linux-kernel - RE: 2.6.20.7 mss negotiation and path mtu discovery mostly broken?

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Message-ID: <7CCD07160348804497EF29E9EA5560D7020F6206@exchtewks2.starentnetworks.com>
Date:	Wed, 25 Apr 2007 10:31:23 -0400
From:	"Ristuccia, Brian" <bristuccia@...rentnetworks.com>
To:	"Andi Kleen" <andi@...stfloor.org>
Cc:	<linux-kernel@...r.kernel.org>, <netdev@...r.kernel.org>
Subject: RE: 2.6.20.7 mss negotiation and path mtu discovery mostly broken?

> > 08:39:55.493029 IP 12.33.234.69.35026 > 10.2.10.254.22: S 
> > 2768979373:2768979373(
> > 0) win 5840 <mss 1460,sackOK,timestamp 3873837730 0,nop,wscale 2>
> > 08:39:55.493119 IP 10.2.10.254.22 > 12.33.234.69.35026: S
> > 963242385:963242385(0)
> >  ack 2768979374 win 17896 <mss 8960,sackOK,timestamp 413751
> 
> The MSS clamp for sending to 10.2.10.254.22 is 8960. MSS is 
> only one way -- each uses what the other tells it.
> 

Right - except that 10.2.10.254 keeps sending to 12.33.234.69 with an
increasingly large MSS, even though 12.33.234.69 has asked for no larger
than 1460.

> > In the following dump, the system eventually gets in a 
> state where it 
> > oscillates between sendng undeliverable 2896 byte packets and 
> > deliverable 1448 byte ones.
> 
> This should only happen on PMTU expire, which is normally ~15mins.
> Perhaps you misconfigured it manually using sysctl.
> 

This is /proc/sys/net/ipv4/route/mtu_expires? It's 600.

-- 
Brian Ristuccia

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