Link State protocols SPF trigger and delay algorithm impact on IGP micro-loops

Link State IGP protocols are based on a topology database on which a SPF (Shortest Path First) algorithm like Dijkstra is implemented to find the optimal routing paths. Specifications like IS-IS () propose some optimization of the route computation (See Appendix C.1) but not all the implementations are following those not mandatory optimizations. We will call SPF trigger, the events that would lead to a new SPF computation based on the topology. Link State IGP protocols, like OSPF () and IS-IS (), are using plenty of timers to control the router behavior in case of churn : SPF delay, PRC delay, LSP generation delay, LSP flooding delay, LSP retransmission interval ... Some of those timers are standardized in protocol specification, some are not especially the SPF computation related timers. For non standardized timers, implementations are free to implement it in any way. For some standardized timer, we can also see that rather than using static configurable values for such timer , implementations may offer dynamically adjusted timers to help controlling the churn. We will call SPF delay, the timer that exists in most implementations that specifies the required delay before running SPF computation after a SPF trigger is received. A micro-loop is a packet forwarding loop that may occur transiently among two or more routers in a hop-by-hop packet forwarding paradigm. We can observe that these micro-loops are formed when two routers do not update their Forwarding Information Base (FIB) for a certain prefix at the same time. The micro-loop phenomenon is described in . Some micro-loop mitigation techniques have been defined by IETF (e.g. , ) but are not implemented due to complexity or are not providing a complete mitigation. In multi vendor networks, using different implementations of a link state protocol may favor micro-loops creation during convergence time due to discrepancies of timers. Service Providers are already aware to use similar timers for all the network as best practice, but sometimes it is not possible due to limitation of implementations. This document will present why it sounds important for service provider to have consistent implementations of Link State protocols across vendors. We are particularly analyzing the impact of using different Link State IGP implementations in a single network in regards of micro-loops. The analysis is focused on the SPF triggers and SPF delay algorithm in a first step. This document is only stating the problem, and defining some work items but its not intended to provide a solution.

A ---- B | | 10 | | 10 | | C ---- D | 2 | Px Px Figure 1 In the figure above, A uses primarily the AC link to reach C. When the AC link fails, IGP convergence occurs. If A converges before B, A will forward traffic to C through B, but as B as not converged yet, B will loop back traffic to A, leading to a micro-loop. The micro-loop appears due to the asynchronous convergence of nodes in a network when a event occurs. Multiple factors (and combination of these factors) may increase the probability for a micro-loop to appear : delay of failure notification : the more B is advised of the failure later than A, the more a micro-loop may appear. SPF delay : most of the implementations supports a delay for the SPF computation to try to catch as many events as possible. If A uses a SPF delay timer of x msec and B uses a SPF delay timer of y msec and x < y, B would start converging after A leading to a potential micro-loop. SPF computation time : mostly a matter of CPU power and optimizations like incremental SPF. If A computes SPF faster than B, there is a chance for a micro-loop to appear. CPUs are today faster enough to consider SPF computation time as negligeable (order of msec in a large network). RIB and FIB prefix insertion speed or ordering : highly implementation dependant. This document will focus on analysis SPF delay (and associated triggers).

Depending of the change advertised in LSP/LSA, the topology may be affected or not. An implementation can decide to not run SPF (and only run IP reachability) if the advertised change is not affecting topology. Different strategies exists to trigger SPF : Always run full SPF whatever the change to process. Run only Full SPF when required : e.g. if a link fails, a local node will run an SPF for its local LSP update. If the LSP from the neighbor (describing the same failure) is received after SPF has started, the local node can decide that a new full SPF is not required as the topology has not change. If topology does not change, only recompute reachability. As pointed in , SPF optimization are not mandatory in specifications, leading to multiple strategies to be implemented.

Implementations of link state routing protocols use different strategies to delay SPF : Two steps. Exponential backoff.

The SPF delay is managed by four parameters : Rapid delay : amount of time to wait before running SPF. Rapid runs : amount of consecutive SPF runs that can run using rapid delay. When amount is exceeded router moves to slow delay. Slow delay : amount of time to wait before running SPF. Wait time : amount of time to wait without events before going back to rapid delay. Example : Rapid delay = 50msec, Rapid runs = 3, Slow delay = 1sec, Wait time = 2sec

SPF delay time ^ | | SD- | x xx x | | | RD- | x x x x | +---------------------------------> Events | | | | || | | < wait time >

The algorithm has two mode : fast mode and backoff mode. In backoff mode, the SPF delay is increasing exponentially at each run. The SPF delay is managed by four parameters : First delay : amount of time to wait before running SPF. This delay is used only when SPF is in fast mode. Incremental delay : amount of time to wait before running SPF. This delay is used only when SPF is in backoff mode and increments exponentially at each SPF run. Maximum delay : maximum amount of time to wait before running SPF. Wait time : amount of time to wait without events before going back to fast mode. Example : First delay = 50msec, Incremental delay = 50msec, Maximum delay = 1sec, Wait time = 2sec

SPF delay time ^ MD- | xx x | | | | | | x | | | | x | FD- | x x x ID | +---------------------------------> Events | | | | || | | < wait time > FM->BM -------------------->FM

