On Thu, Nov 15, 2012 at 9:16 AM, Brian E Carpenter < brian.e.carpen...@gmail.com> wrote:
> On 14/11/2012 22:44, james woodyatt wrote: > > On Nov 14, 2012, at 13:34 , Mikael Abrahamsson <swm...@swm.pp.se> wrote: > >> I've always seen it to be solved via some kind of source based routing > automatically discovered between the ISP routers. > > > > > > My point is that it isn't sufficient to handle this problem at just the > routers. At a minimum, the *hosts* need to be told which default router to > use with each source prefix. > > Of course. I suggested this when MIF started out, and it's a MIF > issue still. > The hosts do already select the correct router depending on the prefix. They're tied together. An RA contains a prefix and router address. That's what the host keeps in memory. If it's two RA's one router becomes default, the other more specific. The host/applications will also only use one prefix at a time, thus always send the packets down the correct path. In my experience it was always the same prefix, the one that got registered first (if no preference was setup in the advertising router). If the host is connected via two or more interfaces (so we're in MIF area now), there will always be one preferred prefix, and interface, and the outgoing routing will work. If applications are able to chose a specific prefix (e.g. VPN) they usually implement some source routing on the host anyways, and send the packets to the router registered with that prefix. If you have an application listening for incoming connections, then it's important that the application is smart enough to use the address on which the incoming connection arrived as source address, and not the default one which might be on a different prefix to avoid asynchronous communication. As far as I know this is already common practice. So I don't really see a problem here, unless you want to do some fancy load sharing and play ping pong with source addresses. If there's only one interface which connects to a multihomed router, the principle is the same, but the multihomed router must be able to perform source address routing, and forward the packets down the right path. And this is something I'm not too sure about routers can currently do. Mat
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