Network Working Group                                        B. Decraene
Internet-Draft                                                    Orange
Intended status: Standards Track                            July 3, 2014
Expires: January 4, 2015

               Back-off SPF algorithm for link state IGP


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

   This 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

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [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 January 4, 2015.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

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.  The algorithm is proposed based on its
   popularity on existing implementations and its large deployed base.
   It's not implied that this algorithm is the best.  Implementations
   may offer alternative optional algorithms.

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2.  Exponential back off algorithm

   This backoff algorithm introduces a delay between the event
   triggering a new RIB computation and the start of the computation.

   The initial wait time is set to INITIAL_WAIT.

   Subsequent wait times are exponentially delayed by INCREMENTAL_WAIT,
   reaching the maximum value MAX_WAIT.

   If no new trigger is received for two times MAX_WAIT_TIME, the delay
   is set back to INITIAL_WAIT.

   The back off algorithm makes no difference regarding the type of
   computation performed to compute the updated RIB.  For example no
   distinction is made between a full SPF, an incremental SPF or a PRC

3.  Parameters

   INITIAL_WAIT SHOULD be configurable from 0 ms to at least 5 s.

   INCREMENTAL_WAIT SHOULD be configurable from 0 ms to at least 5 s.

   MAX_WAIT SHOULD be configurable from 0 ms to at least 10 s.

   In this version of the draft, it's proposed to not define default
   values because such values are subject to change over time as
   hardware and software improve and as customers requirements increase.
   In addition, such timers may be very network dependant.

4.  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) RFC
   5715 [RFC5715] RFC 6976 [RFC6976] draft.litkowski-rtgwg-spf-uloop-pb-
   statement [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 RFC 5715 [RFC5715] or RFC 6976

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5.  IANA Considerations

   No IANA actions required.

6.  Security considerations

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

7.  Acknowledgements

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

8.  References

8.1.  Normative References

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

8.2.  Informative References

              Litkowski, S., "Link State protocols SPF trigger and delay
              algorithm impact on IGP microloops", draft-litkowski-
              rtgwg-spf-uloop-pb-statement-00 (work in progress), June

              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.

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Author's Address

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


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