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Network Working Group                                         Alex Zinin
Internet Draft                                                 Abhay Roy
Expiration Date: September 2001                              Liem Nguyen
File name: draft-ietf-ospf-oob-resync-01.txt               Cisco Systems
                                                           February 2001

                OSPF Out-of-band LSDB resynchronization

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet Drafts are working documents of the Internet Engineering
   Task Force (IETF), its Areas, and its Working Groups. Note that other
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   OSPF is a link-state intra-domain routing protocol used in IP
   networks. 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. OSPF
   standard, however, does not allow routers to do so without actually
   changing the topology view of the network.  This memo describes a
   mechanism to perform such form of out-of-band LSDB synchronization.

1 Motivation

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INTERNET DRAFT            OSPF OOB LSDB Resync             February 2001

   According to the OSPF standard [RFC2328], after two OSPF routers have
   established an adjacency (the neighbor FSMs have reached Full state),
   routers announce the adjacency states in their router-LSAs.
   Asynchronous flooding algorithm ensures 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 in some cases. In this
   document, we provide the means for so-called out-of-band (OOB) LSDB

   The described mechanism can be used in a number of situations
   including those where the routers are picking the adjacencies up
   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

   The format of the OSPF Database Description 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 set.

   Also, two new fields are added to the neighbor data structure. The
   first field indicates 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 TLV (see [LLS]). Routers
   set this bit to indicate their capability to support the described

2.1 The LR bit

   A new bit, called LR (LR stands for LSDB Resynchronization) is
   introduced to the LLS Extended Options TLV (see [LLS]).  The value of
   the bit is TBD (0x00000001 is the temporarily used value, see Figure
   1).  Routers set LR bit to announce OOB LSDB resynchronization

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

                        Figure 1. The Options field

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INTERNET DRAFT            OSPF OOB LSDB Resync             February 2001

   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.

   OOBResync flag is set when the router is initiating the OOB LSDB
   resynchronization (see Section 2.7 for more details).

   Routers clear OOBResync flag on the following conditions.

        o    The neighbor data structure is first created

        o    The neighbor FSM transitions to any state lower than

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

        o    The neighbor FSM reaches state Full

   Note that OOBResync flag may have 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

   It is important to mention that operation of 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

Zinin, Roy, Nguyen                                              [Page 3]

INTERNET DRAFT            OSPF OOB LSDB Resync             February 2001

   LSB resynchronization.

   The format of type-2 (DBD) OSPF packets is changed to include a flag
   indicating 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                          -+
          |                                                               |
          +-                                                             -+
          |                                                               |
          +-                                                             -+
          |                                                               |
          |                              ...                              |

   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 R-bit
   clear is received for a neighbor with active OOBResync flag, the OOB
   LSDB resynchronization process is cancelled and normal LSDB synchron-
   ization procedure is initiated.

   When a DBD packet is received with 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.

Zinin, Roy, Nguyen                                              [Page 4]

INTERNET DRAFT            OSPF OOB LSDB Resync             February 2001

   1)   If the R-bit is set, do the following

       o    If bits I, M, and MS are set and the state of the neighbor
            FSM is Full and OOBResync flag is not set, the packet is
            accepted, the OOBResync flag is set and the FSM is put into
            ExStart state.

       o    Otherwise, if OOBResync flag is set and the state of the
            neighbor FSM is ExStart, Exchange, or Loading, the packet is
            processed just as described in [RFC2328].

       o    Otherwise, if neighbor state is Full and the receiving
            router was the Slave in the LSDB exchange process, it must
            be ready to identify duplicate DBDs with R-bit set from the
            master and resend the acknowledging packet.

       o    Otherwise (the OOBResync flag is off, or the state is not
            Full, or the packet is not a duplicate), a SeqNumberMismatch
            is generated for the neighbor FSM that causes transition to
            state ExStart.

   2)   Otherwise (the R-bit is not set) do the following

       o    If OOBResync flag for the neighbor is set, OOBResync flag is
            cleared and a SeqNumberMismatch event is generated for the
            neighbor FSM.

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

   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 limitting 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, the neighboring
   routers experience problems in LSDB resynchronization.  The request-
   ing 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.

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INTERNET DRAFT            OSPF OOB LSDB Resync             February 2001

2.5 Neighbor State Treatment

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

   o    FSM state transitioning from and to the Full state with
        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, pro-
        vided that OOBResync flag is set for the neighbor. (Flooding and
        MaxAge-LSA-specific procedures should not check the state of
        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 Options field of the neighbor data struc-
   ture).  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 Compatibility Issues

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

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

4 Security Considerations

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

5 Acknowledgements

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

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INTERNET DRAFT            OSPF OOB LSDB Resync             February 2001

6 References

     J. Moy. OSPF version 2. Technical Report RFC 2328, Internet
     Engineering Task Force, 1998.  ftp://ftp.isi.edu/in-

[LLS] Zinin, Friedman, Roy, Nguyen, Yeung, "OSPF Link-local Signaling",
     draft-ietf-ospf-lls-00.txt, Work in progress.

7 Authors' addresses

     Alex Zinin                        Abhay Roy
     Cisco Systems                     Cisco Systems
     150 W. Tasman Dr.                 170 W. Tasman Dr.
     San Jose,CA 95134                 San Jose,CA 95134
     USA                               USA
     E-mail: azinin@cisco.com          E-mail: akr@cisco.com

     Liem Nguyen
     7025 Kit Creek Rd.
     Research Triangle Park, NC 27709
     e-mail: lhnguyen@cisco.com

Zinin, Roy, Nguyen                                              [Page 7]

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