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1.2.22 Optimization of transiting traffic
Starting with IRP 3.7 IRP introduces optimization of transiting traffic capabilities. Optimization of transiting traffic is an enhancement of Inbound bandwidth optimization.
Optimization of transiting traffic relies on the same method of influencing Internet-wide routing – manipulating best path selection by increasing AS Path length for a prefix carrying traffic on an undesirable interface. The most secure and effective way of AS Path manipulation is to apply these changes on a router facing a provider. Edge routers prepend routes according to designated communities.
Besides being an enhancement of Inbound optimization, transit prefixes are governed by a different set of constraints:
- The number of potential routes is very large and only some will/should be targeted for optimization. For this IRP uses filters by ASN/prefix allowing network administrators to configure what segments of the Internet to focus IRP’s attention to.
- The improvements are visible on the Internet and excessive route changes can be flagged by external monitoring services as flapping or route instability. IRP protects against this by rate limiting number of route changes through a specific provider.
- Once a route has been improved all its traffic might be diverted away from this network and no new statistics will be available to make further inferences. In such cases IRP reverts old transit improvements during network’s off-peak hour by either decreasing the number of advertised prepends or withdrawing the improvement altogether (configurable).
- Transit improvements apply to the same prefixes as do outbound improvements. The outbound improvements are withdrawn in order to avoid the risk of contradictory routing decisions.
Implementing optimization of transiting traffic introduces a series of risks and some inherent drawbacks, for example:
- Potential blackholing of traffic when all alternative routes are withdrawn. IRP implements a protection against this by traversing all RIB-in entries for improved transit prefixes and confirming that the route is still being announced by other providers besides IRP. Still there can be a short time period between confirmations when all alternative routes have been withdrawn and IRP did not yet get a chance to re-confirm this. Attempting to reduce this time period by setting up a higher frequency of confirmations leads to increased load on the router and a tradeoff needs to be made for this. For example when the number of providers is quite large the probability that a route will be withdrawn through all of them is quite small and thus the frequency of confirmations can be reduced too.
- Additional CPU load on edge router(s) for servicing mandatory SNMP requests that check alternative route presence and BGP Best Path selection inside IRP.
- Working with missing BGP attributes that is not available over SNMP.
- Strict upper limits on the number of possible Inbound improvements are imposed by the trade-off required to reduce excessive router’s CPU load.
Refer Optimization of transiting traffic for details.