Network Working Group                                        B. Decraene
Internet-Draft                                                    Orange
Intended status: Standards Track                            S. Litkowski
Expires: September 20, 2018                      Orange Business Service
                                                              H. Gredler
                                                             RtBrick Inc
                                                               A. Lindem
                                                           Cisco Systems
                                                             P. Francois

                                                               C. Bowers
                                                  Juniper Networks, Inc.
                                                          March 19, 2018

            SPF Back-off Delay algorithm for link state IGPs


   This document defines a standard algorithm to temporarily postpone or
   'back-off' link-state IGP Shortest Path First (SPF) computations.
   This reduces the computational load and churn on IGP nodes when
   multiple temporally close network events trigger multiple SPF

   Having one standard algorithm improves interoperability by reducing
   the probability and/or duration of transient forwarding loops during
   the IGP convergence when the IGP reacts to multiple temporally close
   IGP events.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in
   [BCP14] [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on September 20, 2018.

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   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  High level goals  . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Definitions and parameters  . . . . . . . . . . . . . . . . .   4
   4.  Principles of SPF delay algorithm . . . . . . . . . . . . . .   5
   5.  Specification of the SPF delay state machine  . . . . . . . .   6
     5.1.  State Machine . . . . . . . . . . . . . . . . . . . . . .   6
     5.2.  State . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     5.3.  Timers  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     5.4.  FSM Events  . . . . . . . . . . . . . . . . . . . . . . .   8
   6.  Parameters  . . . . . . . . . . . . . . . . . . . . . . . . .  10
   7.  Partial Deployment  . . . . . . . . . . . . . . . . . . . . .  11
   8.  Impact on micro-loops . . . . . . . . . . . . . . . . . . . .  11
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   10. Security considerations . . . . . . . . . . . . . . . . . . .  12
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  12
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  12
     12.2.  Informative References . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

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1.  Introduction

   Link state IGPs, such as IS-IS [ISO10589-Second-Edition], OSPF
   [RFC2328] and OSPFv3 [RFC5340], perform distributed route computation
   on all routers in the area/level.  In order to have consistent
   routing tables across the network, such distributed computation
   requires that all routers have the same version of the network
   topology (Link State DataBase (LSDB)) and perform their computation
   essentially at the same time.

   In general, when the network is stable, there is a desire to trigger
   a new Shortest Path First (SPF) computation as soon as a failure is
   detected in order to quickly route around the failure.  However, when
   the network is experiencing multiple failures over a short period of
   time, there is a conflicting desire to limit the frequency of SPF
   computations, which would allow a reduction in control plane
   resources used by IGPs and all protocols/subsystems reacting on the
   attendant route change, such as LDP [RFC5036], RSVP-TE [RFC3209], BGP
   [RFC4271], Fast ReRoute computations (e.g., Loop Free Alternates
   (LFA) [RFC5286]), FIB updates, etc.  This also reduces network churn
   and, in particular, reduces the side effects such as micro-loops
   [RFC5715] that ensue during IGP convergence.

   To allow for this, IGPs usually implement an SPF Back-off Delay
   algorithm that postpones or backs-off the SPF computation.  However,
   different implementations have chosen different algorithms.  Hence,
   in a multi-vendor network, it's not possible to ensure that all
   routers trigger their SPF computation after the same delay.  This
   situation increases the average and maximum differential delay
   between routers completing their SPF computation.  It also increases
   the probability that different routers compute their FIBs based on
   different LSDB versions.  Both factors increase the probability and/
   or duration of micro-loops as discussed in Section 8.

   To allow multi-vendor networks to have all routers delay their SPF
   computations for the same duration, this document specifies a
   standard algorithm.

2.  High level goals

   The high level goals of this algorithm are the following:

   o  Very fast convergence for a single event (e.g., link failure).

   o  Paced fast convergence for multiple temporally close IGP events
      while IGP stability is considered acceptable.

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   o  Delayed convergence when IGP stability is problematic.  This will
      allow the IGP and related processes to conserve resources during
      the period of instability.

   o  Always try to avoid different SPF_DELAY Section 3 timer values
      across different routers in the area/level.  This requires
      specific consideration as different routers may receive IGP
      messages at different interval or even order, due to differences
      both in the distance from the originator of the IGP event and in
      flooding implementations.

