Network Working Group                                             J. Moy
Internet Draft                                   Sycamore Networks, Inc.
Expiration Date: July 2001                                 February 2001
File name: draft-ietf-ospf-hitless-restart-00.txt

                          Hitless OSPF Restart

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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    This memo documents an enhancement to the OSPF routing protocol,
    whereby an OSPF router can stay on the forwarding path even as its
    OSPF software is restarted. This is called "hitless restart" or
    "non-stop forwarding". A restarting router may not be capable of
    adjusting its forwarding in a timely manner when the network
    topology changes. In order to avoid the possible resulting routing
    loops the procedure in this memo automatically terminates when such
    a topology change is detected. The restart procedure is also
    backward-compatible, reverting to standard OSPF processing when one
    or more of the restarting router's neighbors do not support the
    enhancements in this memo. Proper network operation during a hitless
    restart makes assumptions upon the operating environment of the
    restarting router; these assumptions are also documented.

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Table of Contents

    1        Overview ............................................... 2
    2        Operation of restarting router ......................... 3
    2.1      Entering hitless restart ............................... 3
    2.2      Exiting hitless restart ................................ 5
    3        Operation of helper neighbor ........................... 6
    3.1      Entering helper mode ................................... 6
    3.2      Exiting helper mode .................................... 7
    4        Backward compatibility ................................. 7
    5        Notes .................................................. 7
             References ............................................. 8
    A        Grace-LSA format ....................................... 9
             Security Considerations ............................... 10
             Authors' Addresses .................................... 10

1.  Overview

    Today many Internet routers implement a separation of control and
    forwarding functions. Certain processors are dedicated to control
    and management tasks such as OSPF routing, while other processors
    perform the data forwarding tasks. This separation creates the
    possibility of maintaining a router's data forwarding capability
    while the router's control software is restarted/reloaded. We call
    such a possibility "hitless restart" or "non-stop forwarding".

    The problem that the OSPF protocol presents to hitless restart is
    that, under normal operation, OSPF intentionally routes around a
    restarting router while it rebuilds its link-state database. OSPF
    avoids the restarting router to minimize the possibility of routing
    loops and/or black holes caused by lack of database synchronization.
    Avoidance is accomplished by have the router's neighbors reissue
    their LSAs, omitting links to the restarting router.

    However, if (a) the network topology remains stable and (b) the
    restarting router is able to keep its forwarding table(s) across the
    restart, it would be safe to keep the restarting router on the
    forwarding path. This memo documents an enhancement to OSPF that
    makes such hitless restart possible, and one that automatically
    reverts back to standard OSPF for safety when network topology
    changes are detected.

    In a nutshell, the OSPF enhancements for hitless restart are as
    follows. The router attempting a hitless restart originates link-
    local Opaque-LSAs, herein called Grace-LSAs, announcing the
    intention to perform a hitless restart, and asking for a "grace
    period". During the grace period its neighbors continue to announce
    the restarting router in their LSAs as if it were fully adjacent

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    (i.e., OSPF neighbor state Full), but only if the network topology
    remains static (i.e, the contents of the LSAs in the link-state
    database having LS types 1-5,7 remain unchanged; simple refreshes
    are allowed).

    There are two roles being played by OSPF routers during hitless
    restart. First there is the router that is being restarted. The
    operation of this router during hitless restart, including how the
    router enters and leaves hitless restart, is the subject of Section
    2.  Then there are the router's neighbors, which must cooperate in
    order for the restart to be hitless. During hitless restart we say
    that the neighbors are executing in "helper mode". Section 3 covers
    the responsibilities of a router executing in helper mode, including
    entering and leaving helper mode.

2.  Operation of restarting router

    After the router restarts/reloads, it must change its OSPF
    processing somewhat until it re-establishes full adjacencies with
    all its previously fully-adjacent neighbors. This time period,
    between the restart/reload and the reestablishment of adjacencies,
    is called "hitless restart". During hitless restart:

     (1)   The restarting router does not originate LSAs with LS types
           1-5,7. Instead, the restarting router wants the other routers
           in the OSPF domain to calculate routes using the LSAs that it
           had originated prior to its restart, in order to maintain
           forwarding through the restart.

     (2)   The restarting router doesn't run its OSPF routing
           calculations, instead using the forwarding table(s) that it
           had built prior to the restart.

    Otherwise, the restarting router operates the same as any other OSPF
    router. It discovers neighbors using OSPF's Hello protocol, elects
    Designated and Backup Designated Routers, performs the Database
    Exchange procedure to initially synchronize link-state databases
    with its neighbors, and maintains this synchronization through

    The processes of entering hitless restart, and of exiting hitless
    restart (either successfully or not) are covered in the following

    2.1.  Entering hitless restart

        The router (call it Router X) is informed of the desire for its
        hitless restart when an appropriate command is issued by the

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        network operator. The network operator may also specify the
        length of the grace period, or the necessary grace period may be
        calculated by the router's OSPF software.

