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Internet Engineering Task Force (IETF) B. DecraeneRequest for Comments: 8405 OrangeCategory: Standards Track S. LitkowskiISSN: 2070-1721 Orange Business Service H. Gredler RtBrick Inc. A. Lindem Cisco Systems P. Francois C. Bowers Juniper Networks, Inc. June 2018Shortest Path First (SPF) Back-Off Delay Algorithm for Link-State IGPsAbstract 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 computations. 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.Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available inSection 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained athttps://www.rfc-editor.org/info/rfc8405.Decraene, et al. Standards Track [Page 1]

RFC 8405              SPF Back-Off Delay Algorithm             June 2018Copyright Notice   Copyright (c) 2018 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (https://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .21.1.  Requirements Language . . . . . . . . . . . . . . . . . .32.  High-Level Goals  . . . . . . . . . . . . . . . . . . . . . .33.  Definitions and Parameters  . . . . . . . . . . . . . . . . .44.  Principles of the SPF Delay Algorithm . . . . . . . . . . . .55.  Specification of the SPF Delay State Machine  . . . . . . . .65.1.  State Machine . . . . . . . . . . . . . . . . . . . . . .65.2.  States  . . . . . . . . . . . . . . . . . . . . . . . . .75.3.  Timers  . . . . . . . . . . . . . . . . . . . . . . . . .75.4.  FSM Events  . . . . . . . . . . . . . . . . . . . . . . .76.  Parameters  . . . . . . . . . . . . . . . . . . . . . . . . .97.  Partial Deployment  . . . . . . . . . . . . . . . . . . . . .108.  Impact on Micro-loops . . . . . . . . . . . . . . . . . . . .119.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .1110. Security Considerations . . . . . . . . . . . . . . . . . . .1111. References  . . . . . . . . . . . . . . . . . . . . . . . . .1111.1.  Normative References . . . . . . . . . . . . . . . . . .1111.2.  Informative References . . . . . . . . . . . . . . . . .11   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .13   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .13Decraene, et al.             Standards Track                    [Page 2]

RFC 8405 SPF Back-Off Delay Algorithm June 20181. Introduction Link-state IGPs, such as IS-IS [ISO10589], 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 to the attendant route change, such as LDP [RFC5036], RSVP-TE [RFC3209], BGP [RFC4271], Fast Reroute computations (e.g., Loop-Free Alternates (LFAs) [RFC5286]), FIB updates, etc. This also reduces network churn and, in particular, reduces 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 chose 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 inSection 8. This document specifies a standard algorithm to allow multi-vendor networks to have all routers delay their SPF computations for the same duration.1.1. Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described inBCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.Decraene, et al. Standards Track [Page 3]

RFC 8405 SPF Back-Off Delay Algorithm June 20182. 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, o delayed convergence when IGP stability is problematic (this will allow the IGP and related processes to conserve resources during the period of instability), and o avoidance of having different SPF_DELAY timer values (Section 3) across different routers in the area/level. This requires specific consideration as different routers may receive IGP messages at different intervals, or even in different orders, due to differences both in the distance from the originator of the IGP event and in flooding implementations.3. Definitions and Parameters IGP event: The reception or origination of an IGP LSDB change requiring a new routing table computation. Some 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 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, or 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 (such as a node failure or Shared Risk Link Group (SRLG) failure) that leads to multiple IGP events, e.g., 50-100 milliseconds.Decraene, et al. Standards Track [Page 4]

RFC 8405 SPF Back-Off Delay Algorithm June 2018 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 (such as router failure or SRLG failure), e.g., 1 second. It's mostly dependent on failure detection time variation between all routers that are adjacent to the failure. Additionally, it may depend on the different IGP implementations/parameters across the network and their relation to the origination and flooding of link state advertisements. HOLDDOWN_INTERVAL: The time required with no received IGP event before considering the IGP to be stable again and allowing the SPF_DELAY to be restored to INITIAL_SPF_DELAY, e.g., a HOLDDOWN_INTERVAL of 3 seconds. The HOLDDOWN_INTERVAL MUST be defaulted or configured to be longer than the TIME_TO_LEARN_INTERVAL.4. Principles of the SPF Delay Algorithm For the 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 by delaying the initial routing computation for a small interval, 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: the IGP can then 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 (seeSection 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, which is represented by LONG_SPF_DELAY. This SPF delay is used until no IGP events are received for HOLDDOWN_INTERVAL.Decraene, et al. Standards Track [Page 5]

