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.

このドキュメントはインターネットコミュニティのためのインターネットBest Current Practicesを指定し、改善のための議論と提案を要求します。このメモの配布は無制限です。

Copyright Notice


Copyright (C) The Internet Society (2005).




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
1. Introduction
1. はじめに

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.

OSPFプロトコルを実行している大規模なネットワークは、時々、リンク状態アドバタイズメント、またはLSAの多数の同時またはほぼ同時更新を経験し得ます。 OSPFトラフィックエンジニアリングの拡張[REF3]が有意にネットワーク内のLSAの数を増加させることができるが使用される場合、これは特に当てはまります。私たちは、LSAの嵐このイベントを呼び出すと、それは予定外の障害や定期保守イベントによって開始することができます。障害は、ハードウェア、ソフトウェア、または自然の中で手続きすることがあります。

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.

LSAストームは、着信パケットが遅延またはドロップさせるルータに高いCPUとメモリの使用率を引き起こします。 (再送タイマ値を超えた)遅延確認応答は、再送信につながる、とネイバールータとの隣接関係の結果はダウンと宣言されている(ルータ・デッドインターバルを超えた)Helloパケットを遅らせます。再送信および追加LSAのオリジネーションは、本質的に、極端な場合には、不安定な状態にネットワークを駆動することができる、正のフィードバックループを引き起こし、さらにCPUとメモリの使用状況をもたらします。

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.

再送タイマーのデフォルト値は5秒で、ルータ-deadインターバルのそれは40秒です。しかし、最近になって大幅にOSPF収束時間を短縮することに多くの関心がありました。こんにちは(サブ秒)その計画の一環として、はるかに短いとルータデッド間隔は、[REF4]を提案されています。 Helloパケットは、LSAの嵐によって引き起こされるネットワークの輻輳時のルータ、デッドインターバルを超えて遅延させるためにこのようなシナリオでは、それは可能性が高くなります。

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.

ネットワークの拡張性と安定性を向上させるために、我々は重要なOSPFパケットを優先し、輻輳を回避するための手順をお勧めします。お薦めの詳細は、シミュレーション研究は、[Ref13]渋滞現象とその影響を定量化することに報告されているセクション2に記載されています。また、提言のいくつかの研究と、彼らは実際にOSPFプロトコルを使用して、ネットワークの拡張性と安定性を改善することを示しています。 [Ref13]エディタや著者の一人を接触させることによって、リクエストに応じて利用可能です。

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 beyond the RouterDeadInterval, and take other congestion avoidance steps. The recommendations are unordered except that Recommendation 2 is to be implemented only if Recommendation 1 is not implemented.


(1) Classify all OSPF packets in two classes: a "high priority" class comprising OSPF Hello packets and Link State Acknowledgement packets, and a "low priority" class comprising all other packets. The classification is accomplished by examining the OSPF packet header. While receiving a packet from a neighbor and while transmitting a packet to a neighbor, try to process a "high priority" packet ahead of a "low priority" packet.

OSPF Helloパケットおよびリンクステート確認応答パケットを含む「優先度の高い」クラス、および他のすべてのパケットを含む「低優先度」クラス:(1)二つのクラス内のすべてのOSPFパケットを分類します。分類は、OSPFパケットのヘッダを調べることによって達成されます。ネイバーからのパケットを受信して​​いる間や隣人にパケットを送信しながら、先に「低優先度」のパケットの「優先度の高い」パケットを処理しようとします。

       The prioritized processing while transmitting may cause OSPF
       packets from a neighbor to be received out of sequence.  If
       Cryptographic Authentication (AuType = 2) is used (as specified
       in [Ref1]), then successive received valid OSPF packets from a
       neighbor need to have a non-decreasing "Cryptographic sequence
       number".  To comply with this requirement, we recommend that in
       case Cryptographic Authentication (AuType = 2) is used [Ref1],
       prioritized processing not be done at the transmitter.  This will
       avoid packets arriving at the receiver out of sequence.  However,
       after security processing at the receiver (including sequence
       number checking) is complete, the OSPF packets may be kept in a
       "high priority" queue or a "low priority" queue based on their
       class and processed accordingly.  The benefit of prioritized
       processing is clearly higher in the absence of Cryptographic
       Authentication since in that case prioritization can be
       implemented both at the transmitter and at the receiver.
       However, even with Cryptographic Authentication it will be
       beneficial to have prioritization only at the receiver (following
       security processing).

