At Wed, 13 Apr 2005 14:13:34 -0700, Bob Hinden wrote: > > Here is a proposal (rough) based loosely on Fred Baker's proposal and > subsequent discussion on the list: > > Arbitrary use of Internet anycast addresses is not recommended. There > are known complications and hazards when using them in their full > generality [ANYCST]. Specific usage guidelines are:
Ok so far, although "arbitrary" is in the eye of the beholder.... > 1) Anycast may be used for simple query response applications > (for example DNS) where all nodes serving the anycast > address will respond with the same information and the packets > are limited in size so path mtu discovery is not needed. Not quite right. DNS packets might be big enough to need fragmentation. But there isn't any useful way to do path MTU discovery in DNS/UDP anyway (very high client/server ratio, idempotent lookup protocal, and the big packets are the responses going from the server to the client, so the overhead of performing PMTU discovery and keeping the necessary state on the server almost certainly outweighs cost of just performing the query again when fragments get dropped). So the criteria here are: a) Short-lived session (typically two packet UDP exchange, but some argue that even a fast 7 packet TCP exchange is ok, so long as it's fast); b) Path MTU either not needed or not useful for reasons having nothing to do with use of anycast. > 2) Anycast may be used for applications where anycast is used to > rendezvous with a server and subsequently learn a stable unicast > address for further communication. Ok. > 3) Except as described in 1) and 2) above an anycast address must > not be used as the source address of an IPv6 packet. Too strong. I'd be ok with "should not". > 4) Except as described in 1) and 2) an anycast address must not be > assigned to an IPv6 host, that is, it may be assigned to an IPv6 > router only. I realize that this is just continuation of a fine old IPv6 tradition, but it has never made any sense to me. What's so special about routers in this context? Why is it ok when a router does it but not when a host does it? As near as I can tell, the real historical answer to this is that, once upon a time, somebody thought that router discovery might use anycast, back before the current RA protocol. If there's a strong technical justification for treating routers and hosts differently here, please make it (here and in the text), but absent such a justification I'd urge dropping point (4) entirely. > Another approach is to write a separate document that relaxes the rules and > describes the issues in more depth than we might want to add here. This > would keep the current limits in the address architecture (going forward to > Draft standard) and have the new document start at Proposed standard. I'd rather just fix the current doc and let the ongoing work happen in GROW. If the GROW work results in enough IPv6-specific stuff that there's a need for a new IPv6 WG document, fine, but so far I am not seeing it. Anycast is mostly an IP issue, not an IPv6 or IPv4 issue. -------------------------------------------------------------------- IETF IPv6 working group mailing list [email protected] Administrative Requests: https://www1.ietf.org/mailman/listinfo/ipv6 --------------------------------------------------------------------
