From owner-freebsd-net@FreeBSD.ORG Wed Jul 18 13:05:06 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 3CC3616A400 for ; Wed, 18 Jul 2007 13:05:06 +0000 (UTC) (envelope-from Stephen.Clark@seclark.us) Received: from smtpout06.prod.mesa1.secureserver.net (smtpout06-04.prod.mesa1.secureserver.net [64.202.165.227]) by mx1.freebsd.org (Postfix) with SMTP id 02C4413C494 for ; Wed, 18 Jul 2007 13:05:05 +0000 (UTC) (envelope-from Stephen.Clark@seclark.us) Received: (qmail 18243 invoked from network); 18 Jul 2007 13:05:05 -0000 Received: from unknown (24.144.77.243) by smtpout06-04.prod.mesa1.secureserver.net (64.202.165.227) with ESMTP; 18 Jul 2007 13:05:05 -0000 Message-ID: <469E0FFF.8070802@seclark.us> Date: Wed, 18 Jul 2007 09:05:03 -0400 From: Stephen Clark User-Agent: Mozilla/5.0 (X11; U; Linux 2.2.16-22smp i686; en-US; m18) Gecko/20010110 Netscape6/6.5 X-Accept-Language: en-us, en MIME-Version: 1.0 To: karels@karels.net References: <200707150237.l6F2bAgZ011098@redrock.karels.net> In-Reply-To: <200707150237.l6F2bAgZ011098@redrock.karels.net> Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Cc: freebsd-net@FreeBSD.org, Robert Watson , Julian Elischer , Bill Moran , Sten Daniel Soersdal 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: Stephen.Clark@seclark.us List-Id: Networking and TCP/IP with FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 18 Jul 2007 13:05:06 -0000 Mike Karels wrote: >>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 > > > So was any decision reached on this issue - will FreeBSD changed to accept a packet on an interface that is larger than the mtu on that interface? Steve -- "They that give up essential liberty to obtain temporary safety, deserve neither liberty nor safety." (Ben Franklin) "The course of history shows that as a government grows, liberty decreases." (Thomas Jefferson)