3.  Definitions and parameters

   IGP events: The reception or origination of an IGP LSDB change
   requiring a new routing table computation.  Examples are a topology
   change, a prefix change and a metric change on a link or prefix.
   Note that locally triggering a routing table computation is not
   considered as an IGP event since other IGP routers are unaware of
   this occurrence.

   Routing table computation, in this document, is scoped to the IGP.
   So this is the computation of the IGP RIB, performed by the IGP,
   using the IGP LSDB.  No distinction is made between the type of
   computation performed. e.g., full SPF, incremental SPF, Partial Route
   Computation (PRC): the type of computation is a local consideration.
   This document may interchangeably use the terms routing table
   computation and SPF computation.

   SPF_DELAY: The delay between the first IGP event triggering a new
   routing table computation and the start of that routing table
   computation.  It can take the following values:

    INITIAL_SPF_DELAY: A very small delay to quickly handle a single
    isolated link failure, e.g., 0 milliseconds.

    SHORT_SPF_DELAY: A small delay to provide fast convergence in the
    case of a single component failure (node, Shared Risk Link Group
    (SRLG)..) that leads to multiple IGP events, e.g., 50-100

    LONG_SPF_DELAY: A long delay when the IGP is unstable, e.g., 2
    seconds.  Note that this allows the IGP network to stabilize.

   TIME_TO_LEARN_INTERVAL: This is the maximum duration typically needed
   to learn all the IGP events related to a single component failure
   (e.g., router failure, SRLG failure), e.g., 1 second.  It's mostly
   dependent on failure detection time variation between all routers

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   that are adjacent to the failure.  Additionally, it may depend on the
   different IGP implementations/parameters across the network, related
   to origination and flooding of their link state advertisements.

   HOLDDOWN_INTERVAL: The time required with no received IGP events
   before considering the IGP to be stable again and allowing the
   SPF_DELAY to be restored to INITIAL_SPF_DELAY. e.g. a
   defaulted and configured to be longer than the

4.  Principles of SPF delay algorithm

   For this first IGP event, we assume that there has been a single
   simple change in the network which can be taken into account using a
   single routing computation (e.g., link failure, prefix (metric)
   change) and we optimize for very fast convergence, delaying the
   routing computation by INITIAL_SPF_DELAY.  Under this assumption,
   there is no benefit in delaying the routing computation.  In a
   typical network, this is the most common type of IGP event.  Hence,
   it makes sense to optimize this case.

   If subsequent IGP events are received in a short period of time
   (TIME_TO_LEARN_INTERVAL), we then assume that a single component
   failed, but that this failure requires the knowledge of multiple IGP
   events in order for IGP routing to converge.  Under this assumption,
   we want fast convergence since this is a normal network situation.
   However, there is a benefit in waiting for all IGP events related to
   this single component failure so that the IGP can compute the post-
   failure routing table in a single additional route computation.  In
   this situation, we delay the routing computation by SHORT_SPF_DELAY.

   If IGP events are still received after TIME_TO_LEARN_INTERVAL from
   the initial IGP event received in QUIET state Section 5.1, then the
   network is presumably experiencing multiple independent failures.  In
   this case, while waiting for network stability, the computations are
   delayed for a longer time represented by LONG_SPF_DELAY.  This SPF
   delay is kept until no IGP events are received for HOLDDOWN_INTERVAL.

   Note that in order to increase the consistency network wide, the
   algorithm uses a delay (TIME_TO_LEARN_INTERVAL) from the initial IGP
   event, rather than the number of SPF computation performed.  Indeed,
   as all routers may receive the IGP events at different times, we
   cannot assume that all routers will perform the same number of SPF
   computations.  For example, assuming that the SPF delay is 50 ms,
   router R1 may receive 3 IGP events (E1, E2, E3) in those 50 ms and
   hence will perform a single routing computation.  While another

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   router R2 may only receive 2 events (E1, E2) in those 50 ms and hence
   will schedule another routing computation when receiving E3.

5.  Specification of the SPF delay state machine

   This section specifies the finite state machine (FSM) intended to
   control the timing of the execution of SPF calculations in response
   to IGP events.

5.1.  State Machine

   The FSM is initialized to the QUIET state with all three timers
   timers (SPF_TIMER, HOLDDOWN_TIMER, LEARN_TIMER) deactivated.

   The events which may change the FSM states are an IGP event or the
   expiration of one timer (SPF_TIMER, HOLDDOWN_TIMER, LEARN_TIMER).

   The following diagram briefly describes the state transitions.