(2) If Recommendation 1 cannot be implemented, then reset the inactivity timer for an adjacency whenever any OSPF unicast packet or any OSPF packet sent to AllSPFRouters over a point-to-point link is received over that adjacency instead of resetting the inactivity timer only on receipt of the Hello packet. So OSPF would declare the adjacency to be down only if no OSPF unicast packets or no OSPF packets sent to AllSPFRouters over a point-to-point link are received over that adjacency for a period equaling or exceeding the RouterDeadInterval. The reason for not recommending this proposal in conjunction with Recommendation 1 is to avoid potential undesirable side effects. One such effect is the delay in discovering the down status of an adjacency in a case where no high priority Hello packets are being received but the inactivity timer is being reset by other stale packets in the low priority queue.


(3) Use an exponential backoff algorithm for determining the value of the LSA retransmission interval (RxmtInterval). Let R(i) represent the RxmtInterval value used during the i-th retransmission of an LSA. Use the following algorithm to compute R(i).

(3)LSA再送間隔(RxmtInterval)の値を決定するための指数バックオフアルゴリズムを使用します。 R(i)はLSAのi番目の再送信時に使用されるRxmtInterval値を表すものとします。 R(i)を計算するために、次のアルゴリズムを使用します。

                    R(1) = Rmin
                    R(i+1) = Min(KR(i),Rmax)  for i>=1

where K, Rmin, and Rmax are constants and the function Min(.,.) represents the minimum value of its two arguments. Example values for K, Rmin, and Rmax may be 2, 5, and 40 seconds, respectively. Note that the example value for Rmin, the initial retransmission interval, is the same as the sample value of RxmtInterval in [Ref1].

K、Rminの、及びRmaxが定数と機能最小である場合(。、。)その2つの引数の最小値を表します。 K、Rminの、およびRmaxのための例示的な値は、それぞれ、2,5、及び40秒であってもよいです。 Rminの、初期の再送間隔の値の例は、[Ref1の]でRxmtIntervalのサンプル値と同じであることに留意されたいです。

This recommendation is motivated by the observation that during a network congestion event caused by control messages, a major source for sustaining the congestion is the repeated retransmission of LSAs. The use of an exponential backoff algorithm for the LSA retransmission interval reduces the rate of LSA retransmissions while the network experiences congestion (during which it is more likely that multiple retransmissions of the same LSA would happen). This in turn helps the network get out of the congested state.


(4) Implicit Congestion Detection and Action Based on That: If there is control message congestion at a router, its neighbors do not know about that explicitly. However, they can implicitly detect it based on the number of unacknowledged LSAs to this router. If this number exceeds a certain "high-water mark", then the rate at which LSAs are sent to this router should be reduced progressively using an exponential backoff mechanism but not below a certain minimum rate. At a future time, if the number of unacknowledged LSAs to this router falls below a certain "low-water mark", then the rate of sending LSAs to this router should be increased progressively, again using an exponential backoff mechanism but not above a certain maximum rate. The whole algorithm is given below. Note that this algorithm is to be applied independently to each neighbor and only for unicast LSAs sent to a neighbor or LSAs sent to AllSPFRouters over a point-to-point link.


       U(t) = Number of unacknowledged LSAs to neighbor at time t.
       H = A high-water mark (in units of number of unacknowledged
       L = A low-water mark (in units of number of unacknowledged LSAs).
       G(t) = Gap between sending successive LSAs to neighbor at time t.
       F = The factor by which the above gap is to be increased during
           congestion and decreased after coming out of congestion.
       T = Minimum time that has to elapse before the existing gap
           is considered for change.
       Gmin = Minimum allowed value of gap.
       Gmax = Maximum allowed value of gap.

The equation below shows how the gap is to be changed after a time T has elapsed since the last change: _ | | Min(FG(t),Gmax) if U(t+T) > H G(t+T) = | G(t) if H >= U(t+T) >= L | Max(G(t)/F,Gmin) if U(t+T) < L |_

以下の式は、ギャップはTは最後の変更から経過した時間後に変更する方法を示しています_ | |分(FG(T)、Gmaxの)もしU(T + T)> H G(T + T)= | G(t)はならH> = U(T + T)> = L | MAX(G(T)/ F、Gminの)U(T + T)<Lであれば| _

Min(.,.) and Max(.,.) represent the minimum and maximum values of the two arguments, respectively.


