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rfc:rfc4812

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.

Nguyen, et al. Experimental [Page 1] RFC 4812 OSPF Restart Signaling March 2007

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].

Nguyen, et al. Experimental [Page 2] RFC 4812 OSPF Restart Signaling March 2007

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.

Nguyen, et al. Experimental [Page 3] RFC 4812 OSPF Restart Signaling March 2007

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.

Nguyen, et al. Experimental [Page 4] RFC 4812 OSPF Restart Signaling March 2007

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.

Nguyen, et al. Experimental [Page 5] RFC 4812 OSPF Restart Signaling 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

Nguyen, et al. Experimental [Page 6] RFC 4812 OSPF Restart Signaling March 2007

Full Copyright Statement

 Copyright (C) The IETF Trust (2007).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
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 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
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 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Nguyen, et al. Experimental [Page 7]

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