John C Klensin wrote:
[RFC 3461, 3464, and 4409 references only informative]
its appearance in that particular SHOULD does not make the
reference normative -- one does not need to understand RFC 4409
in order to make a competent and conforming implementation of
Implementing 2821bis is one thing, but many readers might be
only interested in understanding how SMTP is supposed to work
today - admins, postmasters, users, and email-arch authors.
Here RFC xxxx about X says that users SHOULD do Y in RFC yyyy.
For that they need to read and understand RFC yyyy, otherwise
they could end up not doing Y without a good excuse.
In the case yyyy = 4409 it is also relevant for implementors,
if what they implement is all in one box, MX, MSA, and final
The cast that 3461 and 3464 are normative is a little
stronger but, if you read the relevant sentence, what is
it basically saying is that, if you find another standard
that is applicable to NDNs and the particular situation,
then you should use it.
The statement in 3.6.3 says "for example", but admittedly
I'm not aware of other "examples". RFC 3461  is also
needed in an ABNF comment:
| the "xtext" syntax  SHOULD be used.
That is as normative as it can get, implementors won't know
when and why they violate this SHOULD without first looking
in RFC 3461.
RFC 3463 appears in the "for example" SHOULD statement, but
also in another SHOULD about extended error codes:
| the system described in RFC 3463  SHOULD be used in
| preference to the invention of new codes.
That is normative for anybody considering new error codes.
Chapters 2.3.7 and 3.5.1 contain further pointers to RFC
3463. The RFC 3464  "for example" may be informative,
but RFC 3461  and RFC 3463  are IMO normative.
OTOH I don't see why RFC 1123 is "normative": It broke a
clear concept of responsibility in RFC 821, its attempt to
simplify RFC 974 wasn't too good after all, and 2821bis
replaces everything in the relevant RFC 1123 chapter 5.
IMO RFC 1123 belongs to the set of informative references
(974, 1047, 1652, 1869, 1870, 2821) obsoleted by 2821bis.