Subject: OSPF Hitless Restart

From: Acee Lindem <[email-masked]>
Date: Wed, 6 Nov 2002 10:34:01 -0500

I've attached draft-ietf-ospf-hitless-restart-04.txt
with the changes discussed in my previous E-mail.
---
Acee

Network Working Group                         J. Moy (Sycamore Networks)
Internet Draft                   Padma Pillay-Esnault (Juniper Networks)
Expiration Date: April 2003      Acee Lindem, Editor  (Redback Networks)
File name: draft-ietf-ospf-hitless-restart-04.txt           October 2002

                          Hitless OSPF Restart
                 draft-ietf-ospf-hitless-restart-04.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

    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 reverts to a normal
    OSPF restart when such a topology change is detected, or 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
    1.1      Acknowledgments ........................................ 3
    2        Operation of restarting router ......................... 3
    2.1      Entering hitless restart ............................... 4
    2.2      When to exit hitless restart ........................... 5
    2.3      Actions on exiting hitless restart ..................... 6
    3        Operation of helper neighbor ........................... 6
    3.1      Entering helper mode ................................... 7
    3.2      Exiting helper mode .................................... 8
    4        Backward compatibility ................................. 9
    5        Unplanned outages ...................................... 9
    6        Interaction with Traffic Engineering .................. 10
    7        Possible Future Work .................................. 10
             References ............................................ 10
    A        Grace-LSA format ...................................... 11
    B        Change log ............................................ 13
             Security Considerations ............................... 14
             Authors' Addresses .................................... 14

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 having 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 a standard OSPF restart 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-

Moy, Pillay-Esnault, Lindem                                     [Page 2]

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    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
    (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; periodic 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.

    1.1.  Acknowledgments

        The authors wish to thank John Drake, Vishwas Manral,
        Kent Wong and Don Goodspeed for their helpful comments.

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.  During this time, the
           restarting router does not modify or flush received self-
           originated LSAs, (see Section 13.4 of [1]) but instead
           accepts them as valid. In particular, the grace-LSAs that the
           restarting router had originated before the restart are left
           in place. Received self-originated LSAs will be dealt with
           when the router exits hitless restart (see Section 2.3).

     (2)   The restarting router runs its OSPF routing calculations, as
           specified in Section 16 of [1]. This is necessary to
           return any OSPF virtual links to operation. However, the
           restarting router does *not* install OSPF routes into the
           system's forwarding table(s), instead relying on the
           forwarding entries that it had installed prior to the
           restart.

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     (3)   If the restarting router determines that it was Designated
           Router on a given segment immediately prior to the restart,
           it elects itself as Designated Router again. The restarting
           router knows that it was Designated Router if, while the
           associated interface is in Waiting state, an Hello packet is
           received from a neighbor listing the router as Designated
           Router.

    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
    flooding.

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

    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
        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 order to avoid
        the restarting router's LSAs from aging out, the grace period
        should not exceed LSRefreshTime (1800 second) [1].

        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
        period.

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

         (2)   The router must note in non-volatile storage the
               cryptographic sequence numbers being used for each
               interface. An alternative is to use the router's
               clock for cryptographic sequence number generation and
               ensure the clock is preserved across restarts (either
               on the same or redundant route processors).
               If neither of these can be guarenteed, it can take up
               to RouterDeadInterval seconds after the restart before
               adjacencies can be reestablished and this would
               force the grace period to be lengthened severely.

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        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. The precise contents of the grace-LSA are
        described in Appendix A.

        A grace-LSA is originated for each of the router's OSPF
        interfaces. 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).

        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. (Note to
        implementors: It may be easiest to simply store the absolute
        time of the end of the grace period).  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.  (Note that prior to the restart, the router
        does not know whether its neighbors are going to cooperate as
        "helpers"; the mere reception of grace-LSAs does not imply
        acceptance of helper responsibilities. This memo assumes that
        the router would want to restart anyway, even if the restart is
        not going to be hitless).

