netdev - Re: TCP and reordering

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Date:	Wed, 28 Nov 2012 00:35:56 -0800
From:	Vijay Subramanian <subramanian.vijay@...il.com>
To:	Saku Ytti <saku@...i.fi>
Cc:	David Miller <davem@...emloft.net>, rick.jones2@...com,
	netdev@...r.kernel.org
Subject: Re: TCP and reordering

>
> My proposal (or question more accurately) was to add 'reorder' counter to
> sockets, which would increment when duplicate ACK is followed by same
> sequence twice.
> Then you could automatically/dynamically delay duplicate acks, as you'd
> start to expect to receive the frames, out-of-order. Giving non-lossy
> reordering links pretty much 100% same performance as non-lossy in-order
> links.

RFC 5681 says that out-of-order packets should be acked immediately.
Please see section 4.2 for detailed reasoning.
It also explains why acks should not be delayed too much.

Also note that reordering is tracked on the sender side using the per
flow variable tp->reordering . This measures the amount of reordering
on the connection so that
fast retransmit and other loss recovery mechanisms are not entered
prematurely. Doesn't this behavior at the  sender already provide the
behavior you seek?

Regards,
Vijay
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