Newsgroups: comp.protocols.tcp-ip
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: Dial up access to Internet facilities
Message-ID: <1990Jun3.020001.11225@utzoo.uucp>
Organization: U of Toronto Zoology
References: <1990May25.163528.14300@ameristar> <9005270423.AA19852@psi.com> <1990May29.191125.9800@portia.Stanford.EDU> <36901@think.Think.COM>
Date: Sun, 3 Jun 90 02:00:01 GMT

In article <36901@think.Think.COM> barmar@nugodot.think.com (Barry Margolin) writes:
>>Indeed, SMTP's assumption that everybody's connected all the time
>>doesn't work well with occasionally-connected hosts.  It would seem
>>that the time is ripe for some sort of extension to SMTP...
>
>No extension is needed: see the TURN command.  However, most SMTP
>implementation don't enable this command for security reasons...

TURN is the wrong tool for the job.  What is needed is the equivalent of
UUCP's callback facility:  A connects to B, says "call me", and hangs up.
B then makes the call -- which eliminates the authentication issue -- and
transfers the traffic.

People interested in infrequently-connected hosts would do well to study
how UUCP software and administrators deal with the problems.  There is
a large body of relevant experience there.

>However, I'm not sure why you need any of this.  What's wrong with the
>MX'ing host periodically trying to connect to the occasionally-connected
>hosts? [which is the way it already works] ...

It has the standard problem of any polling scheme:  frequent attempts eat
resources, infrequent attempts can miss "hitting the window" for a site
which is only connected occasionally.  As with most polling, the real
answer is to have the other end *tell* you "now would be a good time".

SMTP's approach assumes hosts that are on the network more or less
continuously, with partitioning and/or downtime a relatively rare and
transient phenomenon that can be dealt with in simplistic ways.  When
the connection, as opposed to the absence thereof, is rare and transient,
more sophisticated tactics are needed.
-- 
As a user I'll take speed over|     Henry Spencer at U of Toronto Zoology
features any day. -A.Tanenbaum| uunet!attcan!utzoo!henry henry@zoo.toronto.edu