Example values for the various parameters of the algorithm are as follows: H = 20, L = 10, F = 2, T = 1 second, Gmin = 20 ms, Gmax = 1 second.

H = 20、L = 10、F = 2、T = 1秒、Gminの= 20ミリ秒、1秒= Gmaxの次のようなアルゴリズムの様々なパラメータの値の例です。

Recommendations 3 and 4 both slow down LSAs to congested neighbors based on implicitly detecting the congestion, but they have important differences. Recommendation 3 progressively slows down successive retransmissions of the same LSA, whereas Recommendation 4 progressively slows down all LSAs (new or retransmission) to a congested neighbor.


(5) Throttling Adjacencies to Be Brought Up Simultaneously: If a router tries to bring up a large number of adjacencies to its neighbors simultaneously, then that may cause severe congestion due to database synchronization and LSA flooding activities. It is recommended that during such a situation no more than "n" adjacencies should be brought up simultaneously. Once a subset of adjacencies has been brought up successfully, newer adjacencies may be brought up as long as the number of simultaneous adjacencies being brought up does not exceed "n". The appropriate value of "n" would depend on the router processing power, total bandwidth available for control plane traffic, and propagation delay. The value of "n" should be configurable.

(5)スロットル隣接関係を同時に起動することにする:ルータが同時にその隣に隣接の多数を起動しようとした場合、その原因は、データベースの同期とLSAフラッディング活動に深刻な渋滞を引き起こす可能性があります。超えないような状況の際に「n」は隣接関係を同時に育てすることをお勧めします。隣接のサブセットが正常に育てられた後は、新しい隣接関係は限り同時隣接の数が「n」を超えていない育っされているものとして育てていてもよいです。 「N」の適切な値は、ルータの処理能力、制御プレーントラフィックのために利用可能な総帯域幅、及び伝搬遅延に依存します。 「N」の値が設定可能でなければなりません。

       In the presence of throttling, an important issue is the order in
       which adjacencies are to be formed.  We recommend a First Come
       First Served (FCFS) policy based on the order in which the
       request for adjacency formation arrives.  Requests may either be
       from neighbors or self-generated.  Among the self-generated
       requests, a priority list may be used to decide the order in
       which the requests are to be made.  However, once an adjacency
       formation process starts it is not to be preempted except for
       unusual circumstances such as errors or time-outs.

In some of the recommendations above, we refer to point-to-point links. Those references should also include cases where a broadcast network is to be treated as a point-to-point connection from the standpoint of IP routing [Ref5]


3. Security Considerations

This memo does not create any new security issues for the OSPF protocol.


4. Acknowledgments

We would like to acknowledge the support and helpful comments from OSPF WG chairs Rohit Dube, Acee Lindem, and John Moy; Routing Area directors Alex Zinin and Bill Fenner; and IESG reviewers. We acknowledge Vivek Dube, Mitchell Erblich, Mike Fox, Tony Przygienda, and Krishna Rao for comments on previous versions of the document. We also acknowledge Margaret Chiosi, Elie Francis, Jeff Han, Beth Munson, Roshan Rao, Moshe Segal, Mike Wardlow, and Pat Wirth for collaboration and encouragement in our scalability improvement efforts for Link State Protocol-based networks.

私たちは、OSPF WGからのサポートと有用なコメントがのRohitデュベ、ACEE Lindem、そしてジョン・モイの議長を務める承認したいと思います。ルーティングエリアディレクター、アレックスジニンとビルフェナー。そして、IESGの審査。私たちは、ドキュメントの以前のバージョンへのコメントのためのVivekデュベ、ミッチェルErblich、マイク・フォックス、トニーPrzygienda、とクリシュナ・ラオを認めます。また、リンク状態プロトコルベースのネットワークのための私たちのスケーラビリティの改善努力におけるコラボレーションと激励のためにマーガレットChiosi、エリー・フランシス、ジェフ・ハン、ベス・マンソン、ロシャンラオ、モシェ・シーガル、マイクWardlow、そしてパットワースを認めます。

5. Normative References

[Ref1] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.

[Ref1の】モイ、J.、 "OSPFバージョン2"、STD 54、RFC 2328、1998年4月。

[Ref2] Coltun, R., Ferguson, D., and J. Moy, "OSPF for IPv6", RFC 2740, December 1999.

