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Network Working Group                                         Alex Zinin
Internet Draft                                                 Abhay Roy
Expiration Date: July 2001                                   Liem Nguyen
File name: draft-ietf-ospf-oob-resync-00.txt               Cisco Systems
                                                           November 2000



                OSPF Out-of-band LSDB resynchronization
                   draft-ietf-ospf-oob-resync-00.txt


Status of this Memo

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

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Abstract
   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             November 2000


   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
   resynchronization.

   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

   A new OSPF packet type (type 6) is introduced. The function of the
   packet is identical to that of OSPF DBD packet, except that it is
   specifically dedicated for OOB LSDB resynchronization.

   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
   technique.

2.1 The OR bit

   A new bit, called OR (OR stands for Out-of-band 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 OR bit to announce OOB LSDB resynchronization
   capability.

     +---+---+---+---+---+---+---+- -+---+---+---+---+---+---+---+---+
     | * | * | * | * | * | * | * |...| * | * | * | * | * | * | * | OR|
     +---+---+---+---+---+---+---+- -+---+---+---+---+---+---+---+---+

                        Figure 1. The Options field




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   Routers supporting the OOB LSDB resynchronization technique set the
   OR bit in the EO-TLV in the LLS block attached to Hello and DBD
   packets. Note that no bit is set in the 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
             ExStart

        o    The neighbor FSM reaches state Full

   Note that OOBResync flag may be set to TRUE 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 OSPF neighbor FSM is not
   changed by this document. However, depending on the state of the
   OOBResync flag, the router sends either legacy (type-2), or OOB
   (type-6) DBD packets.

2.3 Hello Packets

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

2.4 Type-2 DBD Packets

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

2.5 Type-6 DBD Packets



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   The format of type-6 DBD packets is identical to the format of type-2
   DBD packets, except that the type field is set to value of 6.

   Type-6 packets are sent instead of type-2 packets when the OOBResync
   flag for the specific neighbor is set to TRUE. Type-2 packets are
   considered to be invalid if OOBResync flag is set for the neighbor.

   When a type-6 DBD packet is received and stored value of the Options
   field for that neighbor does have OR bit set, the packet should be
   dropped and a SeqNumberMismatch event should be generated for the
   neighbor.

   Note also, that When a type 2 or type 6 OSPF packet is received and
   saved for later detection of a duplicate packet, the packet type
   should be stored as well.

   Processing of type-6 DBD packets is performed as follows.

   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 neigh-
        bor FSM is ExStart, Exchange, or Loading, the packet is pro-
        cessed just like a type 2 packet.

   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 DDs of both packet types (type 2 and type 6)
        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 gen-
        erated for the neighbor FSM that causes transition to state
        ExStart.

   It is also necessary to limit the time an adjacency can spend in
   ExStart state with OOBResync flag set to RouterDeadInterval seconds.
   If the adjacency does not go out of this state before the timer
   fires, the neighboring router does not process type-6 packets prop-
   erly.  The requesting router may decide to stop trying to resynchron-
   ize the LSDB over this adjacency (if, for example, it can be resyn-
   chronized via another neighbor on the same segment) or to resynchron-
   ize using the legacy method by clearing the OOBResync flag and leav-
   ing the FSM in ExStart state.






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2.6 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
        state.)

2.7 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 OR 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 packets of the
   new type are sent to OOB-incapable routers.

   The OR 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 Russ White, Don Slice, and Alvaro
   Retana for their valuable comments.

6 References



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INTERNET DRAFT            OSPF OOB LSDB Resync             November 2000


[RFC2328]
     J. Moy. OSPF version 2. Technical Report RFC 2328, Internet
     Engineering Task Force, 1998.  ftp://ftp.isi.edu/in-
     notes/rfc2328.txt.

[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
     USA
     e-mail: lhnguyen@cisco.com


























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