netdev - Re: [PATCH 1/2] tcp: Tail loss probe (TLP)

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Date:	Mon, 11 Mar 2013 17:37:28 -0400
From:	Neal Cardwell <ncardwell@...gle.com>
To:	Nandita Dukkipati <nanditad@...gle.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Yuchung Cheng <ycheng@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Netdev <netdev@...r.kernel.org>,
	Ilpo Jarvinen <ilpo.jarvinen@...helsinki.fi>,
	Tom Herbert <therbert@...gle.com>
Subject: Re: [PATCH 1/2] tcp: Tail loss probe (TLP)

On Mon, Mar 11, 2013 at 4:00 PM, Nandita Dukkipati <nanditad@...gle.com> wrote:
> This patch series implement the Tail loss probe (TLP) algorithm described
> in http://tools.ietf.org/html/draft-dukkipati-tcpm-tcp-loss-probe-01. The
> first patch implements the basic algorithm.
>
> TLP's goal is to reduce tail latency of short transactions. It achieves
> this by converting retransmission timeouts (RTOs) occuring due
> to tail losses (losses at end of transactions) into fast recovery.
> TLP transmits one packet in two round-trips when a connection is in
> Open state and isn't receiving any ACKs. The transmitted packet, aka
> loss probe, can be either new or a retransmission. When there is tail
> loss, the ACK from a loss probe triggers FACK/early-retransmit based
> fast recovery, thus avoiding a costly RTO. In the absence of loss,
> there is no change in the connection state.
>
> PTO stands for probe timeout. It is a timer event indicating
> that an ACK is overdue and triggers a loss probe packet. The PTO value
> is set to max(2*SRTT, 10ms) and is adjusted to account for delayed
> ACK timer when there is only one oustanding packet.
>
> TLP Algorithm
>
> On transmission of new data in Open state:
>   -> packets_out > 1: schedule PTO in max(2*SRTT, 10ms).
>   -> packets_out == 1: schedule PTO in max(2*RTT, 1.5*RTT + 200ms)
>   -> PTO = min(PTO, RTO)
>
> Conditions for scheduling PTO:
>   -> Connection is in Open state.
>   -> Connection is either cwnd limited or no new data to send.
>   -> Number of probes per tail loss episode is limited to one.
>   -> Connection is SACK enabled.
>
> When PTO fires:
>   new_segment_exists:
>     -> transmit new segment.
>     -> packets_out++. cwnd remains same.
>
>   no_new_packet:
>     -> retransmit the last segment.
>        Its ACK triggers FACK or early retransmit based recovery.
>
> ACK path:
>   -> rearm RTO at start of ACK processing.
>   -> reschedule PTO if need be.
>
> In addition, the patch includes a small variation to the Early Retransmit
> (ER) algorithm, such that ER and TLP together can in principle recover any
> N-degree of tail loss through fast recovery. TLP is controlled by the same
> sysctl as ER, tcp_early_retrans sysctl.
> tcp_early_retrans==0; disables TLP and ER.
>                  ==1; enables RFC5827 ER.
>                  ==2; delayed ER.
>                  ==3; TLP and delayed ER. [DEFAULT]
>                  ==4; TLP only.
>
> The TLP patch series have been extensively tested on Google Web servers.
> It is most effective for short Web trasactions, where it reduced RTOs by 15%
> and improved HTTP response time (average by 6%, 99th percentile by 10%).
> The transmitted probes account for <0.5% of the overall transmissions.
>
> Signed-off-by: Nandita Dukkipati <nanditad@...gle.com>
> ---

Acked-by: Neal Cardwell <ncardwell@...gle.com>

neal
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