From owner-freebsd-net@FreeBSD.ORG Wed Jan 12 20:00:37 2005 Return-Path: Delivered-To: freebsd-net@freebsd.org Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125]) by hub.freebsd.org (Postfix) with ESMTP id E936116A4CF for ; Wed, 12 Jan 2005 20:00:37 +0000 (GMT) Received: from postal3.es.net (postal3.es.net [198.128.3.207]) by mx1.FreeBSD.org (Postfix) with ESMTP id A6AB343D45 for ; Wed, 12 Jan 2005 20:00:37 +0000 (GMT) (envelope-from oberman@es.net) Received: from ptavv.es.net ([198.128.4.29]) by postal3.es.net (Postal Node 3) with ESMTP (SSL) id IBA74465; Wed, 12 Jan 2005 12:00:37 -0800 Received: from ptavv (localhost [127.0.0.1]) by ptavv.es.net (Tachyon Server) with ESMTP id B058C5D07; Wed, 12 Jan 2005 12:00:36 -0800 (PST) To: JINMEI Tatuya / =?ISO-2022-JP?B?GyRCP0BMQEMjOkgbKEI=?= In-reply-to: Your message of "Wed, 12 Jan 2005 12:31:19 +0900." Date: Wed, 12 Jan 2005 12:00:36 -0800 From: "Kevin Oberman" Message-Id: <20050112200036.B058C5D07@ptavv.es.net> cc: freebsd-net@freebsd.org cc: nesg@es.net Subject: Re: IPv6 TCP transfers are hanging X-BeenThere: freebsd-net@freebsd.org X-Mailman-Version: 2.1.1 Precedence: list List-Id: Networking and TCP/IP with FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 12 Jan 2005 20:00:38 -0000 > Date: Wed, 12 Jan 2005 12:31:19 +0900 > From: JINMEI Tatuya / =?ISO-2022-JP?B?GyRCP0BMQEMjOkgbKEI=?= > > >>>>> On Tue, 11 Jan 2005 14:01:29 -0800, > >>>>> "Kevin Oberman" said: > > > I think I have found a problem with TCP when run over IPv6. > > I set my MSS for TCP to 1460 to allow a full 1500 byte MTU to be > > utilized on my systems. (Yes, I see that this does break some things > > like communicating via links where PMTUD is blocked and one or more > > links restrict MTU to some size less than 1500 bytes. > > > What I am specifically seeing is a packet being sent out with a TCP > > length of 1460. While this is fine for IPv4, it's too back for IPv6 and, > > as you might expect, the far end never receives this packet. > > Two questions to clarify things: > > 1. Which version of FreeBSD are you using? Shows up on 4.11-Release, 4-stable (about a week old), and 5.3-Stable (about a month old) > 2. How did you set the MSS? sysctl.conf More information. I lowered the value of mssdflt to 1400 on both systems and I am still seeing attempts to send 1460 byte packets. These, of course, never make it to the far side of the 1500 byte MTU link. I am now guessing that PMTUD is finding the MTU to be 1500 and calculating an MSS of 1460 event hough IPv6 requires 20 added bytes of header. Also, the problem seems to show up over ssh connections. I have never seen it over any other link, but I don't use many other things over IPv6. I have never had a problem with gkrellmd running over IPv6. (gkrellmd is the network daemon for the gkrellm system monitor.) Here is the packet that is failing: 08:20:12.680313 aaa.es.net.ssh > bbb.es.net.54854: . 10145:11573(1428) ack 31040 win 58548 [flowlabel 0x6ae53] (len 1460, hlim 64) I am also seeing a number of slightly shorter packets which work fine: 08:19:54.222301 bbb.es.net.54854 > aaa.es.net.ssh: . [tcp sum ok] 23616:25020(1404) ack 5073 win 32844 [flowlabel 0x19ed] (len 1436, hlim 64) I cannot confirm the problem occurring when the source is a V5 system. It is possible that only V4 systems are doing this. -- R. Kevin Oberman, Network Engineer Energy Sciences Network (ESnet) Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab) E-mail: oberman@es.net Phone: +1 510 486-8634