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Re: dummy as IMQ replacement

To: Thomas Graf <tgraf@xxxxxxx>
Subject: Re: dummy as IMQ replacement
From: Andy Furniss <andy.furniss@xxxxxxxxxxxxx>
Date: Tue, 01 Feb 2005 01:02:12 +0000
Cc: jamal <hadi@xxxxxxxxxx>, netdev@xxxxxxxxxxx, Nguyen Dinh Nam <nguyendinhnam@xxxxxxxxx>, Remus <rmocius@xxxxxxxxxxxxxx>, Andre Tomt <andre@xxxxxxxx>,, Damion de Soto <damion@xxxxxxxxxxxx>
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Thomas Graf wrote:
Or dropping packets. TCP will adjust itself either way; at least
thats true according to this formula [rfc3448] (originally derived from
Reno, but people are finding it works fine with all other variants of

The throughput equation is:

  X =  ----------------------------------------------------------
       R*sqrt(2*b*p/3) + (t_RTO * (3*sqrt(3*b*p/8) * p * (1+32*p^2)))


     X is the transmit rate in bytes/second.
     s is the packet size in bytes.
     R is the round trip time in seconds.
     p is the loss event rate, between 0 and 1.0, of the number of loss
       events as a fraction of the number of packets transmitted.
     t_RTO is the TCP retransmission timeout value in seconds.
     b is the number of packets acknowledged by a single TCP

WRT policers I never figured out where you would put the effects of playing with the burst size parameter and it's effects with few/many connections and any burstiness caused into an equasion like that.


Agreed, this was my first attempt and my current code is still based on
this. I'm trying to avoid a retransmit battle, therefore I try to
delay packets if possible with the hope that it's either just a peak
or the slow down is fast enough. I use a simplified RED and
tcp_xmit_retransmit_queue() input to avoid flick flack effects which
works pretty well for bulky streams. A burst buffer takes care
of interactive traffic with peaks but this doesn't work perfectly fine
yet. Overall, my attempt works pretty well if the other side uses
reno/bic and quite well for westwood and vegas. The problem is not that
it doesn't work at all but achieving a certain _stable_ rate is very
difficult, the delta of the requested and real rate is up to 25% depending
on the constancy of the rtt and wether they follow one of the proposed
tcp cc algorithms. The cc guessing code helps a bit but isn't very

This sounds cool. For me in someways I think it could be nicer (in the case of shaping from the wrong end of a slow link) to delay the real packets - that way the tcps of the clients get to see the smoothed version of the traffic and you can delay udp aswell.

How intelligent and how much, if any, per connection state do you/could you keep? I think being able to set a class that behaves as full before it is, removing the s from sfq, de piggybacking acks and singling out and handling slowstart connections specially could really help the world of shaping from the wrong end of slow links.

There's always playing with rwin, but maybe that's abit OTT :-)


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