Network Working Group Alex Zinin Internet Draft Alcatel Expiration Date: April 2005 October 2004 File name: draft-zinin-microloop-analysis-00.txt Analysis and Minimization of Microloops in Link-state Routing Protocols draft-zinin-microloop-analysis-00.txt Status of this Memo This document is an Internet-Draft and is subject to all provisions of section 3 of RFC 3667. By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she become aware will be disclosed, in accordance with RFC 3668. Internet Drafts are working documents of the Internet Engineering Task Force (IETF), its Areas, and its Working Groups. Note that other groups may also distribute working documents as Internet Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than a "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Abstract Link-state routing protocols (e.g. OSPF or IS-IS) are known to converge to a loop-free state within a finite period of time after a failure. It is normal, however, to observe short-term loops during the period of failure propagation, route recalculation, and forwarding table update, due to the asynchronous nature of link-state protocol operation. This document provides an analysis of formation of such microloops and suggests simple mechanisms to minimize them. Zinin [Page 1] INTERNET DRAFT IGP Microloop Analysis & Minimization October 2004 1 Introduction Link-state routing protocols, such as [OSPF] and [ISIS] converge to a loop-free state within a finite period of time after a failure. Additional failures postpone the convergence, but do not get in its way. During the period of convergence, however, link-state protocols exhibit short-term routing table inconsistencies caused by the protocol's asynchronous nature. These incornsistencies may cause short-term packet loops, also known as microloops. For example, see a sample network in Figure 1. +--+ 1 +--+ |A |---------|B | +--+ +--+ | \ 10 | 5| ------ |1 | \ | +--+ 10 \+--+ |E |---------|C | +--+ +--+ \_ / 5 \ /1 (failure) +--+ |D | +--+ Figure 1. Microloop example We are interested in routers A and B and their best paths towards D. Before failure, B's best path to D is B-C-D with cost 2, and A's best path is A-B-C-D with cost 3. When link C-D fails, both C and D announce their link state information with link C-D missing. Within finite period of time, both A and B shall receive the topology update and converge on it, installing new best paths: A-E-D (10) for A, and B-A-E-D (11) for B. However, if, due to timing differences, B calculates and installs its new best path through A before A has a chance to switch from B to E, a microloop will form between A and B for the duration of time required for A to complete its routing table update. Similar microloops may form when other topological changes happen in the network. For example, a new link or a node is added, a link cost is changed, etc. In summary, whenever a topological change in the network results in changes of the shortest path three (SPT) for more than one node, it is possible for the network to exhibit temporary loops. Zinin [Page 2] INTERNET DRAFT IGP Microloop Analysis & Minimization October 2004 This document provides an analysis of microloop formation. Specifically, we categorize different types of reconvergence scenarios, and explore their properties. We then show that in certain scenaiors microloops do not form, in others they can be eliminated using simple techniques described in this document, and define scenarios where more sophisticated loop avoidance mechanisms may be necessary. 2 Analysis To analyse the behavior of a network during reconvergence, we look at a given router and its neighbors before failure and during the transition to the new routes. More specifically, we analyse whether switching to the new routing information can result in loop formation or not. 2.1 Terminology The following terms are used in the draft. Downstream neighbor Neighbor N of router S is considered S's downstream neighbor for destination D, if Dopt(N, D) < Dopt(S, D) Primary neighbor Neighbor N of router S is considered S's primary neighbor for destination D, if N provides the shortest path to D according to the SPF calculation. Loop-free neighbor Neighbor N of router S is considered S's loop-free neighbor for destination D, if Dopt(N, D) < Dopt(N, S) + Dopt(S, D). Note that a loop-free neighbor may be, for example, router's primary before or after failure. 2.2 Next hop safety condition We start the analysis with the following observation: When router X learns about a topology change and starts using neighbor Y as its new primary neighbor for a given destination, a microloop between X and Y can only form if the topology before failure of topology after failure are such that Y uses X as its primary neighbor for the same destination. Indeed, if the topologies before and after failure are such that Y does not use X as it's next hop, then there is no moment in time before Y learned about the failure or after it learned about it Zinin [Page 3] INTERNET DRAFT IGP Microloop Analysis & Minimization October 2004 when it would forward traffic to X. Hence, at least one of the two topologies must be such that Y uses X as its next hop for a microloop between X and Y to form. Based on the above, we can