Recall that the proposed rate-based congestion control framework (see Figure 4) is composed of a number of functions that intercommunicate via several congestion control fields in every network packet, as was shown in Figure 5. Any specific function that meets the framework's requirements can be used as one of the framework's functions. Thus there could be any number of transport protocols, sustainable rate measurement functions, and so on.
In this thesis I have described several specific functions to be used in the congestion control framework. Several of these functions have parameters which affect their operation. In this chapter we will explore how the possible values of these parameters affect the functions and also the overall performance of the framework where these functions are used.
The two major function instantiations which have been described are TRUMP, a rate-based Transport protocol, and RBCC, a Sustainable Rate Measurement function. The implementations of RBCC and TRUMP have parameters which can be altered to change their behaviour. RBCC has the following parameters:
The TRUMP transport protocol has one parameter:
The set of possible parameter values form a 5-dimensional space with an infinite number of 5-tuples. We will explore some of the 5-space by keeping 4 parameters constant and varying a fifth.
Most of abstracted measurements presented in this chapter are the same as for the previous chapter where TRUMP/RBCC, TCP Reno and TCP Vegas were compared in the 500 pseudo-random scenarios. The abbreviations given below will be used in the table presented in this chapter.
For each measurement, the median value and the 90% range will be given. The only exception to this is the lost result. The median value is not given, as this is nearly always 0. Instead, the total number of packets lost will be given.
Rate Quench packets were described in Section 8.4: If a data packet causes a flow's rate to be lowered in a router's ABT, a Rate Quench packet is returned to the source with the Return Rate field set to the flow's new rate in the ABT.
This congestion avoidance mechanism causes traffic flows to throttle back their transmission faster than the usual mechanism of obtaining the new rate via acknowledgment packets, which takes one round trip. The overall effect should be to lower network congestion, as traffic flows are transmitting data at an incorrect rate for less time. However, the Rate Quench packets form a traffic flow themselves, and as a network becomes more congested, more Rate Quench packets are generated, which may lead to even more congestion.
The effect of Rate Quench packets were examined in the 500 pseudo-random
scenarios described in the previous chapter. The following table shows the
measurements for these scenarios where Rate Quench packets were
generated/not generated, and where the remaining four parameters have the
values [selacksize=1, revupdates=no, thresh=5,
|Measurement||No Quenches||Quench Packets|
|Median||90% Range||Median||90% Range|
It is very difficult to distinguish between the two parameter values. Highest queue lengths are smaller, but average queue lengths are slightly higher. Utilisation is negligibly higher, and all other results are equal. We must look at the total number of packets lost to see any significant difference. With no quenches, 551 packets are lost over the 500 scenarios. With Rate Quench packets used, only 411 packets are lost.
The use of Rate Quench appears to lower overall packet loss without affecting other network characteristics such as utilisation or end-to-end variance. Given the results above, I would recommend that Rate Quench packets be used in any implementation of the rate-based congestion control framework.
A reverse ABT update occurs when the router updates its ABT table when a packet's Return_Rate is less than the flow's rate in the table, or when the flow's bottleneck has altered the Return_Rate.
Allowing a router to perform reverse ABT updates provides it with `downstream' information from packets flowing upstream. Without reverse ABT updates, the router only receives downstream congestion information after:
It would appear that reverse ABT updates should help the network infrastructure react more quickly to overall network congestion. The results below indicate, however, that in fact reverse ABT updates increase network congestion.
The following table shows the measurements for the 500 pseudo-random scenarios where reverse ABT updates were allowed/disallowed, and where the remaining four parameters have the values [selacksize=1, quench=no, thresh=5, alpha=0.75].
|Measurement||No Reverse||Reverse Updates|
|Median||90% Range||Median||90% Range|
Reverse updates have no significant effect end-to-end standard deviation. However, reverse updates cause queue sizes for the most congested router, and for all routers, to be larger. Utilisation across the most-used links decreases, and there are more router ABT updates as expected. Overall packet loss is the same.
