Internet Engineering Task Force (IETF) M. Shand Request for Comments: 6976 Individual Contributor Category: Informational S. Bryant ISSN: 2070-1721 S. Previdi C. Filsfils Cisco Systems P. Francois Institute IMDEA Networks O. Bonaventure Universite catholique de Louvain July 2013
Framework for Loop-Free Convergence Using the Ordered Forwarding Information Base (oFIB) Approach
Abstract
This document describes an illustrative framework of a mechanism for use in conjunction with link-state routing protocols that prevents the transient loops that would otherwise occur during topology changes. It does this by correctly sequencing the forwarding information base (FIB) updates on the routers.
This mechanism can be used in the case of non-urgent (management action) link or node shutdowns and restarts or link metric changes. It can also be used in conjunction with a fast reroute mechanism that converts a sudden link or node failure into a non-urgent topology change. This is possible where a complete repair path is provided for all affected destinations.
After a non-urgent topology change, each router computes a rank that defines the time at which it can safely update its FIB. A method for accelerating this loop-free convergence process by the use of completion messages is also described.
The technology described in this document has been subject to extensive simulation using pathological convergence behavior and real network topologies and costs. However, the mechanisms described in this document are purely illustrative of the general approach and do not constitute a protocol specification. This document represents a snapshot of the work of the Routing Area Working Group at the time of publication and is published as a document of record. Further work is needed before implementation or deployment.
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Status of This Memo
This document is not an Internet Standards Track specification; it is published for informational purposes.
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are a candidate for any level of Internet Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6976.
Copyright Notice
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This document describes an illustrative framework of a mechanism for use in conjunction with link-state routing protocols that prevents the transient loops that would otherwise occur during topology changes. It does this by correctly sequencing the forwarding information base (FIB) updates on the routers.
At the time of publication there is no demand to deploy this technology; however, in view of the subtleties involved in the design of extensions for loop-free convergence routing protocols, the Routing Area Working Group considered it desirable to publish this document to place on record the design consideration of the ordered FIB (oFIB) approach.
The mechanisms presented in this document are purely illustrative of the general approach and do not constitute a protocol specification. This document represents a snapshot of the work of the working group at the time of publication and is published as a document of record. Additional work is needed to specify the necessary routing protocol extensions necessary to support this IP fast reroute (FRR) method before implementation or deployment.
With link-state protocols, such as IS-IS [ISO10589] and OSPF [RFC2328], each time the network topology changes, some routers need to modify their forwarding information bases (FIBs) to take into account the new topology. Each topology change causes a convergence phase. During this phase, routers may transiently have inconsistent FIBs, which may lead to packet loops and losses, even if the reachability of the destinations is not compromised after the topology change. Packet losses and transient loops can also occur in the case of a link down event implied by a maintenance operation, even if this operation is predictable and not urgent. When the link- state change is a metric update and when a new link is brought up in the network, there is no direct loss of connectivity, but transient packet loops and loss can still occur.
In this document, a distinction is made between urgent and non-urgent network events. Urgent events are those that arise from unpredictable network outages (such as node or link failures) that are traditionally resolved through the convergence of routing protocols or by protection mechanisms reliant on fault detection and reporting (such as through Operations, Administration, and Maintenance). Non-urgent events are those that arise from
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predictable events such as the controlled shutdown of network resources by a management system, or the modification of network parameters (such as routing metrics). Typically, non-urgent events can be planned around, while urgent events must be handled by dynamic systems. All network events, both urgent and non-urgent, may lead to transient packet loops and loss.
For example, in Figure 1, if the link between X and Y is shut down by an operator, packets destined to X can loop between R and Y when Y has updated its FIB while R has not yet updated its FIB, and packets destined to Y can loop between X and S if X updates its FIB before S. According to the current behavior of IS-IS and OSPF, this scenario will happen most of the time because X and Y are the first routers to be aware of the failure, so that they will update their FIBs first.
It should be noted that the loops can occur remotely from the failure, not just adjacent to it.
[RFC5715] provides an introduction to a number of loop-free convergence methods, and readers unfamiliar with this technology are recommended to read it before studying this document in detail. Note that in common with other loop-free convergence methods, oFIB is only capable of providing loop-free convergence in the presence of a single failure.
