netdev - Re: [PATCH net-next v2] tcp: avoid reducing cwnd when ACK+DSACK is received

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Message-ID: <1420730396.5947.55.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Thu, 08 Jan 2015 07:19:56 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Sébastien Barré <sebastien.barre@...ouvain.be>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	Gregory Detal <gregory.detal@...ouvain.be>,
	Nandita Dukkipati <nanditad@...gle.com>,
	Yuchung Cheng <ycheng@...gle.com>,
	Neal Cardwell <ncardwell@...gle.com>
Subject: Re: [PATCH net-next v2] tcp: avoid reducing cwnd when ACK+DSACK is
 received

On Thu, 2015-01-08 at 13:20 +0100, Sébastien Barré wrote:
> When the peer has delayed ack enabled, it may reply to a probe with an
> ACK+D-SACK, with ack value set to tlp_high_seq. In the current code,
> such ACK+DSACK will be missed and only at next, higher ack will the TLP
> episode be considered done. Since the DSACK is not present anymore,
> this will cost a cwnd reduction.
> 
...
>  net/ipv4/tcp_input.c | 30 +++++++++++++-----------------
>  1 file changed, 13 insertions(+), 17 deletions(-)
> 
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 075ab4d..cf63a29 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -3363,29 +3363,25 @@ static void tcp_replace_ts_recent(struct tcp_sock *tp, u32 seq)
>  static void tcp_process_tlp_ack(struct sock *sk, u32 ack, int flag)
>  {
>  	struct tcp_sock *tp = tcp_sk(sk);
> -	bool is_tlp_dupack = (ack == tp->tlp_high_seq) &&
> -			     !(flag & (FLAG_SND_UNA_ADVANCED |
> -				       FLAG_NOT_DUP | FLAG_DATA_SACKED));
>  

No idea why you removed this bool, it really helped to understand the
code. This makes your patch looks more complex than needed.

>  	/* Mark the end of TLP episode on receiving TLP dupack or when
>  	 * ack is after tlp_high_seq.
> +	 * With delayed acks, we may also get a regular ACK+DSACK, in which
> +	 * case we don't want to reduce the cwnd either.
>  	 */
> -	if (is_tlp_dupack) {
> +	if (((ack == tp->tlp_high_seq) &&
> +	     !(flag & (FLAG_SND_UNA_ADVANCED |
> +		       FLAG_NOT_DUP | FLAG_DATA_SACKED))) ||
> +	    (!before(ack, tp->tlp_high_seq) && (flag & FLAG_DSACKING_ACK))) {
>  		tp->tlp_high_seq = 0;
> -		return;
> -	}


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