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          +---->|                   |<-------------------+
          |     |      QUIET        |                    |
          +-----|                   |<---------+         |
      7:        +-------------------+          |         |
      SPF_TIMER           |                    |         |
      expiration          |                    |         |
                          | 1: IGP event       |         |
                          |                    |         |
                          v                    |         |
                +-------------------+          |         |
          +---->|                   |          |         |
          |     |    SHORT_WAIT     |----->----+         |
          +-----|                   |                    |
      2:        +-------------------+  6: HOLDDOWN_TIMER |
      IGP event           |               expiration     |
      8: SPF_TIMER        |                              |
         expiration       |                              |
                          | 3: LEARN_TIMER               |
                          |    expiration                |
                          |                              |
                          v                              |
                +-------------------+                    |
          +---->|                   |                    |
          |     |     LONG_WAIT     |------------>-------+
          +-----|                   |
       4:       +-------------------+  5: HOLDDOWN_TIMER
       IGP event                          expiration
       9: SPF_TIMER expiration

                          Figure 1: State Machine

5.2.  State

   The naming and semantics of each state corresponds directly to the
   SPF delay used for IGP events received in that state.  Three states
   are defined:

   QUIET: This is the initial state, when no IGP events have occurred
   for at least HOLDDOWN_INTERVAL since the previous routing table
   computation.  The state is meant to handle link failures very

   SHORT_WAIT: State entered when an IGP event has been received in
   QUIET state.  This state is meant to handle single component failure
   requiring multiple IGP events (e.g., node, SRLG).

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   LONG_WAIT: State reached after TIME_TO_LEARN_INTERVAL.  In other
   words, state reached after TIME_TO_LEARN_INTERVAL in state
   SHORT_WAIT.  This state is meant to handle multiple independent
   component failures during periods of IGP instability.

5.3.  Timers

   SPF_TIMER: The FSM timer that uses the computed SPF delay.  Upon
   expiration, the Route Table Computation (as defined in Section 3) is

   HOLDDOWN_TIMER: The FSM timer that is (re)started whan an IGP event
   is received and set to HOLDDOWN_INTERVAL.  Upon expiration, the FSM
   is moved to the QUIET state.

   LEARN_TIMER: The FSM timer that is started when an IGP event is
   recevied while the FSM is in the QUIET state.  Upon expiration, the
   FSM is moved to the LONG_WAIT state.

5.4.  FSM Events

   This section describes the events and the actions performed in

   Transition 1: IGP event, while in QUIET state.

   Actions on event 1:

   o  If SPF_TIMER is not already running, start it with value



   o  Transition to SHORT_WAIT state.

   Transition 2: IGP event, while in SHORT_WAIT.

   Actions on event 2:


   o  If SPF_TIMER is not already running, start it with value

   o  Remain in current state.

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   Transition 3: LEARN_TIMER expiration.

   Actions on event 3:

   o  Transition to LONG_WAIT state.

   Transition 4: IGP event, while in LONG_WAIT.

   Actions on event 4:


   o  If SPF_TIMER is not already running, start it with value

   o  Remain in current state.

   Transition 5: HOLDDOWN_TIMER expiration, while in LONG_WAIT.

   Actions on event 5:

   o  Transition to QUIET state.

   Transition 6: HOLDDOWN_TIMER expiration, while in SHORT_WAIT.

   Actions on event 6:

   o  Deactivate LEARN_TIMER.

   o  Transition to QUIET state.

   Transition 7: SPF_TIMER expiration, while in QUIET.

   Actions on event 7:

   o  Compute SPF.

   o  Remain in current state.

   Transition 8: SPF_TIMER expiration, while in SHORT_WAIT.

   Actions on event 8:

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   o  Compute SPF.

   o  Remain in current state.

   Transition 9: SPF_TIMER expiration, while in LONG_WAIT.

   Actions on event 9:

   o  Compute SPF.

   o  Remain in current state.

