From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Brownell Subject: Re: 2.6.11-rc2 TCP ignores PMTU ICMP (Re: Linux 2.6.11-rc2) Date: Thu, 27 Jan 2005 11:28:48 -0800 Message-ID: <200501271128.48411.david-b@pacbell.net> References: <200501232251.42394.david-b@pacbell.net> <200501251054.37053.david-b@pacbell.net> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: David Ford , linux-kernel@vger.kernel.org, netdev@oss.sgi.com Return-path: To: Janos Farkas In-Reply-To: Content-Disposition: inline Sender: netdev-bounce@oss.sgi.com Errors-to: netdev-bounce@oss.sgi.com List-Id: netdev.vger.kernel.org On Thursday 27 January 2005 1:02 am, Janos Farkas wrote: > On 2005-01-25 at 10:54:36, David Brownell wrote: > > On Tuesday 25 January 2005 10:35 am, David Ford wrote: > > > PMTU bug -- or better said, bad firewall admin who blocks all ICMP. > > > > PMTU bug, sure -- but one that came late in RC2. Remember: same firewall > > in both cases, but only RC2 breaks. The ICMP packet has landed in > > the RC2 system, which ignores it. 2.6.10 handled it correctly... I > > suspect one of the TCP cleanups borked this. And in fact, I'm pretty sure RC1 handled it fine too. This failure appeared in a BK pull I did a couple days before RC2, and installing it clobbered a "more virgin" RC1 install; so it's hard to verify. It took a while to track the failure mode down to being PMTU related, but the only TCP-related changes I noticed in what I pulled sure seemed like cleanups (removing some "duplicated" state). > > My current workaround is "ifconfig eth0 mtu 1492" but that's not > > something I'd expect to keep. > > Indeed, I had to shuffle my machines around a bit to get a proof that > something is broken, but now I can confirm the above with a connection > to cvs.sourceforge.net: Thanks for confirming it wasn't just me ... I confess I'm a bit surprised more folk haven't reported this yet! Your symptoms are exactly like those I saw, just with a different mission-critical application: CVS, not SMTP. Did you happen to notice whether CVS pulls worked, when pushes (like this) failed? - Dave > ... > > (the PPPoE gateway notifies the host about the PMTU issue, one for each > segment) > > 08:05:45.265330 IP (tos 0xc0, ttl 128, id 17970, offset 0, flags [none], length: 576) 192.168.59.1 > 192.168.59.10: icmp 556: 66.35.250.207 unreachable - need to frag (mtu 1492) > 08:05:45.265453 IP (tos 0xc0, ttl 128, id 17971, offset 0, flags [none], length: 576) 192.168.59.1 > 192.168.59.10: icmp 556: 66.35.250.207 unreachable - need to frag (mtu 1492) > > (the source host sends the full frame again...?) > > 08:05:45.770331 IP (tos 0x0, ttl 64, id 21544, offset 0, flags [DF], length: 1500) 192.168.59.10.29612 > 66.35.250.207.2401: . [tcp sum ok] 499:1947(1448) ack 526 win 1728 > 08:05:45.770900 IP (tos 0xc0, ttl 128, id 17972, offset 0, flags [none], length: 576) 192.168.59.1 > 192.168.59.10: icmp 556: 66.35.250.207 unreachable - need to frag (mtu 1492) > > (and looping goes on..) > > 08:05:46.783148 IP (tos 0x0, ttl 64, id 21546, offset 0, flags [DF], length: 1500) 192.168.59.10.29612 > 66.35.250.207.2401: . [tcp sum ok] 499:1947(1448) ack 526 win 1728 > 08:05:46.783752 IP (tos 0xc0, ttl 128, id 17973, offset 0, flags [none], length: 576) 192.168.59.1 > 192.168.59.10: icmp 556: 66.35.250.207 unreachable - need to frag (mtu 1492) > 08:05:48.808861 IP (tos 0x0, ttl 64, id 21548, offset 0, flags [DF], length: 1500) 192.168.59.10.29612 > 66.35.250.207.2401: . [tcp sum ok] 499:1947(1448) ack 526 win 1728 > 08:05:48.809882 IP (tos 0xc0, ttl 128, id 17974, offset 0, flags [none], length: 576) 192.168.59.1 > 192.168.59.10: icmp 556: 66.35.250.207 unreachable - need to frag (mtu 1492) > > ... >