Path: utzoo!attcan!utgpu!news-server.csri.toronto.edu!mailrus!cs.utexas.edu!smoot
From: smoot@cs.utexas.edu (Smoot Carl-Mitchell)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Question on forwarding broadcasts
Message-ID: <13003@cs.utexas.edu>
Date: 29 Sep 90 15:25:42 GMT
References: <1990Sep28.143945.7440@ultra.com> <70667@sgi.sgi.com>
Organization: Texas Internet Consulting
Lines: 29

In article <70667@sgi.sgi.com> vjs@rhyolite.wpd.sgi.com (Vernon Schryver) writes:
>
>What about using any of the zillions of routing protocols instead
>of static, default routes?  Even RIP should work.
>
>I thought some people say directed broadcasts are or should be illegal.
>
>
>Vernon Schryver,  Silicon Graphics,   vjs@sgi.com

Routing has little to do with this problem.  It looks like a variation of
the famous broadcast storm problem inherent in 4.2BSD based TCP/IP
implementations.  The rule is a host which is not configured as a router
should *NEVER* forward an IP packet which has a different destination
address.  4.2BSD has the unfortunate feature of having IP forwarding turned on
for any host.  This has since been fixed in 4.3BSD.

Obviously if more than one host forwards such a misdirected broadcast then
network meltdown can occur very rapidly.  An amusing variation on this
problem is to get a packet sniffer and send an ICMP echo request packet
using the Ethernet and IP broadcast address at about 0.5 sec intervals and
watch the growth in ICMP traffic as all the hosts on the ethernet reply to
this request all at once.  Needless to say on a CSMA/CD network the collison
rate also goes thru the roof.  The corollary rule is never build an application
which sends a broadcast packet which expects more than one host to reply to
the broadcast.
-- 
Smoot Carl-Mitchell, Texas Internet Consulting
smoot@tic.com, smoot@cs.utexas.edu