netdev - Re: Question about LRO/GRO and TCP acknowledgements

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Date:	Sun, 12 Jun 2011 11:07:12 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Joris van Rantwijk <joris@...isvr.nl>
Cc:	Ben Hutchings <bhutchings@...arflare.com>, netdev@...r.kernel.org
Subject: Re: Question about LRO/GRO and TCP acknowledgements

Le dimanche 12 juin 2011 à 09:51 +0200, Joris van Rantwijk a écrit :
> On 2011-06-12, Ben Hutchings <bhutchings@...arflare.com> wrote:
> > LRO implementations (and GRO) are expected to put the actual segment
> > size in skb_shared_info(skb)->gso_size on the aggregated skb.  TCP
> > will then use that rather than the aggregated payload size when
> > deciding whether to defer an ACK.
> 
> Thanks. I see that indeed gso_size is being used for MSS calculations
> instead of the total GRO size.
> 
> However, I'm not sure that this completely answers my question.
> I am not so much concerned about quick ACK vs delayed ACK.
> Instead, I'm looking at the total number of ACKs transmitted.
> The sender depends on the _number_ of ACKs to update its congestion
> window.
> 


> As far as I can see, current code will send just one ACK per coalesced
> GRO bundle, while the sender expects one ACK per two segments.
> 

One ACK carries an implicit ack for _all_ previous segments. If sender
only 'counts' ACKs, it is a bit dumb...


10:05:02.755146 IP 192.168.20.110.57736 > 192.168.20.108.53563: SWE 96444459:96444459(0) win 14600 <mss 1460,sackOK,timestamp 12174491 
0,nop,wscale 8>
10:05:02.755242 IP 192.168.20.108.53563 > 192.168.20.110.57736: SE 1849523184:1849523184(0) ack 96444460 win 14480 <mss 1460,sackOK,tim
estamp 15334585 12174491,nop,wscale 7>
10:05:02.755310 IP 192.168.20.110.57736 > 192.168.20.108.53563: . ack 1 win 58 <nop,nop,timestamp 12174491 15334585>
10:05:02.755369 IP 192.168.20.110.57736 > 192.168.20.108.53563: . 1:1449(1448) ack 1 win 58 <nop,nop,timestamp 12174491 15334585>
10:05:02.755417 IP 192.168.20.108.53563 > 192.168.20.110.57736: . ack 1449 win 136 <nop,nop,timestamp 15334585 12174491>
10:05:02.755428 IP 192.168.20.110.57736 > 192.168.20.108.53563: P 1449:8689(7240) ack 1 win 58 <nop,nop,timestamp 12174491 15334585>
10:05:02.755476 IP 192.168.20.108.53563 > 192.168.20.110.57736: . ack 8689 win 159 <nop,nop,timestamp 15334585 12174491>
10:05:02.755482 IP 192.168.20.110.57736 > 192.168.20.108.53563: . 8689:13033(4344) ack 1 win 58 <nop,nop,timestamp 12174491 15334585>
10:05:02.755529 IP 192.168.20.108.53563 > 192.168.20.110.57736: . ack 13033 win 181 <nop,nop,timestamp 15334585 12174491>
10:05:02.755535 IP 192.168.20.110.57736 > 192.168.20.108.53563: . 13033:14481(1448) ack 1 win 58 <nop,nop,timestamp 12174491 15334585>
10:05:02.755582 IP 192.168.20.108.53563 > 192.168.20.110.57736: . ack 14481 win 204 <nop,nop,timestamp 15334585 12174491>
10:05:02.755588 IP 192.168.20.110.57736 > 192.168.20.108.53563: P 14481:16385(1904) ack 1 win 58 <nop,nop,timestamp 12174491 15334585>
10:05:02.755635 IP 192.168.20.108.53563 > 192.168.20.110.57736: . ack 16385 win 227 <nop,nop,timestamp 15334585 12174491>
10:05:02.755641 IP 192.168.20.110.57736 > 192.168.20.108.53563: . 16385:23625(7240) ack 1 win 58 <nop,nop,timestamp 12174491 15334585>
10:05:02.755689 IP 192.168.20.108.53563 > 192.168.20.110.57736: . ack 23625 win 249 <nop,nop,timestamp 15334585 12174491>
10:05:02.755695 IP 192.168.20.110.57736 > 192.168.20.108.53563: P 23625:26521(2896) ack 1 win 58 <nop,nop,timestamp 12174491 15334585>
10:05:02.755742 IP 192.168.20.108.53563 > 192.168.20.110.57736: . ack 26521 win 272 <nop,nop,timestamp 15334585 12174491>
10:05:02.755750 IP 192.168.20.110.57736 > 192.168.20.108.53563: P 26521:33761(7240) ack 1 win 58 <nop,nop,timestamp 12174491 15334585>
10:05:02.755796 IP 192.168.20.108.53563 > 192.168.20.110.57736: . ack 33761 win 295 <nop,nop,timestamp 15334585 12174491>
10:05:02.755802 IP 192.168.20.110.57736 > 192.168.20.108.53563: . 33761:39553(5792) ack 1 win 58 <nop,nop,timestamp 12174491 15334585>
10:05:02.755849 IP 192.168.20.108.53563 > 192.168.20.110.57736: . ack 39553 win 259 <nop,nop,timestamp 15334585 12174491>



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