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

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Date:	Fri, 19 Dec 2008 16:46:44 +0300
From:	Alexey Kuznetsov <kuznet@....inr.ac.ru>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	Petr Tesarik <ptesarik@...e.cz>,
	Ilpo J??rvinen <ilpo.jarvinen@...sinki.fi>,
	davem@...emloft.net, netdev@...r.kernel.org
Subject: Re: [PATCH] tcp: make urg+gso work for real this time

Hello!

> Indeed they do have more quirks.  I only saw it after reading it
> again, but they always use snd_nxt instead of the current sequence
> number:

Their snd_nxt and our snd_nxt are different things.
Analog of our snd_nxt used to be called snd_max.

BSD snd_nxt points to current transmission head,
which is rewinded to snd_una when it starts to retransmit.

> 	if (SEQ_GT(tp->snd_up, tp->snd_nxt)) {
> 		u_int32_t urp = tp->snd_up - tp->snd_nxt;
> 		if (urp > IP_MAXPACKET)
> 			urp = IP_MAXPACKET;
> 		th->th_urp = htons((u_int16_t)urp);
> 		th->th_flags |= TH_URG;

Obviously, somewhere around this place they do th->th_seq = tp->snd_nxt.

BTW, that line of code which I sent was logically (not practically) wrong,
Dave spotted this instantly. :-) Logically correct way would be
to check against future snd_nxt which will be advanced
after the packet is submitted. Practically, it does not matter,
the difference is only for jumbo frames, which are inhibited
for tcp.

Alexey
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