netdev - Re: [PATCH] tcp: make urg+gso work for real this time

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Message-Id: <20081225.171146.208898156.davem@davemloft.net>
Date:	Thu, 25 Dec 2008 17:11:46 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	ilpo.jarvinen@...sinki.fi
Cc:	herbert@...dor.apana.org.au, kuznet@....inr.ac.ru,
	ptesarik@...e.cz, netdev@...r.kernel.org
Subject: Re: [PATCH] tcp: make urg+gso work for real this time

From: "Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi>
Date: Thu, 18 Dec 2008 21:45:13 +0200 (EET)

> No, would you use that 0xffff hack with gso you would create even worse 
> problem in case of reordering / losses. And that's just because the very 
> scenario still remains fully unsolved. I'm not sure if you understood my 
> scenario at all (I was hopeful that you did :-))? It's basically that 
> (with the terms of this hack) some segments in a super-skb would need to 
> set 0xffff while some < 0xffff. ...But you just can't send too large urg 
> ptr in any segment or you're asking for trouble.

Hmmm, since URG is relative it means that GSO (or in-hardware TSO)
would need to adjust the URG offset for each segment.

But if we stick this 0xffff thing there, it _can't_ do that properly
because it cannot know where the correct URG pointer actually is.

In fact, the straightforward URG offset adjust one would expect a TSO
engine to make would corrupt the URG pointer when we put this
"infinity" value there.

The URG pointer might be >= 0xffff from the initial sequence number
of the starting TSO segment, but the URG offset might be in range
of one of the non-initial MSS segmented frames.

Anyways, we skip TSO/GSO for any time URG is indicated making all of
this particular concern moot. ;-)  But if at some point we want to
allow it, we'd need to consider this problem.

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