From owner-freebsd-hackers Thu Mar 26 17:38:29 1998 Return-Path: Received: (from majordom@localhost) by hub.freebsd.org (8.8.8/8.8.8) id RAA01550 for freebsd-hackers-outgoing; Thu, 26 Mar 1998 17:38:29 -0800 (PST) (envelope-from owner-freebsd-hackers@FreeBSD.ORG) Received: from implode.root.com (implode.root.com [198.145.90.17]) by hub.freebsd.org (8.8.8/8.8.8) with ESMTP id RAA01453 for ; Thu, 26 Mar 1998 17:38:17 -0800 (PST) (envelope-from root@implode.root.com) Received: from implode.root.com (localhost [127.0.0.1]) by implode.root.com (8.8.5/8.8.5) with ESMTP id RAA28420; Thu, 26 Mar 1998 17:37:52 -0800 (PST) Message-Id: <199803270137.RAA28420@implode.root.com> To: Open Systems Networking cc: hackers@FreeBSD.ORG Subject: Re: mssdflt size 512? or 536? In-reply-to: Your message of "Thu, 26 Mar 1998 19:12:28 EST." From: David Greenman Reply-To: dg@root.com Date: Thu, 26 Mar 1998 17:37:52 -0800 Sender: owner-freebsd-hackers@FreeBSD.ORG Precedence: bulk X-Loop: FreeBSD.ORG (re-cc'd to hackers - hope that's okay) >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 To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message