[Ref2の】Coltun、R.、ファーガソン、D.、およびJ.モイ、 "IPv6のためのOSPF"、RFC 2740、1999年12月。

6. Informative References

[Ref3] Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering (TE) Extensions to OSPF Version 2", RFC 3630, September 2003.

、RFC 3630、2003年9月[REF3]カッツ、D.、Kompella、K.、およびD.ヨン、 "OSPFバージョン2へのトラフィックエンジニアリング(TE)の拡張"。

[Ref4] C. Alaettinoglu, V. Jacobson and H. Yu, "Towards Millisecond IGP Convergence", Work in Progress.

"ミリ秒IGPコンバージェンスに向けて" [REF4] C. Alaettinoglu、V. Jacobson氏およびH.ユー、進行中で働いて。

[Ref5] N. Shen, A. Lindem, J. Yuan, A. Zinin, R. White and S. Previdi, "Point-to-point operation over LAN in link-state routing protocols", Work in Progress.

【REF5】N.シェン、A. Lindem、J.元、A.ジニン、R.ホワイト及びS. Previdi、「リンクステートルーティングプロトコルにおけるLAN経由ポイントツーポイント操作」、ProgressのWork。

[Ref6] Pappalardo, D., "AT&T, customers grapple with ATM net outage", Network World, February 26, 2001.

[Ref6] Pappalardo、D.、ネットワークの世界、2001年2月26日には、 "AT&Tは、顧客がATMネット停電に取り組みます"。

[Ref7] "AT&T announces cause of frame-relay network outage," AT&T Press Release, April 22, 1998.

[Ref7]は、 "AT&Tは、フレームリレーネットワーク障害の原因を発表し、" AT&Tプレスリリース、1998年4月22日。

[Ref8] Cholewka, K., "MCI Outage Has Domino Effect", Inter@ctive Week, August 20, 1999.

[Ref8] Cholewka、K.は、インター@ ctiveウィーク、1999年8月20日 "MCIの停止は、ドミノ効果があります"。

[Ref9] Jander, M., "In Qwest Outage, ATM Takes Some Heat", Light Reading, April 6, 2001.

"クエストの停止で、ATMは、いくつかの熱を奪う" [Ref9] Jander、M.、光読書、2001年4月6日。

[Ref10] A. Zinin and M. Shand, "Flooding Optimizations in Link-State Routing Protocols", Work in Progress.

[Ref10] A.ジニンとM.シャンド、「リンクステートルーティングプロトコルにおける洪水の最適化」が進行中で働いています。

[Ref11] Pillay-Esnault, P., "OSPF Refresh and Flooding Reduction in Stable Topologies", RFC 4136, July 2005.

[Ref11] Pillay-Esnault、P.、 "OSPFリフレッシュと安定したトポロジでのフラッディング削減"、RFC 4136、2005年7月。

[Ref12] G. Ash, G. Choudhury, V. Sapozhnikova, M. Sherif, A. Maunder, V. Manral, "Congestion Avoidance & Control for OSPF Networks", Work in Progress.

[Ref12] G.アッシュ、G.チョードリー、V。Sapozhnikova、M.シェリフ、A. Maunder、V. Manral、 "OSPFネットワークの輻輳回避とコントロール"、ProgressのWork。

[Ref13] G. Choudhury, G. Ash, V. Manral, A. Maunder and V. Sapozhnikova, "Prioritized Treatment of Specific OSPF Packets and Congestion Avoidance: Algorithms and Simulations", AT&T Technical Report, August 2003.

[Ref13] G.チョードリー、G.アッシュ、V. Manral、A. MaunderとV. Sapozhnikova、 "特定のOSPFパケットの優先処理と輻輳回避:アルゴリズムとシミュレーション"、AT&Tテクニカルレポート、2003年8月。

[Ref14] Nichols, K., Blake, S., Baker, F., and D. Black, "Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers", RFC 2474, December 1998.

[Ref14]ニコルズ、K.、ブレイク、S.、ベイカー、F.、およびD.黒、 "IPv4とIPv6ヘッダーとの差別化されたサービス分野(DS分野)の定義"、RFC 2474、1998年12月。

Appendix A. LSA Storm: Causes and Impact

付録A. LSAの嵐:原因と影響

An LSA storm may be initiated due to many reasons. Here are some examples:


(a) one or more link failures due to fiber cuts,


(b) one or more router failures for some reason, e.g., software crash or some type of disaster (including power outage) in an office complex hosting many routers,


(c) link/router flapping,


(d) requirement of taking down and later bringing back many routers during a software/hardware upgrade,


(e) near synchronization of the periodic 1800-second LSA refreshes of a subset of LSAs,


(f) refresh of all LSAs in the system during a change in software version,


(g) injecting a large number of external routes to OSPF due to a procedural error,


(h) Router ID changes causing a large number of LSA re-originations (possibly LSA purges as well depending on the implementation).


