From owner-freebsd-questions@FreeBSD.ORG Sun Jul 1 00:00:53 2007 Return-Path: X-Original-To: freebsd-questions@freebsd.org Delivered-To: freebsd-questions@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [69.147.83.52]) by hub.freebsd.org (Postfix) with ESMTP id 46ACA16A46C for ; Sun, 1 Jul 2007 00:00:53 +0000 (UTC) (envelope-from cpghost@cordula.ws) Received: from fw.farid-hajji.net (fw.farid-hajji.net [213.146.115.42]) by mx1.freebsd.org (Postfix) with ESMTP id CEC3713C487 for ; Sun, 1 Jul 2007 00:00:52 +0000 (UTC) (envelope-from cpghost@cordula.ws) Received: from epia-2.farid-hajji.net (epia-2 [192.168.254.11]) by fw.farid-hajji.net (Postfix) with ESMTP id 254B3DFB1D; Sun, 1 Jul 2007 02:00:56 +0200 (CEST) Message-ID: <4686EF2A.6000408@cordula.ws> Date: Sun, 01 Jul 2007 02:02:50 +0200 From: cpghost User-Agent: Thunderbird 2.0.0.4 (X11/20070616) MIME-Version: 1.0 To: Modulok References: <128191.89983.qm@web57414.mail.re1.yahoo.com> <64c038660706301605o7cdcd24fvb03af4167d5ca49@mail.gmail.com> In-Reply-To: <64c038660706301605o7cdcd24fvb03af4167d5ca49@mail.gmail.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: freebsd-questions@freebsd.org, chloe K Subject: Re: MTU question X-BeenThere: freebsd-questions@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: User questions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sun, 01 Jul 2007 00:00:53 -0000 Modulok wrote: > On 6/30/07, chloe K wrote: >> Hi all >> >> ls our network provider insists to set the mtu to 1600? but I can >> only set >> the freebsd as 1500. >> >> ls there any network issue? >> >> thank you > > A value of 1600 is not standard compliant. A value of 1500 is the > largest MTU for standard ethernet, as stated in RFC 894 - "A Standard > for the Transmission of IP Datagrams over Ethernet Networks", and RFC > 1191 - "Path MTU discovery." Perhaps your provider is confused? > > RFC 894: > "...the maximum length of an IP datagram sent over an Ethernet is 1500 > octets." That's correct, but... perhaps they're not using Ethernet? Some layer 2 technologies also support larger frames, e.g. 4k. However, any router can (and generally will) fragment an IP packet into many IP packets so that they fit into smaller MTUs (provided the don't fragment bit is not set). Those packets will then be routed independently of each other, and usually reassembled at the end point by the TCP stack of the receiving machine. > As far as overriding this to a higher value, you may be restricted in > doing so by your hardware, as stated in the ifconfig(8) manpage, > "...Not all interfaces support setting the MTU, and some interfaces > have range restrictions." > > -Modulok- There are also some other reasons to avoid jumbograms and stick to Ethernet's 1500 MTU (or less, as specified by the ISP, often if they tunnel their traffic to their upstream provider). -cpghost. -- Cordula's Web. http://www.cordula.ws/