Hi,
[Document author hat on]
Interesting discussion. Sorry for not responding to this thread, but I have been dealing with a family matter and wasn't staying current with email.
I think there is general agreement that we can relax the anycast rules specified the address architecture. The relevant text from Section 2.6 is currently:
There is little experience with widespread, arbitrary use of Internet anycast addresses, and some known complications and hazards when using them in their full generality [ANYCST]. Until more experience has been gained and solutions are specified, the following restrictions are imposed on IPv6 anycast addresses:
o An anycast address must not be used as the source address of an
IPv6 packet. o An anycast address must not be assigned to an IPv6 host, that
is, it may be assigned to an IPv6 router only.The intent of this text was to not prohibit their usage for all time, but to wait until there was more experience and the usage was specified. This may well be now.
I agree with the discussion that we shouldn't relax the rules without some additional text that summarizes the issues. The cases where we think it is good to use anycast are specific and we still want to limit general usage.
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:
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. 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. 3) Except as described in 1) and 2) above an anycast address must
not be used as the source address of an IPv6 packet. 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.Comments and suggestions welcome.
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.
Comments?
Bob
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