The goal of this document is to describe a mechanism that sequences the router FIB updates to maintain consistency throughout the network. By correctly setting the FIB change order, no looping or packet loss can occur. This mechanism may be applied to the case of managed link-state changes, i.e., link metric change, manual link down/up, manual router down/up, and managed state changes of a set of links attached to one router. It may also be applied to the case
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where one or more network elements are protected by a fast reroute mechanism (FRR) [RFC5714] [RFC4090]. The mechanisms that are used in the failure case are exactly the same as those used for managed changes. For simplicity, this document makes no further distinction between managed and unplanned changes.
It is assumed in the description that follows that all routers in the routing domain are oFIB capable. This can be verified in an operational network by having the routers report oFIB capability using the IGP. Where non-oFIB-capable routers exist in the network, normal convergence would be used by all routers. The operation of mixed-mode networks is for further study.
The technology described in this document has been subject to extensive simulation using pathological convergence behavior and real network topologies and costs. A variant of the technology described here has been experimentally deployed in a production network.
This section provides an overview of the required ordering of the FIB updates. A more detailed analysis of the rerouting dynamics and correctness proofs of the mechanism can be found in [refs.PFOB07].
For simplicity, the correct ordering for single link changes are described first. The document then builds on this to demonstrate that the same principles can be applied to more complex scenarios such as line-card or node changes.
First, consider the non-urgent failure of a link (i.e., where an operator or a network management system (NMS) shuts down a link, thereby removing it from the currently active topology) or the increase of a link metric by the operator or NMS. In this case, a router R must not update its FIB until all other routers that send traffic via R and the affected link have first updated their FIBs.
The following argument shows that this rule ensures the correct order of FIB changes when the link X->Y is shut down or its metric is increased.
An "outdated" FIB entry for a destination is defined as being a FIB entry that still reflects the shortest path(s) in use before the topology change. Once a packet reaches a router R that has an outdated FIB entry for the packet destination, then, provided the
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oFIB ordering is respected, the packet will continue to X only traversing routers that also have an outdated FIB entry for the destination. The packet thus reaches X without looping and will be forwarded to Y via X->Y (or in the case of FRR, the X->Y repair path) and will reach its destination.
Since it can be assumed that the original topology was loop-free, Y will never use the link Y->X to reach the destination, and hence the path(s) between Y and the destination are guaranteed to be unaffected by the topology change. It therefore follows that the packet arriving at Y will reach its destination without looping.
Since it can also be assumed that the new topology is loop-free, by definition a packet cannot loop while being forwarded exclusively by routers with an updated FIB entry.
In other words, when the oFIB ordering is respected, if a packet reaches an outdated router, it can never subsequently reach an updated router, and it cannot loop because from this point on it will only be forwarded on the consistent path that was used before the event. If it does not reach an outdated router, it will only be forwarded on the loop-free path that will be used after the convergence.
According to the proposed ordering, X will be the last router to update its FIB. Once it has updated its FIB, the link X->Y can actually be shut down (or the repair removed).
If the link X-Y is bidirectional, a similar process must be run to order the FIB update for destinations using the link in the direction Y->X. As has already been shown, no packet ever traverses the X-Y link in both directions, and hence the operation of the two ordering processes is orthogonal.
In the case of link up events or metric decreases, a router R must update its FIB before all other routers that will use R to reach the affected link.
The following argument shows that this rule ensures the correct order of FIB changes when the link X->Y is brought into service or its metric is decreased.
Firstly, when a packet reaches a router R that has already updated its FIB, all the routers on the path from R to X will also have updated their FIB, so that the packet will reach X and be forwarded along X->Y, ultimately reaching its destination.
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Secondly, a packet cannot loop between routers that have not yet updated their FIB. This proves that no packet can loop.
The following sections describe the required ordering for single events that may manifest as multiple link events. For example, the failure of a router may be notified to the rest of the network as the individual failure of all its attached links. The means of identifying the event type from the collection of received link events is described in Section 3.1.
In the case of the non-urgent shutdown of a router, a router R must not update its FIB until all other routers that send traffic via R and the affected router have first updated their FIBs.
