From owner-freebsd-hackers Thu Jan 30 14:43:29 2003 Delivered-To: freebsd-hackers@freebsd.org Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125]) by hub.freebsd.org (Postfix) with ESMTP id BACE637B401 for ; Thu, 30 Jan 2003 14:43:27 -0800 (PST) Received: from puffin.mail.pas.earthlink.net (puffin.mail.pas.earthlink.net [207.217.120.139]) by mx1.FreeBSD.org (Postfix) with ESMTP id EEF0243F9B for ; Thu, 30 Jan 2003 14:43:26 -0800 (PST) (envelope-from tlambert2@mindspring.com) Received: from pool0073.cvx22-bradley.dialup.earthlink.net ([209.179.198.73] helo=mindspring.com) by puffin.mail.pas.earthlink.net with asmtp (SSLv3:RC4-MD5:128) (Exim 3.33 #1) id 18eNOv-0000qC-00; Thu, 30 Jan 2003 14:43:18 -0800 Message-ID: <3E39AA2D.FCD12EC6@mindspring.com> Date: Thu, 30 Jan 2003 14:41:49 -0800 From: Terry Lambert X-Mailer: Mozilla 4.79 [en] (Win98; U) X-Accept-Language: en MIME-Version: 1.0 To: Daniel Ellard Cc: freebsd-hackers@freebsd.org, Audsin Subject: Re: Changing the Maximum Segment Size (MSS) of Kame MIP6 Free BSD4.4 References: Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit X-ELNK-Trace: b1a02af9316fbb217a47c185c03b154d40683398e744b8a4ad621738ff9386f286c9c773bfa224c7350badd9bab72f9c350badd9bab72f9c350badd9bab72f9c Sender: owner-freebsd-hackers@FreeBSD.ORG Precedence: bulk List-ID: List-Archive: (Web Archive) List-Help: (List Instructions) List-Subscribe: List-Unsubscribe: X-Loop: FreeBSD.ORG Daniel Ellard wrote: > > man ifconfig > > /mtu > > The original question asked about the TCP MSS, not the MTU. Looking > at ifconfig isn't going to help. Actually, the original question is about how to cause the creation of fragments, for the purposes of testing. The MSS question is kind of based on a loose assumption about where baby frags come from, which isn't quite correct. > Note that the actual MSS is negotiated; if both ends can't support the > same value, the smaller is chosen. This is the problem. The MSS can be negotiated up to the MTU size from the default, or down, to match an intermediate hop. The only thing changing the default will do for you is increase the negotiation time. > For MTUs, in case that's really what you meant, it's even more strict > and depending on how the transport layer is implemented, it may be > impossible (or reckless) to increase the MTU. For example, on > ethernet many switches simply do not support an MTU larger than 1500. This is the one thing he *can* hard-code, via ifconfig. MSS is not hard-codeable. There's an outside chance that he wanted the MTU larger (e.g. 9k for "jumbograms" on gigabit hardware). Actually, hardware MSS negotiation doesn't work between some gigabit cards, and has to be set manually on both ends to get the higher number. But since he was talking about fargs vs. no frags, it's a good bet that what he wanted was: MTU X MTU X/3 host1 <----------------> router <----------------> host 2 So that frags are created when transmitting data from host1 to host2. -- Terry To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message