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RFC 4811 - OSPF Out-of-Band Link State Database (LSDB) Resynchro

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Network Working Group                                          L. Nguyen
Request for Comments: 4811                                        A. Roy
Category: Informational                                    Cisco Systems
                                                                A. Zinin
                                                              March 2007

     OSPF Out-of-Band Link State Database (LSDB) Resynchronization

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).


   OSPF is a link-state intra-domain routing protocol used in IP
   networks.  Link State Database (LSDB) synchronization in OSPF is
   achieved via two methods -- initial LSDB synchronization when an OSPF
   router has just been connected to the network and asynchronous
   flooding that ensures continuous LSDB synchronization in the presence
   of topology changes after the initial procedure was completed.  It
   may sometime be necessary for OSPF routers to resynchronize their
   LSDBs.  The OSPF standard, however, does not allow routers to do so
   without actually changing the topology view of the network.

   This memo describes a vendor-specific mechanism to perform such a
   form of out-of-band LSDB synchronization.  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. The LR-Bit .................................................3
      2.2. OSPF Neighbor Data Structure ...............................3
      2.3. Hello Packets ..............................................4
      2.4. DBD Packets ................................................4
      2.5. Neighbor State Treatment ...................................7
      2.6. Initiating OOB LSDB Resynchronization ......................7
   3. Backward Compatibility ..........................................7
   4. Security Considerations .........................................7
   5. IANA Considerations .............................................7
   6. References ......................................................8
      6.1. Normative References .......................................8
      6.2. Informative References .....................................8
   Appendix A.  Acknowledgements ......................................9

1.  Introduction

   According to the OSPF standard [RFC2328], after two OSPF routers have
   established an adjacency (the neighbor Finite State Machines (FSMs)
   have reached Full state), routers announce the adjacency states in
   their router-Link State Advertisements (LSAs).  Asynchronous flooding
   algorithm ensures that routers' LSDBs stay in sync in the presence of
   topology changes.  However, if routers need (for some reason) to
   resynchronize their LSDBs, they cannot do that without actually
   putting the neighbor FSMs into the ExStart state.  This effectively
   causes the adjacencies to be removed from the router-LSAs, which may
   not be acceptable if the desire is to prevent routing table flaps
   during database resynchronization.  In this document, we provide the
   means for so-called out-of-band (OOB) LSDB resynchronization.

   The described mechanism can be used in a number of situations
   including those where the routers are picking up the adjacencies
   after a reload.  The process of adjacency preemption is outside the
   scope of this document.  Only the details related to LSDB
   resynchronization are mentioned herein.

2.  Proposed Solution

   With this Out-of-Band Resynchronization Solution, the format of the
   OSPF Database Description (DBD) packet is changed to include a new
   R-bit indicating OOB LSDB resynchronization.  All DBD packets sent
   during the OOB resynchronization procedure are sent with the R-bit

   Also, two new fields are added to the neighbor data structure.  The
   first field indicates a neighbor's OOB resynchronization capability.
   The second indicates that OOB LSDB resynchronization is in process.
   The latter field allows OSPF implementations to utilize the existing
   neighbor FSM code.

   A bit is occupied in the Extended Options (EO) TLV (see [RFC4813]).
   Routers set this bit to indicate their capability to support the
   described technique.

2.1.  The LR-Bit

   A new bit, called LR (LR stands for LSDB Resynchronization), is
   introduced to the LLS Extended Options TLV (see [RFC4813]).  The
   value of the bit is 0x00000001; see Figure 1.  See the "IANA
   Considerations" section of [RFC4813] for more information on the
   Extended Options bit definitions.  Routers set the LR-bit to announce
   OOB LSDB resynchronization capability.

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

                       Figure 1.  The Options Field

   Routers supporting the OOB LSDB resynchronization technique set the
   LR-bit in the EO-TLV in the LLS block attached to both Hello and DBD
   packets.  Note that no bit is set in the standard OSPF Options field,
   neither in OSPF packets nor in LSAs.

2.2.  OSPF Neighbor Data Structure

   A field is introduced into OSPF neighbor data structure, as described
   below.  The name of the field is OOBResync, and it is a flag
   indicating that the router is currently performing OOB LSDB
   resynchronization with the neighbor.

   The OOBResync flag is set when the router is initiating OOB LSDB
   resynchronization (see Section 2.6 for more details).

   Routers clear the OOBResync flag on the following conditions:

    o  The neighbor data structure is first created.

    o  The neighbor FSM transitions to any state lower than ExStart.

    o  The neighbor FSM transitions to the ExStart state because a DBD
       packet with the R-bit clear has been received.

    o  The neighbor FSM reaches the state Full.

   Note that the OOBResync flag may have a TRUE value only if the
   neighbor FSM is in states ExStart, Exchange, or Loading.  As
   indicated above, if the FSM transitions to any other state, the
   OOBResync flag should be cleared.

   It is important to mention that operation of the OSPF neighbor FSM is
   not changed by this document.  However, depending on the state of the
   OOBResync flag, the router sends either normal DBD packets or DBD
   packets with the R-bit set.

