Prioritized Treatment of Specific OSPF Version 2 Packets and Congestion Avoidance
RFC 4222

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Network Working Group                                  G. Choudhury, Ed.
Request for Comments: 4222                                          AT&T
BCP: 112                                                    October 2005
Category: Best Current Practice

            Prioritized Treatment of Specific OSPF Version 2
                    Packets and Congestion Avoidance

Status of This Memo

   This document specifies an Internet Best Current Practices for the
   Internet Community, and requests discussion and suggestions for
   improvements.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This document recommends methods that are intended to improve the
   scalability and stability of large networks using Open Shortest Path
   First (OSPF) Version 2 protocol.  The methods include processing OSPF
   Hellos and Link State Advertisement (LSA) Acknowledgments at a higher
   priority compared to other OSPF packets, and other congestion
   avoidance procedures.

Table of Contents

   1. Introduction...................................................2
   2. Recommendations................................................3
   3. Security Considerations........................................6
   4. Acknowledgments................................................6
   5. Normative References...........................................6
   6. Informative References.........................................7
   Appendix A. LSA Storm: Causes and Impact..........................8
   Appendix B. List of Variables and Values.........................10
   Appendix C. Other Recommendations and Suggestions................11

Choudhury, Ed.           Best Current Practice                  [Page 1]
RFC 4222                 Prioritized Treatment              October 2005

1.  Introduction

   In this document, OSPF refers to OSPFv2 [Ref1].  The scalability and
   stability improvement techniques described here may also apply to
   OSPFv3 [Ref2], but that will require further study and operational
   experience.

   A large network running OSPF protocol may occasionally experience the
   simultaneous or near-simultaneous update of a large number of link
   state advertisements, or LSAs.  This is particularly true if OSPF
   traffic engineering extension [Ref3] is used that may significantly
   increase the number of LSAs in the network.  We call this event an
   LSA storm and it may be initiated by an unscheduled failure or a
   scheduled maintenance event.  The failure may be hardware, software,
   or procedural in nature.

   The LSA storm causes high CPU and memory utilization at the router
   causing incoming packets to be delayed or dropped.  Delayed
   acknowledgments (beyond the retransmission timer value) result in
   retransmissions, and delayed Hello packets (beyond the router-dead
   interval) result in neighbor adjacencies being declared down.  The
   retransmissions and additional LSA originations result in further CPU
   and memory usage, essentially causing a positive feedback loop,
   which, in the extreme case, may drive the network to an unstable
   state.

   The default value of the retransmission timer is 5 seconds and that
   of the router-dead interval is 40 seconds.  However, recently there
   has been a lot of interest in significantly reducing OSPF convergence
   time.  As part of that plan, much shorter (sub-second) Hello and
   router-dead intervals have been proposed [Ref4].  In such a scenario,
   it will be more likely for Hello packets to be delayed beyond the
   router-dead interval during network congestion caused by an LSA
   storm.

   In order to improve the scalability and stability of networks, we
   recommend steps for prioritizing critical OSPF packets and avoiding
   congestion.  The details of the recommendations are given in Section
   2.  A simulation study is reported in [Ref13] that quantifies the
   congestion phenomenon and its impact.  It also studies several of the
   recommendations and shows that they indeed improve the scalability
   and stability of networks using OSPF protocol.  [Ref13] is available
   on request by contacting the editor or one of the authors.

Choudhury, Ed.           Best Current Practice                  [Page 2]
RFC 4222                 Prioritized Treatment              October 2005

   Appendix A explains in more detail LSA storm scenarios, their impact,
   and points out a few real-life examples of control-message storms.
   Appendix B provides a list of variables used in the recommendations
   and their example values.  Appendix C provides some further
   recommendations and suggestions with similar goals.

2.  Recommendations

   The recommendations below are intended to improve the scalability and
   stability of large networks using OSPF protocol.  During periods of
   network congestion, they would reduce retransmissions, avoid an
   adjacency to be declared down due to Hello packets being delayed