Avi Freedman <freedman@netaxs.com> wrote:

Anyway, sorry to be snide - any suggestions you have about algorithms for exchanging routing data between (or inside of) providers that also takes into account link size and perhaps hop-count, as opposed to just AS-PATH, are welcome at the next IETF meeting.

The problem with physical metrics is that they're pretty much useless in the real world, where network administrators often want to have traffic to go along suboptimal ways to balance the load. Obviously, any "automagic" load-sensitive system must have a global knowledge, and long-term statistics on traffic patterns to be able to simulate the behaviour of "gut feeling" of the engineers. Regarding BGP metrics -- there's a problem with a single "universal" metric in an exterior routing protocol, namely the absense of a single authority to assign those metrics. Different networks may have very much confiliciting interests in how to assign BGP metrics. Some time ago i wrote a draft (which didn't get any support and quietly expired) regarding BGP metrics -- the idea was to have a vector of metrics (one per AS hop) instead of a single scalar metric; and allow network operators to calculate weighted sums to obtain the locally significant scalar metrics. I.e. a network operator may decide for himself how to interpret BGP metrics coming from different AS-es (or by different paths). The draft also describes a mechanism for scaling IGP metrics to generate BGP metrics (thus replacing MEDs with a generic mechanism). It also allows "fudging" the weighted sum with an exit-specific constant, thus superceding local prefs. In other words, the proposed metric system is generic enough to make MEDs, LOCAL_PREFs, and many cases of use of communities, a mere particular cases; so they can be obsoleted. The default metric for an AS-hop is 1.0, and the default weight vector is all-ones, so the default behaviour is identical to that of the current BGP. If there's any interest i can dig the draft out and place it on my www server. --vadim