2.3.  Hello Packets

   Routers capable of performing OOB LSDB resynchronization should
   always set the LR-bit in their Hello packets.

2.4.  DBD Packets

   Routers supporting the described technique should always set the LR-
   bit in the DBD packets.  Since the Options field of the initial DBD
   packet is stored in corresponding neighbor data structure, the LR-bit
   may be used later to check if a neighbor is capable of performing OOB
   LSDB resynchronization.

   The format of type 2 (DBD) OSPF packets is changed to include a flag
   indicating the OOB LSDB resynchronization procedure.  Figure 2
   illustrates the new packet format.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |   Version #   |       2       |         Packet length         |
   |                          Router ID                            |
   |                           Area ID                             |
   |           Checksum            |             AuType            |
   |                       Authentication                          |
   |                       Authentication                          |
   |         Interface MTU         |    Options    |0|0|0|0|R|I|M|MS
   |                     DD sequence number                        |
   |                                                               |
   +-                                                             -+
   |                                                               |
   +-                      An LSA Header                          -+
   |                                                               |
   +-                                                             -+
   |                                                               |
   +-                                                             -+
   |                                                               |
   |                              ...                              |

                      Figure 2.  Modified DBD Packet

   The R-bit in OSPF type 2 packets is set when the OOBResync flag for
   the specific neighbor is set to TRUE.  If a DBD packets with the R-
   bit clear is received for a neighbor with active OOBResync flag, the
   OOB LSDB resynchronization process is canceled and normal LSDB
   synchronization procedure is initiated.

   When a DBD packet is received with the R-bit set and the sender is
   known to be OOB-incapable, the packet should be dropped and a
   SeqNumber-Mismatch event should be generated for the neighbor.

   Processing of DBD packets is modified as follows:

    1.  If the OOBResync flag for the neighbor is set (the LSDB
        resynchronization process has been started) and the received DBD
        packet does not have the R-bit set, ignore the packet and
        generate a SeqNumberMismatch event for the neighbor FSM.

    2.  Otherwise, if the OOBResync flag for the neighbor is clear and
        the received DBD packet has the R-bit set, perform the following

        *  If the neighbor FSM is in state Full and bits I, M, and MS
           are set in the DBD packet, set the OOBResync flag for the
           neighbor, put the FSM in ExStart state, and continue
           processing the DBD packet as described in [RFC2328].

        *  Otherwise, ignore received DBD packet (no OOB DBD packets are
           allowed with OOBResync flag clear and FSM in state other than
           Full).  Also, if the state of the FSM is Exchange or higher,
           generate a SeqNumberMismatch event for the neighbor FSM.

    3.  Otherwise, process the DBD packet as described in [RFC2328].

   During normal processing of the initial OOB DBD packet (with bits R,
   I, M, and MS set), if the receiving router is selected to be the
   Master, it may speed up the resynchronization process by immediately
   replying to the received packet.

   It is also necessary to limit the time an adjacency can spend in
   ExStart, Exchange, and Loading states with OOBResync flag set to a
   finite period of time (e.g., by limiting the number of times DBD and
   link state request packets can be retransmitted).  If the adjacency
   does not proceed to Full state before the timeout, it is indicative
   that the neighboring router cannot resynchronize its LSDB with the
   local router.  The requesting router may decide to stop trying to
   resynchronize the LSDB over this adjacency (if, for example, it can
   be resynchronized via another neighbor on the same segment) or to
   resynchronize using the legacy method by clearing the OOBResync flag
   and leaving the FSM in ExStart state.  The neighboring router may
   decide to cancel the OOB procedure for the neighbor.

2.5.  Neighbor State Treatment

   An OSPF implementation supporting the described technique should
   modify the logic consulting the state of a neighbor FSM as described

    o  FSM state transitioning from and to the Full state with the
       OOBResync flag set should not cause origination of a new version
       of router-LSA or network-LSA.

    o  Any explicit checks for the Full state of a neighbor FSM for the
       purposes other than LSDB synchronization and flooding should
       treat states ExStart, Exchange, and Loading as state Full,
       provided that OOBResync flag is set for the neighbor.  (Flooding
       and MaxAge-LSA-specific procedures should not check the state of
       the OOBResync flag, but should continue consulting only the FSM

2.6.  Initiating OOB LSDB Resynchronization

   To initiate out-of-band LSDB resynchronization, the router must first
   make sure that the corresponding neighbor supports this technology
   (by checking the LR-bit in the Options field of the neighbor data
   structure).  If the neighboring router is capable, the OOBResync flag
   for the neighbor should be set to TRUE and the FSM state should be
   forced to ExStart.

3.  Backward Compatibility

   Because OOB-capable routers explicitly indicate their capability by
   setting the corresponding bit in the Options field, no DBD packets
   with the R-bit set are sent to OOB-incapable routers.

   The LR-bit itself is transparent for OSPF implementations and does
   not affect communication between routers.

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.

Appendix A.  Acknowledgments

   The authors would like to thank Acee Lindem, 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
   EMail: lhnguyen@cisco.com

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

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

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