Sam,

Sam Wilson wrote: It's an odd feature of that RFC that it's published as Informational rather then Proposed Standard,

Which RFC are you referring to? 2373 is in the standards track and so is draft-ietf-ipngwg-addr-arch-v3-11.txt which is going to obsolete 2373 soon.

...

[RFC2373] Network Working Group R. Hinden Request for Comments: 2373 Nokia Obsoletes: 1884 S. Deering Category: Standards Track Cisco Systems

and the only reference to the RFC 2119 terms "MUST", "MUST NOT" etc. is in the Introduction. It specifically does not say: "Interface IDs are REQUIRED to be 64 bits long and to be constructed in IEEE EUI-64 format [EUI64]." Perhaps someone who is further into the the IETF process can say whether this was an oversight or if the latitude was intended.

I can't speak for the authors in terms of intent, but the fact of the matter is that if it said "REQUIRED" there would be no compliant implementation today, which would forbid the standards track. This very point has actually been appealed recently, so regardless of the intent the latitude is necessary as of today. This an egg-and-chicken problem: there are a lot of legacy tunnels that use /127, and software developers don't want to enforce the /64 boundary because it would force legacy networks to renumber, which can't happen overnight so the likely result is that almost no one would use the updated software which is no good for a protocol under development such as IPv6. This is largely a user education problem, because the old reflexes of using a /30 or a /31 that are justified in v4 are not required anymore for v6. Michel.