    2.2.  When to exit hitless restart

        A Router X exits hitless restart when any of the following
        occurs:

         (1)   Router X has reestablished all its adjacencies. Router X
               can determine this by examining the router-LSAs that it
               had last originated before the restart (called the "pre-
               restart router-LSA"), and, on those segments where the
               router is Designated Router, the pre-restart network-
               LSAs. These LSAs will have been received from the helping
               neighbors, and need not have been stored in non-volatile
               storage across the restart. All previous adjacencies will
               be listed as type-1 and type 2 links in the router-LSA,
               and as neighbors in the body of the network-LSA.

         (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

Moy, Pillay-Esnault, Lindem                                     [Page 5]

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               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.

    2.3.  Actions on 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 updating its forwarding table(s)
        based on the current contents of the link-state database. In
        particular, the following actions should be performed when
        exiting, either successfully or unsuccessfully, hitless restart.

         (1)   The router should reoriginate its router-LSAs for all
               attached areas, to make sure they have the correct
               contents.

         (2)   The router should reoriginate network-LSAs on all
               segments where it is Designated Router.

         (3)   The router reruns its OSPF routing calculations (Section
               16 of [1]), this time installing the results into the
               system forwarding table, and originating summary-LSAs,
               Type-7 LSAs and AS-external-LSAs as necessary.

         (4)   Any remnant entries in the system forwarding table that
               were installed before the restart, but that are no longer
               valid, should be removed.

         (5)   Any received self-originated LSAs that are no longer
               valid should be flushed.

         (6)   Any grace-LSAs that the router had originated should be
               flushed.

3.  Operation of helper neighbor

    The helper relationship is per network segment.  As a "helper
    neighbor" on a segment S for a restarting router X, router Y has
    several 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 an adjacency to X over network segment S,
    regardless of the adjacency's current synchronization state. This

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    logic affects the contents of both router-LSAs and network-LSAs, and
    also depends on the type of network segment S (see Sections 12.4.1.1
    through 12.4.1.5 and Section 12.4.2 of [1]). When helping over a
    virtual link, the helper must also continue to set bit V in its
    router-LSA for the virtual link's transit area (Section 12.4.1 of
    [1]).

    Also, if X was the Designated Router on network segment S when the
    helping relationship began, Y maintains X as Designated router until
    the helping relationship is terminated.

    3.1.  Entering helper mode

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

         (1)   Y currently has a full adjacency with X (neighbor state
               Full) over the associated network segment. On broadcast,
               NBMA and Point-to-MultiPoint segments, the neighbor
               relationship with X is identified by the IP interface
               address in the body of the grace-LSA (see Appendix A). On
               all other segment types X is identified by the grace-
               LSA's Advertising Router field.

         (2)   There have been no changes in content to the link-state
               database (LS types 1-5,7) since router X restarted. This
               is determined as follows. Router Y examines the link-
               state retransmission list for X over the associated
               network segment. If there are any LSAs with LS types
               1-5,7 on the list, then they all must be periodic
               refreshes. If there are instead LSAs on the list whose
               contents have changed (see Section 3.3 of [8]), Y must
               refuse to enter helper mode.

         (3)   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).

         (4)   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, c) only help on software reloads/upgrades, or d) never
               act as a helper for certain specific routers (specified
               by OSPF Router ID).

        There is one exception to the above requirements. If Y was
        already helping X on the associated network segment, the new

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        grace-LSA should be accepted and the grace period should be
        updated accordingly.

        Note that Router Y may be helping X on some network segments,
        and not on others. However, that circumstance will probably lead
        to the premature termination of X's hitless restart, as Y will
        not continue to advertise adjacencies on the segments where it
        is not helping (see Section 2.2).

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

    3.2.  Exiting helper mode

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

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

         (2)   The grace-LSA's grace period expires.

