netdev - Re: [PATCH][BUG] tcp: advertise MSS requested by user

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Message-Id: <20080828.024617.117112334.davem@davemloft.net>
Date:	Thu, 28 Aug 2008 02:46:17 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	virtualphtn@...il.com
Cc:	netdev@...r.kernel.org, lachlan.andrew@...il.com
Subject: Re: [PATCH][BUG] tcp: advertise MSS requested by user

From: Tom Quetchenbach <virtualphtn@...il.com>
Date: Tue, 26 Aug 2008 11:11:52 -0700

> I'm trying to use the TCP_MAXSEG option to setsockopt() to set the MSS
> for both sides of a bidirectional connection.
> 
> man tcp says: "If this option is set before connection establishment, it
> also changes the MSS value announced to the other end in the initial
> packet."
> 
> However, the kernel only uses the MTU/route cache to set the advertised
> MSS. That means if I set the MSS to, say, 500 before calling connect(),
> I will send at most 500-byte packets, but I will still receive 1500-byte
> packets in reply.
> 
> This is a bug, either in the kernel or the documentation.

I think this got broken by accident several years ago, shows how many
people use this feature :)

We used to use ->mss_clamp to determine the MSS option value,
but along the way when all of the code using ->advmss et al.
was added it didn't get translated correctly.

Thanks for your patch, I'll mull over this and put in the fix
once I understand exactly how this got broken.
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