At first, the result appears to be counter-intuitive: the reverse congestion information should help routers to determine better rates for traffic flows. This should lead to better network utilisation. Instead, there is higher queue lengths and less utilisation. A hypothesis for this follows. Reverse ABT updates cause more router table changes, which cause more rate changes in the sources. Source rates fluctuate more, and so the network is slightly more unstable and higher congestion results.
Given the result, I would recommend that reverse ABT updates not be used in RBCC. This reduces the overhead of the RBCC on network routers, and keeps router queue lengths slightly lower, with no detrimental effect on network performance.
One of the framework's design goals, given in Section 4.2, is that `A congestion control scheme should strive to keep queue lengths at length 1'. The threshold causes each router to scale the bandwidth of an output interface by if there are more than packets queued for transmission on that interface. This is a form of congestion avoidance, as the scaled bandwidth lowers traffic flows' rates through the interface, which will help bring the interface's queue length back to length 1.
If is small, than the scaling occurs before the queue size is large. This will help to maintain the queue length near 1, but will cause more router ABT updates, and will also cause more traffic flow rate fluctuations, as the queue length will cross the threshold very often.
If is large (close to the maximum number of packets which can be queued on the interface), then the threshold is rarely crossed, no congestion avoidance is performed, and high queue lengths are permitted.
The effect of threshold values of 2, 3, 4, 5, 7 and 10 packets were examined in the 500 pseudo-random scenarios, where an interface is able to queue up to 15,000 octets (10 data packets). The remaining four parameters have the values [selacksize=1, quench=no, revupdates=no, alpha=0.75]. The following tables give the median and 90% range results for each threshold value.
From the tables, it is apparent that all of the measurements, except the number of congestion cache misses and ABT updates, increases as increases. Router queue lengths increase as the threshold is raised. As a byproduct of this, utilisation in the congested router increases as there are more packets ready for transmission. Because of the higher router queue lengths, end-to-end delays increase, and the higher occupancy in queue lengths causes higher end-to-end variance. More packets are lost as higher queue lengths are tolerated. However, because the threshold is crossed less at higher values of , the number of ABT updates decreases.
The tables show that a lower threshold value provides better congestion control in the rate-based framework. The only drawbacks are a higher number of ABT updates, and a slightly lower network utilisation. If this form of congestion avoidance was to be implemented, I would recommend a value of between 3 and 6 packets, inclusive.
Hand-in-hand with the threshold value is the scaling factor . When the threshold is crossed, an interface's total bandwidth is scaled by and its ABT is recalculated. can take values between 0.0 (exclusive) and 1.0 (inclusive). Values of near 1.0 have little scaling effect, and should provide little congestion avoidance. Values near 0.0 clamp traffic flows much below their ideal rate (until short-term congestion is reduced), and should have a negative effect on the overall network utilisation.
The effect of values of 0.125, 0.25, 0.375, 0.5, 0.625, 0.75 and 0.875 were examined in the 500 pseudo-random scenarios. The remaining four parameters have the values [selacksize=1, quench=no, revupdates=no, thresh=5]. The following two tables give the median and 90% range results for each value.
Low values of lower the utilisation of the highest utilised link as predicted. Router queue lengths increase and packet loss also increases as approaches 1.0. The number of ABT updates is not substantially affected by any value of .
However, as approaches 0.0, router queue lengths increase and packet loss increases dramatically. From the results given in the tables above, I would recommend values in the range 0.375 to 0.5, where the / congestion avoidance scheme is implemented.
Another design goal for the rate-based congestion control framework, given in Section 4.2, is that retransmission schemes such as Selective Acknowledgment should be used, as they retransmit packets only when necessary. This gives a parameter in the framework: the number of data packets that a Selective Acknowledgment can acknowledge.
A higher number of data packets acknowledged per ack packet lowers the required rate for the acknowledgment traffic flow, and thus lowers the load on the network. However, the drawback of a higher number of data packets acknowledged per ack packet is that the framework's congestion information takes longer to reach the source, as the information is delayed until a `full' selective acknowledgment packet is transmitted. Another drawback to Selective Acknowledgments is the complexity that they add to a transport protocol.
