Seems to me, given the above wording, 2460 says Path MTU is a SHOULD, not a MAY. Note that the MAY is about _not_ implementing it (in some situations), not a "MAY" implement it in some subset of the comment cases.
I.e, if node-requirements says MAY, I think that is a downgrade from the SHOULD in 2460 as quoted above. I don't think this document should be doing that.
I agree that the node requirements document should follow what 2460 says.
Taking a new look at a the text, it does indeed sound more like a "SHOULD normally implement but MAY not implement in some cases".
RECOMMENDED is a synonym for SHOULD, and upper/lower case usually (?) should not matter when interpreting protocol requirements.
Suggested node requirements text change:
4.3.1 Path MTU Discovery - RFC1981
Path MTU Discovery [RFC-1981] MAY be supported. It is expected that most implementations will indeed support this, although the possible exception cases are sufficient that the used of "SHOULD" is not justified. The rules in RFC 2460 MUST be followed for packet fragmentation and reassembly.
=>
4.3.1 Path MTU Discovery - RFC1981
Path MTU Discovery [RFC-1981] SHOULD be supported, though minimal implementations MAY choose to not support it and avoid large packets. The rules in RFC 2460 MUST be followed for packet fragmentation and reassembly.
--Jari
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