Prioritized Treatment of Specific OSPF Version 2 Packets and Congestion Avoidance
RFC 4222
Document | Type |
RFC - Best Current Practice
(October 2005; No errata)
Also known as BCP 112
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Author | G Choudhury | ||
Last updated | 2015-10-14 | ||
Stream | Internet Engineering Task Force (IETF) | ||
Formats | plain text html pdf htmlized (tools) htmlized bibtex | ||
Stream | WG state | (None) | |
Document shepherd | No shepherd assigned | ||
IESG | IESG state | RFC 4222 (Best Current Practice) | |
Action Holders |
(None)
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Consensus Boilerplate | Unknown | ||
Telechat date | |||
Responsible AD | Bill Fenner | ||
Send notices to | <rohit@utstar.com> |
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 delayedShow full document text