Revised Default Values for the BGP 'Minimum Route Advertisement Interval'

The proper functioning of the routing protocol is of great importance to the Internet. Issues regarding matters of its stability and convergence have been documented widely, such as in , and . One such issue is the effect of 'Minimum Route Advertisement Interval' (MRAI).

The Minimum Route Advertisement Interval (MRAI) timer is specified in RFC4271. This timer acts to rate-limit updates, on a per-destination basis. suggests values of 30s and 5s for this interval for eBGP and iBGP respectively. The MRAI must also be applied to withdrawals according to RFC4271, a change from the earlier RFC1771. Some implementations apply this rate-limiting on a per-peer basis, presumably an adequate approximation. Some implementations apply it to withdrawal methods (often called "WRATE" in the literature). Some implementations do not apply MRAI at all.

The MRAI timer serves to suppress messages which BGP would otherwise send out to describe transitory states, and so allow BGP to converge with significantly fewer messages sent. This beneficial effect of the MRAI timer, in terms of # of messages, increases as the timer is increased until an optimum value is reached, after which the beneficial effect stabilises. In terms of convergence time, a similar beneficial effect is seen as the MRAI increases to the same optimum value. However as the timer value is increased past the optimum, the convergence time increases again linearly - the scale of this increase is worse with WRATE. The optimum MRAI timer value is dependent on several factors, most particularly the topology. The optimum value will differ between different subsets of the Internet.

As the MRAI helps eliminate some updates, it interacts with flap-damping. The lower the MRAI timer, the greater the risk of crossing below the threshold of the optimum value. So with a lower value, there is an increased number of updates somewhere within the BGP system, and hence an increased risk of paths being dampened, which otherwise would not. So, in presence of significant flap-damping deployment and given the uncertainty of what the optimum is, it is reasonable to err towards selecting a value of the MRAI timer significantly higher than the optimum. However, given that flap-damping increasingly is discouraged in Internet routing, this particular need to be conservative in the choice of MRAI timer value may be less important.

The current recommended value of 30s may be far higher than is optimal, based on observations of certain parameters related to the topology of the Internet. In it is suggested that the optimal value may be between 5s ('semi-safe') to 15s ('safe'). The estimation of the 'safe' value here is of no relevance if WRATE is universally deployed, as in such a case the 'semi-safe' value and 'safe' value are the same. Further empirical work by the same authors suggests that the optimal, Internet MRAI may be below 5s. Further, and argue that operational conditions (e.g. different routers using different MRAI values) mean the MRAI is having an adverse effect even on the number of messages sent, and so further exacerbating convergence problems in the global BGP system, such as path hunting. The document goes further still and argues that MRAI be deprecated in favour of some better way of damping BGP UPDATES, however there are no clear proposals before the IDR as of this writing for such changes to BGP.

Though there is an optimum value for the MRAI, it's unlikely that it can be determined empirically or otherwise for the general Internet. It may even not be possible, as the optimum MRAI will differ for different subsets of the Internet. Some degree of guesstimation at a reasonable value for the MRAI is required, which is an exercise in risk; whether to err towards fast convergence at the risk of a disproportionate increase in BGP messaging, or to err to the side of an optimal number of messages at the expense of convergence. Arguably, economising on bandwidth and control-plane processing power is today less important than the convergence time of BGP, than in times past. Presuming this, any new recommendations for the MRAI should seek to err slightly to the side of convergence, rather than erring towards minimising BGP traffic. Further, if we assume most implementations apply the MRAI to withdrawals, then the Internet BGP topology effectively is WRATE-enabled, and suggests there is even less benefit to erring toward a higher MRAI. The most definite risk of lowering the MRAI is the increased risk of flap-damping, if the value is set too much below the optimum. Therefore, taking into account estimations of that optimum is required. That said, at least one BGP implementation by default does not apply any MRAI at all.