Using a proof similar to that for link failure, it can be shown that no loops will occur if this ordering is respected [refs.PFOB07].
In the case of a router being brought into service, a router R must update its FIB BEFORE all other routers that WILL use R to reach the affected router.
A proof similar to that for link up shows that no loops will occur if this ordering is respected [refs.PFOB07].
The failure of a line card involves the failure of a set of links, all of which have a single node in common, i.e., the parent router. The ordering to be applied is the same as if it were the failure of the parent router.
In a similar way, the restoration of an entire line card to service as a single event can be treated as if the parent router were returning to service.
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As has been described, a single event such as the failure or restoration of a single link, single router, or line card may be notified to the rest of the network as a set of individual link change events. It is necessary to deduce from this collection of link-state notifications the type of event that has occurred in the network and hence the required ordering.
When a link change event is received that impacts the receiving router's FIB, the routers at the near and far end of the link are noted.
If all events received within some hold-down period (the time that a router waits to acquire a set of Link State Packets (LSPs) that should be processed together) have a single router in common, then it is assumed that the change reflects an event (line-card or router change) concerning that router.
In the case of a link change event, the router at the far end of the link is deemed to be the common router.
All ordering computations are based on treating the common router as the root for both link and node events.
There are some events (for example, a subsequent failure with conflicting repair requirements occurring before the ordered FIB process has completed) that cannot be correctly processed by this mechanism. In these cases, it is necessary to ensure that convergence falls back to the conventional mode of operation (see Section 6).
In all cases, it is necessary to wait some hold-down period after receiving the first notification to ensure that all routers have received the complete set of link-state notifications associated with the single event.
At any time, if a link change notification is received that would have no effect on the receiving router's FIB, then it may be ignored.
If no other event is received during the hold-down time, the event is treated as a link event. Note that the IGP reverse connectivity check means that only the first failure event or second up event has an effect on the FIB.
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If an event that is received within the hold-down period does NOT reference the common router (R), then, in this version of the specification, normal convergence is invoked immediately (see Section 6).
Network reconvergence using the ordered FIB approach takes longer than the normal reconvergence process. Where the failure is protected by an FRR mechanism, this additional delay in convergence causes no packet loss. When the sudden failure of a link or a set of links that are not protected using an FRR mechanism occurs, the failure must be processed using the conventional (faster) mode of operation to minimize packet loss during reconvergence.
In summary, an ordered FIB process is applicable if the set of link state notifications received between the first event and the hold- down period reference a common router R, and one of the following assertions is verified:
o The set of notifications refers to link down events concerning protected links and metric increase events.
o The set of notifications refers to link up events and metric decrease events.
This section describes how the required ordering is computed.
This computation required the introduction of the concept of a reverse Shortest Path Tree (rSPT). The rSPT uses the cost towards the root (rather than from it) and yields the best paths towards the root from other nodes in the network [IPFRR-TUNNELS].
To respect the proposed ordering, routers compute a rank that will be used to determine the time at which they are permitted to perform their FIB update. In the case of a failure event rooted at router Y or an increase of the metric of link X->Y, router R computes the rSPT in the topology before the failure (rSPT_old) rooted at Y. This rSPT gives the shortest paths to reach Y before the failure. The branch of the rSPT that is below R corresponds to the set of shortest paths to R that are used by the routers that reach Y via R.
The rank of router R is defined as the depth (in number of hops) of this branch. In the case of Equal Cost Multipath (ECMP), the maximum depth of the ECMP path set is used.
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Router R is required to update its FIB at time
T0 + H + (rank * MAX_FIB)
where T0 is the arrival time of the Link State Packet containing the topology change, H is the hold-down time, and MAX_FIB is a network- wide constant that reflects the maximum time required to update a FIB irrespective of the change required. The value of MAX_FIB is network specific, and its determination is out of the scope of this document. This value must be agreed to by all the routers in the network. This agreement can be performed by using a capability TLV as defined in Appendix B.
All the routers that use R to reach Y will compute a lower rank than R, and hence the correct order will be respected. It should be noted that only the routers that used Y before the event need to compute their rank.
