Problems in datagram socket between 4.2 BSD

[Chris Torek]

> We have connected two 4.2BSD with serial I/O routines from Harvard.
> Operations using TCP(or stream socket) work well,
> but operations using UDP(or datagram socket) - talk, rwho, dsh -
> do not work.  If the size of data to receive is large,
> the local machine seems not to get the data from 
> the remote machine.  In case of the command rwho, 
> it works well only when the remote machine has up to nine users.
> It does not receive at all if more than nine users.

I don't know why you'd have problems with talk or dsh, but rwho uses
broadcast UDP packets, and these are not allowed to fragment.  We
weren't sure why this was so -- perhaps to reduce the load on anyone
receiving uninteresting broadcast packets -- but we went ahead and
removed the restriction, and things work fine.

The restriction is in sys/netinet/ip_output.c, where you will find
a set of lines looking something like this:

		/* don't allow broadcast messages to be fragmented */
		if (ip->ip_len > ifp->if_mtu) {
			error = EMSGSIZE;
			goto bad;
		}

If you simply remove this check, the kernel will merrily fragment
broadcast messages, and you can ``rwho'' with a practially infinite
number of users.  (Whether you can run anything else with as many
users is an entirely different question.)
-- 
(This line accidently left nonblank.)

In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (301) 454-7690
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