S ---- E | | 10 | | 10 | | D ---- A | 2 Px Figure 2 In the diagram above, we consider a flow of packet from S to D. We consider that S is using optimized SPF triggering (Full SPF is triggered only when necessary), and two steps SPF delay (rapid=150ms,rapid-runs=3, slow=1s). As implementation of S is optimized, Partial Reachability Computation (PRC) is available. We consider the same timers as SPF for delaying PRC. We consider that E is using a SPF trigger strategy that always compute Full SPF and exponential backoff strategy for SPF delay (start=150ms, inc=150ms, max=1s) We also consider the following sequence of events (note : the timescale does not intend to represent a real router timescale where jitters are introduced to all timers) : t0=0 ms : a prefix is declared down in the network. We consider this event to happen at time=0. 200ms : the prefix is declared as up. 400ms : a prefix is declared down in the network. 1000ms : S-D link fails. Time Network Event Router S events Router E events t0=0Prefix DOWN 10msSchedule PRC (in 150ms)Schedule SPF (in 150ms) 160msPRC startsSPF starts 161msPRC ends 162msRIB/FIB starts 163msSPF ends 164msRIB/FIB starts 175msRIB/FIB ends 178msRIB/FIB ends 200msPrefix UP 212msSchedule PRC (in 150ms) 214msSchedule SPF (in 150ms) 370msPRC starts 372msPRC ends 373msSPF starts 373msRIB/FIB starts 375msSPF ends 376msRIB/FIB starts 383msRIB/FIB ends 385msRIB/FIB ends 400msPrefix DOWN 410msSchedule PRC (in 300ms)Schedule SPF (in 300ms) 710msPRC startsSPF starts 711msPRC ends 712msRIB/FIB starts 713msSPF ends 714msRIB/FIB starts 716msRIB/FIB endsRIB/FIB ends 1000msS-D link DOWN 1010msSchedule SPF (in 150ms)Schedule SPF (in 600ms) 1160msSPF starts 1161msSPF ends 1162msMicro-loop may start from hereRIB/FIB starts 1175msRIB/FIB ends 1612msSPF starts 1615msSPF ends 1616msRIB/FIB starts 1626msMicro-loop endsRIB/FIB ends In the table above, we can see that due to discrepancies in SPF management, after multiple events (different types of event), SPF delays are completely misaligned between nodes leading to long micro-loop creation. The same issue can also appear with only single type of events as displayed below : Time Network Event Router S events Router E events t0=0Link DOWN 10msSchedule SPF (in 150ms)Schedule SPF (in 150ms) 160msSPF startsSPF starts 161msSPF ends 162msRIB/FIB starts 163msSPF ends 164msRIB/FIB starts 175msRIB/FIB ends 178msRIB/FIB ends 200msLink DOWN 212msSchedule SPF (in 150ms) 214msSchedule SPF (in 150ms) 370msSPF starts 372msSPF ends 373msSPF starts 373msRIB/FIB starts 375msSPF ends 376msRIB/FIB starts 383msRIB/FIB ends 385msRIB/FIB ends 400msLink DOWN 410msSchedule SPF (in 150ms)Schedule SPF (in 300ms) 560msSPF starts 561msSPF ends 562msMicro-loop may start from hereRIB/FIB starts 568msRIB/FIB ends 710msSPF starts 713msSPF ends 714msRIB/FIB starts 716msMicro-loop endsRIB/FIB ends 1000msLink DOWN 1010msSchedule SPF (in 1s)Schedule SPF (in 600ms) 1612msSPF starts 1615msSPF ends 1616msMicro-loop may start from hereRIB/FIB starts 1626msRIB/FIB ends 2012msSPF starts 2014msSPF ends 2015msRIB/FIB starts 2025msMicro-loop endsRIB/FIB ends

In order to enhance the current LinkState IGP behavior, authors would encourage working on standardization of some behaviors. Authors are proposing the following work items : Standardize SPF trigger strategy. Standardize computation timer scope : single timer for all computation operations, separated timers ... Standardize "slowdown" timer algorithm including its association to a particular timer : authors of this document does not presume that the same algorithm must be used for all timers. Using the same event sequence as in figure 2, we may expect fewer and/or shorter micro-loops using standardized implementations. Time Network Event Router S events Router E events t0=0Prefix DOWN 10msSchedule PRC (in 150ms)Schedule SPF (in 150ms) 160msPRC startsPRC starts 161msPRC ends 162msRIB/FIB startsPRC ends 163msRIB/FIB starts 175msRIB/FIB ends 176msRIB/FIB ends 200msPrefix UP 212msSchedule PRC (in 150ms) 213msSchedule PRC (in 150ms) 370msPRC startsPRC starts 372msPRC ends 373msRIB/FIB startsPRC ends 374msRIB/FIB starts 383msRIB/FIB ends 384msRIB/FIB ends 400msPrefix DOWN 410msSchedule PRC (in 300ms)Schedule PRC (in 300ms) 710msPRC startsPRC starts 711msPRC endsPRC ends 712msRIB/FIB starts 713msRIB/FIB starts 716msRIB/FIB endsRIB/FIB ends 1000msS-D link DOWN 1010msSchedule SPF (in 150ms)Schedule SPF (in 150ms) 1160msSPF starts 1161msSPF endsSPF starts 1162msMicro-loop may start from hereRIB/FIB startsSPF ends 1163msRIB/FIB starts 1175msRIB/FIB ends 1177msMicro-loop endsRIB/FIB ends As displayed above, there could be some other parameters like router computation power, flooding timers that may also influence micro-loops. In the figure 5, we consider E to be a bit slower than S, leading to micro-loop creation. Despite of this, we expect that by aligning implementations at least on SPF trigger and SPF delay, service provider may reduce number or duration of micro-loops.

This document does not introduce any security consideration.

Authors would like to thank Mike Shand for his useful comments.

This document has no action for IANA.