Re: [PATCH 1/3] net: TCP thin-stream detection
From: Andreas Petlund
Date: Thu Oct 29 2009 - 09:51:47 EST
Den 28. okt. 2009 kl. 04.09 skrev William Allen Simpson:
Andreas Petlund wrote:
+/* Determines whether this is a thin stream (which may suffer fromThis bothers me a bit. Having just looked at your Linux presentation,
+ * increased latency). Used to trigger latency-reducing mechanisms.
+static inline unsigned int tcp_stream_is_thin(const struct
+ return tp->packets_out < 4;
and not (yet) read your papers, it seems much of your justification
with 1 packet per RTT. Here, you seem to be concentrating on 4,
because many implementations quickly ramp up to 4.
The limit of 4 packets in flight is based on the fact that less than 4
packets in flight makes fast retransmissions impossible, thus limiting
the retransmit options to timeout-retransmissions. The criterion is
therefore as conservative as possible while still serving its purpose.
If further losses occur, the exponential backoff will increase latency
further. The concept of using this limit is also discussed in the
Internet draft for Early Retransmit by Allman et al.:
But there's a fair amount of experience showing that ramping to 4 is
problematic on congested paths, especially wireless networks. Fast
retransmit in that case would be disastrous.
First, the modifications implemented in the patch is explicitly
enabled only for applications where the developer knows that streams
will be thin, thus only a small subset of the streams will apply the
modifications. Second, experiments we have performed to try to map the
effect on a congested bottleneck both with and without the
modifications show that no measurable effect is recorded.
Graphs presenting results from experiments performed to analyse
latency and fairness issues can be found here:
Once upon a time, I worked on a fair number of interactive games a
or so ago. And agree that this can be a problem, although I've never
been a fan of turning off the Nagle algorithm. My solution has always
been a heartbeat, rather than trying to shoehorn this into TCP.
The beginning of this patch was an analysis of game traffic from the
Norwegian game company Funcom. They use TCP for all their MMOGs as
does, for example, Blizzard for WoW. Our analysis showed that many
players experienced extreme latencies, and the source of this was
tracked to the effects that we discuss here. As long as a wide range
of time-dependent applications choose to use TCP, and we can improve
conditions for their needs without jeopardising other functionality,
we think that this will add value to the TCP stack.
Also, I've not seen any discussion on the end-to-end interest list.
It will be enlightening to have a discussion on end-to-end about this
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