On Tue, 6 Nov 2001, Robert Elz wrote: > | Router B plugged in the network with Router A (e.g. cross-over cable). > | Neither has anything configured or set up yet on this link. > | 3ffe:ffff::1/127 address is added to Router A; now it DAD's for > | 3ffe:ffff::1 (normal address) and, being a router in the subnet, also > | 3ffe:ffff::0, and succeeds. > > My point was that it must not. That is, if there are to be two nodes > on a link, and there are two addresses available, it is positively insane > for one of the two nodes to take both of them.
Sure.. > If it wants (either via some config file, or just random chance) to pick > the xxx0 address, then it gets to be the target of the anycast. If it > doesn't, then it doesn't... > > It can't pick the xxx1 address for itself, and then go ahead and also > pick the xxx0 address to be anycast. If an I-D needs to be written to > make this (self-evident) truth clear, then I guess we should write one. > > I know that the normal use of anycast is for everyone eligible to assign > themselves the address, but there's nothing in the definition that requires > that - this just happens to be a scenario where that is inappropriate > behaviour. .. but as you noted, this is different from the "normal" use of anycast. This operational/technical restriction has not been discussed anywhere (AFAICS). It'd be pretty optimistic to assume every implementation would do the "right" thing (contrary to "do it by the spec"). > | The fact remains, as shown in the scenario above, that on router-router > | links, using /127 is impossible unless subnet-router anycast addresses are > | restricted or disabled. > > No, go back to my first message - it isn't impossible at all. It just > needs rational considered implementations. Sure. This approach would case the following, I suppose: 1) (at least almost) everybody implements by the spec (thinking of "normal procedure") 2) some implementation, later, notices this problem, discards it by giving operational advice; "don't do /127 ptp" 3) some implementation, later, notices this problem, fixes it by a few rational rules, but if the other end of the pipe is non-rational, everything can still break. 4) there will be non-rational implementations forever I believe, the only thing there is to nail this for good is to either/and: - make it widely known that this should be operationally avoided - specify the rational behaviour of anycast assignment in this case > If anything, what we have now when systems use /127 for p2p links is > the use of the subnet-router anycast address ready made (even though > most probably the nodes don't know that's what they're doing, and in the > rare cases when one end is a host rather than a router, the host might > have the subnet-router address inappropriately). Everything is working > just like it should. This is an illusion that will break. Nobody that I know of implements subnet-router anycast address yet. (just checked Linux, FreeBSD 4.4, Cisco), so this is not yet a problem that it would be one day. -- Pekka Savola "Tell me of difficulties surmounted, Netcore Oy not those you stumble over and fall" Systems. Networks. Security. -- Robert Jordan: A Crown of Swords -------------------------------------------------------------------- IETF IPng Working Group Mailing List IPng Home Page: http://playground.sun.com/ipng FTP archive: ftp://playground.sun.com/pub/ipng Direct all administrative requests to [EMAIL PROTECTED] --------------------------------------------------------------------