RFC 8405              SPF Back-Off Delay Algorithm             June 2018   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 computations 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   milliseconds, router R1 may receive three IGP events (E1, E2, E3) in   those 50 milliseconds and hence will perform a single routing   computation, while another router R2 may only receive two events (E1,   E2) in those 50 milliseconds 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   (SPF_TIMER, HOLDDOWN_TIMER, and LEARN_TIMER) deactivated.   The events that may change the FSM states are an IGP event or the   expiration of one timer (SPF_TIMER, HOLDDOWN_TIMER, or LEARN_TIMER).   The following diagram briefly describes the state transitions.Decraene, et al.             Standards Track                    [Page 6]

RFC 8405              SPF Back-Off Delay Algorithm             June 2018                     +-------------------+               +---->|                   |<-------------------+               |     |      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 Machine5.2.  States   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 last routing table   computation.  The state is meant to handle link failures very   quickly.   SHORT_WAIT: This is the state entered when an IGP event has been   received in QUIET state.  This state is meant to handle a single   component failure requiring multiple IGP events (e.g., node, SRLG).Decraene, et al.             Standards Track                    [Page 7]

RFC 8405              SPF Back-Off Delay Algorithm             June 2018   LONG_WAIT: This is the 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: This is the FSM timer that uses the computed SPF delay.   Upon expiration, the routing table computation (as defined inSection 3) is performed.   HOLDDOWN_TIMER: This is the FSM timer that is (re)started when an IGP   event is received and set to HOLDDOWN_INTERVAL.  Upon expiration, the   FSM is moved to the QUIET state.   LEARN_TIMER: This is the FSM timer that is started when an IGP event   is received 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   response.   Transition 1: IGP event while in QUIET state   Actions on event 1:   o  If SPF_TIMER is not already running, start it with value      INITIAL_SPF_DELAY.   o  Start LEARN_TIMER with TIME_TO_LEARN_INTERVAL.   o  Start HOLDDOWN_TIMER with HOLDDOWN_INTERVAL.   o  Transition to SHORT_WAIT state.   Transition 2: IGP event while in SHORT_WAIT   Actions on event 2:   o  Reset HOLDDOWN_TIMER to HOLDDOWN_INTERVAL.   o  If SPF_TIMER is not already running, start it with value      SHORT_SPF_DELAY.   o  Remain in current state.Decraene, et al.             Standards Track                    [Page 8]

RFC 8405              SPF Back-Off Delay Algorithm             June 2018   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  Reset HOLDDOWN_TIMER to HOLDDOWN_INTERVAL.   o  If SPF_TIMER is not already running, start it with value      LONG_SPF_DELAY.   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.Decraene, et al.             Standards Track                    [Page 9]

RFC 8405 SPF Back-Off Delay Algorithm June 2018 Transition 8: SPF_TIMER expiration while in SHORT_WAIT Actions on event 8: 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 level. They MAY be configurable on a per IGP LSDB basis (e.g., IS-IS level, OSPF area, or IS-IS Level 1 area). All the delays (INITIAL_SPF_DELAY, SHORT_SPF_DELAY, LONG_SPF_DELAY, TIME_TO_LEARN_INTERVAL, and HOLDDOWN_INTERVAL) SHOULD be configurable with a granularity of a millisecond. They MUST be configurable with a granularity of at least a tenth of a second. The configurable range for all the parameters SHOULD be from 0 milliseconds to at least 6000 milliseconds. The HOLDDOWN_INTERVAL MUST be defaulted or configured to be longer than the TIME_TO_LEARN_INTERVAL. If this SPF Back-Off 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: INITIAL_SPF_DELAY 50 ms SHORT_SPF_DELAY 200 ms LONG_SPF_DELAY 5000 ms TIME_TO_LEARN_INTERVAL 500 ms HOLDDOWN_INTERVAL 10000 ms In order to satisfy the goals stated inSection 2, operators are RECOMMENDED to configure delay intervals such that INITIAL_SPF_DELAY <= SHORT_SPF_DELAY and SHORT_SPF_DELAY <= LONG_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 as determined primarily by the number of IP prefixes advertised in the IGP, the frequency and numberDecraene, et al. Standards Track [Page 10]