        In preparation for the hitless restart, Router X must perform
        the following actions before its software is restarted/reloaded.
        Note that common OSPF shutdown procedures are *not* performed,
        since we want the other OSPF routers to act as if Router X
        remains in continuous service. For example, Router X does not
        flush its locally originated LSAs, since we want them to remain
        in other routers' link-state databases throughout the restart

         (1)   Router X must ensure that its forwarding table(s) is/are
               up-to-date and will remain in place across the restart.

         (2)   Router X must resign any Designated Router (DR) or Backup
               Designated Router duties that it currently has. It does
               this by sending Hellos with Designated Router Priority
               set to 0. Resigning DR duties ensures that flooding works
               unimpeded across restarts, and that the DR/Backup will
               not change *after* the Grace-LSA is generated, which
               would be interpreted as a topology change and would
               terminate the hitless restart procedure prematurely.

         (3)   The router must note in non-volatile storage the
               cryptographic sequence numbers being used for each
               interface. Otherwise it will take up to
               RouterDeadInterval seconds after the restart before it
               can start to reestablish its adjacencies, which would
               force the grace period to be lengthened severely.

        Router X then originates the grace-LSAs. These are link-local
        Opaque-LSAs (see Appendix A). Their LS Age field is set to 0,
        and the requested grace period (in seconds) is inserted into the
        body of the grace-LSA. A grace-LSA is originated for each of the
        router's OSPF interfaces. However, a grace-LSA need not be
        originated for an interface if either a) the interface has no
        fully adjacent neighbors or b) the interface is of type point-
        to-point and a grace-LSA has already been sent to the attached
        neighbor on another interface. If Router X wants to ensure that
        its neighbors receive the grace-LSAs, it should retransmit the
        grace-LSAs until they are acknowledged (i.e, perform standard
        OSPF reliable flooding of the grace-LSAs). If one or more fully
        adjacent neighbors do not receive grace-LSAs, they will more
        than likely cause premature termination of the hitless restart
        procedure (see Section 4).

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        After the grace-LSAs have been sent, the router should store the
        fact that it is performing hitless restart along with the length
        of the requested grace period in non-volatile storage. The OSPF
        software should then be restarted/reloaded, and when the
        reloaded software starts executing the hitless restart
        modifications in Section 2 above are followed.

    2.2.  Exiting hitless restart

        On exiting "hitless restart", the reloaded router reverts back
        to completely normal OSPF operation, reoriginating LSAs based on
        the router's current state and recalculating its forwarding
        table(s) based on the current contents of the link-state
        database. The router exits hitless restart when any of the
        following occurs:

         (1)   Router X has reestablished all its adjacencies. Router X
               can determine this by building (but not installing or
               flooding) its router-LSA, based on the current router
               state, and comparing it to the router-LSA that it had
               last originated before the restart (called the "pre-
               restart router-LSA"). If the contents are the same, all
               adjacencies have been reestablished.

         (2)   Router X receives an LSA that is inconsistent with its
               pre-restart router-LSA. For example, X receives a router-
               LSA originated by router Y that does not contain a link
               to X, even though X's pre-start router-LSA did contain a
               link to Y. This indicates that either a) Y does not
               support hitless restart, b) Y never received the grace-
               LSA or c) Y has terminated its helper mode for some
               reason (Section 3.2).

         (3)   The grace period expires.

         (4)   Router X gets a valid hitless restart request (grace-LSA)
               from another router.  A router cannot both simultaneously
               attempt hitless restart and help a neighboring router
               undergo hitless restart, because the neighboring router
               must be monitoring the network state for changes
               throughout the entire restart period.

        When it exits hitless restart, the reloaded router should flush
        any grace-LSAs that it had originated.

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3.  Operation of helper neighbor

    As a "helper neighbor" for a router X undergoing hitless restart,
    router Y has two duties. It monitors the network for topology
    changes, and as long as there are none, continues to its advertise
    its LSAs as if X had remained in continuous OSPF operation. This
    means that Y's LSAs continue to list all adjacencies to X that were
    full (OSPF neighbor state Full) when the grace-LSA was first
    received, regardless of their current sycnchronization state. This
    logic affects the contents of both router-LSAs and network-LSAs, and
    also depends on the type of interface associated with the (possibly
    former) adjacency (see Sections through and
    Section 12.4.2 of [Ref1]).

    3.1.  Entering helper mode

        When a router Y receives a grace-LSA from router X, it enters
        helper mode for X as long as all the following checks pass:

         (1)   There have been no changes in content to the link-state
               database (LS types 1-5,7) since the beginning of the
               grace period specified by the grace-LSA. The grace period
               began N seconds ago, where N is the current LS age of the

         (2)   The grace period has not yet expired. This means that the
               LS age of the grace-LSA is less than the grace period
               specified in the body of the grace-LSA (Appendix A).

         (3)   Local policy allows Y to act as the helper for X.
               Examples of configured policies might be a) never act as
               helper, b) never allow the grace period to exceed a Time
               T, or c) never act as a helper for certain specific
               routers (specified by OSPF Router ID).

        Note that Router Y only needs to receive a single grace-LSA from
        X, even if X and Y attach to multiple common segments.  The data
        in the first valid grace-LSA received is used to indicate the
        beginning and the end of the grace period -- all subsequent
        grace-LSAs received from X are ignored. This first grace-LSA is
        referred to below as simply "the grace-LSA from X".

