Network Working Group M. Shand Request for Comments: 5306 L. Ginsberg Obsoletes: 3847 Cisco Systems Category: Standards Track October 2008 Restart Signaling for IS-IS Status of This Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.
AbstractThis document describes a mechanism for a restarting router to signal to its neighbors that it is restarting, allowing them to reestablish their adjacencies without cycling through the down state, while still correctly initiating database synchronization. This document additionally describes a mechanism for a restarting router to determine when it has achieved Link State Protocol Data Unit (LSP) database synchronization with its neighbors and a mechanism to optimize LSP database synchronization, while minimizing transient routing disruption when a router starts. This document obsoletes RFC 3847.
1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Conventions Used in This Document . . . . . . . . . . . . . . 4 3. Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3.1. Timers . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3.2. Restart TLV . . . . . . . . . . . . . . . . . . . . . . . 5 3.2.1. Use of RR and RA Bits . . . . . . . . . . . . . . . . 6 3.2.2. Use of the SA Bit . . . . . . . . . . . . . . . . . . 8 3.3. Adjacency (Re)Acquisition . . . . . . . . . . . . . . . . 8 3.3.1. Adjacency Reacquisition during Restart . . . . . . . . 9 3.3.2. Adjacency Acquisition during Start . . . . . . . . . . 11 3.3.3. Multiple Levels . . . . . . . . . . . . . . . . . . . 12 3.4. Database Synchronization . . . . . . . . . . . . . . . . . 13 3.4.1. LSP Generation and Flooding and SPF Computation . . . 14 188.8.131.52. Restarting . . . . . . . . . . . . . . . . . . . . 14 184.108.40.206. Starting . . . . . . . . . . . . . . . . . . . . . 16 4. State Tables . . . . . . . . . . . . . . . . . . . . . . . . . 16 4.1. Running Router . . . . . . . . . . . . . . . . . . . . . . 17 4.2. Restarting Router . . . . . . . . . . . . . . . . . . . . 18 4.3. Starting Router . . . . . . . . . . . . . . . . . . . . . 19 5. Security Considerations . . . . . . . . . . . . . . . . . . . 19 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20 7. Manageability Considerations . . . . . . . . . . . . . . . . . 20 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 20 9. Normative References . . . . . . . . . . . . . . . . . . . . . 21
RFC1195] [ISO10589] is a link state intra-domain routing protocol. Normally, when an IS-IS router is restarted, temporary disruption of routing occurs due to events in both the restarting router and the neighbors of the restarting router. The router that has been restarted computes its own routes before achieving database synchronization with its neighbors. The results of this computation are likely to be non-convergent with the routes computed by other routers in the area/domain. Neighbors of the restarting router detect the restart event and cycle their adjacencies with the restarting router through the down state. The cycling of the adjacency state causes the neighbors to regenerate their LSPs describing the adjacency concerned. This in turn causes a temporary disruption of routes passing through the restarting router. In certain scenarios, the temporary disruption of the routes is highly undesirable. This document describes mechanisms to avoid or minimize the disruption due to both of these causes. When an adjacency is reinitialized as a result of a neighbor restarting, a router does three things: 1. It 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). 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 complete set of Complete Sequence Number PDUs (CSNPs), 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. The third action above minimizes the number of LSPs that must be exchanged and, if made reliable, provides a means of determining when the LSP databases of the neighboring routers have been synchronized. This is desirable whether or not the router is being restarted (so that the overload bit can be cleared in the router's own LSP, for example).
This document 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. This document additionally describes a mechanism for a restarting router to determine when it has achieved LSP database synchronization with its neighbors and a mechanism to optimize LSP database synchronization and minimize transient routing disruption when a router starts. It is assumed that the three-way handshake [RFC5303] is being used on Point-to-Point circuits. BCP 14, [RFC2119]. 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 resumption of control function operations. 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 commenced operations for the first time or has resumed operations, 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.
An instance of the timer T2 is maintained for each LSP database (LSPDB) present in the system, i.e., for a Level 1/2 system, there will be an instance of the timer 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 the timer 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 IS-IS Hellos (IIHs) containing a restart TLV with the Restart Acknowledgement (RA) set and an indication that the neighbor has an adjacency in the "UP" state to the restarting router. NOTE: The timer T3 is only used by a restarting router.
Flags (1 octet) 0 1 2 3 4 5 6 7 +--+--+--+--+--+--+--+--+ | Reserved |SA|RA|RR| +--+--+--+--+--+--+--+--+ RR - Restart Request RA - Restart Acknowledgement SA - Suppress adjacency advertisement (Note: Remaining fields are required when the RA bit is set.) Remaining Time (2 octets) Remaining holding time (in seconds) Restarting Neighbor System ID (ID Length octets) The System ID of the neighbor to which an RA refers. Note: Implementations based on earlier versions of this document may not include this field in the TLV when the RA is set. In this case, a router that is expecting an RA on a LAN circuit SHOULD assume that the acknowledgement is directed at the local system.
