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RFC 4812 - OSPF Restart Signaling


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Network Working Group                                          L. Nguyen
Request for Comments: 4812                                        A. Roy
Category: Informational                                    Cisco Systems
                                                                A. Zinin
                                                          Alcatel-Lucent
                                                              March 2007

                         OSPF Restart Signaling

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   OSPF is a link-state intra-domain routing protocol used in IP
   networks.  Routers find new and detect unreachable neighbors via the
   Hello subprotocol.  Hello OSPF packets are also used to ensure two-
   way connectivity within time.  When a router restarts its OSPF
   software, it may not know its neighbors.  If such a router sends a
   Hello packet on an interface, its neighbors are going to reset the
   adjacency, which may not be desirable in certain conditions.

   This memo describes a vendor-specific mechanism that allows OSPF
   routers to inform their neighbors about the restart process.  Note
   that this mechanism requires support from neighboring routers.  The
   mechanism described in this document was proposed before Graceful
   OSPF Restart, as described in RFC 3623, came into existence.  It is
   implemented/supported by at least one major vendor and is currently
   deployed in the field.  The purpose of this document is to capture
   the details of this mechanism for public use.  This mechanism is not
   an IETF standard.

Table of Contents

   1. Introduction ....................................................2
   2. Proposed Solution ...............................................2
      2.1. Sending Hello Packets with the RS-bit Set ..................3
      2.2. Receiving Hello Packets with the RS-Bit Set ................3
      2.3. Ensuring Topology Stability ................................4
   3. Backward Compatibility ..........................................4
   4. Security Considerations .........................................4
   5. IANA Considerations .............................................4
   6. References ......................................................5
      6.1. Normative References .......................................5
      6.2. Informative References .....................................5
   Appendix A.  Acknowledgements ......................................6

1.  Introduction

   While performing a graceful restart of OSPF software [RFC3623],
   routers need to prevent their neighbors from resetting their
   adjacencies.  However, after a reload, routers may not be aware of
   the neighbors they had adjacencies with in their previous
   incarnations.  If such a router sends a Hello packet on an interface
   and this packet does not list some neighbors, those neighbors will
   reset the adjacency with the restarting router.

   This document describes a technique that allows restarting routers to
   inform their neighbors that they may not know about some neighbors
   yet and the absence of some router IDs in the Hello packets should be
   ignored.

2.  Proposed Solution

   With this Restart Signaling Solution, a new bit, called RS (restart
   signal), is introduced into the Extended Options (EO) TLV in the
   Link-Local Signaling (LLS) block (see [RFC4813]).  The value of this
   bit is 0x00000002; see Figure 1 below.

   +---+---+---+---+---+---+---+- -+---+---+---+---+---+---+---+---+
   | * | * | * | * | * | * | * |...| * | * | * | * | * | * | RS| LR|
   +---+---+---+---+---+---+---+- -+---+---+---+---+---+---+---+---+

                  Figure 1.  Bits in Extended Options TLV

   For a definition of the LR-bit, see [RFC4811].

2.1.  Sending Hello Packets with the RS-bit Set

   OSPF routers should set the RS-bit in the EO-TLV attached to a Hello
   packet when it is not known that all neighbors are listed in this
   packet, but the restarting router wants them to preserve their
   adjacencies.  The RS-bit must not be set in Hello packets longer than
   RouterDeadInterval seconds.

2.2.  Receiving Hello Packets with the RS-Bit Set

   When an OSPF router receives a Hello packet containing the LLS block
   with the EO-TLV that has the RS-bit set, the router should skip the
   two-way connectivity check with the announcing neighbor (i.e., the
   router should not generate a 1-WayReceived event for the neighbor if
   it does not find its own router ID in the list of neighbors as
   described in Section 10.5 of [RFC2328]), provided that the neighbor
   Finite State Machine (FSM) for this neighbor is in the Full state.

   The router should also send a unicast Hello back to the sender in
   reply to a Hello packet with RS-bit set.  This is to speed up
   learning of previously known neighbors.  When sending such a reply
   packet, care must be taken to ensure that the RS-bit is clear in it.

   Two additional fields are introduced in the neighbor data structure:
   RestartState flag and ResyncTimeout timer.  RestartState flag
   indicates that a Hello packet with the RS-bit set has been received
   and the local router expects its neighbor to go through the Link
   State Database (LSDB) resynchronization procedure using [RFC4811].
   ResyncTimeout is a single-shot timer limiting the delay between the
   first seen Hello packet with the RS-bit set and initialization of the
   LSDB resynchronization procedure.  The length of ResyncTimeout timer
   is RouterDeadInterval seconds.

   When a Hello packet with the RS-bit set is received and RestartState
   flag is not set for the neighbor, the router sets RestartState flag
   and starts ResyncTimeout timer.  If ResyncTimeout expires,
   RestartState flag is cleared and a 1-WayReceived event is generated
   for the neighbor.  If, while ResyncTimeout timer is running, the
   neighbor starts LSDB resynchronization procedure using [RFC4811],
   ResyncTimeout timer is canceled.  The router also clears RestartState
   flag on completion of the LSDB resynchronization process.

   Two or more routers on the same segment cannot have Hello packets
   with the RS-bit set at the same time, as can be the case when two or
   more routers restart at about the same time.  In such a scenario, the
   routers should clear the RestartState flag, cancel the ResyncTimeout
   timer, and generate a 1-WayReceived event.

2.3.  Ensuring Topology Stability

   Under certain circumstances, it might be desirable to stop announcing
   the restarting router as fully adjacent if this may lead to possible
   routing loops.  In order to provide this functionality, a
   configurable option is provided on the neighboring routers that
   instructs the OSPF process to follow the logics described below.

   When an OSPF router schedules a routing table calculation due to a
   change in the contents of its LSDB, it should also reset all
   adjacencies with restarting routers (those with RestartState set to
   TRUE) by clearing the RestartState neighbor flags, canceling
   ResyncTimeout timers (if running), and generating the 1-WayReceived
   events for the neighbor FSMs.

3.  Backward Compatibility

   The described technique requires cooperation from neighboring
   routers.  However, if neighbors do not support this technique, they
   will just reset the adjacency.

4.  Security Considerations

   The described technique does not introduce any new security issues
   into the OSPF protocol.

5.  IANA Considerations

   Please refer to the "IANA Considerations" section of [RFC4813] for
   more information on the Extended Options bit definitions.

6.  References

6.1.  Normative References

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.

   [RFC3623]  Moy, J., Pillay-Esnault, P., and A. Lindem, "Graceful OSPF
              Restart", RFC 3623, November 2003.

6.2.  Informative References

   [RFC4813]  Friedman, B., Nguyen, L., Roy, A., Yeung, D., and A.
              Zinin, "OSPF Link-Local Signaling", RFC 4813, March 2007.

   [RFC4811]  Nguyen, L., Roy, A., and A. Zinin, "OSPF Out-of-Band Link
              State Database (LSDB) Resynchronization", RFC 4811, March
              2007.

Appendix A.  Acknowledgments

   The authors would like to thank John Moy, Russ White, Don Slice, and
   Alvaro Retana for their valuable comments.

Authors' Addresses

   Liem Nguyen
   Cisco Systems
   225 West Tasman Drive
   San Jose, CA  95134
   USA
   EMail: lhnguyen@cisco.com

   Abhay Roy
   Cisco Systems
   225 West Tasman Drive
   San Jose, CA  95134
   USA
   EMail: akr@cisco.com

   Alex Zinin
   Alcatel-Lucent
   Mountain View, CA
   USA
   EMail: alex.zinin@alcatel-lucent.com

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