In the case of a link or router up event rooted at Y or a link metric decrease affecting link Y->W, a router R must have a rank that is higher than the rank of the routers that it will use to reach Y, according to the rule described in Section 2. Thus, the rank of R is the number of hops between R and Y in its renewed Shortest Path Tree. When R has multiple equal-cost paths to Y, the rank is the length in hops of the longest ECMP path to Y.
Router R is required to update its FIB at time
T0 + H + (rank * MAX_FIB)
It should be noted that only the routers that use Y after the event have to compute a rank, i.e., only the routers that have Y in their SPT after the link-state change.
The mechanism described above is conservative and hence may be relatively slow. The purpose of this section is to describe a method of accelerating the controlled convergence in such a way that ordered loop-free convergence is still guaranteed.
In many cases, a router will complete its required FIB changes in a time much shorter than MAX_FIB, and in many other cases, a router will not have to perform any FIB change at all.
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This section describes the use of completion messages to speed up the convergence by providing a means for a router to inform those routers waiting for it that it has completed any required FIB changes. When a router has been advised of completion by all the routers for which it is waiting, it can safely update its own FIB without further delay. In most cases, this can result in a sub-second reconvergence time, which is comparable with a normal convergence time.
Routers maintain a waiting list of the neighbors from which a completion message must be received. Upon reception of a completion message from a neighbor, a router removes this neighbor from its waiting list. Once its waiting list becomes empty, the router is allowed to update its FIB immediately even if its ranking timer has not yet expired. Once this is done, the router sends a completion message to the neighbors that are waiting for it to complete. Those routers are listed in a list called the Notification List. Completion messages contain an identification of the event to which they refer.
Note that, since this is only an optimization, any loss of completion messages will result in the routers waiting their defined ranking time, and hence the loop-free properties will be preserved.
5.1. Construction of the Waiting List and Notification List
Consider a link or node down event rooted at router Y or the cost increase of the link X->Y. A router R will compute rSPT_old(Y) to determine its rank. When doing this, R also computes the set of neighbors that R uses to reach the failing node or link, and the set of neighbors that are using R to reach the failing node or link. The notification list of R is equal to the former set, and the waiting list of R is equal to the latter.
Note that R could include all its neighbors in the notification list except those in the waiting list; this would have no impact on the correctness of the protocol but would be unnecessarily inefficient.
Consider a link or node up event rooted at router Y or the cost decrease of the link Y->X. A router R will compute its new SPT (SPT_new(R)). The waiting list is the set of next-hop routers that R uses to reach Y in SPT_new(R).
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In a simple implementation, the notification list of R is all the neighbors of R excluding those in the waiting list. This may be further optimized by computing rSPT_new(Y) to determine those routers that are waiting for R to complete.
In circumstances where a router detects that it is dealing with incomplete or inconsistent link-state information, or when a further topology event is received before completion of the current ordered FIB update process, it may be expedient to abandon the controlled convergence process. A number of possible fallback mechanisms are described in Appendix A. This mechanism is referred to as "Abandoning All Hope" (AAH). The state machine defined in the body of this document does not make any assumption about which fallback mechanism will be used.
An implementation must be capable of interworking with the model of an oFIB state machine described in this section.
An oFIB-capable router maintains an oFIB state value, which is one of: OFIB_STABLE, OFIB_HOLDING_DOWN, OFIB_HOLDING_UP, OFIB_ABANDONED, or OFIB_ONGOING.
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An oFIB-capable router maintains a timer, Hold_down_timer. An oFIB- capable router is configured with a value referred to as HOLD_DOWN_DURATION. This configuration can be performed manually or using Appendix B.
An oFIB-capable router maintains a timer, rank_timer.
OFIB_STABLE is the state of a router that is not currently involved in any convergence process. This router is ready to process an event by applying oFIB.
EVENT: Reception of a Link State Packet that describes an event of the type link X--Y down or metric increase and is to be processed using oFIB.
ACTION:
Set state to OFIB_HOLDING_DOWN.
Start Hold_down_timer.
ofib_current_common_set = {X,Y}.
Compute rank with respect to the event, as defined in Section 4.
Store the waiting list and notification list for X--Y obtained from the rank computation.
EVENT: Reception of a Link State Packet that describes an event of the type link X--Y up or metric decrease and is to be processed using oFIB.
