On Tue, 5 Apr 2005, Bruce Lilly wrote:
Bruce's comments have been very carefully thought through, and I mostly
agree with what he says.
o Architecturally, all messages must pass through at least one
Gateway (which may be implemented in conjunction with an MTA, MDA,
or even MUA). [this is necessary to accomplish MIME
message/partial reassembly as detailed in RFC 1344]. As reassembly
(RFC 2046) is a rather complex process that involves message
modification, the operative unit is unquestionably a gateway [it
might be useful to differentiate different types of gateways,
though that is not done in the simple diagram above or in the
draft]
I was under the impression that message/partial reassembly is often
implemented by the rMUA rather than some transport-level entity between
the sender and recipient. There's no requirement for a gateway. I don't
think it's helpful to describe the MIME functionality in an MUA as
something to do with gatewaying because the MUA is not a transport-level
entity. It's only necessary for gateways to implement MIME in order for
them to properly handle the mismatch between Internet email and some other
email system, so the MIME handling is a secondary feature rather than
something fundamental to gateways.
o filtering operates at the Application layer rather than transport
layer
Why? I presume you mean filtering implemented by Sieve or some equivalent,
which can occur in the MDA or MUA. The boundary between transport and
application becomes rather blurry here, so I'm interested in where you
draw the line and why. Note also that this kind of filtering can be used
to implement aliasing and mailing list expansion, which you later say is
a transport level function implemented in the MDA (and I agree with this).
o filtering must occur after message reassembly (Again see RFC 1344
and see also http://www.kb.cert.org/vuls/id/836088 to see what can
happen when that architectural principle is ignored)
Not necessarily; this is only required if the filtering is performing some
content-sensitive security function, which is not usually the case for
sieve-style filters. I think it's important to distinguish between sieve
filtering (on behalf of users) and security filtering performed within the
MTS.
o aliasing and list expansion (RFCs 822, 2822)
Don't forget 1123 :-)
Tony.
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