On Tue, Jul 29, 2008 at 10:05 PM, Gerrit Renker <gerrit@xxxxxxxxxxxxxx> wrote: > This improves the receiver RTT sampling algorithm so that it tries harder to get > as many RTT samples as possible, using several optimisations. > > Applicability and background > ---------------------------- > The algorithm uses timestamps and differences of the CCval window counter to > guess the RTT, using concepts presented in RFC 4340, 8.1. > > There exist 4 cases for the CCVal difference: > * == 0: less than RTT/4 passed since last packet -- unusable sample; > * > 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 so far tried to optimise by storing away the > candidate and then re-trying next time. This had the following problems: > * a large number of samples is needed to smooth out the given inaccuracies; > * the sender may not be sending enough packets to warrant a "next time"; > * hence it is better to use suboptimal samples whenever possible. > As a consequence, the algorithm now stores away the current sample only if the > difference is 0. > > A realistic example to illustrate the failure of the (previous) algorithm is MP3 > streaming, where packets are sent at a rate of less than one packet per RTT. > Which means that suitable samples may be 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%), 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. > This means an almost five-fold increase in the number of samples. > > Signed-off-by: Gerrit Renker <gerrit@xxxxxxxxxxxxxx> Great work. This should make a real improvement. Acked-by: Ian McDonald <ian.mcdonald@xxxxxxxxxxx> -- To unsubscribe from this list: send the line "unsubscribe dccp" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
