Network Working Group                                        B. Decraene
Internet-Draft                                                    Orange
Intended status: Standards Track                           March 9, 2015
Expires: September 10, 2015

               Back-off SPF algorithm for link state IGP


   This document defines a standard algorithm to back-off link-state IGP
   SPF computations.

   Having one standardized algorithm improves interoperability by
   reducing the probability and/or duration of transient forwarding
   loops during the IGP convergence in the area/level when the network
   reacts to multiple consecutive events.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on September 10, 2015.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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1.  Introduction

   Link state IGP, such as IS-IS [ISO10589-Second-Edition] and OSPF
   [RFC2328], performs distributed computation on all nodes of the area/
   level.  In order to have consistent routing tables across the
   network, such distributed computation requires that all routers have
   the same vision of the network (Link State DataBase (LSDB)) and
   perform their computation at the same time.

   In general, when the network is stable, there is a desire to compute
   the new SPF as soon as the failure is known, in order to quickly
   route around the failure.  However, when the network is experiencing
   multiple consecutive failures over a short period of time, there is a
   desire to limit the frequency of SPF computations.  Indeed, this
   allow reducing the control plane resources used by IGP and all
   protocols/sub system reacting on it such as LDP, RSVP-TE, BGP, Fast
   ReRoute computations, FIB updates..., reduce the churn on nodes and
   in the network, in particular reduce side effects such as micro-loops
   which may happen during each IGP convergence.

   To allow for this, some back-off algorithm have been implemented.
   Different implementations choose different algorithms, hence in a
   multi-vendor network, it's not possible to enforce that all routers
   triggers their SPF computation after the same waiting delay.  This
   situation increases the average differential delay between routers
   end of RIB computation.  It also increases the probability that
   different routers compute their RIB based on a different LSDB.  Both
   increases the probability and/or duration of micro-loops.

   To allow for multi-vendors networks having all the routers delaying
   their SPF for the same duration, this document specifies a
   standardized algorithm.  Implementations may offer alternative
   optional algorithms.

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2.  High level goals

   The high level goals of this algorithm are the following:

   o  Very fast convergence for single simple events (link failure).

   o  Fast convergence in general while the IGP stability is considered
      under control.

   o  A long delay when the IGP stability is considered out of control,
      in order to let all related process calm down.

   o  At any time, try to avoid using different SPF_TIMERS values for
      nodes in the area/level.  Even though not all nodes will receive
      IGP message at the same time (due to difference in distance from
      the source and due to different flooding implementations on the
      path from the source).

3.  Definitions and parameters

   IGP events: An LSDB change requiring a new RIB computation (topology
   change, prefix change, metric change).  No distinction is done
   between the type of computation performed (e.g. full SPF, incremental
   SPF, PRC).  The type of computation is a local consideration.

   The SPF_DELAY timer can take the following values:

    INITIAL_WAIT: a very small delay to quickly handle link failure.
    e.g. 0 millisecond.

    FAST_WAIT: a small delay to have a fast convergence. e.g. 50-100
    millisecond.  Note: we want to be fast, but as this failure requires
    multiple IGP events, being too fast increase the probability to
    receive additional IGP events just after the RIB computation.

    LONG_WAIT: a long delay as IGP is unstable. e.g. 2 seconds.  Note:
    let's bring calm in the IGP.

   The TIME_TO_CONVERGE timer is the time to learn all the IGP events
   related to a single failure (e.g. node failure, SRLG failure). e.g. 1
   second.  It's mostly dependent on variation of failure detection
   times between all nodes which are neighbour to the failure, and then
   may depend on different flooding algorithms of nodes in the network.

   The HOLD_DOWN timer is the time needed with no IGP events received,
   before considering that the IGP is quiet again and we can set the
   SPF_DELAY back to INITAL_WAIT. e.g. 5 seconds.

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4.  Principle of SPF delay algorithm

   The first IGP event is handled very quickly (INITIAL_WAIT) in order
   to be very reactive for the first event if it only needs one IGP
   event (e.g. link failure, prefix change).

