Date: Thu, 26 Mar 1998 17:37:52 -0800 From: David Greenman <dg@root.com> To: Open Systems Networking <opsys@mail.webspan.net> Cc: hackers@FreeBSD.ORG Subject: Re: mssdflt size 512? or 536? Message-ID: <199803270137.RAA28420@implode.root.com> In-Reply-To: Your message of "Thu, 26 Mar 1998 19:12:28 EST." <Pine.BSF.3.95.980326190840.20213A-100000@orion.webspan.net>
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>On Thu, 26 Mar 1998, David Greenman wrote:
>
>> The official non-local network mss default for implementations that
>> don't support Path MTU Discovery is 512. FreeBSD has PMTU Discovery,
>> however, so the default starts at the interface MTU and goes down from
>> there depending on what it learns from the network.
>
>Ok I can understand that. So the sysctl variable is misleading?
>Since my MTU is 1500, default MSS should be 1500 to start and then
>negotiate from 1500 down?
Yes, actually it starts at (1500-tcp/ip header size)=1460. It's even more
complicated than this; for inbound connections, it's actually the lower of
{offered mss, interface MTU}, with the tcp_mssdflt (the sysctl variable)
being used if the peer offers no mss in the SYN segment. ...and of course,
if there is a route (e.g., the peer has previously connected and a clone
host route is still cached), then that value is used instead of the interface
MTU. ...and if there is a route with the MTU 'lock' flag set, then that is
used as the minimum mss. So for instance if you wanted to disable Path
MTU Discovery, then you'd do something like this with the default route:
route add default 165.113.121.82 -lock -mtu 1500
...which is what I do on wcarchive since we get too many complaints
from people who have broken firewalls and don't know it.
-DG
David Greenman
Core-team/Principal Architect, The FreeBSD Project
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