Re: Who uses directed broadcasts?


Charles Hedrick (aramis.rutgers.edu!athos.rutgers.edu!hedrick@rutgers.edu)
21 Mar 88 08:40:21 GMT


We use directed broadcasts for at least two purposes:

1) Our kinetics Appletalk gateways use them to keep track of each
other or of some hosts that provide services for them. (I'm a bit
vague, since I'm not an expert with Kinetics. I support the main IP
gateways, and I know I had to get directed broadcasts working properly
in order to satisfy our Mac guys.)

2) The cisco gateways that we use have a concept of helper address.
If a net doesn't have any servers on it, you can get a gateway to
forward all requests of a certain class (those that would normally be
used for booting: TFTP, bootp, timed, and named) to a specified
address. FOr reliability reasons, we don't to provide several
servers, so we use a subnet with lots of servers, and use directed
broadcast onto that subnet.

I have also done some experiments with sending messges warning of
gateway reboots etc. by doing rwall to a broadcast addresss for each
of the subnets affected. It seems to work.

In general, I'd say the directed broadcast mechanism is useful, and
that gateways should implement it. Note that the way you know whether
to absorb the packet yourself or forward it onto the subnet is by
whether it arrived as a physical broadcast or not. If it did, then it
has already been broadcast, and you should process it as a host. If
it arrives as a non-broadcast, then the sender wants you to put it out
on the subnet as a broadcast. This must be implemented properly, or
disaster will ensue.



This archive was generated by hypermail 2.0b3 on Thu Mar 09 2000 - 14:41:31 GMT