   If more IGP events are received quickly after, we consider that they
   are related to the same single failure, and handle the IGP events
   relatively quickly (FAST_WAIT) during the time needed to receive all
   the IGP events related to the failure (TIME_TO_CONVERGE).

   If IGP events are still received after this time, then the network is
   presumably experiencing multiple independent failures and the while
   waiting for its stability, the computations are delayed for a longer
   time (LONG_WAIT).

   Note: previous SPF delay algorithms used to count the number of RIB
   computations.  However, as all nodes may receive the LSP events in a
   different way we cannot assume that all nodes will perform the same
   number of SPF computations or that they will schedule them at the
   same time.  For example, assuming that the SPF delay is 50 ms, node
   R1 may receive 3 IGP events (E1, E2, E3) in those 50 ms and hence
   will perform a single routing computation.  While another node R2 may
   only receive 2 events (E1, E2) in those 50ms and hence will schedule
   another routing computation when further receiving E3.  That's why
   this document prefers to define a time limit (TIME_TO_CONVERGE) since
   the first event, rather than a number of routing computations.

5.  Specification of SPF delay algorithm

   When the previous IGP events is more than HOLD_DOWN ago:

   o  The IGP is set to the QUIET state.

   When the IGP is in the QUIET state and an IGP event is received:

   o  The time of this first IGP event is stored in FIRST_EVENT_TIME.

   o  The next RIB computation time is set to LSP receive time +

   o  The IGP is set to the FAST_WAIT state.

   When the IGP is in the FAST_WAIT state and an IGP event is received:

   o  If more than TIME_TO_CONVERGE has passed since FIRST_EVENT_TIME,
      then the IGP is set to the HOLD_DOWN state.

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   o  If the next RIB_computation time is in the past, set the next RIB
      computation time to LSP receive time + FAST_WAIT.

   When the IGP is in the HOLD_DOWN state and an IGP event is received:

   o  If the next RIB_computation time is in the past, set the next RIB
      computation time to LSP receive time + LONG_WAIT.

6.  Impact on micro-loops

   Micro-loops during IGP convergence are due to a non synchronized or
   non ordered update of the forwarding information tables (FIB)
   [RFC5715] [RFC6976] [I-D.litkowski-rtgwg-spf-uloop-pb-statement].
   FIB are installed after multiple steps such as SPF wait time, SPF
   computation, FIB distribution and FIB update.  This document only
   address the first contribution.  This standardized procedure reduces
   the probability and/or duration of micro-loops when the IGP
   experience multiple consecutive events.  It does not remove all
   micro-loops.  However, it is beneficial and its cost seems limited
   compared to full solutions such as [RFC5715] or [RFC6976].

7.  IANA Considerations

   No IANA actions required.

8.  Security considerations

   This document has no impact on the security of the IGP.

9.  Acknowledgements

   We would like to acknowledge Hannes Gredler, Les Ginsberg and Pierre
   Francois for the discussions related to this document.

10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

10.2.  Informative References

              Litkowski, S., "Link State protocols SPF trigger and delay
              algorithm impact on IGP microloops", draft-litkowski-
              rtgwg-spf-uloop-pb-statement-02 (work in progress), March

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              International Organization for Standardization,
              "Intermediate system to Intermediate system intra-domain
              routeing information exchange protocol for use in
              conjunction with the protocol for providing the
              connectionless-mode Network Service (ISO 8473)", ISO/IEC
              10589:2002, Second Edition, Nov 2002.

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.

   [RFC5715]  Shand, M. and S. Bryant, "A Framework for Loop-Free
              Convergence", RFC 5715, January 2010.

   [RFC6976]  Shand, M., Bryant, S., Previdi, S., Filsfils, C.,
              Francois, P., and O. Bonaventure, "Framework for Loop-Free
              Convergence Using the Ordered Forwarding Information Base
              (oFIB) Approach", RFC 6976, July 2013.

Author's Address

   Bruno Decraene
   38 rue du General Leclerc
   Issy Moulineaux cedex 9  92794


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