        A single router is allowed to simultaneously serve as a helper
        for multiple restarting neighbors.

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    3.2.  Exiting helper mode

        Router Y ceases to perform the helper function for its neighbor
        Router X when one of the following events occurs.

         (1)   The grace-LSA originated by X is flushed. This is the
               successful termination of hitless restart.

         (2)   The grace period expires.

         (3)   Router Y receives an LSA with LS types 1-5,7 and whose
               contents have changed. This includes LSAs with no
               previous link-state database instance and the flushing of
               LSAs from the database, but excludes simple LSA
               refreshes. A change in LSA contents indicates a network
               topology change, which forces termination of a hitless

        When router Y exits helper mode for X, Y reoriginates its LSAs
        based on the current state of its Router X adjacencies.

4.  Backward compatibility

    Backward-compatibility with unmodified OSPF routers is an automatic
    consequence of the functionality documented above. If one or more
    neighbors of a router requesting hitless restart are unmodified, or
    if they do not received the grace-LSA, the hitless restart is
    prematurely aborted.

    The unmodified routers will start routing around the restarted
    router X as it performs initial database synchronization, by
    reissuing their LSAs with links to X omitted. These LSAs will be
    interpreted by helper neighbors as a topology change, and by X as an
    LSA inconsistency, in either case aborting hitless restart and
    resuming normal OSPF operation.

5.  Notes

    Note the following details concerning the hitless OSPF restart
    mechanism described in this memo.

    o   DoNotAge is never set in a grace-LSA, even if the grace-LSA is
        flooded over a demand circuit. This is because the grace-LSA's
        LS age field is used to calculate the extent of the grace period
        (see Appendix A).

    o   Grace-LSAs have link-local scope because a) they only need to be
        seen by the router's direct neighbors and b) restricting them to

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        link-local scope makes it easy to detect the illegal
        configuration of two restarting routers being asked to help each
        other (Section 2.2).

    o   It may be noted that the hitless restart mechanisms in this memo
        can also be used for unplanned outages. For example, after a
        crash of its control software, the router may come up and send
        grace-LSAs in an attempt to remain on the forwarding path while
        it regains its control state. This may not be a good idea, as it
        seems unlikely that such a router could guarantee the sanity of
        its forwarding table(s). However, if the router does attempt a
        hitless restart from an unplanned outage, it should at the least
        (a) allow the network operator to turn this feature off and (b)
        attempt to determine when its forwarding tables were last
        updated, setting the beginning of the grace period accordingly
        (this means originating the grace-LSA with LS age equal to the
        time that the forwarding tables were last updated).


    [Ref1]  Moy, J., "OSPF Version 2", RFC 2328, April 1998.

    [Ref2]  Coltun, R., "The OSPF Opaque LSA Option", RFC 2370, July

    [Ref3]  Murphy, S., M. Badger and B. Wellington, "OSPF with Digital
            Signatures", RFC 2154, June 1997.

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A. Grace-LSA format

    The grace-LSA is a link-local scoped Opaque-LSA [Ref2] having Opaque
    Type of TBD1 and Opaque ID equal to TBD2. The grace-LSA is
    originated by a router that wishes to execute a hitless restart of
    its OSPF software. The grace-LSA requests that the router's
    neighbors aid it in its hitless restart by continuing to advertise
    the router as fully adjacent during a specified grace period.

    It is assumed that the grace-LSA has LS age field set to 0 when the
    LSA is first originated; the current value of LS age then indicates
    how long ago the restarting router made its request. The body of the
    LSA contains the length of the grace period in seconds.

        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
       |            LS age             |     Options   |       9       |
       |  Opaque Type  |               Opaque ID                       |
       |                     Advertising Router                        |
       |                     LS sequence number                        |
       |         LS checksum           |             length            |
       |                        Grace Period                           |

    Grace Period
        The number of seconds that the router's neighbors should
        continue to advertise the router as fully adjacent, regardless
        of the the state of database synchronization between the router
        and its neighbors. Since this time period began when grace-LSA's
        LS age was equal to 0, the grace period terminates when either
        a) the LS age of the grace-LSA exceeds the value of Grace Period
        or b) the grace-LSA is flushed. See Section 3.2 for other
        conditions which terminate the grace period.

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    Security Considerations

    One of the ways to attack a link-state protocol such as OSPF is to
    inject false LSAs into, or corrupt existing LSAs in, the link-state
    database.  Injecting a false grace-LSA would allow an attacker to
    spoof a router that, in reality, has been withdrawn from service.
    The standard way to prevent such corruption of the link-state
    database is to secure OSPF protocol exchanges using the Crytographic
    authentication specified in [Ref1]. An even stronger way of securing
    link-state database contents has been proposed in [Ref3].

Authors' Addresses

    J. Moy
    Sycamore Networks, Inc.
    150 Apollo Drive
    Chelmsford, MA 01824
    Phone: (978) 367-2505
    Fax:   (978) 256-4203

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