netdev - Re: Why we discard all rtt samples when only some of the acked skbs have been retransmited in processing ack?

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Date:	Tue, 17 Sep 2013 09:31:58 -0400
From:	Neal Cardwell <ncardwell@...gle.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	LovelyLich <lovelylich@...il.com>,
	Yuchung Cheng <ycheng@...gle.com>,
	Netdev <netdev@...r.kernel.org>
Subject: Re: Why we discard all rtt samples when only some of the acked skbs
 have been retransmited in processing ack?

On Tue, Sep 17, 2013 at 1:11 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Tue, 2013-09-17 at 12:01 +0800, LovelyLich wrote:
>> Hi Eric,
>>
>> In tcp_clean_rtx_queue(), we set the flag FLAG_RETRANS_DATA_ACKED when we
>>
>> encounter one ever retransmited skb A. But if there is one( or more) skb B
>>
>> after this retransmited skb, and we calculate the rtt for skb B. The question
>>
>> is because we have set the flag FLAG_RETRANS_DATA_ACKED, and we will just
>>
>> return in tcp_ack_no_tstamp() !
>>
>> Two questions:
>>
>> 1. if we will just ignore all packets in this ack, we do not need to calculate
>>
>> skb B's rtt sample.
>>
>> 2. what I want to know, even if A's rtt sample is not reliable, but B's rtt
>>
>> sample can be trusted. Why we discard it ?
>>
>>
>>
>> Thanks in advanced.
>>
>
> Good point !
>
> Yuchung, what do you think of following patch ?
>
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 25a89ea..7f12b96 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -2971,7 +2971,7 @@ static int tcp_clean_rtx_queue(struct sock *sk, int prior_fackets,
>         struct sk_buff *skb;
>         u32 now = tcp_time_stamp;
>         int fully_acked = true;
> -       int flag = 0;
> +       int flag = FLAG_RETRANS_DATA_ACKED;
>         u32 pkts_acked = 0;
>         u32 reord = tp->packets_out;
>         u32 prior_sacked = tp->sacked_out;
> @@ -3002,7 +3002,6 @@ static int tcp_clean_rtx_queue(struct sock *sk, int prior_fackets,
>                 if (sacked & TCPCB_RETRANS) {
>                         if (sacked & TCPCB_SACKED_RETRANS)
>                                 tp->retrans_out -= acked_pcount;
> -                       flag |= FLAG_RETRANS_DATA_ACKED;
>                 } else {
>                         ca_seq_rtt = now - scb->when;
>                         last_ackt = skb->tstamp;
> @@ -3013,6 +3012,7 @@ static int tcp_clean_rtx_queue(struct sock *sk, int prior_fackets,
>                                 reord = min(pkts_acked, reord);
>                         if (!after(scb->end_seq, tp->high_seq))
>                                 flag |= FLAG_ORIG_SACK_ACKED;
> +                       flag &= ~FLAG_RETRANS_DATA_ACKED;
>                 }
>
>                 if (sacked & TCPCB_SACKED_ACKED)

I think the existing logic is better than the patch. If we get a
cumulative ACK that covers a retransmitted packet, then any RTT sample
we try to extract is suspect, and likely to be at least 2x too high.

Consider the following common scenario:

t=0: send pkts 1, 2, 3, 4
t=1*RTT: receive dupack with SACK for pkts 2,3,4; fast retransmit  pkt 1
t=2*RTT: receive cumulative ack for all pkts through 4

With the existing logic, because the ACK we get at t=2*RTT covers the
retransmitted pkt 1, we do not attempt to take an RTT sample.

With that proposed patch, when we get the ACK at t=2*RTT we see that
there are non-retransmitted pkts 2,3,4 being ACKed, so we clear the
FLAG_RETRANS_DATA_ACKED bit and take an RTT sample of 2*RTT. But this
is 2x too big, and will distort our RTT sample.

Note that with Yuchung's recent patch to gather RTT samples from
SACKed packets (59c9af4234b0c21a1ed05cf65bf014d0c1a67bfd "tcp: measure
RTT from new SACK"), we will already be extracting essentially all the
RTT samples that we possibly can out of such scenarios with
retransmitted packets (for OSes that support SACK, which is basically
everyone). In the example above, Yuchung's new SACK-based RTT scheme
will correctly take RTT samples at t=1*RTT for the SACKed packets.

neal
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