In addition to the LSAs originated as a direct result of link/router failures, there may be other indirect LSAs as well. One example in MPLS networks is traffic engineering LSAs [Ref3] originated at other links as a result of significant changes in reserved bandwidth. These result from rerouting of Label Switched Paths (LSPs) that went down during the link/router failure. The LSA storm causes high CPU and memory utilization at the router processor causing incoming packets to be delayed or dropped. Delayed acknowledgments (beyond the retransmission timer value) results in retransmissions, and delayed Hello packets (beyond the Router-Dead interval) results in links being declared down. A trunk-down event causes router LSA origination by its end-point routers. If traffic engineering LSAs are used for each link, then that type of LSA would also be originated by the end-point routers and potentially elsewhere as well due to significant changes in reserved bandwidths at other links caused by the failure and reroute of LSPs originally using the failed trunk. Eventually, when the link recovers that would also trigger additional router LSAs and traffic engineering LSAs.

リンク/ルータの障害の直接の結果として生まれたLSAに加えて、同様に他の間接のLSAがあるかもしれません。予約された帯域幅の大きな変化の結果として、他のリンクで発生したMPLSネットワークの一例は、トラフィックエンジニアリングのLSA [REF3]です。ラベルの再ルーティングからのこれらの結果は、リンク/ルータの障害時にダウンしたスイッチパス(LSP)。 LSAストームは、着信パケットが遅延またはドロップさせるルータのプロセッサで高いCPUとメモリの使用率を引き起こします。遅延確認応答(再送タイマ値を超えた)の再送信をもたらし、(ルータ・デッドインターバルを超えた)Helloパケットダウンと宣言されているリンクで結果を遅らせます。トランクダウンイベントは、そのエンドポイントルータによって、ルータLSAの発信を引き起こします。トラフィックエンジニアリングのLSAは、各リンクのために使用されている場合は、LSAのタイプは、エンドポイント・ルータによって発信されるだろうし、潜在的に他の場所にも起因する故障による他のリンクで予約された帯域幅の大幅な変化にして使用して元々のLSPの再ルーティング失敗したトランク。最終的には、リンクが回復したときには、追加ルータLSAとトラフィックエンジニアリングLSAをトリガーします。

The retransmissions and additional LSA originations result in further CPU and memory usage, essentially causing a positive feedback loop. We define the LSA storm size as the number of LSAs in the original storm, not counting any additional LSAs resulting from the feedback loop described above. If the LSA storm is too large, then the positive feedback loop mentioned above may be large enough to indefinitely sustain a large CPU and memory utilization at many routers in the network, thereby driving the network to an unstable state. In the past, network outage events have been reported in IP and ATM networks using link-state protocols such as OSPF, Intermediate System to Intermediate System (IS-IS), Private Network-Network Interface (PNNI), or some proprietary variants. See for example [Ref6-Ref9]. In many of these examples, large-scale flooding of LSAs or other similar control messages (either naturally or triggered by some bug or inappropriate procedure) have been partly or fully responsible for network instability and outage.

再送信および追加LSAのオリジネーションは、本質的に正のフィードバックループを引き起こし、さらにCPUとメモリの使用状況をもたらします。我々は、元の嵐でのLSAの数としてLSAストームサイズを定義し、上記フィードバックループに起因する任意の追加のLSAをカウントしません。 LSAストームが大きすぎる場合には、上記正帰還ループは、それによって不安定な状態にネットワークを駆動する、無期限にネットワークに多くのルータに大きなCPUとメモリ使用率を維持するのに十分な大きさであってもよいです。過去には、ネットワーク障害のイベントは、このような中間システムへのOSPF、中間システム(IS-IS)、プライベートネットワーク - ネットワークインターフェイス(PNNI)、またはいくつかの独自の変種として、リンクステートプロトコルを使用してIPおよびATMネットワークで報告されています。たとえば[Ref6-Ref9]を参照してください。これらの例の多くでは、LSAのまたは他の同様の制御メッセージ(自然に、またはいくつかのバグや不適切な手順によってトリガ)の大規模な洪水は、ネットワークが不安定と停電のために、部分的または完全に担当しています。

