Subject: Re: [PATCH 1/3] net: TCP thin-stream detection

From: Arnd Hannemann <[email-masked]>
Date: Fri, 30 Oct 2009 16:24:02 +0100

[email-masked] schrieb:
> As Ilpo writes, the mechanism we propose is simpler than the ID, and
> slightly more aggressive. The reason why we chose this is as follows: 1)
> The ID and Limited Transmit tries to prevent retransmission timeouts by
> retransmitting more aggressively, thus keeping the congestion window open
> even though congestion may be the limiting factor. If their limiting
> conditions change, they still have higher sending rates available. The
> thin-stream applications are not limited by congestion control. There is
> therefore no motivation to prevent retransmission timeouts in order to
> keep the congestion window open because in the thin-stream scenario, a
> larger window is not needed, but we retransmit early only to reduce
> application-layer latencies. 2) Our suggested implementation is simpler.
> 3) I believe that the reason why the ID has not been implemented in Linux
> is that the motivation did not justify the achieved result. We have
> analysed a wide range of time-dependent applications and found that they
> very often produce thin streams due to transmissions being triggered by
> human interaction. This changes the motivational picture since a thin
> stream is an indicator of time-dependency.

Both mechanism prevent retransmission timeouts, thereby reducing latency.
Who cares, that they were motivated by performance?

I agree, that you are more aggressive, and that your scheme may have
latency advantages, at least for the Limited Transmit case. And there are
probably good reasons for your proposal. But I really think you should
bring your proposal up in IETF TCPM WG. I have the feeling that there are
a lot of corner cases we didn't think of.

One example: Consider standard NewReno non-SACK enabled flow:
For some reasons two data packets get reordered.
The TCP sender will produce a dupACK and an ACK.
The dupACK will trigger (because of your logic) a spurious retransmit.
The spurious retransmit will trigger a dupACK.
This dupACK will again trigger a spurious retransmit.
And this game will continue, unless a packet is dropped by coincidence.

P.S.: The Early-Rexmit ID has not been implemented in Linux,
because our student who was working on that is busy with something
else...

Best regards,
Arnd Hannemann