RFC 8405 SPF Back-Off Delay Algorithm June 2018 of IGP events, and the number of protocol reactions/computations triggered by IGP SPF computation (e.g., BGP, Path Computation Element Communication Protocol (PCEP), Traffic Engineering Constrained SPF (CSPF), and 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). 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 values.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, the 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; over time, 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 using a given SPF Back-Off Delay algorithm with another implementation using a different one.8. Impact on Micro-loops Micro-loops during IGP convergence are due to a non-synchronized or non-ordered update of FIBs [RFC5715] [RFC6976] [SPF-MICRO]. 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 Distributed Tunnels [RFC5715], Synchronized FIB Update [RFC5715], or the ordered FIB approach [RFC6976].Decraene, et al. Standards Track [Page 11]

RFC 8405 SPF Back-Off Delay Algorithm June 20189. IANA Considerations This document has no IANA actions.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. References11.1. 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, <https://www.rfc-editor.org/info/rfc2119>. [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase inRFC2119 Key Words",BCP 14,RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>.11.2. Informative References [ISO10589] International Organization for Standardization, "Information technology -- Telecommunications and information exchange between systems -- 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, November 2002. [RFC2328] Moy, J., "OSPF Version 2", STD 54,RFC 2328, DOI 10.17487/RFC2328, April 1998, <https://www.rfc-editor.org/info/rfc2328>. [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, <https://www.rfc-editor.org/info/rfc3209>.Decraene, et al. Standards Track [Page 12]

RFC 8405              SPF Back-Off Delay Algorithm             June 2018   [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,              <https://www.rfc-editor.org/info/rfc4271>.   [RFC5036]  Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed.,              "LDP Specification",RFC 5036, DOI 10.17487/RFC5036,              October 2007, <https://www.rfc-editor.org/info/rfc5036>.   [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,              <https://www.rfc-editor.org/info/rfc5286>.   [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF              for IPv6",RFC 5340, DOI 10.17487/RFC5340, July 2008,              <https://www.rfc-editor.org/info/rfc5340>.   [RFC5715]  Shand, M. and S. Bryant, "A Framework for Loop-Free              Convergence",RFC 5715, DOI 10.17487/RFC5715, January              2010, <https://www.rfc-editor.org/info/rfc5715>.   [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, <https://www.rfc-editor.org/info/rfc6976>.   [SPF-MICRO]              Litkowski, S., Decraene, B., and M. Horneffer, "Link State              protocols SPF trigger and delay algorithm impact on IGP              micro-loops", Work in Progress,draft-ietf-rtgwg-spf-uloop-pb-statement-07, May 2018.Decraene, et al.             Standards Track                   [Page 13]

RFC 8405              SPF Back-Off Delay Algorithm             June 2018Acknowledgements   We would like to acknowledge Les Ginsberg, Uma Chunduri, Mike Shand,   and Alexander Vainshtein for the discussions and comments related to   this document.Authors' Addresses   Bruno Decraene   Orange   Email: bruno.decraene@orange.com   Stephane Litkowski   Orange Business Service   Email: stephane.litkowski@orange.com   Hannes Gredler   RtBrick Inc.   Email: hannes@rtbrick.com   Acee Lindem   Cisco Systems   301 Midenhall Way   Cary, NC  27513   United States of America   Email: acee@cisco.com   Pierre Francois   Email: pfrpfr@gmail.com   Chris Bowers   Juniper Networks, Inc.   1194 N. Mathilda Ave.   Sunnyvale, CA  94089   United States of America   Email: cbowers@juniper.netDecraene, et al.             Standards Track                   [Page 14]

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