Bruce,
Let me make a few observations, largely, in support of my
esteemed co-author...
--On Tuesday, 15 February, 2005 15:28 -0800 Randall Gellens
<randy(_at_)qualcomm(_dot_)com> wrote:
> Specifically regarding the 4.3 MUST quoted above and the
> reply code, and the necessary two independent
> implementations required for advancement to Draft
> Standard status, do the implementations supporting the
> request to advance to Draft in fact unconditionally
> require authorization (i.e. independent of whether port
> 25 or 587 is used, and regardless of administrative
> configuration other than specifying that the
> implementation is to act as an MSA), and reply with code
> 530 (unspecified extended response code) if
> authorization is lacking?
Both the draft and RFC 2476 allow submission to use 25
instead of 587, but state that "normally" 587 is used, and
allow 25 to be used in order to accommodate implementations
that are hard to configure. Both say that port 587 is
reserved for submission. So, 587 is the normal case for
submission, and 25 is an exceptional case.
But as written, section 4.3 specifies that when acting as an
MSA, authentication MUST (emphasis in original) be used,
regardless of port.
Section 4.3 says "unless it has already independently
established authentication or authorization (such as being
within a protected subnetwork)." It's not uncommon for
organizations to deploy MSAs that treat all internal
connections as authenticated, even if they allow travellers to
connect in using port 587 and in that case require
authentication.
Bruce, if I understand your concern (I may not), let's back up a
bit and look at a bit of history and the usage case. The notion
of a separate submission protocol arose in part out of a problem
that became clear when we were doing 2821. There were a whole
series of things that we would really prefer that MTAs not do
--things that violated the "don't mess with the message"
meta-principle for MTAs but were necessary because clients
weren't able to supply all of the information needed for a valid
2822/822 message, or weren't able to supply it in a form that
would be security-acceptable. So, to face the reality of the
fact that these MTA-fixes were being applied (and were
necessary), 2821 contains a loophole for submission servers to
fix messages up.
That loophole is, however, somewhat bad news, since it doesn't
involve explicit client authorization for message-tampering and
there is no real way to permit such authorization even if one
wanted to do it (similarities to some of the OPES issues here
are not coincidental). "Message Submission" ("Submit" below) is
obviously different: we allowed for extensions to be used with
Submit that are not specified for SMTP (even though no one has
done that -- the mechanism has been shown to work with
extensions that are also specified for SMTP) and the mere fact
of using the Submit protocol says "this is a submission agent,
not a general MTA, and hence gets to use those features".
Now the intent is that Submit be used on its own port, for all
of the obvious reasons. But (i) nothing really prevents it from
being run on _any_ port; such is the nature of IETF applications
protocols and (ii) in particular, it can, explicitly, be run on
Port 25. But, if it is run on port 25, the question
immediately arises as to how you tell it from SMTP, since the
two are deliberately quite similar in their handshakes. The
answer is "administrative or policy decision, reflected in
configurations". No other answer really makes sense. And, if
you get that far, then you need to ask the question of whether
one would want to permit a host whose port 25 is being used to
support Submit to also support SMTP. And the answer is that
the question is meaningless if you are not either using Submit
extensions that are not defined for SMTP (or vice versa) and a
really dumb idea if you are and feel like mixing the two and
distinguishing heuristically, halfway through the protocol
transaction. Such a concurrent implementation on the same port
would actually probably violate both specs, since it would have
to advertise extensions that were not defined for the protocol
that the client was expecting (not that doing so would make a
lot of difference).
So...
Again, "or not" is not allowed by the draft as written;
implementations with an "or not" provision would fail to
comply with the "MUST" (they would comply with a "SHOULD",
however).
My understanding is that implementations are permitted to have
configuration options that allow sites to deploy them in ways
that violate the RFCs, as long as they can be operated in a
conforming manner. For example, it is not uncommon for POP
implementations to have options to have timeouts shorter than
mandated, or to enter UPDATE state on abort.
This has always been the rule, at least since we decided that
specifications, to advance to draft, must exhibit implementation
of all of the features. We don't do code review to verify
conformance and spot possibly-non-conforming capabilities. The
rule is that the implementations must be able to demonstrate
that they can be configured in a way that conforms to the spec.
Nothing else counts as it is, by definition, outside the
specification.
...
From my understanding of the requirements, there are multiple
implementations which comply. This is based on two points:
(1) implementations can be compliant even if they have options
which permit non-conformance; and (2) there must be at least
two implementations for each feature, but it is not required
that any individual implementation meet every SHOULD. I
believe Message Submission meets the criteria on the basis of
either of these points; the fact that it meets it on both is
nice but not required.
Under other circumstances, I might actually quibble about (2),
but (1) is the important point.
> On another matter, admittedly unchanged since RFC 2476,
> there seem to be some undesirable discrepancies between
> submission and non-submission ESMTP regarding extended
> response codes. Draft section 3.4 states that extended
> status code 5.6.2 means "Bad domain or address", whereas
> RFC 3463 assigns that code the semantics "Conversion
> required and prohibited" [RFC 3463 section 3.7]. The
> > corresponding RFC 3463 extended response code for
> domain/address issues would be in the 5.1.XXX range [RFC
>...
To the extent to which this is an issue, it is an issue with RFC
3463 and not with this draft. A real problem would arise only
in the "both protocols, same machine, port 25" case described
above. But I agree with Randy -- the right action is to delete
the specific codes from this specification and leave that to
1893, 3463, and their successors.
> the draft should explicitly say so (the term "gateway"
> does not appear anywhere in the draft). [that would be a
> novel use of the term "gateway" in Internet mail; a
> gateway usually has one side in a non-SMTP environment]
Actually, we discovered in working on RFC 2821 that life wasn't
that easy. First, it has never been the definition, "usually"
or otherwise: at the time 821 was written, the concept of a
mail gateway applied when either one side was in a non-SMTP
environment or when one side was in a non-TCP environment,
whether SMTP was being used or not (or, indeed, when the two
sides didn't share underlying transports, whether either one was
TCP or not).
While I would prefer that "one side in non-SMTP or different
transport" definition, one runs into real-world problems when
the addressing structures differ, whether SMTP and TCP are on
both sides or not. For example, assume that one has a
submission client on a private network using 1918 space. It
submits the message to an MSA which lies at the NAT boundary
between that network and the public Internet. To meet the
requirements of 2821, the MSA (or submission MTA) should modify
or supplement addresses and internal names so that they reflect
public names and addresses. That moves it into gateway
territory, and 2821 was written to permit that case (doesn't
make that I, or the DRUMS WG, liked the case very much, but it
reflects reality).
> Conversely, if MSAs are not always to be considered as
> gateways, then returning errors in response to message
> content is: 1. explicitly counter to the SHOULD NOT of
RFC 2821 section 3.4 (bottom of
> page 18)
> 2. inappropriately associated with "conversion"
> semantics where no conversion is in fact required
> (indeed,
other than adding trace fields,
> tinkering with message data by
non-gateway SMTP receivers is disallowed
> [RFC 2821 section 2.3.8]).
It is not the intent of either this draft or RFC 2476 to
say that MSAs are always gateways; rather, the intent in
both is to recognize the reality that organizations
sometimes see a need to examine and potentially modify
messages submitted using their servers, and to make a clear
distinction between this case, and the
examination/modification of messages being relayed.
Bruce, you are assuming, I think, that every MSA must be
conformant to the MTA requirements of 2821 (gateway or not).
That was never intended to be the case. Indeed, that is part of
the reason why this is a separate protocol (i.e., the "clear
distinction" to which Randy refers to above).
best,
john
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