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From: grog@lemis.de (Greg Lehey)
Organisation: LEMIS, Schellnhausen 2, 36325 Feldatal, Germany
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Subject: IP question: who should return ENOBUFS?
To: hackers@freebsd.org (FreeBSD Hackers)
Date: Thu, 20 Jun 1996 20:03:32 +0200 (MET DST)
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I've just spent a fair amount of time trying to figure out why our
ISDN software jams up if it can't establish a connection.  The
symptoms are that if you can't establish a connection before a certain
number of packets have been sent, the whole interface just returns:

=== root@freebie (/dev/ttyp0) /sys/netinet 11 -> ping 192.109.197.38
PING 192.109.197.38 (192.109.197.38): 56 data bytes
ping: sendto: No buffer space available
ping: wrote 192.109.197.38 64 chars, ret=-1
ping: sendto: No buffer space available

I found this strange, since in the interface code (b_isdnipi.c, line
474) we have the code:

	if (IF_QFULL(&ifp->if_snd))
	{
		if (ifp->if_flags & IFF_RUNNING)
			isdn_output(sc->sc_appl);
		IF_DROP(&ifp->if_snd);
		m_freem(m);
		splx(x);
		ifp->if_oerrors++;
		return (ENOBUFS);
	}

This will drop the first packet *and* not enqueue the last if the
queue is full.  So I set a breakpoint on the IF_DROP, and hey! nothing
happened.  After a bit more investigation, I found this code in
ip_output.c:

	/*
	 * Verify that we have any chance at all of being able to queue
	 *      the packet or packet fragments
	 */
	if ((ifp->if_snd.ifq_len + ip->ip_len / ifp->if_mtu + 1) >=
		ifp->if_snd.ifq_maxlen) {
			error = ENOBUFS;
			goto bad;
	}

I think this is bogus.  It's not present in the BSD/OS version, so I
assume it was added in FreeBSD.  The problem is, it gives you no
possibility of recovery: the queue is full and stays that way.  Can
anybody give me an idea of why it's there, when the interface is
perfectly capable of looking after itself?

Greg