TURN and disconnected SMTP with dynamic IP addresses

["Jeff Stephenson" <jeffstep@xxxxxxxxxxxxx>]

There has been a small amount of discussion on the Disconnected SMTP list
(archive at <http://www.imc.org/ietf-disconn-smtp/>) of how best to dequeue
mail over a dial-up connection with dynamic IP addresses (a situation which
is common with many ISPs).  The ETRN extension works in the case of static
IP addresses, but encounters several problems when dynamic IP addresses are
involved:

1) Dynamic DNS is required in order to give ETRN an A or MX record for the
appropriate domain.
2) Because of latency involved in dynamic DNS, it's possible for ETRN to
attempt to dequeue mail before the DNS updates have been made.  This can
lead to all mail in the queue being NDRd when the server doesn't find the
appropriate records.
3) Another latency problem can occur when the dial-in host issues ETRN and
disconnects.  If another dial-in host gets the IP address which had been
allocated to the first one before DNS has been updated, it's possible for
the server to deliver mail queued for the first host to the second.

The obvious solution to this problem is the now-deprecated TURN command
coupled with client authentication to the server.  In the past, it made
sense not to support TURN because of the security concerns associated with
it.  With client authentication on the horizon, though, those concerns can
now be addressed.  I propose that we move the TURN command out of the
deprecated section of the DRUMS draft and only deprecate its use in the
absence of knowledge of the identity of the client.

<Follow-up should be to the DRUMS list, not the Disconnected SMTP list.>

-- jeff