ACTION:
Set state to OFIB_HOLDING_UP.
Start Hold_down_timer.
ofib_current_common_set = {X,Y}.
Compute rank with respect to the event, as defined in Section 4.
Store the waiting list and notification list for X--Y obtained from the rank computation.
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OFIB_HOLDING_DOWN is the state of a router that is collecting a set of link down or metric increase Link State Packets to be processed together using controlled convergence.
EVENT: Reception of a Link State Packet that describes an event of the type link up or metric decrease and can be processed using oFIB.
ACTION:
Set state to OFIB_ABANDONED.
Reset Hold_down_timer.
Trigger AAH mechanism.
EVENT: Reception of a Link State Packet that describes an event of the type link A--B down or metric increase and can be processed using oFIB.
If ofib_current_common_set is empty, then there is no longer a node in common in all the pending link-state changes.
Set state to OFIB_ABANDONED.
Reset Hold_down_timer.
Trigger AAH mechanism.
If ofib_current_common set is not empty, update the waiting list and notification list as defined in Section 4. Note that in the case of a single link event, the Link State Packet received when the router is in this state describes the state change of the other direction of the link; hence, no changes will be made to the waiting and notification lists.
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EVENT: Hold_down_timer expires.
ACTION:
Set state to OFIB_ONGOING.
Start rank_timer with computed rank.
EVENT: Reception of a completion message.
ACTION: Remove the sender from the waiting list associated with the event identified in the completion message.
OFIB_HOLDING_UP is the state of a router that is collecting a set of link up or metric decrease Link State Packets to be processed together using controlled convergence.
EVENT: Reception of a Link State Packet that describes an event of the type link down or metric increase and is to be processed using oFIB.
ACTION:
Set state to OFIB_ABANDONED.
Reset Hold_down_timer.
Trigger AAH mechanism.
EVENT: Reception of a Link State Packet that describes an event of the type link A--B up or metric decrease and is to be processed using oFIB.
If ofib_current_common_set is empty, then there is no longer a common node in the set of pending link-state changes.
Set state to OFIB_ABANDONED.
Reset Hold_down_timer.
Trigger AAH mechanism.
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If ofib_current_common set is not empty, update the waiting list and notification list as defined in Section 4. Note that in the case of a single link event, the Link State Packet received when the router is in this state describes the state change of the other direction of the link; hence, no changes will be made to the waiting and notification lists.
EVENT: Reception of a completion message.
ACTION: Remove the sender from the waiting list associated with the event identified in the completion message.
OFIB_ONGOING is the state of a router that is applying the ordering mechanism with respect to the set of Link State Packets collected when in OFIB_HOLDING_DOWN or OFIB_HOLDING_UP state.
EVENT: rank_timer expires or waiting list becomes empty.
ACTION:
Perform FIB updates according to the change.
Send completion message to each member of the notification list.
Set state to OFIB_STABLE.
EVENT: Reception of a completion message.
ACTION: Remove the sender from the waiting list.
EVENT: Reception of a Link State Packet describing a link-state change event.
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OFIB_ABANDONED is the state of a router that has fallen back to fast convergence due to the reception of Link State Packets that cannot be dealt with together using oFIB.
EVENT: Reception of a Link State Packet describing a link-state change event.
A system for recording the dynamics of the convergence process needs to be deployed in order to make a post hoc diagnosis of the reconvergence. The sensitivity of applications to any packet reordering introduced by the delayed convergence process will need to be studied. However, these needs apply to any loop-free convergence method and are not specific to the ordered FIB method described in this document.
This document requires only minor modifications to existing routing protocols and therefore does not add significant additional security risks. However, a full security analysis would need to be provided within the protocol-specific specifications proposed for deployment.
Security considerations related to timer values set by routers are noted in Appendix B.4.
[IPFRR-TUNNELS] Bryant, S., Filsfils, C., Previdi, S., and M. Shand, "IP Fast Reroute using tunnels", Work in Progress, November 2007.
[ISO10589] International Organization for Standardization, "Intermediate system to Intermediate system intra-domain routing information exchange protocol for use in conjunction with the protocol for providing the connectionless-mode Network Service (ISO 8473)", ISO/IEC 10589:2002, Second Edition, November 2002.
