netdev - Re: [net-next-2.6 PATCH v6 4/7 RFC] TCPCT part 1d: define TCP cookie option, extend existing struct's

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Message-Id: <20091117.042250.28821014.davem@davemloft.net>
Date:	Tue, 17 Nov 2009 04:22:50 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	ilpo.jarvinen@...sinki.fi
Cc:	eric.dumazet@...il.com, william.allen.simpson@...il.com,
	netdev@...r.kernel.org, joe@...ches.com
Subject: Re: [net-next-2.6 PATCH v6 4/7 RFC] TCPCT part 1d: define TCP
 cookie option, extend existing struct's

From: "Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi>
Date: Tue, 17 Nov 2009 14:18:57 +0200 (EET)

> On Mon, 16 Nov 2009, David Miller wrote:
> 
>> From: Eric Dumazet <eric.dumazet@...il.com>
>> Date: Mon, 16 Nov 2009 23:26:04 +0100
>> 
>> > So adding DATA to SYN packets might be problematic for part of our tcp 
>> > stack. 
>> 
>> I can almost guarentee it won't work.  For one thing getting a SACK
>> response to a SYN+DATA packet will explode quite nicely for one thing.
> 
> Now I'm really lost??? How can you get SACKs for that in the first 
> place since they are either lost or delivered in unison???

Ideally, you're probably right.

However, it seems to me that the receiver can do whatever it likes
with it's receive queue when it's under memory pressure.

It can chop packets up, partially free bits, and then send a SACK
block back to you for the parts it tried to free.

If you'll recall, I wanted to put some tough restrictions into what is
allowed with SACK so that we could optimize things on the sender side.
But there was resistence and therefore we have to keep allowing all
kinds of silly situations, the one we're talking about here merely
being one of them :-)
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