Date: Sun, 03 Aug 2008 14:16:54 -0700 From: perryh@pluto.rain.com To: nvass@teledomenet.gr, derek@computinginnovations.com, keramida@ceid.upatras.gr Cc: freebsd-questions@freebsd.org Subject: Re: setting the other end's TCP segment size Message-ID: <48962046.334w0KWDk7nStfQ/%perryh@pluto.rain.com> In-Reply-To: <200807311027.37878.nvass@teledomenet.gr> References: <488fe865.x7NyNic2A5pcZPCL%perryh@pluto.rain.com> <6.0.0.22.2.20080730155021.024dd828@mail.computinginnovations.com> <87abfzxbbu.fsf@kobe.laptop> <200807311027.37878.nvass@teledomenet.gr>
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> > >> Is there a simple way for a FreeBSD system to cause its > > >> peer to use a transmit segment size of, say, 640 bytes -- > > >> so that the peer will never try to send a packet larger > > >> than that? > > >> > > >> I'm trying to get around a network packet-size problem. > > >> In case it matters, the other end is SunOS 4.1.1 on a > > >> sun3, and I've been unable to find a way to limit its > > >> packet size directly. ... > > > Each tcp conversation can have it's own size set along > > > with a bunch of other params. > > > > Good point. The TCP_MAXSEG can reduce the maximum segment > > size for a single TCP connection to something smaller than > > the interface MTU :) That would be OK, provided I could somehow arrange for it to apply to all conversations with this particular destination (which is what the next item seems to do :) > Just adding that MTU can be set per destination with the help > of route(8) and the -mtu modifier. That would be better than setting the local mtu -- which has been causing other problems although it takes care of the original -- and it is a better match to the physical situation. (The culprit is neither the Sun nor the FreeBSD system, but the physical link between the Sun and the hub.) What I haven't been able to come up with is a way of making such a setting permanent. If I've communicated with the Sun recently enough, "netstat -r -W" reports a line like this (some spaces removed, for length, and I've no longer got xl0's mtu set low) Destination Gateway Flags Refs Use Mtu Netif Expire 192.168.200.3 08:00:20:00:a7:a6 UHLW 1 34 1500 xl0 1184 Now if I do # route change 192.168.200.3 -lock -mtu 640 the mtu column changes to 640 and it works fine, but only until the routing entry expires. Adding -static makes no difference -- the entry still expires and loses the mtu specification. I've been unable to come up with a route command that will *create* an entry like that (vs modifying an existing one), nor that will transform a transient entry into a permanent one.
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