From owner-freebsd-net@FreeBSD.ORG Sun Jul 15 02:40:24 2007 Return-Path: X-Original-To: freebsd-net@FreeBSD.org Delivered-To: freebsd-net@FreeBSD.org Received: from mx1.freebsd.org (mx1.freebsd.org [69.147.83.52]) by hub.freebsd.org (Postfix) with ESMTP id 8187516A400; Sun, 15 Jul 2007 02:40:24 +0000 (UTC) (envelope-from karels@redrock.karels.net) Received: from redrock.karels.net (redrock.karels.net [206.196.45.2]) by mx1.freebsd.org (Postfix) with ESMTP id 355A413C471; Sun, 15 Jul 2007 02:40:23 +0000 (UTC) (envelope-from karels@redrock.karels.net) Received: from redrock.karels.net (localhost.karels.net [127.0.0.1]) by redrock.karels.net (8.13.8/8.13.6) with ESMTP id l6F2bAgZ011098; Sat, 14 Jul 2007 21:37:10 -0500 (CDT) (envelope-from karels@redrock.karels.net) Message-Id: <200707150237.l6F2bAgZ011098@redrock.karels.net> To: Robert Watson From: Mike Karels In-reply-to: Your message of Sun, 15 Jul 2007 00:46:27 +0100. <20070715003156.B94899@fledge.watson.org> Date: Sat, 14 Jul 2007 21:37:10 -0500 Sender: karels@karels.net Cc: Stephen.Clark@seclark.us, Sten Daniel Soersdal , Julian Elischer , Bill Moran , freebsd-net@FreeBSD.org Subject: Re: 6.2 mtu now limits size of incomming packet X-BeenThere: freebsd-net@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list Reply-To: karels@karels.net List-Id: Networking and TCP/IP with FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sun, 15 Jul 2007 02:40:24 -0000 > A related change that should probably be discussed if we want to think more > about asymmetry in maximum transmission unit is this one: > ---------------------------- > revision 1.98 > date: 2006/06/26 17:54:53; author: andre; state: Exp; lines: +2 -0 > In syncache_respond() do not reply with a MSS that is larger than what > the peer announced to us but make it at least tcp_minmss in size. > Sponsored by: TCP/IP Optimization Fundraise 2005 > ---------------------------- > In this change, we cap the advertised MSS in SYN/ACK to the received > advertised MSS, which presumably avoids an extra PMTU round trip if jumbograms > are enabled on the receiving endpoint. However, it also prevents use of > larger packet sizes if asymmetric MTU is supported. I think I suggested after > this was committed that we at least add an administrative twiddle to > enable/disable this mode of operation, but don't see one in there currently. > Does the Secure Computing scenario use TCP in this way, and is the potential > win in avoiding a PMTU round-trip worth disallowing asymmetric MSS at the TCP > layer? In our case, TCP isn't aware of the MRU, and bases its MSS on the MTU values. However, I don't see any reason for TCP to cap the MSS at the received MSS. If the other end doesn't want to receive more than 1024 bytes, that's no reason to refuse to accept more. Mike