Path: utzoo!attcan!uunet!lll-winken!lll-lcc!ames!pasteur!ucbvax!ulysses!smb
From: smb@ulysses.homer.nj.att.com (Steven Bellovin)
Newsgroups: comp.dcom.lans
Subject: Re: Can ethernet TCP/IP lock up?
Keywords: I don't know; someone says it can
Message-ID: <10303@ulysses.homer.nj.att.com>
Date: 20 May 88 00:44:37 GMT
References: <299@fedeva.UUCP> <21674@amdcad.AMD.COM> <21680@amdcad.AMD.COM>
Organization: AT&T Bell Laboratories, Murray Hill
Lines: 29

This afternoon, we had some sort of network lockup that could have been
a two-point failure.  Ulysses (a Sun-3/280) suddenly started muttering
``ie1: Ethernet jammed''.  The lights on the transceiver showed
continuous receive, as if someone were indeed talking continuously.
I've only seen something like this once before, and I deliberately
applied the technique I discovered accidentally last time:  I
unterminated the coax.  That caused the jabberer to see a collision,
and hence to shut up.  The network recovered immediately, and
everything was able to talk once again.

The particular net in question is a very difficult one to debug.  It's
our backbone, and consists of a very short segment of coax with lots of
repeaters to other segments.  Some of those repeaters and transceivers
are ancient; our segment may be connected via a 5 or 6 year-old 3Com
transceiver.  I have no idea which host was misbehaving; it could even
have been Ulysses, since the repeater may have isolated such a failing
segment.

All that, right on the heels of this discussion (and while a network
equipment sales rep was in my office, trying to sell me a network
management gizmo!) got me thinking.  There is in general *no way to
know* if a jabber-detect has failed -- there is no standard diagnostic
for it!  Thus, the second failure (of a controller) can happen at any
time in the future; the two don't have to be coincident.  (As has been
noted, some jabber circuits are redundant, but not necessarily all of
them.)


			--Steve Bellovin
			ulysses!smb, smb@ulysses.att.com