[LF-TIMERS] Atlas, A., Bryant, S., and M. Shand, "Synchronisation of Loop Free Timer Values", Work in Progress, February 2008.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
[RFC4090] Pan, P., Swallow, G., and A. Atlas, "Fast Reroute Extensions to RSVP-TE for LSP Tunnels", RFC 4090, May 2005.
[RFC5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework", RFC 5714, January 2010.
[RFC5715] Shand, M. and S. Bryant, "A Framework for Loop-Free Convergence", RFC 5715, January 2010.
[refs.PFOB07] Francois, P. and O. Bonaventure, "Avoiding transient loops during the convergence of link-state routing protocols", IEEE/ACM Transactions on Networking, Vol. 15, No. 6, pp. 1280-1292, December 2007, <http://dx.doi.org/10.1109/TNET.2007.902686>.
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Appendix A. Candidate Methods of Safely Abandoning Loop-Free Convergence (AAH)
IP Fast Reroute [RFC5714] and loop-free convergence techniques [RFC5715] can deal with single topology change events, multiple correlated change events, and in some cases even certain uncorrelated events. However, in all cases, there are events that cannot be dealt with, and the mechanism needs to quickly revert to normal convergence. This is known as "Abandoning All Hope" (AAH).
This appendix describes the outcome of a design study into the AAH problem and is included here to trigger discussion on the trade-offs between complexity and robustness in the AAH solution space.
The "hold-down timer only" AAH method uses a hold-down to acquire a set of LSPs that should be processed together. On expiry of the local hold-down timer, the router begins processing the batch of LSPs according to the loop-free prevention algorithm.
There are a number of problems with this simple approach. In some cases, the timer value will be too short to ensure that all the related events have arrived at all routers (perhaps because there was some unexpected propagation delay, or one or more of the events are slow in being detected). In other cases, a completely unrelated event may occur after the timer has expired but before the processing is complete. In addition, since the timer is started at each router on reception of the first LSP announcing a topology change, the actual starting time is dependent upon the propagation time of the first LSP. So, for a subsequent event occurring around the time of the timer expiry, because of variations in propagation delay, it may reach some routers before the timer expires and others after it has expired. In the former case, this LSP will be included in the set of changes to be considered; while in the latter, it will be excluded leading to serious routing inconsistency. In such cases, continuing to operate the loop-free convergence protocol may exacerbate the situation.
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The simple approach to this would be to revert to normal convergence (AAH) whenever an LSP is received after the timer has expired. However, this also has problems for the reasons above and therefore AAH must be a synchronous operation, i.e., it is necessary to arrange that an AAH invoked anywhere in the network causes ALL routers to invoke AAH.
It is also necessary to consider the means of exiting the AAH state. Again, the simplest method is to use a timer. However, while in AAH state, any topology changes that are previously received or subsequently received should be processed immediately using the traditional convergence algorithms, i.e., without invoking controlled convergence. If the exit from the AAH state is not correctly synchronized, a new event may be processed by some routers immediately (as AAH), while those that have already left AAH state will treat it as the first of a new batch of changes and attempt controlled convergence. Thus, both entry and exit from the AAH state need to be synchronized. A method of achieving this is described in Appendix A.3.
Like the simple timer AAH method, the "AAH messages" method uses a hold-down to acquire a set of LSPs that should be processed together. On expiry of the local hold-down timer, the router begins processing the batch of LSPs according to the loop-free prevention algorithm. This is the same behavior as the hold-down timer only method. However, if any router, having started the loop-free convergence process receives an LSP that would trigger a topology change, it locally abandons the controlled convergence process and sends an AAH message to all its neighbors. This eventually triggers all routers to abandon the controlled convergence. The routers remain in AAH state (i.e., processing topology changes using normal "fast" convergence), until a period of quiescence has elapsed. The exit from AAH state is synchronized by using a two-step process. To achieve the required synchronization, two additional messages are required, AAH and AAH ACK. The AAH message is reliably exchanged between neighbors using the AAH ACK message. These could be implemented as a new message within the routing protocol or carried in existing routing hello messages. Two types of state machines are needed -- a per-router AAH state machine and a per-neighbor AAH state machine (PNSM). These are described below.