define a safety condition for neighbor Y of router X that has just learned about a topology change. Note that the condition must satisfy the topological criteria above, and be non-recursive, i.e. not lead to loops if both X and Y follow it. Next-hop safety condition: After a topology change, it is safe for router X to switch to neigbor Y as its next-hop for a specific destination if the path through Y satisfies both of the following criteria: 1. X considered Y as its loop-free neighbor based on the topology before change AND 2: X considers Y as its downstream neighbor based on the topology after change. The first requirement ensures that Y has not been forwarding traffic to X before the change occured and both X and Y used old topology. The second requirement makes sure Y does not forward traffic to X when Y learns the new topology. The difference in the two characteristics is there to make sure that X and Y do not recursively consider each other as safe next-hops when they learn about the failure. 3.2 Transition types Here, we analyse different types of scenarios that a given router may find itself in after learning about a topology change. For each topological change, the network will have three major types of nodes categorized by the degree of safety of their old primary, new primary, and other neighbors. Type A Routers whose new primary next-hops after the topology change are safe and transition to them will not create a microloop. Two subtypes are recognized: A1: Routers whose SPT hasn't been affected by the topology change, and hence their primaries haven't changed. Zinin [Page 4] INTERNET DRAFT IGP Microloop Analysis & Minimization October 2004 A2: Routers whose new primary satisfies the safety condition Type B Routers whose new primary next-hops after the topology change do not satisfy the safety condition, but that have at least one other neighbor that does. Note that such a neighbor can be the router's old primary (type B1) or a neighbor that is nei- ther old nor new primary (type B2). Type C Routers that have no neighbor that satisfies the safety condi- tion. It is clear that type-A routers can immediately switch to their new primary next hops once they are calculated after the topology change. It can also be shown that if type-B routers do not immediately switch to their new primaries, but use their safe next-hops for some time, switching to the new primaries later will not create loops, provided that their downstream routers have also switched to the safe hops or have already switched to the new primaries. The following section defines this mechanism more formally. 3.3 Basic loop prevention mechanism On discovering the topology change and calculating the new SPT, each router checks the safety status of each neighbor using the condition in section 3.1, calculates the new routes, and applies the following logic for each route depending on the type of role it finds itself in: Type A: The new route can be installed immediately. Type B: The new route is installed with one or more temporary next- hops that satisfy the safety condition. The router uses these safe next-hops for time DELAY_TYPEB, and only then installs the new primary next-hops. Type C: Installation of the new route is delayed by time DELAY_TYPEC. Zinin [Page 5] INTERNET DRAFT IGP Microloop Analysis & Minimization October 2004 Until then, the router continues to use its old next-hops. DELAY_TYPEB and DELAY_TYPEC are two configurable constants. They should be configured with the same value on all routers in the net- work and such that following equation is true: DELAY_TYPEB > DELAY_TYPEC > fault-propagation-time where fault-propagation-time is the time it is expected to take routers in the network to propagate new link-state information, cal- culate the new SPT, check the safety condition of the neighbors, and install required FIB entries. Suggested default values for DELAY_TYPEB and DELAY_TYPEC are 4 and 2 seconds correspondingly. Note that if routers in the network already implement [IPFRR], this algorithm adds minimal complexity. Its other advantage is that it does not require explicit ordering or signaling between routers. Each router determines its type relative to the experienced failure and decides how to transition. The above algorithm minimizes the probability of loop formation. More specifically, loops will only be possible when two neighboring routers both experience the type C condition after the topology change. Appendix A shows that transitions between A-A, A-B, A-C, and B-C routers are loop-free. 2.6 Coverage analysis 3 Security Considerations Appendix A. Loop formation analysis S is the calculating router discovering the failure through a link- state update. P is the old primary, NP is the new primary. BF: <------ [P]----------------[S]----------------[NP] ...>? AF: Zinin [Page 6] INTERNET DRAFT IGP Microloop Analysis & Minimization October 2004 ------> [P]----------------[S]----------------[NP] ?