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22338f09bd
linux/net/dccp
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Gerrit Renker 22338f09bd dccp tfrc: Increase number of RTT samples 
This improves the receiver RTT sampling algorithm so that it tries harder to get
as many RTT samples as possible. 

The algorithm is based the concepts presented in RFC 4340, 8.1, using timestamps
and the CCVal window counter. There exist 4 cases for the CCVal difference:
 * == 0: less than RTT/4 passed since last packet -- unusable;
 *  > 4: (much) more than 1 RTT has passed since last packet -- also unusable;
 * == 4: perfect sample (exactly one RTT has passed since last packet);
 * 1..3: sub-optimal sample (between RTT/4 and 3*RTT/4 has passed).

In the last case the algorithm tried to optimise by storing away the candidate
and then re-trying next time. The problem is that
 * a large number of samples is needed to smooth out the inaccuracies of the
   algorithm;
 * the sender may not be sending enough packets to warrant a "next time";
 * hence it is better to use suboptimal samples whenever possible.
The algorithm now stores away the current sample only if the difference is 0.

Applicability and background
----------------------------
A realistic example is MP3 streaming where packets are sent at a rate of less
than one packet per RTT, which means that suitable samples are absent for a
very long time.

The effectiveness of using suboptimal samples (with a delta between 1 and 4) was
confirmed by instrumenting the algorithm with counters. The results of two 20
second test runs were:
 * With the old algorithm and a total of 38442 function calls, only 394 of these
   calls resulted in usable RTT samples (about 1%), and 378 out of these were
   "perfect" samples and 28013 (unused) samples had a delta of 1..3.
 * With the new algorithm and a total of 37057 function calls, 1702 usable RTT
   samples were retrieved (about 4.6%), 5 out of these were "perfect" samples.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
2008-09-04 07:45:42 +02:00
..
ccids dccp tfrc: Increase number of RTT samples 2008-09-04 07:45:42 +02:00
ackvec.c dccp ccid-2: Separate option parsing from CCID processing 2008-09-04 07:45:37 +02:00
ackvec.h dccp ccid-2: Separate option parsing from CCID processing 2008-09-04 07:45:37 +02:00
ccid.c dccp: Auto-load (when supported) CCID plugins for negotiation 2008-09-04 07:45:31 +02:00
ccid.h dccp: Return-value convention of hc_tx_send_packet() 2008-09-04 07:45:38 +02:00
dccp.h dccp: Clamping RTT values 2008-09-04 07:45:41 +02:00
diag.c dccp ccid-2: Phase out the use of boolean Ack Vector sysctl 2008-09-04 07:45:31 +02:00
feat.c dccp ccid-2: Use feature-negotiation to report Ack Ratio changes 2008-09-04 07:45:32 +02:00
feat.h dccp: Debugging functions for feature negotiation 2008-09-04 07:45:32 +02:00
input.c dccp: Clamping RTT values 2008-09-04 07:45:41 +02:00
ipv4.c dccp: Cleanup routines for feature negotiation 2008-09-04 07:45:26 +02:00
ipv6.c dccp: Cleanup routines for feature negotiation 2008-09-04 07:45:26 +02:00
ipv6.h Remove obsolete #include <linux/config.h> 2006-06-30 19:25:36 +02:00
Kconfig dccp ccid-2: Remove old infrastructure 2008-09-04 07:45:37 +02:00
Makefile dccp: Policy-based packet dequeueing infrastructure 2008-09-04 07:45:39 +02:00
minisocks.c dccp: Fix the adjustments to AWL and SWL 2008-09-04 07:45:35 +02:00
options.c dccp ccid-2: Separate option parsing from CCID processing 2008-09-04 07:45:37 +02:00
output.c dccp: Policy-based packet dequeueing infrastructure 2008-09-04 07:45:39 +02:00
probe.c dccp: Reduce noise in output and convert to ktime_t 2008-09-04 07:45:36 +02:00
proto.c dccp tfrc: Let dccp_tfrc_lib do the sampling work 2008-09-04 07:45:41 +02:00
qpolicy.c dccp qpolicy: Parameter checking of cmsg qpolicy parameters 2008-09-04 07:45:40 +02:00
sysctl.c dccp: Initialisation and type-checking of feature sysctls 2008-09-04 07:45:32 +02:00
timer.c dccp ccid-3: Runtime verification of timer resolution 2008-09-04 07:45:40 +02:00