a more recent sequence number) which were present in the neighbors' databases at the time of (re)starting. LSPs that arrived in a neighbor's database after the time of (re)starting may, or may not, be present, but the normal operation of the update process will guarantee that they will eventually be received. At this point the local database is deemed to be "synchronized". Since LSPs mentioned in the CSNP(s) with a zero remaining lifetime are not recorded, and those with a short remaining lifetime are deleted from the list when the lifetime expires, cancellation of the timer T2 will not be prevented by waiting for an LSP that will never arrive. 4.4.1 LSP generation and flooding and SPF computation The operation of a router starting, as opposed to restarting is somewhat different. These two cases are dealt with separately below. 4.4.1.1. Restarting In order to avoid causing unnecessary routing churn in other routers, it is highly desirable that the own LSPs generated by the restarting system are the same as those previously present in the network (assuming no other changes have taken place). It is important therefore not to regenerate and flood the LSPs until all the adjacencies have been re-established and any information required for propagation into the local LSPs is fully available. Ideally, the information should be loaded into the LSPs in a deterministic way, such that the same information occurs in the same place in the same LSP (and hence the LSPs are identical to their Shand, Ginsberg Expires Sep 2003 [Page 10] INTERNET DRAFT IS-IS restart Mar 2003 previous versions). If this can be achieved, the new versions will not even cause SPF to be run in other systems. However, provided the same information is included in the set of LSPs (albeit in a different order, and possibly different LSPs), the result of running the SPF will be the same and will not cause churn to the forwarding tables. In the case of a restarting router, none of the router's LSPs are transmitted, nor are the router's own forwarding tables updated while the timer T3 is running. Redistribution of inter-level information must be regenerated before this router's LSP is flooded to other nodes. Therefore the Level-n non-pseudonode LSP(s) should not be flooded until the other level's T2 timer has expired and its SPF has been run. This ensures that any inter-level information that should be propagated can be included in the Level-n LSP(s). During this period, if one of the router's own (including pseudonodes) LSPs is received, which the local router does not currently have in its own database, it is NOT purged. Under normal operation, such an LSP would be purged, since the LSP clearly should not be present in the global LSP database. However, in the present circumstances, this would be highly undesirable, because it could cause premature removal of an own LSP -- and hence churn in remote routers. Even if the local system has one or more own LSPs (which it has generated, but not yet transmitted) it is still not valid to compare the received LSP against this set, since it may be that as a result of propagation between Level 1 and Level 2 (or vice versa) a further own LSP will need to be generated when the LSP databases have synchronized. During this period a restarting router SHOULD send CSNPs as it normally would. Information about the router's own LSPs MAY be included, but if it is included it MUST be based on LSPs which have been received, not on versions which have been generated(but not yet transmitted). This restriction is necessary to prevent premature removal of an LSP from the global LSP database. When the timer T2 expires or is cancelled indicating that synchronization for that level is complete, the SPF for that level is run in order to derive any information which is required to be propagated to another level, but the forwarding tables are not yet updated. Once the other level's SPF has run and any inter-level propagation has been resolved, the 'own' LSPs can be generated and flooded. Any 'own' LSPs which were previously ignored, but which are not part of the current set of 'own' LSPs (including pseudonodes) should then be purged. Note that it is possible that a Designated Router change may have taken place, and consequently the router should purge those Shand, Ginsberg Expires Sep 2003 [Page 11] INTERNET DRAFT IS-IS restart Mar 2003 pseudonode LSPs which it previously owned, but which are now no longer part of its set of pseudonode LSPs. When all the T2 timers have expired or been cancelled, the timer T3 is cancelled and the local forwarding tables are updated. If the timer T3 expires before all the T2 timers have expired or been cancelled, this indicates that the synchronization process is taking longer than minimum holding time of the neighbors. The router's own LSP(s) for levels which have not yet completed their first SPF computation are then flooded with the overload bit set to indicate that the router's LSPDB is not yet synchronized (and other routers should therefore not compute routes through this router). Normal operation of the update process resumes and the local forwarding tables are updated. In order to prevent the neighbor's adjacencies from expiring, IIHs with the normal interface value for the holding time are transmitted over all interfaces with neither RR nor RA set in the restart TLV. This will cause the neighbors to refresh their adjacencies. The own LSP(s) will continue to have the overload bit set until timer T2 has expired or been cancelled. 4.4.1.2. Starting In the case of a starting router, as soon as each adjacency is established, and before any CSNP exchanges, the router's own zeroth LSP is transmitted with the overload bit set. This prevents other routers from computing routes through the router until it has reliably acquired the complete set of LSPs. The overload bit remains set in subsequent transmissions of the zeroth LSP (such as will occur if a previous copy of the routers LSP is still present in the network) while any timer T2 is running. When all the T2 timers have been cancelled, the own LSP(s) MAY be regenerated with the overload bit clear (assuming the router isn't in fact overloaded, and there is no other reason, such as incomplete BGP convergence, to keep the overload bit set), and flooded as normal. Other 'own' LSPs (including pseudonodes) are generated and flooded as normal, irrespective of the timer T2. The SPF is also run as normal and the RIB and FIB updated as routes become available. To avoid the possible formation of temporary blackholes the starting router sets the SA bit in the restart TLV (as described in 4.3.2) in all IIHs that it sends. When all T2 timers have been cancelled, the starting router MUST transmit IIHs with the SA bit clear. Shand, Ginsberg Expires Sep 2003 [Page 12] INTERNET DRAFT IS-IS restart Mar 2003 5. Security Considerations This memo does not create any new security issues for the IS-IS protocol. Security considerations for the base IS-IS protocol are covered in [2] and [3]. 6. References 1 Bradner, S., "The Internet Standards Process -- Revision 3", BCP 9, RFC 2026, October 1996. 2 Callon, R., "OSI IS-IS for IP and Dual Environment," RFC 1195, December 1990. 3 ISO, "Intermediate system to Intermediate system routeing information exchange protocol for use in conjunction with the Protocol for providing the Connectionless-mode Network Service (ISO 8473)," ISO/IEC 10589:2002, Second Edition. 4 Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997 5 Katz, D., "Three-Way Handshake for IS-IS Point-to-Point Adjacencies", RFC 3373, September 2002 7. Acknowledgments The authors would like to acknowledge contributions made by Radia Perlman, Mark Schaefer, Naiming Shen, Nischal Sheth, Russ White, and Rena Yang. 8. Authors' Addresses Mike Shand Cisco Systems 250 Longwater Avenue, Reading, Berkshire, RG2 6GB UK Phone: +44 208 824 8690 Email: mshand@cisco.com Les Ginsberg Cisco Systems 510 McCarthy Blvd. Milpitas, Ca. 95035 USA Email: ginsberg@cisco.com Shand, Ginsberg Expires Sep 2003 [Page 13]