<... To analyze possible loop formation, we need to check the following: 1) if it is possible for P to start forwarding packets to S before S switches to NP 2) if it is possible for NP to be forwarding packets back to S before or after S starts using it Assumptions are that type-As switch-over to NP immediately, and type- Bs and type-Cs wait certain amount of time so that: DELAY_TYPEB > DELAY_TYPEC > fault-propagation-time 1. S is type A: BF analysis: 1.1 If P is another type-A, then S cannot be its new primary, since S has not been P's LFA before (since it's been fwd'ing through P). Hence, P will not route through S AF, and the will be no loops between P and S. 1.2 If P is a type-B, then S hasn't been P's LF neighbor BF, and P will not forward through S at least for DELAY_TYPEB, which gives S enough time to switch to NP. After DELAY_TYPEB P may start using S as it's new primary. 1.3 If P is a type-C, then it hasn't been forwarding traffic to S BF, and will not use S as its new primary at least for DELAY_TYPEC, which should give S enough time to switch to NP. 1.4 Consequently, no loops will form between a type-A node and it's old primary before the type-A nodes switches to its new primary. AF analysis: 1.5 Regardless of its type, NP has not been forwarding packets to S BF and will not do so AF by definition of type-A. 1.6 Consequently, no loops will form between a type-A node and it's new primary before or after the type-A nodes switches to it. 2. S is type B: Zinin [Page 7] INTERNET DRAFT IGP Microloop Analysis & Minimization October 2004 BF analysis: 2.1 If P is a type-A, then similarly to 1.1 above, there will be no routes between P and S. 2.2 If P is another type-B, then similarly to 1.2, S will not be used by P for at least DELAY_TYPEB, and S will have enough time to switch to its safe hops or NP. 2.3 If P is a type-C, then similarly to 1.3, S hasn't been receiv- ing traffic from P BF, and will not AF for at least DELAY_TYPEC, which should give S enough time to switch to its safe hops or NP. 2.4 Consequently, no loops will form between a type-B node and it's old primary before the type-B nodes switches to its new primary. AF analysis: 2.5 If NP is a type-A, then because of the DELAY_TYPEB NP must have had enough time to switch to its new NP, which cannot be S by defi- nition of SPT considering that NP is S's new nexthop in the SPT AF. 2.6 If NP is another type-B, then because of DELAY_TYPEB, NP must have had enough time to switch from its old primary and can equally likely be routing through either its safe hops, or its new primary. Neither of the two can be S by definition of a downstream node (for safe hops) and SPT (for new primary). 2.7 If NP is a type-C, then because DELAY_TYPEB > DELAY_TYPEC, NP must have had enough time to switch to its new primary, which can't be S by definition of SPT and considering that NP is S's nexthop in the SPT AF. 2.8 Consequently, no loops will form between a type-B node and it's new primary before or after the type-A nodes switches to it. 3. S is type C: BF analysis: 3.1 If P is a type-A, then similarly to 1.1 before, S has not been P's LF neighbor before and hence won't be its new primary, so no loops will form between P and S. 3.2 If P is a type-B, then similarly to 1.2, S will not be used by P for at least DELAY_TYPEB, and because DELAY_TYPEB > DELAY_TYPEC, S will have enough time to switch to NP. Zinin [Page 8] INTERNET DRAFT IGP Microloop Analysis & Minimization October 2004 3.3 If P is another type-C, then it hasn't been using S as its pri- mary BF, but it is possible for P to consider S as its new primary AF and to install routes before S after their DELAY_TYPEC expires. Hence, a microloop is possible between P and S. 3.4 Consequently, a microloop between a type-C node and its old primary is possible only if the old primary is also a type-C node and it considers S as its new primary AF. Note that DELAY_TYPEC only delays probably loop formation, but does not increase its duration, as both neighboring routers are using the same delay. AF analysis: 3.5 If NP is a type-A, then because of the DELAY_TYPEC NP must have had enough time to switch to its new NP, which cannot be S by defi- nition of SPT considering that NP is S's new nexthop in the SPT AF. 3.6 If NP is a type-B, then because of DELAY_TYPEC, NP must have had enough time to switch to its safe hops, which can't be S by definition of a downstream node and considering that NP is S's new SPT next-hop. 3.7 If NP is another type-C, a loop is possible if S's DELAY_TYPEC expires before that on NP and NP has been using S as its primary BF. 3.8 Consequently, a microloop between a type-C node and its new primary is possible only if the new primary is also a type-C node and S was NP's primary BF. 4. Given the above analysis, it can be noted that, for a given fail- ure, presence of single type-C nodes in the network does not create microloops. It is the C-C combination that introduces this potential. Acknowledgements The author would like to thank Don Fedyk, Chris Martin, Mike Shand, and Alex Audu, for the discussion on this idea. Special thanks go to Alia Atlas who, among other things, was instrumental in fine-tuning the safety condition. References [Ref1] Ref1 full Author's Address Zinin [Page 9] INTERNET DRAFT IGP Microloop Analysis & Minimization October 2004 Alex Zinin Alcatel 701 E Middlefield Rd Mountain View, CA 94043 E-mail: zinin@psg.com IPR Disclaimer The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. 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