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Re: Recipient is offline

2011-09-03 08:55:05

On 9/2/2011 8:43 AM, Tony Finch wrote:
There's also ODMR / ATRN - RFC 2645.


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.




[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

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