On 9/2/2011 8:43 AM, Tony Finch wrote:
There's also ODMR / ATRN - RFC 2645.
Right.
The model for the Internet's email infrastructure is always-connected,
supporting an active push to the recipient's machine at the sender's initiative.
Unavailability is supported in terms of unplanned outages, not basic
connectivity limitations such as being connected only part of the time.
uucp-based Usenet was a very early example of a service designed for
partial-connectivity. CSNet[1][2] served as the first example within the
Internet (Arpanet) mail model, with remote sites dialing up when they could
afford to.
SMTP was developed shortly after this and we included efforts to get a "give me
mail that you are holding for me" mechanism to work[3]: The
periodically-connected site signals that it is now available to receive waiting
mail. Like the sequence that produced the MX record, finding the right design
for the TURN mechanism took a few iterations. In this case, getting the
security details right was the major challenge. Although the first effort was
deprecated, later efforts are still standards-track. (I don't know why RFC 5321
fails to mention ATRN, since it as an enhancement over ETRN.)
Here's the essential bit: Occasional connectivity fits into the Internet mail
model only as a last-hop, receive-side issue. There must be an always-connected
server working on behalf of the sometimes-connected receiver and holding mail
for it. Hence, use of ODMR is a 'local' matter. Do not expect random, remote
servers to participate.
d/
[1] http://isoc.org/wp/newsletter/?p=1098
[2] http://bbiw.net/recent.html#PostelCSNet
[3] It was interesting presenting the SMTP "working group" with the challenge
of this operational style, since it was entirely foreign to the existing world
of the Arpanet. Folks were not all that receptive to it until Vint Cerf, then
at Arpa and always looking for ways to justify funding worthy projects, suddenly
said "submarines!" At that point, everybody grok'd the operational model that
leaves availability for delivery as a choice of the receiver.
--
Dave Crocker
Brandenburg InternetWorking
bbiw.net