On 10-mrt-2006, at 12:53, Igor Gashinsky wrote:
I'm specificly talking about the case where the client is PA, and server is PI. So, how do you get the server to realize that the the client, which connected from ip-uunet, is also ip-cogent (which is something that BGPknows in v4, and is very important for outbound TE), and do so without adding an enormous amount of state/latency/overhead?
Simple: you don't. And when you say this is possible in v4 with BGP, you mean that this is possible with v4 in BGP if the _client_ is multihomed with PI space. If this means you're saying that for large content networks, it's a requirement that all multihomed clients should have PI space, then I'm afraid nothing we can do is going to make you very happy... Also note that if they weren't multihomed, part of them would still try to reach you over your expensive connection, so this is a fact of life in general and doesn't have much to do with multihoming.
A very basic problem is that committing to the initial source/dest address choice happens very early in the process, and almost unavoidably before there is any communication. This means that it's not possible to determine information that depends on the specific path properties for the different paths between the two hosts in question. This is a roundabout way of saying that it's very hard if not impossible to make a client connect to a server over a link that's cheaper for the server if part of the internet _can_ connect over that cheaper link while another part of the internet can't or can't cheaply. (I.e., if the server peers with some networks while the rest of the world is reachable over more expensive transit: the clients have no way of knowing whether a certain path to the server is over peering or over transit as seen from the server.)
So I think it's probably best to assume that this specific problem will be dealt with in routing rather than end-to-end. Note that this doesn't necessarily require the use of PI: a server could also take address space from two or more ISPs and announce their more specifics to their peers, so regardless of the destination chosen by a client, if the server is reachable over peering, this will happen because of the more specifics.