netdev - Re: [PATCH v3 net-next 2/2] tcp: Add Redundant Data Bundling (RDB)

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Message-ID: <1454445306.7627.204.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Tue, 02 Feb 2016 12:35:06 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Bendik Rønning Opstad <bro.devel@...il.com>
Cc:	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
	Yuchung Cheng <ycheng@...gle.com>,
	Neal Cardwell <ncardwell@...gle.com>,
	Andreas Petlund <apetlund@...ula.no>,
	Carsten Griwodz <griff@...ula.no>,
	Pål Halvorsen <paalh@...ula.no>,
	Jonas Markussen <jonassm@....uio.no>,
	Kristian Evensen <kristian.evensen@...il.com>,
	Kenneth Klette Jonassen <kennetkl@....uio.no>,
	Bendik Rønning Opstad 
	<bro.devel+kernel@...il.com>
Subject: Re: [PATCH v3 net-next 2/2] tcp: Add Redundant Data Bundling (RDB)

On Tue, 2016-02-02 at 20:23 +0100, Bendik Rønning Opstad wrote:
> RDB is a mechanism that enables a TCP sender to bundle redundant
> (already sent) data with TCP packets containing new data. By bundling
> (retransmitting) already sent data with each TCP packet containing new
> data, the connection will be more resistant to sporadic packet loss
> which reduces the application layer latency significantly in congested
> scenarios.
> 
> The main functionality added:
> 
>   o Loss detection of hidden loss events: When bundling redundant data
>     with each packet, packet loss can be hidden from the TCP engine due
>     to lack of dupACKs. This is because the loss is "repaired" by the
>     redundant data in the packet coming after the lost packet. Based on
>     incoming ACKs, such hidden loss events are detected, and CWR state
>     is entered.
> 
>   o When packets are scheduled for transmission, RDB replaces the SKB to
>     be sent with a modified SKB containing the redundant data of
>     previously sent data segments from the TCP output queue.

Really this looks very complicated.

Why not simply append the new skb content to prior one ?

skb_still_in_host_queue(sk, prior_skb) would also tell you if the skb is
really available (ie its clone not sitting/waiting in a qdisc on the
host)

Note : select_size() always allocate skb with SKB_WITH_OVERHEAD(2048 -
MAX_TCP_HEADER) available bytes in skb->data.

Also note that tcp_collapse_retrans() is very similar to your needs. You
might simply expand it.