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RX = Reception TX = Transmission TX-AAH = Send "go to TX-AAH" to all other PNSMs.
Per-Router State Table
Operation of the per-router state machine is as follows:
Operation of this state machine under normal topology change involves only states: Quiescent (Q), Hold-down (Hold) and Controlled Convergence (CC). The remaining states are associated with an AAH event.
The resting state is Quiescent. When the router in the Quiescent state receives an LSP indicating a topology change, which would normally trigger an SPF, it starts the hold-down timer and changes state to Hold-down. It normally remains in this state, collecting additional LSPs until the hold-down timer expires. Note that all routers must use a common value for the hold-down timer. When the hold-down timer expires, the router then enters Controlled
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Convergence (CC) state and executes the CC mechanism to reconverge the topology. When the CC process has completed on the router, the router re-enters the Quiescent state.
If this router receives a topology-changing LSP whilst it is in the CC state, it enters AAH state and sends a "go to TX-AAH" command to all per-neighbor state machines; this causes each per-neighbor state machine to signal this state change to its neighbor. Alternatively, if this router receives an AAH message from any of its neighbors whilst in any state except AAH, it starts the AAH timer and enters the AAH state. The per-neighbor state machine corresponding to the neighbor from which the AAH was received executes the RX AAH action (which causes it to send an AAH ACK), while the remainder of neighbors are sent the "go to TX-AAH" command. The result is that the AAH is acknowledged to the neighbor from which it was received and propagated to all other neighbors. On entering AAH state, all CC timers are expired, and normal convergence takes place.
Whilst in the AAH state, LSPs are processed in the traditional manner. Each time an LSP is received, the AAH timer is restarted. In an unstable network, ALL routers will remain in this state for some time, and the network will behave in the traditional uncontrolled convergence manner.
When the AAH timer expires, the router enters AAH-hold state and starts the AAH-hold timer. The purpose of the AAH-hold state is to synchronize the transition of the network from AAH to Quiescent. The additional state ensures that the network cannot contain a mixture of routers in both AAH and Quiescent states. If, whilst in AAH-hold state the router receives a topology changing LSP, it re-enters AAH state and commands all per-neighbor state machines to "go to TX-AAH". If, whilst in AAH-hold state, the router receives an AAH message from one of its neighbors, it re-enters the AAH state and commands all other per-neighbor state machines to "go to TX-AAH". Note that the per-neighbor state machine receiving the AAH message will autonomously acknowledge receipt of the AAH message. Commanding the per-neighbor state machine to "go to TX-AAH" is necessary, because routers may be in a mixture of Quiescent, Hold-down, and AAH-hold states, and it is necessary to rendezvous the entire network back to AAH state.
When the AAH-hold timer expires, the router changes to Quiescent and is ready for loop-free convergence.
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There is one instance of the per-neighbor state machine (PNSM) for each neighbor within the convergence control domain.
The normal state is IDLE.
On command ("go to TX-AAH") from the router state machine, the state machine enters TX-AAH state, transmits an AAH message to its neighbor, and starts a timer.
On receipt of an AAH ACK in state TX-AAH, the state machine cancels the timer and enters IDLE state.
In state IDLE, any AAH ACK message received is ignored.
On expiry of the timer in state TX-AAH, the state machine transmits an AAH message to the neighbor and restarts the timer. (The timer cannot expire in any other state.)
In any state, receipt of an AAH causes the state machine to transmit an AAH ACK and enter the IDLE state.
Note that for correct operation the state machine must remain in state TX-AAH until an AAH ACK or an AAH is received or until the state machine is deleted. Deletion of the per-neighbor state machine occurs when routing determines that the neighbor has gone away or when the interface goes away.
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When routing detects a new neighbor, it creates a new instance of the per-neighbor state machine in state IDLE. The consequent generation of the router's own LSP will then cause the router state machine to execute the LSP receipt actions that, if necessary, will result in the new per-neighbor state machine receiving a "go to TX-AAH" command and transitioning to TX-AAH state.
Appendix B. Synchronization of Loop-Free Timer Values
This appendix provides the reader with access to the design considerations originally described in [LF-TIMERS].
