Traffic Management for the Available Bit Rate (ABR) Service in ...
Traffic Management for the Available Bit Rate (ABR) Service in ... Traffic Management for the Available Bit Rate (ABR) Service in ...
MSS is 9140 bytes, the maximum TCP thoughput is 87% of ABR throughput, and this is the value used to compare the total TCP throughput against. Observe that the e ciency achieved in all cases is high (above 90%) in spite of the high variation in ABR capacity. Also observe that the total TCP throughput is higher (as well as the e ciency) for TCP MSS = 9140 bytes in all cases. The maximum queue length is controlled to about three times the feedback delay (or one round trip time) worth of queue. The feedback delay for this con guration is 10 ms, which corresponds to (10 ms) (367 cells/ms) = 3670 cells worth of queue when the network is on the average overloaded by a factor of 2 (as is the case with TCP). The round-trip time for this con guration is 30 ms. The queue length is higher when the mean per-source rate is lower (i.e., when the average ABR rate is higher). This is explained as follows. Whenever there is variation in capacity, the switch algorithm may make errors in estimating the average capacity and may overallocate rates temporarily. When the average ABR capacity is higher, each error in allocating rates will result in a larger backlog of cells to be cleared than for the corresponding case when the average ABR capacity is low. The combination of these backlogs may result in a larger maximum queue before the long-term queue reduction mechanism of the switch algorithm reduces the queues. 8.18.2 Comparison with ON-OFF VBR Results Recall that in section 8.16 we have studied the behavior of TCP over ABR in the presence of ON-OFF VBR sources. We studied ranges of ON-OFF periods from 1 ms through 100 ms. Further, we had looked at results where the ON period was not equal to the OFF period. The worst cases were seen in the latter simulations. 325
However, with modi cations to ERICA+ and a larger averaging interval we found that the maximum switch queue length was 5637 cells. This experiment has a duty cycle of 0.7 and a period of 20ms i.e., the ON time was 14 ms and the o time was 6 ms. Since we use the same switch algorithm parameters in this study, we can perform a comparison of the two studies. We observe that, even after the introduction of the long-range dependent VBR model, the queues do not increase substantially (beyond one round trip worth of queues) and the e ciency remains high (around 90%). This is because the ERICA+ switch algorithm has been re ned and tuned to handle variation in the ABR capacity and ABR demand. These re nements allow the convergence of the ABR queues, without compromising on the e ciency. 8.18.3 Satellite simulations with Short Feedback Delay In this section and the next, we repeat the experiments with some links being satellite links. In the rst set of simulations, we replace the bottleneck link shared by 15 sources with a satellite link as shown in Figure 8.17. The links from the second switch to the destination nodes are 1 km each. The total round trip time is 550 ms, but the feedback delay remains 10 ms. Table 8.12 and Table 8.13 (similar to Tables 8.10 and 8.11) show the maximum switch queue length, the total TCP throughput, VBR throughput, ABR throughput, and e ciency for three combinations of the mean and standard deviation. Table 8.12 is for TCP MSS = 512 bytes, while Table 8.13 is for TCP MSS = 9140 bytes. Note that the TCP startup time in this con guration is large because the round trip time (550 ms) is large and TCP requires multiple round trips to be able to use its 326
- Page 301 and 302: Window Size in bytes vfive-tcp/opti
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- Page 305 and 306: 8.13 Summary of TCP/IP performance
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- Page 315 and 316: to equalize rates for fairness, and
- Page 317 and 318: QB = Link bandwidth (RT T ; T ) and
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- Page 327 and 328: a modi ed version of the ERICA algo
- Page 329 and 330: ABR is better than UBR in these (en
- Page 331 and 332: problems. During the ON time, the V
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- Page 335 and 336: like ERICA+ which uses the queueing
- Page 337 and 338: minimum fairshare is low. This may
- Page 339 and 340: 8.17 E ect of Long-Range Dependent
- Page 341 and 342: terminology) are called \Presentati
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- Page 349 and 350: For the video sources, we choose me
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- Page 365 and 366: call such a switch a \VS/VD switch"
- Page 367 and 368: Figure 9.2: Per-class queues in a n
- Page 369 and 370: which arises is where the rate calc
- Page 371 and 372: 9.2 The ERICA Switch Scheme: Renota
- Page 373 and 374: The unknowns in the above equations
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- Page 379 and 380: # VC Rate VC Input Rate Input Rate
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However, with modi cations to ERICA+ and a larger averag<strong>in</strong>g <strong>in</strong>terval we found<br />
that <strong>the</strong> maximum switch queue length was 5637 cells. This experiment has a duty<br />
cycle of 0.7 and a period of 20ms i.e., <strong>the</strong> ON time was 14 ms and <strong>the</strong> o time was 6<br />
ms. S<strong>in</strong>ce we use <strong>the</strong> same switch algorithm parameters <strong>in</strong> this study, we can per<strong>for</strong>m<br />
a comparison of <strong>the</strong> two studies.<br />
We observe that, even after <strong>the</strong> <strong>in</strong>troduction of <strong>the</strong> long-range dependent VBR<br />
model, <strong>the</strong> queues do not <strong>in</strong>crease substantially (beyond one round trip worth of<br />
queues) and <strong>the</strong> e ciency rema<strong>in</strong>s high (around 90%). This is because <strong>the</strong> ERICA+<br />
switch algorithm has been re ned and tuned to handle variation <strong>in</strong> <strong>the</strong> <strong>ABR</strong> capacity<br />
and <strong>ABR</strong> demand. These re nements allow <strong>the</strong> convergence of <strong>the</strong> <strong>ABR</strong> queues,<br />
without compromis<strong>in</strong>g on <strong>the</strong> e ciency.<br />
8.18.3 Satellite simulations with Short Feedback Delay<br />
In this section and <strong>the</strong> next, we repeat <strong>the</strong> experiments with some l<strong>in</strong>ks be<strong>in</strong>g<br />
satellite l<strong>in</strong>ks. In <strong>the</strong> rst set of simulations, we replace <strong>the</strong> bottleneck l<strong>in</strong>k shared by<br />
15 sources with a satellite l<strong>in</strong>k as shown <strong>in</strong> Figure 8.17. The l<strong>in</strong>ks from <strong>the</strong> second<br />
switch to <strong>the</strong> dest<strong>in</strong>ation nodes are 1 km each. The total round trip time is 550 ms,<br />
but <strong>the</strong> feedback delay rema<strong>in</strong>s 10 ms.<br />
Table 8.12 and Table 8.13 (similar to Tables 8.10 and 8.11) show <strong>the</strong> maximum<br />
switch queue length, <strong>the</strong> total TCP throughput, VBR throughput, <strong>ABR</strong> throughput,<br />
and e ciency <strong>for</strong> three comb<strong>in</strong>ations of <strong>the</strong> mean and standard deviation. Table 8.12<br />
is <strong>for</strong> TCP MSS = 512 bytes, while Table 8.13 is <strong>for</strong> TCP MSS = 9140 bytes.<br />
Note that <strong>the</strong> TCP startup time <strong>in</strong> this con guration is large because <strong>the</strong> round<br />
trip time (550 ms) is large and TCP requires multiple round trips to be able to use its<br />
326
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