6.  Parameters

   All the parameters MUST be configurable at the protocol instance
   granularity.  They MAY be configurable at the area/level granularity.
   the millisecond granularity.  They MUST be configurable at least at
   the tenth of second granularity.  The configurable range for all the
   parameters SHOULD at least be from 0 milliseconds to 60 seconds.  The
   HOLDDOWN_INTERVAL MUST be defaulted or configured to be longer than

   If this SPF backoff algorithm is enabled by default, then in order to
   have consistent SPF delays between implementations with default
   configuration, the following default values SHOULD be implemented:

   In order to satisfy the goals stated in Section 2, operators are
   RECOMMENDED to configure delay intervals such that INITIAL_SPF_DELAY

   When setting (default) values, one should consider the customers and
   their application requirements, the computational power of the
   routers, the size of the network, and, in particular, the number of
   IP prefixes advertised in the IGP, the frequency and number of IGP
   events, the number of protocols reactions/computations triggered by
   IGP SPF computation (e.g., BGP, PCEP, Traffic Engineering CSPF, Fast
   ReRoute computations).  Note that some or all of these factors may
   change over the life of the network.  In case of doubt, it's
   RECOMMENDED that timer intervals should be chosen conservatively
   (i.e., longer timer values).

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   For the standard algorithm to be effective in mitigating micro-loops,
   it is RECOMMENDED that all routers in the IGP domain, or at least all
   the routers in the same area/level, have exactly the same configured

7.  Partial Deployment

   In general, the SPF Back-off Delay algorithm is only effective in
   mitigating micro-loops if it is deployed, with the same parameters,
   on all routers in the IGP domain or, at least, all routers in an IGP
   area/level.  The impact of partial deployment is dependent on the
   particular event, topology, and the algorithm(s) used on other
   routers in the IGP area/level.  In cases where the previous SPF Back-
   off Delay algorithm was implemented uniformly, partial deployment
   will increase the frequency and duration of micro-loops.  Hence, it
   is RECOMMENDED that all routers in the IGP domain or at least within
   the same area/level be migrated to the SPF algorithm described herein
   at roughly the same time.

   Note that this is not a new consideration as over times, network
   operators have changed SPF delay parameters in order to accommodate
   new customer requirements for fast convergence, as permitted by new
   software and hardware.  They may also have progressively replaced an
   implementation with a given SPF Back-off Delay algorithm by another
   implementation with a different one.

8.  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.ietf-rtgwg-spf-uloop-pb-statement].  FIBs
   are installed after multiple steps such as flooding of the IGP event
   across the network, SPF wait time, SPF computation, FIB distribution
   across line cards, and FIB update.  This document only addresses the
   contribution from the SPF wait time.  This standardized procedure
   reduces the probability and/or duration of micro-loops when IGPs
   experience multiple temporally close events.  It does not prevent all
   micro-loops.  However, it is beneficial and is less complex and
   costly to implement when compared to full solutions such as [RFC5715]
   or [RFC6976].

9.  IANA Considerations

   No IANA actions required.

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10.  Security considerations

   The algorithm presented in this document does not compromise IGP
   security.  An attacker having the ability to generate IGP events
   would be able to delay the IGP convergence time.  The LONG_SPF_DELAY
   state may help mitigate the effects of Denial-of-Service (DOS)
   attacks generating many IGP events.

11.  Acknowledgements

   We would like to acknowledge Les Ginsberg, Uma Chunduri, Mike Shand
   and Alexander Vainshtein for the discussions and comments related to
   this document.

12.  References

12.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

12.2.  Informative References

              Litkowski, S., Decraene, B., and M. Horneffer, "Link State
              protocols SPF trigger and delay algorithm impact on IGP
              micro-loops", draft-ietf-rtgwg-spf-uloop-pb-statement-06
              (work in progress), January 2018.

              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,
              DOI 10.17487/RFC2328, April 1998,

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   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
              and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
              Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,

   [RFC5036]  Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed.,
              "LDP Specification", RFC 5036, DOI 10.17487/RFC5036,
              October 2007, <>.

   [RFC5286]  Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for
              IP Fast Reroute: Loop-Free Alternates", RFC 5286,
              DOI 10.17487/RFC5286, September 2008,

   [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
              for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,

   [RFC5715]  Shand, M. and S. Bryant, "A Framework for Loop-Free
              Convergence", RFC 5715, DOI 10.17487/RFC5715, 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, DOI 10.17487/RFC6976, July
              2013, <>.

Authors' Addresses

   Bruno Decraene


   Stephane Litkowski
   Orange Business Service


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   Hannes Gredler
   RtBrick Inc


   Acee Lindem
   Cisco Systems
   301 Midenhall Way
   Cary, NC  27513


   Pierre Francois


   Chris Bowers
   Juniper Networks, Inc.
   1194 N. Mathilda Ave.
   Sunnyvale, CA  94089


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