"Restart mode" and the adjacency holding time is refreshed -- otherwise, the holding time is not refreshed. The "remaining time" transmitted according to (b) below MUST reflect the actual time after which the adjacency will now expire. Receipt of a normal IIH with the RR bit reset will clear the "Restart mode" state. This procedure allows the restarting router to cause the neighbor to maintain the adjacency long enough for restart to successfully complete, while also preventing repetitive restarts from maintaining an adjacency indefinitely. Whether or not an adjacency is marked as being in "Restart mode" has no effect on adjacency state transitions. b. immediately (i.e., without waiting for any currently running timer interval to expire, but with a small random delay of a few tens 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, in the case of Point-to-Point adjacencies having updated the "Point-to-Point Three-Way Adjacency" 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. If the corresponding interface is a LAN interface, then the Restarting Neighbor System ID SHOULD be set to the System ID of the router from which the IIH with the RR bit set was received. This is required to correctly associate the acknowledgement and holding time in the case where multiple systems on a LAN restart at approximately the same time. This IIH SHOULD be transmitted before any LSPs or SNPs are transmitted as a result of the receipt of the original IIH. c. if the corresponding interface is a Point-to-Point interface, or if the receiving router has the highest LnRouterPriority (with the highest source MAC (Media Access Control) address breaking ties) among those routers to which the receiving router has an adjacency in state "UP" on this interface whose IIHs contain the restart TLV, excluding adjacencies to all routers which are considered in "Restart mode" (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.
ISO10589], Annex C.2.5, Step 0:b) "pre-load TENT with the local adjacency database", the suppressed adjacency MUST NOT be loaded into TENT.
Section 3.2.1 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 the timer T2 for each LSPDB, and for each interface (and in the case of a LAN circuit, for each level) starts the timer T1 and transmits an IIH containing the restart TLV with the RR bit set. On a Point-to-Point circuit, the restarting router SHOULD set the "Adjacency Three-Way State" to "Init", because the receipt of the acknowledging IIH (with RA set) MUST cause the adjacency to enter the "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 with 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 initialization). Upon expiry of the timer T1, it is restarted and the IIH is retransmitted as above. When a restarting router receives an IIH a local adjacency is established as usual, and if the IIH contains a restart TLV with the RA bit set (and on LAN circuits with a Restart Neighbor System ID that matches that of the local system), the receipt of the acknowledgement over that interface is noted. When the RA bit is set and the state of the remote adjacency is "UP", then the timer T3 is set to the minimum of its current value and the value of the "Remaining Time" field in the received IIH.
On a Point-to-Point link, 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. However, since no CSNP is guaranteed to be received over this interface, the timer T1 is cancelled immediately without waiting for a complete set of CSNPs. Synchronization may therefore be deemed complete even though there are some LSPs which are held (only) by this neighbor (see Section 3.4). In this case, we also want to be certain that the neighbor will reinitialize the adjacency in order to guarantee that the SRMflags have been set on its database, thus ensuring eventual LSPDB synchronization. This is guaranteed to happen except in the case where the Adjacency Three-Way State in the received IIH is "UP" and the Neighbor Extended Local Circuit ID matches the extended local circuit ID assigned by the restarting router. In this case, the restarting router MUST force the adjacency to reinitialize by setting the local Adjacency Three-Way State to "DOWN" and sending a normal IIH. In the case of a LAN interface, receipt of an IIH not containing the restart TLV is unremarkable since synchronization can still occur so long as at least one of the non-restarting neighboring routers on the LAN supports restart. Therefore, T1 continues to run in this case. If none of the neighbors on the LAN are restart capable, T1 will eventually expire after the locally defined number of retries. 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 three-way handshake information. The presence of "Neighbor Extended Local Circuit ID" information that does not match the value currently in use by the local system is ignored (since the IIH may have been transmitted before the neighbor had received the new value 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 source neighbor information (e.g., SNPAAddress) is recorded and used for adjacency establishment and maintenance as normal. When BOTH a complete set of CSNPs (for each active level, in the case of a Point-to-Point circuit) and an acknowledgement have been received over the interface, the timer T1 is cancelled. Once the timer T1 has 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 Section 220.127.116.11, the SPF would never be run. Therefore, timer T1 is cancelled after some predetermined number of expirations (which MAY be 1). Section 3.2.2 is followed. Upon starting, a router 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. Upon 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). Upon receipt of an IIH with the RR bit set (regardless of whether or not the SA bit is set), the behavior described in Section 3.2.1 is followed.
