Microloop prevention by introducting a local convergence delay
draft-litkowski-rtgwg-uloop-delay-01

Abstract

This document describes a mechanism for link-state routing protocols to prevent a subset of transient loops during topology changes. It introduces a two-step convergence by introducing a delay between the network wide convergence and the node advertising the failure. As the network wide convergence is delayed in case of down events, this mechanism can be used for planned maintenance or for unplanned outage provided a fast reroute mechanism is used in conjunction to convert a failure into a less urgent topology change.

Simulation using real network topologies and costs, with pathological convergence behaviour, have been performed. While the proposed mechanism does not provide a complete solution, it is simple and it solves an interesting fraction of the transient loops in the analyzed SP topologies.

Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

Status of This Memo

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Copyright Notice

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Table of Contents

• 1. Introduction
• 2. Overview of the solution
• 3. Specification
• 3.1. Definitions
• 3.2. Current IGP reactions
• 3.3. Local delay
• 3.3.1. Link down event
• 3.3.2. Link up event
• 4. Use case
• 4.1. Applicable case
• 4.2. Non applicable case
• 5. Applicability
• 5.1. Topological applicability
• 6. Deployment considerations
• 7. Security Considerations
• 8. Acknowledgements
• 9. IANA Considerations
• 10. References
• 10.1. Normative References
• 10.2. Informative References
• Authors' Addresses

1. Introduction

     A ------ B
     |        |
     |        |
     D--------C        All the links have a metric of 1 except BC=5

     Figure 1	 
		

In figure 1, upon link AD down event, for the destination A, if D updates its forwarding entry before C, a transient forwarding loop occurs between C and D. We have a similar loop for link up event, if C updates its forwarding entry A before D.

2. Overview of the solution

This document defines a two-step convergence initiated by the router detecting the failure and advertising the topological changes in the IGP. This introduces a delay between the convergence of the local router compared to the network wide convergence. This delay is positive in case of "down" events and negative in case of "up" events.

This ordered convergence, is similar to the ordered FIB proposed defined in [I-D.ietf-rtgwg-ordered-fib], but limited to only one hop distance. The proposed mechanism reuses also some concept described in [I-D.ietf-rtgwg-microloop-analysis] with some limitation and improvements. As a consequence, it can only eliminate the loops between the node local to the event and its neighbors. In the SP topologies that were analyzed, this can avoid a high number of transient loops. On the other hand, as this mechanism is local to the router, it can be deployed incrementally with incremental benefit.

3. Specification

3.1. Definitions

This document will refer to the following existing IGP timers:

• LSP_GEN_TIMER: to batch multiple local events in one single local LSP update. It is often associated with damping mechanism to slowdown reactions by incrementing the timer when multiple consecutive events are detected.
• SPF_TIMER: to batch multiple events in one single computation. It is often associated with damping mechanism to slowdown reactions by incrementing the timer when the IGP is instable.
• IGP_LDP_SYNC_TIMER: defined in [RFC5443] to give LDP some time to establish the session and learn the MPLS labels before the link is used.

This document introduces the following two new timers :

• ULOOP_DELAY_DOWN_TIMER: slowdown the network wide IGP convergence in case of link down events.
• ULOOP_DELAY_UP_TIMER: slowdown the local node convergence in case of link up events.

3.2. Current IGP reactions

Upon a change of status on an adjacency/link, the existing behavior of the router advertising the event is the following:

1. UP/Down event is notified to IGP.
2. IGP processes the notification and postpones the reaction in LSP_GEN_TIMER msec.
3. Upon LSP_GEN_TIMER expiration, IGP updates its LSP/LSA and floods it.
4. SPF is scheduled in SPF_TIMER msec.
5. Upon SPF_TIMER expiration, SPF is computed and RIB/FIB are updated.

3.3. Local delay

3.3.1. Link down event

Upon an adjacency/link down event, this document introduces a change in step 5 in order to delay the local convergence compared to the network wide convergence: the node SHOULD delay the forwarding entry updates by ULOOP_DELAY_DOWN_TIMER. Such delay SHOULD only be introduced if all the LSDB modifications processed are only reporting down local events. Note that determining that all topological change are only local down events requires analyzing all modified LSP/LSA as a local link or node failure will typically be notified by multiple nodes. If a subsequent LSP/LSA is received/updated and a new SPF computation is triggered before the expiration of ULOOP_DELAY_DOWN_TIMER, then the same evaluation SHOULD be performed.

As as result of this addition, routers local to the failure will converge slower than remote routers.

3.3.2. Link up event

Upon an adjacency/link up event, this document introduces the following change in step 3 where the node SHOULD:

• Firstly build a LSP/LSA with the new adjacency but setting the metric to MAX_METRIC. It SHOULD flood it but not compute the SPF at this time.
• Then build the LSP/LSA with the target metric but SHOULD delay the flooding of this LSP/LSA by SPF_TIMER + ULOOP_DELAY_UP_TIMER. MAX_METRIC is equal to MaxLinkMetric (0xFFFF) for OSPF and 2^24-2 (0xFFFFFE) for IS-IS.
• Then continue with next steps (SPF computation) without waiting for the expiration of the above timer. In other word, only the flooding of the LSA/LSP is delayed, not the local SPF computation.

As as result of this addition, routers local to the failure will converge faster than remote routers.

If this mechanism is used in cooperation with "LDP IGP Synchronization" as defined in [RFC5443] then the mechanism defined in RFC 5443 is applied first, followed by the mechanism defined in this document. More precisely, the procedure defined in this document is applied once the LDP session is considered "fully operational" as per [RFC5443].

4. Use case

As previously stated, the mechanism only avoids the forwarding loops on the links between the node local to the failure and its neighbor. Forwarding loops may still occur on other links.

4.1. Applicable case

     A ------ B ----- E
     |              / |
     |             /  |
 G---D------------C   F        All the links have a metric of 1 

     Figure 2	 
		

Let us consider the traffic from G to F. The primary path is G->D->C->E->F. When link CE fails, if C updates its forwarding entry for F before D, a transient loop occures.

By implementing the mechanism defined in this document on C, when the CE link fails, C delays the update of his forwarding entry to F, in order to let some time for D to converge. When the timer expires on C, forwarding entry to F is updated. There is no transient forwarding loop on the link CD.

Note that C should implement a protection mechanism during the convergence delay in order to not increase the loss of traffic.

4.2. Non applicable case

     A ------ B ----- E --- H
     |                      |
     |                      |
 G---D--------C ------F --- J ---- K    
 
 All the links have a metric of 1 except BE=15    

     Figure 3	 
		

Let us consider the traffic from G to K. The primary path is G->D->C->F->J->K. When the CF link fails, if C updates its forwarding entry to K before D, a transient loop occures between C and D.

By implementing the mechanism defined in this document on C, when the link CF fails, C delays the update of his forwarding entry to K, letting time for D to converge. When the timer expires on C, forwarding entry to F is updated. There is no transient forwarding loop between C and D. However, a transient forwarding loop may still occur between D and A. In this scenario, this mechanism is not enough to address all the possible forwarding loops. However, it does not create additional traffic loss. Besides, in some cases -such as when the nodes update their FIB in the following order C, A, D, for example because the router A is quicker than D to converge- the mechanism may still avoid the forwarding loop that was occuring.

5. Applicability

Simulations have been run on multiple service provider topologies. So far, only link down event have been tested.

5.1. Topological applicability