In [Ref13], a simulation model is used to show that there is a certain LSA storm size threshold above which the network may show unstable behavior caused by a large number of retransmissions, link failures due to missed Hello packets, and subsequent link recoveries. It is also shown that the LSA storm size causing instability may be substantially increased by providing prioritized treatment to Hello and LSA Acknowledgment packets and by using an exponential backoff algorithm for determining the LSA retransmission interval. If it is not possible to prioritize Hello packets, then resetting the inactivity timer on receiving any valid OSPF packets can also provide the same benefit. Furthermore, if we prioritize Hello packets, then even when the network operates somewhat above the stability threshold, links are not declared down due to missed Hellos. This implies that even though there is control plane congestion due to many retransmissions, the data plane stays up and no new LSAs are originated (besides the ones in the original storm and the refreshes). These observations support the first three recommendations in Section 2. The authors of this document have also done simulations to verify that the other recommendations in Section 2 help avoid congestion and allow a graceful exit from a congested state.


One might argue that the scalability issue of large networks should be solved solely by dividing the network hierarchically into multiple areas so that flooding of LSAs remains localized within areas. However, this approach increases the network management and design complexity and may result in less optimal routing between areas. Also, Autonomous System External (ASE) LSAs are flooded throughout the AS, and it may be a problem if there are large numbers of them. Furthermore, a large number of summary LSAs may need to be flooded across areas, and their numbers would increase significantly if multiple Area Border Routers are employed for the purpose of reliability. Thus, it is important to allow the network to grow towards as large a size as possible under a single area.


The recommendations in the document are synergistic with a broader set of scalability and stability improvement proposals. [Ref10] proposes flooding overhead reduction in case more than one interface goes to the same neighbor. [Ref11] proposes a mechanism for greatly reducing LSA refreshes in stable topologies.

ドキュメントの推奨事項は、拡張性と安定性改善提案の広いセットと相乗的です。 [Ref10]複数のインターフェイスが同じ隣人になった場合に浸水のオーバーヘッドの削減を提案しています。 [Ref11]大幅に安定したトポロジーでLSAのリフレッシュを低減するための機構を提案しています。

[Ref12] proposes a wide range of congestion control and failure recovery mechanisms (some of those ideas are covered in this document, but [Ref12] has other ideas not covered here).


Appendix B. List of Variables and Values


F = The factor by which the gap between sending successive LSAs to a neighbor is to be increased during congestion and decreased after coming out of congestion (used in Recommendation 4). Example value is 2.

F =隣人に連続するLSAを送信との間のギャップは、輻輳時に増加し、輻輳を出た後に減少するれる因子(勧告4で使用されます)。値の例は、2です。

G(t) = Gap between sending successive LSAs to a neighbor at time t (used in Recommendation 4).


Gmax = Maximum allowed value of gap between sending successive LSAs to a neighbor (used in Recommendation 4). Example value is 1 second.

GMAX =最大は(勧告4で使用)ネイバーに連続するLSAを送信間のギャップの値を可能にしました。値の例は1秒です。

Gmin = Minimum allowed value of gap between sending successive LSAs to a neighbor (used in Recommendation 4). Example value is 20 ms.

GMIN =最小(勧告4で使用)ネイバーに連続するLSAを送信間のギャップの値を可能にしました。値の例は、20ミリ秒です。

H = A high-water mark (in units of number of unacknowledged LSAs). Exceeding this mark would trigger a potential increase in the gap between sending successive LSAs to a neighbor. (used in Recommendation 4). Example value is 20.

Hは、(未確認のLSAの数の単位で)高水位標を=。このマークを超えると隣人に連続したLSAを送信間のギャップの潜在的な増加を誘発するだろう。 (勧告4で使用されます)。値の例は20です。

K = A multiplicative constant used in increasing the RxmtInterval value used during successive retransmissions of the same LSA (used in Recommendation 3). Example value is 2.


L = A low-water mark (in units of number of unacknowledged LSAs) Dropping below this mark would trigger a potential decrease in the gap between sending successive LSAs to a neighbor. (used in Recommendation 4). Example value is 10.