         (3)   A change in link-state database contents indicates a
               network topology change, which forces termination of a
               hitless restart.  Specifically, if router Y installs a
               new LSA in its database with LS types 1-5,7 and having
               the following two properties, it should cease helping X.
               The two properties of the LSA are a) the contents of the
               LSA have changed; this includes LSAs with no previous
               link-state database instance and the flushing of LSAs
               from the database, but excludes periodic LSA refreshes
               (see Section 3.3 of [8]), and b) the LSA would have
               been flooded to X, had Y and X been fully adjacent. As an
               example of the second property, if Y installs a changed
               AS-external-LSA, it should not terminate a helping
               relationship with a neighbor belonging to a stub area, as
               that neighbor would not see the AS-external-LSA in any
               case. An implementation MAY provide a configuration
               option to disable link-state database options from
               terminating hitless restart. Such an option will,
               however, increase the risk of routing loops and
               black holes.

        When router Y exits helper mode for X on a given network
        segment, it reoriginates its LSAs based on the current state of
        its adjacency to Router X over the segment. In detail, Y takes
        the following actions: (a) Y recalculates the Designated Router
        for the segment, (b) Y reoriginates its router-LSA for the
        segment's OSPF area, (c) if Y is Designated Router for the
        segment, it reoriginates the network-LSA for the segment and (d)
        if the segment was a virtual link, Y reoriginates its router-LSA

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        for the virtual link's transit area.

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 converts
    to a normal OSPF restart.

    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 reverting to normal OSPF
    operation.

5.  Unplanned outages

    The hitless restart mechanisms in this memo can be used for
    unplanned outages. (Examples of unplanned outages include the crash
    of a router's control software, an unexpected switchover to a
    redundant control processor, etc). However, implementors and network
    operators should note that attempting hitless restart from an
    unplanned outage may not be a good idea, owing to the router's
    inability to properly prepare for the restart (see Section 2.1). In
    particular, it seems unlikely that a router could guarantee the
    sanity of its forwarding table(s) across an unplanned restart. In
    any event, implementors providing the option to recover hitlessly
    from unplanned outages must allow a network operator to turn the
    option off.

    In contrast to the procedure for planned restart/reloads that was
    described in Section 2.1, a router attempting hitless restart after
    an unplanned outage must originate grace-LSAs *after* its control
    software resumes operation. The following points must be observed
    during this grace-LSA origination.

    o   The grace-LSAs must be originated and sent *before* the
        restarted router sends any OSPF Hello Packets. On broadcast
        networks, this LSA must be flooded to the AllSPFRouters
        multicast address (224.0.0.5) since the restarting router is
        not aware of its previous DR state.

    o   The grace-LSAs are encapsulated in Link State Update Packets and
        sent out all interfaces, even though the restarted router has no
        adjacencies and no knowledge of previous adjacencies.

    o   To improve the probability that grace-LSAs be delivered, an
        implementation may send them a number of times (see for example
        the Robustness Variable in [8]).

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    o   The restart reason in the grace-LSAs must be set to unknown(0).
        This enables the neighbors to decide whether they want to help
        the router through an unplanned restart.

6.  Interaction with Traffic Engineering

    The operation of the Traffic Engineering Extensions to OSPF [4]
    during OSPF Hitless Restart is specified in [6].

7.  Possible Future Work

    Devise a less conservative algorithm for graceful restart
    helper termination that provides a comparable level of
    black hole and routing loop avoidance.

Normative References

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

    [2]  Coltun, R., "The OSPF Opaque LSA Option", RFC 2370, July
         1998.

Informative References

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

    [4]  Katz, D., D. Yeung and K. Kompella, "Traffic Engineering
         Extensions to OSPF", work in progress.

    [5]  Coltun, R., V. Fuller and P. Murphy, "The OSPF NSSA Option",
         work in progress.

    [6]  Kompella, K., et. al., "Routing Extensions in Support of
         Generalized MPLS", work in progress.

