craig@NNSC.NSF.NET (Craig Partridge) (11/12/90)
There were a couple of questions about dial-up IP behavior that I'd like
to respond to (by the way, all this stuff was covered in a paper on dial-up
IP that Leo Lanzillo and I gave at the 1989 Winter USENIX).
Connection set up time:
Someone pointed out that many TCPs limit their open timeout to 30 seconds,
which means dialing a link has to be done in less than 30 seconds. This is
a known problem -- it turns out that you often find that dial-up IP connects
the link just as your SYN times out (recall that 30 seconds includes delays
before you get to the dial-up gateway, so even a dial-up IP that can connect,
in say, 15 seconds, often loses). However Host Requirements now require
TCPs to have a much longer timeout, so if your TCP still does 30 seconds,
beat up on your vendor.
SMTP:
There was a question about j-random host trying to SMTP to your system
which is behind a dial-up IP link. If the link can be brought up at any time,
then assuming your TCP is HRRFC conformant, no problem. However, some
sites limit the times their link can be brought up to minimize phone costs
(e.g. only at evening and weekend rates). In these situations, people use
MX records to forward their mail to a particular machine, and then have a
program that tickles the remote mailer to deliver to them. (This is NOT
done using TURN; rather the final destination machine telnets to a special
port, which causes the SMTP daemon to start up -- this avoids the security
problems of TURN).
Craig