Impact of Shortest Path First (SPF) Trigger and Delay Strategies on IGP Micro-loops
RFC 8541

Document Type RFC - Informational (March 2019; No errata)
Last updated 2019-03-20
Replaces draft-litkowski-rtgwg-spf-uloop-pb-statement
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Internet Engineering Task Force (IETF)                      S. Litkowski
Request for Comments: 8541                       Orange Business Service
Category: Informational                                      B. Decraene
ISSN: 2070-1721                                                   Orange
                                                            M. Horneffer
                                                        Deutsche Telekom
                                                              March 2019

    Impact of Shortest Path First (SPF) Trigger and Delay Strategies
                           on IGP Micro-loops

Abstract

   A micro-loop is a packet-forwarding loop that may occur transiently
   among two or more routers in a hop-by-hop packet-forwarding paradigm.

   This document analyzes the impact of using different link state IGP
   implementations in a single network with respect to micro-loops.  The
   analysis is focused on the Shortest Path First (SPF) delay algorithm
   but also mentions the impact of SPF trigger strategies.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   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).  Not all documents
   approved by the IESG are candidates for any level of Internet
   Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8541.

Litkowski, et al.             Informational                     [Page 1]
RFC 8541              SPF Impact on IGP Micro-loops           March 2019

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 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 Contents

   1. Introduction ....................................................3
   2. Problem Statement ...............................................4
   3. SPF Trigger Strategies ..........................................6
   4. SPF Delay Strategies ............................................6
      4.1. Two-Step SPF Delay .........................................7
      4.2. Exponential Back-Off Delay .................................7
   5. Mixing Strategies ...............................................9
   6. Benefits of Standardized SPF Delay Behavior ....................13
   7. Security Considerations ........................................14
   8. IANA Considerations ............................................14
   9. References .....................................................14
      9.1. Normative References ......................................14
      9.2. Informative References ....................................15
   Acknowledgements ..................................................15
   Authors' Addresses ................................................15

Litkowski, et al.             Informational                     [Page 2]
RFC 8541              SPF Impact on IGP Micro-loops           March 2019

1.  Introduction

   Link state IGP protocols are based on a topology database on which
   the SPF algorithm is run to find a consistent set of non-looping
   routing paths.

   Specifications like IS-IS [RFC1195] propose some optimizations of the
   route computation (see Appendix C.1 of [RFC1195]), but not all
   implementations follow those non-mandatory optimizations.

   In this document, we refer to the events that lead to a new SPF
   computation based on the topology as "SPF triggers".

   Link state IGP protocols, like OSPF [RFC2328] and IS-IS [RFC1195],
   use multiple timers to control the router behavior in case of churn:
   SPF delay, Partial Route Computation (PRC) delay, Link State Packet
   (LSP) generation delay, LSP flooding delay, and LSP retransmission
   interval.

   Some of the values and behaviors of these timers are standardized in
   protocol specifications, and some are not.  The SPF computation-
   related timers have generally remained unspecified.

   Implementations are free to implement non-standardized timers in any
   way.  For some standardized timers, implementations may offer
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