Re: Broadcast Packets Evil?

On Mon, 22 May 2006, in the Usenet newsgroup comp.os.linux.networking, in
article <GAmcg.885$vd6.720@xxxxxxxxxxxxxxxx>, Rick Jones wrote:

Given the difficulty of imagining 16.7 million hosts all on the same
side of a router I suspect that an issue with 16.7 million hosts
trying to respond to a broadcast ICMP echo request is moot.

Probably true - we're using /22s and that's bad enough. What I was trying
to do was show the _concept_ of the problem.

Is the response to a broadcast ping really considered a "storm" these
days? I thought that a broadcast storm was supposed to be
self-sustaining.

To a minor extent, it depends on the network topology. If you have 10
hosts on a switch, the bandwidth of the switch should sustain the connection,
and they'll just be a momentary spike in network traffic. If you have an
unswitched network, or it is large, then people start stepping on each other,
backing off, stepping on each other, backing off - Lather, Rinse, Repeat.

After the first TCP segment or so, there would be a bunch of RST's and
the attempt would get nowhere.

Exactly. That's why I asked the O/P what he was trying to accomplish.

And besides, that ass-u-me's that TCP even allows one to try to send to
a broadcast address in the first place.

Of course it does. The network stack only knows the broadcast address of
the local network. There is no reliable mechanism that identifies every
broadcast address for every network in the world. The router at the
destination LAN _may_ forward the packet locally (see RFC1812 Section
5.3.5 et seq.), or it may not. The source network, and intermediate routers
are not likely to know anything other than a destination IP. A ppp (or
other point-to-point) link has no concept of the peers network mask, though
that peers LAN side stack should know better. However nothing prohibits
the broadcast address as a destination - see RFC1122 Section 3.2.1 et seq.

Old guy
.

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