    [7]  Moy, J., "Extending OSPF to Support Demand Circuits", RFC
         1793, April 1995.

    [8]  Fenner, W., "Internet Group Membership Protocol, Version 2",
         RFC 2236, November 1997.

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A. Grace-LSA format

    The grace-LSA is a link-local scoped Opaque-LSA [2] having Opaque
    Type of 3 and Opaque ID equal to 0. Grace-LSAs are originated by a
    router that wishes to execute a hitless restart of its OSPF
    software. A 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.

    Each 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 is TLV-
    encoded. The TLV-encoded information includes the length of the
    grace period, the reason for the hitless restart and, when the
    grace-LSA is associated with a broadcast, NBMA or Point-to-
    MultiPoint network segment, the IP interface address of the
    restarting router.

        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       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |       3       |                    0                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Advertising Router                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     LS sequence number                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         LS checksum           |             length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +-                            TLVs                             -+
       |                             ...                               |

    The format of the TLVs within the body of a grace-LSA is the same as
    the TLV format used by the Traffic Engineering Extensions to OSPF
    [4]. The TLV header consists of a 16-bit Type field and a 16-bit
    length field, and is followed by zero or more bytes of value. The
    length field indicates the length of the value portion in bytes. The
    value portion is padded to four-octet alignment, but the padding is
    not included in the length field. For example, a one byte value
    would have the length field set to 1, and three bytes of padding
    would be added to the end of the value portion of the TLV.

    The following is the list of TLVs that can appear in the body of a
    grace-LSA.

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    o   Grace Period (Type=1, length=4).  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. This TLV must always appear in a grace-LSA.

    o   Hitless restart reason (Type=2, length=1). Encodes the reason
        for the router restart, as one of the following: 0 (unknown), 1
        (software restart), 2 (software reload/upgrade) or 3 (switch to
        redundant control processor). This TLV must always appear in a
        grace-LSA.

    o   IP interface address (Type=3, length=4). The router's IP
        interface address on the subnet associated with the grace-LSA.
        Required on broadcast, NBMA and Point-to-MultiPoint segments,
        where the helper uses the IP interface address to identify the
        restarting router (see Section 3.1).

    DoNotAge is never set in a grace-LSA, even if the grace-LSA is
    flooded over a demand circuit [7]. This is because the grace-LSA's
    LS age field is used to calculate the extent of the grace period.

    Grace-LSAs have link-local scope because they only need to be seen
    by the router's direct neighbors.

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B. Change Log (To be removed prior to publication)

  Changes from 02 to 03 version:

     1. Add Padma Pillay-Esnault and Acee Lindem as authors to help
        finish up the draft.

  Changes from 03 to 04 version:

     1. Add change log (Appendix B).
     2. Document that the grace period is restricted to
        LSRefreshTime (Section 2.1).
     3. Document an alternative to saving cryptographic sequence
        numbers in non-volatile storage (Section 2.1).
     4. Document that an implementation may disable graceful
        restart helper termination when the link-state database
        changes (Section 3.2).
     5. In the case of an unplanned restart, document that
        grace LSAs should be flooded to AllSPFRouters on
        broadcast networks (Section 5).
     6. Remove MOSPF from future work. Add Vishwas's suggested
        technique for less conservative helper mode termination as
        possible future work (Section 7).
     7. Change references and citations to meet prevailing IETF
        standards.

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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
    Cryptographic authentication specified in [1]. An even stronger
    way of securing link-state database contents has been proposed in
    [3].

Authors' Addresses

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

    Padma Pillay-Esnault
    Juniper Networks
    1194 N, Mathilda Avenue
    Sunnyvale, CA 94089-1206
    Email: [email-masked]

    Acee Lindem
    Redback Networks
    102 Carric Bend Court
    Cary, NC 27519
    Email: [email-masked]

Moy, Pillay-Esnault, Lindem                                    [Page 14]