Path: utzoo!attcan!uunet!cs.utexas.edu!tut.cis.ohio-state.edu!ucbvax!SIRIUS.CC.UTEXAS.EDU!spurgeon
From: spurgeon@SIRIUS.CC.UTEXAS.EDU (Charles Spurgeon)
Newsgroups: comp.protocols.tcp-ip
Subject: Small TTL values evaporate in large networks
Message-ID: <8905241815.AA15718@sirius.cc.utexas.edu>
Date: 24 May 89 18:15:44 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 17

Here's one for the ``Netstoppers Notebook.''

We've recently resolved a number of mail failures at UTexas by
noticing that the TCP in older UNIX boxes (BSD 4.2, SunOS 3.2) was
setting the TTL to 15 in outgoing packets.

The network path to some sites at MIT consists of 17 hops from Austin
and the TTL was expiring before the packet got to the destination
address!  This kept the mail from getting through.  If you notice
consistent mail failures to a distant address you might want to check
on the outgoing TTL.  It appears that BSD 4.3 sets the TTL to 30 in
outgoing TCP.

While cognizant of the need to keep TCP segment lifetimes on a short
chain one does wonder what a good value for the TCP TTL might be?
With hop counts of around 20 already being seen, the new ``standard''
of 30 doesn't inspire confidence...