John C Klensin wrote:
I'd make a further distinction and claim that even many email
gateways don't rise to the level (or sink to the depths) of what
we usually think about as ALGs. First of all, I would contend
that an SMTP system that accepts an SMTP-and-MIME conformant
message that arrives over an IPv6-based TCP connection and that
is sent back out, using an unchanged address, over an IPv4-based
TCP connection is performing a standard relay function, not even
a mail gateway one (I think that is consistent with Dave's
comment above).
Entirely consistent.
Gateways do transformations at their own level of the stack. Anything at a
lower layer is essentially transparent, given the basic relaying task.
Simply put, if it isn't transforming the rfc2822 object, it isn't a gateway.
But even a mail gateway that, e.g., needs to do
some rewriting of addresses to get from the systems on one side
to the systems on the other still isn't in the ALG business as
I, at least, usually understand it. The transformations are
Address translation, format translation, possibly other mappings. But yeah,
addresses seem to be the biggies.
mechanical, they don't require any external information, and
they don't inherently depend on the underlying transport
technology. So I would suggest that some, but not all, email
gateways are ALGs... and that no relay is.
mumble. probably. mumble.
ps. No one will be surprised that I've documented this issue
in the email
oops. didn't add the specific reference:
I meant the Internet Mail Architecture draft, of course:
<http://bbiw.net/specifications/draft-crocker-email-arch-08.html>
On the other hand, they might be surprised that we seem to be
agreeing <grin>
Particularly given that it is about email technology...
d/
--
Dave Crocker
Brandenburg InternetWorking
bbiw.net
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