The TRUMP protocol, implemented in REAL, allows between 1 and 16 data packets to be acknowledged in each acknowledgment packet. The effect of selective acknowledgment sizes 1, 2, 4, 8 and 16 data packets were examined in the 500 pseudo-random scenarios. The remaining four parameters have the values [quench=no, revupdates=no, thresh=5, alpha=0.75]. The following two tables give the median and 90% range results for each selective acknowledgment size.
|Median||Size 1||Size 2||Size 4||Size 8||Size 16|
|90% Range||Size 1||Size 2||Size 4||Size 8||Size 16|
The tables show that higher selective acknowledgment size increase packet loss, queue lengths and ABT updates. End-to-end variance also increases, and this is due to the extra delays in acknowledging several data packets. Surprisingly, a selective acknowledgment size of one appears slightly worse than a size of two: router queue sizes are increased, and network utilisation is lowered across the most congested link. However, the best packet loss results are obtained for a selective acknowledgment size of one.
The effects on other measurements are not significant. Given that the results indicate some problems with high selective acknowledgment sizes on queue lengths, packet loss, end-to-end variance and most congested link utilisation, I would suggest that small selective acknowledgment sizes be used. Taking into account the packet loss results and the complexity of implementing a Selective Acknowledgment scheme, I would recommend that selective acknowledgments not be used in the rate-based congestion control scheme, and that transport protocols implement a `One data packet, one acknowledgment packet' Acknowledgment scheme.
Rate Quenches also help to mitigate the effect of network delays, as was shown in Scenario 9 in Chapter 9. As large selective acknowledgments add delays to the network, then I would also highly recommend the use of Rate Quenches where large selective acknowledgment sizes are employed.
If there is no room to queue a newly-arrived packet in a router, one or more packets must be dropped by the router. The function which determines which packet(s) are dropped is the Packet Dropping Function. The following dropping functions are available in the REAL network simulator:
In fact, more than one packet may be dropped in the last four schemes so that enough room is made available to queue the newly-arrived packet. For example, several small acknowledgment packets may need to be dropped to queue a newly-arrived data packet.
In order to distinguish between the results of the five schemes, each was run on those randomly-generated scenarios where packets were lost with TRUMP/RBCC. Other parameter values were set at [selacksize=1, revupdates=no, quench=no, thresh=5, alpha=0.75].
The two schemes with the best combined packet loss/queue length results are Decongest First and Drop Tail. Decongest First has better queue lengths, network utilisation and end-to-end variance than Drop Tail. Despite this, I would argue for the use of Drop Tail as the preferred packet loss mechanism, as it is simple and imposes less overhead on routers than Decongest First.
The parameters available within the implemented functions of the rate-based congestion control framework do influence the characteristics of the framework, congestion and otherwise. At least two of the five parameters described above, the threshold and the scaling factor , have a significant negative impact on the framework's congestion control operation for certain values. Other parameters have some impact, but in general the framework works quite well for all their values.
If the congestion control framework was deployed with the function instantiations examined in this chapter, I would recommend the following parameter values:
A 5-tuple of parameters was chosen from the range of recommended parameters above: Rate Quench packets, no reverse updates, Drop Tail as the packet dropping mechanism, a threshold value of 4 packets, and an value of 0.4. The following table gives the results in the 500 random scenarios, for these parameters against the 5-tuple used in Chapter 10.
|Measurement||Ch. 10 Parameters||Recommended Parameters|
|Median||90% Range||Median||90% Range|
The recommended parameter values give a 47% drop in packet loss, with a lowering of the queue average in the most congested routers. Link utilisation for the most congested link is slightly lower, as is the end-to-end variance. With the threshold lower, congestion avoidance occurs more frequently, and so the number of ABT updates has increased. The result is a small increase in the amount of RBCC work performed which gives a marked improvement in packet loss, offset by a slight lowering in link utilisation.
By tuning the parameters available with TRUMP and RBCC, the rate-based congestion control framework achieves 600 times fewer packets losses than TCP Reno, and 270 times fewer packets losses than TCP Vegas, in the 500 randomly-generated scenarios. Overall, throughput and network utilisation is improved, end-to-end standard deviation is lower, and the extra workload on the intermediate routers appears small. The framework provides excellent congestion control for connectionless packet-switched networks.