netdev - Re: [PATCH 0/9]: tcp-2.6 patchset

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Date:	Tue, 29 May 2007 14:15:27 -0700
From:	Stephen Hemminger <shemminger@...ux-foundation.org>
To:	"Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi>"@smtp.osdl.org
Cc:	David Miller <davem@...emloft.net>, Baruch Even <baruch@...en.org>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH 0/9]: tcp-2.6 patchset

On Tue, 29 May 2007 23:58:39 +0300 (EEST)
"Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi> wrote:

> On Tue, 29 May 2007, Stephen Hemminger wrote:
> 
> > On Tue, 29 May 2007 23:07:00 +0300 (EEST)
> > "Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi> wrote:
> > 
> > > On Tue, 29 May 2007, Stephen Hemminger wrote:
> > > 
> > > > Since we don't invoke congestion control modules until after the SYN
> > > > handshake this is not a problem.
> > > 
> > > Just curious, do you mean that cc modules cannot measure, e.g., initial
> > > RTT through this mechanism (though they could do that in init() cb then
> > > I suppose)... Or do you mean that they are called already for the ACK
> > > that completes the SYN handshake and therefore its skb is being cleaned 
> > > from the queue right now (this is the case I above refer to)? 
> > > In the first case the decrementer code is NOP. If the latter, then it
> > > is just interface specification question, i.e., if SYNs are treated as 
> > > zero or one in num_acked for the pkts_acked callback (I have no opinion
> > > on this but was just trying to make sure cc modules get what they
> > > expect :-)). 
> > 
> > We don't switch a socket out of Reno until after the initial handshake.
> 
> ...It's still not very clear to me what exactly your "after" means (both 
> here and in your earlier description), i.e., whether clean_rtx_queue call 
> that cleans SYN skb from the queue happens before or after the switch out 
> of reno or not... If I understand the code correctly, this specific 
> clean_rtx_queue call happens after "your after" but I could be 
> misunderstanding the current 3-way handshake code. :-)

The call to clean_rtx_queue and congestion control routine happens here:
    tcp_ack
       tcp_clean_rtx_queue
          cong->pkts_acked

Setup of congestion control happens here:
    tcp_ack
    tcp_rcv_syn_sent_state_process
      tcp_init_congestion_control

or here:
    tcp_ack
    tcp_rcv_state_process (case ack of TCP_SYN_RECV)
       tcp_init_congestion_control
     
Another benefit of this, is that when tcp_init_congestion_control is called
things like the srtt and initial sequence number have already been setup.

> > As an interface, it makes sense to keep the API with the SYN counting
> > as a packet.
> 
> Ok, this one answers the remaining question concerning the patch, here
> is it without the decrementer for SYN case (which IMHO wasn't very 
> beautiful looking anyway :-)).
> 
> Dave, please consider this to net-2.6. It could be a stable candidate 
> as well, haven't tested yet if it applies cleanly to stable:


-- 
Stephen Hemminger <shemminger@...ux-foundation.org>
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