When an IIH is received by the starting router and the IIH contains a restart TLV with the RA bit set (and on LAN circuits with a Restart Neighbor System ID that matches that of the local system), the receipt of the acknowledgement over that interface is noted. On a Point-to-Point link, 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. Since the neighbor will have reinitialized the adjacency, this guarantees that SRMflags have been set on its database, thus ensuring eventual LSPDB synchronization. However, since no CSNP is guaranteed to be received over this interface, the timer T1 is cancelled immediately without waiting for a complete set of CSNPs. Synchronization may therefore be deemed complete even though there are some LSPs that are held (only) by this neighbor (see Section 3.4). In the case of a LAN interface, receipt of an IIH not containing the restart TLV is unremarkable since synchronization can still occur so long as at least one of the non-restarting neighboring routers on the LAN supports restart. Therefore, T1 continues to run in this case. If none of the neighbors on the LAN are restart capable, T1 will eventually expire after the locally defined number of retries. The usual operation of the update process will ensure that synchronization is eventually achieved. When BOTH a complete set of CSNPs (for each active level, in the case of a Point-to-Point 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. Timer T1 is cancelled after some predetermined number of expirations (which MAY be 1). When the T2 timer(s) are cancelled or expire, transmission of "normal" IIHs (with RR, RA, and SA bits clear) will begin.
On a Point-to-Point interface, only a single IIH (indicating support for both levels) is required, but it MUST perform the CSNP synchronizations independently for each level. Section 3.3.1 or Section 3.3.2. In addition to normal processing of the CSNPs, the set of LSPIDs contained in the first complete set of CSNPs received over each interface is recorded, together with their remaining lifetime. In the case of a LAN interface, a complete set of CSNPs MUST consist of CSNPs received from neighbors that are not restarting. 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 an LSP arrives 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 the timer 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) that 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.
router's 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 router's 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 that have been received, not on versions that 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 that 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 router's own LSPs can be generated and flooded. Any own LSPs that were previously ignored, but that are not part of the current set of own LSPs (including pseudonodes), MUST then be purged. Note that it is possible that a Designated Router change may have taken place, and consequently the router SHOULD purge those pseudonode LSPs that it previously owned, but that 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 the minimum holding time of the neighbors. The router's own LSP(s) for levels that 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 therefore other routers MUST 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 router's own LSP(s) will continue to have the overload bit set until timer T2 has expired or been cancelled.
Section 3.3.2) in all IIHs that it sends. When all T2 timers have been cancelled, the starting router MUST transmit IIHs with the SA bit clear. ISO10589] and [RFC5303]. The states named in the columns of the tables below are a mixture of states that are specific to a single adjacency (ADJ suppressed, ADJ Seen RA, ADJ Seen CSNP) and states that are indicative of the state of the protocol instance (Running, Restarting, Starting, SPF Wait). Three state tables are presented from the point of view of a running router, a restarting router, and a starting router.
Section 3.2.1c Note 2: If Restart Mode clear
If the RR bit is set in a false IIH, neighbors who receive such an IIH will continue to maintain an existing adjacency in the "UP" state and may (re)send a complete set of CSNPs. While the latter action is wasteful, neither action causes any disruption in correct protocol operation. If the RA bit is set in a false IIH, a (re)starting router that receives such an IIH may falsely believe that there is a neighbor on the corresponding interface that supports the procedures described in this document. In the absence of receipt of a complete set of CSNPs on that interface, this could delay the completion of (re)start procedures by requiring the timer T1 to time out the locally defined maximum number of retries. This behavior is the same as would occur on a LAN where none of the (re)starting router's neighbors support the procedures in this document and is covered in Sections 3.3.1 and 3.3.2. If an SA bit is set in a false IIH, this could cause suppression of the advertisement of an IS neighbor, which could either continue for an indefinite period or occur intermittently with the result being a possible loss of reachability to some destinations in the network and/or increased frequency of LSP flooding and SPF calculation. The possibility of IS-IS PDU spoofing can be reduced by the use of authentication as described in [RFC1195] and [ISO10589], and especially the use of cryptographic authentication as described in [RFC5304].
[ISO10589] ISO, "Intermediate System to Intermediate System intra- domain routeing information exchange protocol for use in conjunction with the protocol for providing the connectionless-mode network service (ISO 8473)", International Standard 10589:2002, Second Edition, 2002. [RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and dual environments", RFC 1195, December 1990. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC5303] Katz, D., Saluja, R., and D. Eastlake 3rd, "Three-Way Handshake for IS-IS Point-to-Point Adjacencies", RFC 5303, October 2008. [RFC5304] Li, T. and R. Atkinson, "IS-IS Cryptographic Authentication", RFC 5304, October 2008.
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