Most of the loop-free convergence mechanisms [RFC5715] require one or more convergence delay timers that must have a duration that is consistent throughout the routing domain. This time is the worst- case time that any router will take to calculate the new topology and to make the necessary changes to the FIB. The timer is used by the routers to know when it is safe to transition between the loop-free convergence states. The time taken by a router to complete each phase of the loop-free transition will be dependent on the size of the network and the design and implementation of the router. Therefore, it can be expected that the optimum delay will need to be tuned from time to time as the network evolves. Manual configuration of the timer is fraught for two reasons. Firstly, it is always difficult to ensure that the correct value is installed in all of the routers. Secondly, if any change is introduced into the network that results in a need to change the timer (for example, due to a change in hardware or software version), then all of the routers need to be reconfigured to use the new timer value. Therefore, it is desirable that a means be provided by which the convergence delay timer can be automatically synchronized throughout the network.
The timer synchronization mechanism must have the following properties:
o The convergence delay time must be consistent amongst all routers that are converging on the new topology.
o The convergence delay time must be the highest delay required by any router in the new topology.
o The mechanism must increase the delay when a new router that requires a higher delay than is currently in use is introduced to the network.
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o When the router that had the longest delay requirements is removed from the topology, the convergence delay timer value must, within some reasonable time, be reduced to the longest delay required by the remaining routers.
o It must be possible for a router to change the convergence delay timer value that it requires.
o A router that is in multiple routing areas or is running multiple routing protocols may signal a different loop-free convergence delay for each area and for each protocol.
How a router determines the time that it needs to execute each convergence phase is an implementation issue and outside the scope of this specification. However, a router that dynamically determines its proposed timer value must do so in such a way that it does not cause the synchronized value to continually fluctuate.
A new information element is introduced into the routing protocol that specifies the maximum time (in milliseconds) that the router will take to calculate the new topology and to update its FIB as a result of any topology change.
When a topology change occurs, the longest convergence delay time required by any router in the new topology is used by the loop-free convergence mechanism.
If a routing protocol message is issued that changes the convergence delay timer value but does not change the topology, the new timer value must be taken into consideration during the next loop-free transition but must not instigate a loop-free transition.
If a routing protocol message is issued that changes the convergence timer value and changes the topology, a loop-free transition is instigated, and the new timer value is taken into consideration.
The loop-free convergence mechanism should specify the action to be taken if a timer change (only) message and a topology change message are independently generated during the hold-off time. A suitable action would be to take the same action that would be taken if two uncorrelated topology changes occurred in the network.
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All routers that support loop-free convergence must advertise a loop- free convergence delay time. The loop-free convergence mechanism must specify the action to be taken if a router does not advertise a convergence delay time.
B.4. Security Considerations Related to Router Timer Values
If an abnormally large timer value is proposed by a router, then there is a danger that the loop-free convergence process will take an excessive amount of time. If during that time the routing protocol signals the need for another transition, the loop-free transition will be abandoned and the default best-case (traditional) convergence mechanism used.
It is still undesirable that the routers select a convergence delay time that has an excessive value. The maximum value that can be specified in the LSP or Link State Advertisement (LSA) is limited (through the use of a 16-bit field) to about 65 seconds. When sufficient implementation experience is gained, an architectural constant will be specified as the upper limit of the convergence delay timer.
Authors' Addresses
Mike Shand Individual Contributor
EMail: imc.shand@googlemail.com
Stewart Bryant Cisco Systems 10 New Square, Bedfont Lakes Feltham, Middlesex TW18 8HA United Kingdom
EMail: stbryant@cisco.com
Stefano Previdi Cisco Systems Via Del Serafico 200 00142 Roma Italy
EMail: sprevidi@cisco.com
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Clarence Filsfils Cisco Systems Brussels Belgium
EMail: cfilsfil@cisco.com
Pierre Francois Institute IMDEA Networks Avda. del Mar Mediterraneo, 22 Leganese 28918 Spain
EMail: pierre.francois@imdea.org
Olivier Bonaventure Universite catholique de Louvain Place Ste Barbe, 2 Louvain-la-Neuve 1348 Belgium