Re: limiting outbound packet size

• Date: Fri, 17 Jan 1997 07:26:34 PST
• From:
  Alex Stewart <riche@crl.com>
• Cc: moo-cows@parc.xerox.com
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• In-Reply-To: <Pine.SUN.3.95.970115161428.7792B-100000@ren.us.itd.umich.edu> from "Christopher Unkel" at Jan 15, 97 01:30:21 pm

Sigh.. I guess I should have checked my MOO-Cows mail a little earlier this
week..

> In short, there's probably no MOOserver-level fix.

This was the conclusion I had come to as well (I am the wone Dave was quoting
about the reason for DU's problems.  I have since come to the conclusion that
this is only part of the reason for our problems, but even solving just this
part would help a lot).

> However, you can probably reconfigure your ethernet interface with a
> smaller MTU to avoid the problem; there is probably an mtu option on the
> ifconfig command (I can't find a machine with matching OS version so I
> can't confirm this.) that could be used to keep your ethernet interface
> from sending large packets.  Doing so would hurt local-network performance
> but might solve your other problems. 

No, this won't help at all.. in fact it would just make things worse (Dave, if
you're listening, DON'T have SRI do this).  The problem here is that Dave
didn't adequately describe what's really going on which is causing the problem.

The problem is not due to the MOO's machine sending out ethernet packets which
are too large, the problem is what happens when it sends out _IP_ packets which
are too large (larger than the MTU of the network (1500)).  When an IP packet
which is larger than the MTU is sent, the IP layer breaks this packet down into
"fragments", causing one IP packet to be sent as multiple ethernet (or
whatever) frames.  The multiple network frames are then reassembled at the
other end of things into the original IP packet.

However, the problem we're experiencing appears to be at least partially due to
a router at our ISP (well, several routers, it appears) which are overburdened
and are having problems passing certain types of traffic without losing bits of
it.  These routers seem to be able to pass single-packet network frames ok, but
when they encounter an IP packet which has been fragmented into multiple
frames, it won't get through (or won't get through complete), and the
connection just stops while TCP keeps retrying (and keeps failing) to send it
through.

This is why I was attempting to reduce the size of the IP packets which DU was
sending to under 1500 (to keep the IP packet size under the MTU of the network,
and thus avoid fragmentation).  For obvious reasons, therefore, reducing the
MTU of the local ethernet adapter would only make the situation worse, as it
would cause even more packets to be fragmented and thus die.

BTW, it is also quite clear to me that the problems DU is experiencing are not
due to local OS bugs, but due to network congestion or router misconfiguration
at the ISP, and therefore applying various SunOS patches to the machine are
unlikely to change much (though, admittedly, they are a good thing to do in
general)

-R
-------------------------------------------------------------------------------
     Alex Stewart - riche@crl.com - Richelieu @ Diversity University MOO
                         http://www.crl.com/~riche
           "For the world is hollow, and I have touched the sky."

• Re: limiting outbound packet size
• From: Jay Carlson <nop@nop.com>

• Re: limiting outbound packet size
• From: Christopher Unkel <cunkel@us.itd.umich.edu>

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