Lは、隣接する連続したLSAを送信間のギャップの潜在的な減少をトリガするこのマークを下回る(未確認のLSAの数の単位で)低ウォーターマークを=。 (勧告4で使用されます)。値の例は10です。

n = Upper limit on the number of adjacencies to be brought up simultaneously (used in Recommendation 5).

(勧告5で使用される)を同時に起動されるべき隣接の数にN =上限。

R(i) = RxmtInterval value used during the i-th retransmission of an LSA (used in Recommendation 3).

R(勧告3で使用)LSAのi番目の再送信時に使用される式(I)= RxmtInterval値。

Rmax = The maximum allowed value of RxmtInterval (used in Recommendation 3). Example value is 40 seconds.


Rmin = The minimum allowed value of RxmtInterval (used in Recommendation 3). Example value is 5 seconds.


T = Minimum time that has to elapse before the existing gap between sending successive LSAs to a neighbor is considered for change (used in Recommendation 4). Example value is 1 second.

T =隣人に連続するLSAを送信間の既存のギャップが(勧告4で使用される)を変更するために考慮される前に経過しなければならない最小時間。値の例は1秒です。

U(t) = Number of unacknowledged LSAs to a neighbor at time t (used in Recommendation 4).


Appendix C. Other Recommendations and Suggestions


(1) Explicit Marking: In Section 2, we recommended that OSPF packets be classified to "high" and "low" priority classes based on examining the OSPF packet header. In some cases (particularly in the receiver), this examination may be computationally costly. An alternative would be the use of different TOS/Precedence field settings for the two priority classes. [Ref1] recommends setting the TOS field to 0 and the Precedence field to 6 for all OSPF packets. We recommend this same setting for the "low" priority OSPF packets and a different setting for the "high" priority OSPF packets in order to be able to classify them separately without having to examine the OSPF packet header. Two examples are given below:

(1)明示的なマーキング:第2節では、我々は、OSPFパケットがOSPFパケットヘッダを検査に基づいて「高」と「低」優先クラスに分類することを推奨。 (特に、受信機で)いくつかのケースでは、この検査は、計算コストがかかるかもしれません。代替は、二つの優先度クラスごとに異なるTOS / Precedenceフィールドの設定を使用することであろう。 [Ref1の全てのOSPFパケットのTOSフィールド0へと優先順位フィールドに6を設定することをお勧めします。私たちは、OSPFパケットのヘッダーを調べることなく、それらを別々に分類することができるようにするために「高」優先度のOSPFパケットのための「低」優先OSPFパケットに対して、この同じ設定と異なる設定をお勧めします。 2つの例を以下に示します。

       Example 1: For "low" priority packets, set TOS field to 0 and
                  Precedence field to 6, and for "high" priority packets
                  set TOS field to 4 and Precedence field to 6.

Example 2: For "low" priority packets, set TOS field to 0 and Precedence field to 6, and for "high" priority packets set TOS field to 0 and Precedence field to 7.


Note that the TOS/Precedence bits have been redefined by Diffserv (RFC 2474, [Ref14]). Also note that the different TOS/Precedence field settings suggested above only need to be agreed among the systems on the link. This recommendation is not needed to be followed if it is easy to examine the OSPF packet header and thereby separately classify "high" and "low" priority packets.

TOS /優先順位ビットはDiffservの(RFC 2474、[Ref14])によって再定義されていることに留意されたいです。また、上記の提案異なるTOS / Precedenceフィールドの設定は、リンクだけでシステム間で合意する必要があることに注意してください。 OSPFパケットヘッダを検査することが容易であり、それによって別々に「高」と「低」優先度のパケットを分類する場合は、この推奨に従うことがする必要はありません。

(2) Further Prioritization of OSPF Packets: Besides the packets designated as "high" priority in Recommendation 1 of Section 2, there may be a need for further priority separation among the "low" priority OSPF packets. We recommend the use of three priority classes: "high", "medium" and "low". While receiving a packet from a neighbor and while transmitting a packet to a neighbor, try to process a "high priority" packet ahead of "medium" and "low" priority packets and a "medium" priority packet ahead of "low priority" packets. The "high" priority packets are as designated in Recommendation 1 of Section 2. We provide below two candidate examples for "medium" priority packets. All OSPF packets not designated as "high" or "medium" priority are "low" priority. If Cryptographic Authentication (AuType = 2) is used (as specified in [Ref1]), then prioritized treatment is to be provided only at the receiver and after security processing, but not at the transmitter since that may cause packets to arrive out of sequence and violate the requirements of "Autype = 2".

