From: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Date: Thu, 4 Sep 2008 05:30:19 +0000 (+0200)
Subject: dccp ccid-3: Preventing Oscillations
X-Git-Tag: v2.6.28-rc1~717^2~305^2
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=a3cbdde8e9c38b66b4f13ac5d6ff1939ded0ff20;p=pandora-kernel.git

dccp ccid-3: Preventing Oscillations

This implements [RFC 3448, 4.5], which performs congestion avoidance behaviour
by reducing the transmit rate as the queueing delay (measured in terms of
long-term RTT) increases.

Oscillation can be turned on/off via a module option (do_osc_prev) and via sysfs
(using mode 0644), the default is off.

Overflow analysis:
------------------
 * oscillation prevention is done after update_x(), so that t_ipi <= 64000;
 * hence the multiplication "t_ipi * sqrt(R_sample)" needs 64 bits;
 * done using u64 for sqrt_sample and explicit typecast of t_ipi;
 * the divisor, R_sqmean, is non-zero because oscillation prevention is first
   called when receiving the second feedback packet, and tfrc_scaled_rtt() > 0.

A detailed discussion of the algorithm (with plots) is on
http://www.erg.abdn.ac.uk/users/gerrit/dccp/notes/ccid3/sender_notes/oscillation_prevention/

The algorithm has negative side effects:
  * when allowing to decrease t_ipi (leads to a large RTT) and
  * when using it during slow-start;
both uses are therefore disabled.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
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