netdev - Re: [PATCH net-next V2] tcp: Fix bug when gap in rcv sequence is filled

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Date:	Sat, 7 Apr 2012 11:58:09 -0700
From:	Vijay Subramanian <subramanian.vijay@...il.com>
To:	Neal Cardwell <ncardwell@...gle.com>
Cc:	chetan loke <loke.chetan@...il.com>, netdev@...r.kernel.org,
	davem@...emloft.net, ilpo.jarvinen@...sinki.fi
Subject: Re: [PATCH net-next V2] tcp: Fix bug when gap in rcv sequence is filled

Thanks for the review Neal.

>
> If you have some packet traces documenting that there is actually a
> bug here, can you please share them?
>
No. My claim was based only on an audit of the code.

> AFAICT the code correctly generates immediate ACKs in these cases by:
>
> (a) having __tcp_ack_snd_check() call tcp_send_ack() to send an
> immediate ack if out_of_order_queue is non-empty.
>
> (b) having tcp_data_queue set icsk_ack.pingpong to 0 when
> out_of_order_queue transitions from non-empty to empty
>

I noted that  icsk_ack.pingpong is set to 0 in event (b) and also when
data is added to ofo queue
but I completely missed the check in __tcp_ack_snd_check().


>
> .798 B>A . 50001:60001(10000) ack 1001 win 20000
> .838 A>B . ack 60001 win 10000
> .848 B>A . 61001:62001(1000) ack 1001 win 20000
> .848 A>B . ack 60001 win 10000 <nop,nop,sack 1 {61001:62001}>
> .858 B>A . 63001:64001(1000) ack 1001 win 20000
> .858 A>B . ack 60001 win 10000 <nop,nop,sack 2 {63001:64001}{61001:62001}>
> .868 B>A . 60001:61001(1000) ack 1001 win 20000
> .868 A>B . ack 62001 win 10000 <nop,nop,sack 1 {63001:64001}>
>

For your trace it is clear acks are sent correctly and there is no bug.
I should have checked the behavior on the wire. Sorry for the noise.

Thanks,
Vijay
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