(2)OSPFパケットの更なる優先順位付け:第2の勧告1の「高」優先順位として指定パケットに加えて、「低」優先OSPFパケットのうち、さらに優先分離する必要があるかもしれません。 「高」、「中」「低」:私たちは3つの優先クラスを使用することをお勧めします。ネイバーからのパケットを受信して​​いる間や隣人にパケットを送信しながら、先に「低優先度」のパケットの先に「中」「低」優先パケットと「中」優先パケットの「優先度の高い」パケットを処理しようとします。我々は、「中」優先パケットの2つの候補例を下に提供する第2の勧告1で指定される「高」優先度パケットです。 「高」または「中」の優先順位として指定されていないすべてのOSPFパケットは、「低」の優先順位です。 ([Ref1の]で指定されるように)暗号化認証(AuType = 2)が使用される場合、優先順位処理は、受信機でセキュリティ処理後に提供されるべきではなく、送信時とは、パケットが順序が狂って到着する可能性がありのでそして「Autype = 2」の要件に違反します。

       One example of "medium" priority packet is the Database
       Description (DBD) packet from a slave (during the database
       synchronization process) that is used as an acknowledgment.

A second example is an LSA carrying intra-area topology change information (this may trigger SPF calculation and rerouting of Label Switched Paths, so fast processing of this packet may improve OSPF/Label Distribution Protocol (LDP) convergence times). However, if the processing cost of identifying and separately queueing the LSA in this example is deemed to be high, then the implementer may decide not to do it.

第二の例は、エリア内のトポロジ変化情報を搬送LSAである(これは、ラベルのSPF計算および再ルーティングをトリガすることができるので、このパケットの高速処理がOSPF /ラベル配布プロトコル(LDP)の収束時間を改善することができる、パスのスイッチ)。この例のLSAを識別し、別々にキューイングの処理コストが高いとみなされる場合は、その後実装はそれをしないことを決定することができます。

(3) Processing a Large Number of LSA Purges: Occasionally, some events in the network, such as router ID changes, may result in a large number of LSA re-originations and LSA purges. In such a scenario, one may consider processing LSAs in different order, e.g., processing LSA purges ahead of LSA originations. We, however, do not recommend out-of-order LSA processing for several reasons. First, detecting the LSA type ahead of queueing may be computationally expensive. Out-of-order processing may also cause subtle bugs. We do not want to recommend a major change in the LSA processing paradigm for a relatively rare event such as router ID change. However, a router with a changing ID may flush the old LSAs gradually without causing a storm.


Contributing Authors and Their Addresses


In addition to the editor, several people contributed to this document. The names and contact information of all authors are given below.


Anurag S. Maunder Erlang Technology 2880 Scott Boulevard Santa Clara, CA 95052 USA

アヌラーグS. Maunderアーランテクノロジー2880スコット大通りサンタクララ、CA 95052 USA

Phone: (408) 420-7617 EMail:

電話:(408)420-7617 Eメール

Gerald R. Ash AT&T Room D5-2A01 200 Laurel Avenue Middletown, NJ, 07748 USA

ジェラルドR.アッシュAT&TルームD5-2A01 200ローレルアベニューミドルタウン、NJ、USA 07748

Phone: (732) 420-4578 EMail:

電話:(732)420-4578 Eメール

Vishwas Manral Sinett Corp, 2/1 Embassy Icon Annex, Infantry Road, Bangalore 560 001 India

Vishwas Manral Sinett社、2/1大使館アイコン別館、歩兵ロード、バンガロール560 001インド

Phone: +91-(805)-137-7023 EMail:

電話:+ 91-(805)-137-7023 Eメール

Vera D. Sapozhnikova AT&T Room C5-2C29 200 Laurel Avenue Middletown, NJ, 07748 USA

ヴェラ・D. Sapozhnikova AT&TルームC5-2C29 200ローレルアベニューミドルタウン、NJ、USA 07748

Phone: (732) 420-2653 EMail:

電話:(732)420-2653 Eメール

Editor's Address


Gagan L. Choudhury AT&T Room D5-3C21 200 Laurel Avenue Middletown, NJ, 07748 USA

GAGAN L.チョードリーAT&TルームD5-3C21 200ローレルアベニューミドルタウン、NJ、USA 07748

Phone: (732) 420-3721 EMail:

電話:(732)420-3721 Eメール

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