To investigate the effectiveness of the packet retransmission mechanism in recovering packet losses in the proposed flexible optical network, the performance quantities, including the resulting net- work workload, the packet retransmission rate, the system throughput, the retransmission buffer size, the packet loss rate and the overall communication latency, are evaluated under both uniform and non-uniform traffic patterns. Two different retransmission policies are considered: Random Retransmission (RR) and Pre-Reservation Retransmission (PRR), which were explained in Sec- tion 5.4. To explore the influence of the two retransmission techniques on network performance, extensive simulations are carried out in network scenarios with different numbers of allowable re- transmissions B. In the experiments, the parameter B is set to be 0 (no retransmission),1, 2, 3, 4, 5 and ∞ (an infinite number of retransmissions). Note that in the PRR scheme, the maximum num- ber of packet retransmissions required is 1, implying that the maximum value of B in this case is 1. Additionally, the link distance between the clusters and the core OPSs is set to be 25m, resulting in a propagation delay of 125ns. We examine the impact of increasing the link length in a later section.
The performance measurements of the optical network employing the two packet retransmission mechanisms are first evaluated and analysed under uniform traffic pattern. Shown in Figures 5.12 - 5.13 are the resulting network workload, the packet retransmission rate, the network throughput
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(a) Resulting network load versus offered network load ρ
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(b) Packet retransmission rate versus offered network load ρ
Figure 5.12: Network workload and packet retransmission rate under uniform traffic pattern.
and the required retransmission buffer capacity. Numerical results are plotted as a function of the offered network load ρ for different values of B. These graphs give an insight of the effectiveness of the two retransmission strategies in resolving traffic loss under various network traffic loads. Figure 5.12 plots the resulting network workload and the packet retransmission rate versus the offered network load ρ for different retransmission scheme configurations. It is intuitive that in the RR scheme, increasing the number of allowable retransmissions B results in an evident increase in the resultant network workload. This is mainly due to the fact that increasing B leads to more frequent packet retransmissions, as shown in Figure 5.12(b), and as packet retransmissions occupy additional network resources, which in turn causes more frequent packet contentions, the amount of traffic in the network is greatly increased. It is further shown that as the input traffic load grows, the increase of the network workload becomes progressively more significant, and consequently, system overloading occurs. This overloading is indeed the overfilling of the input transmission channels of the optical network rather than the output tranmission channels, due to the PreRes scheduling strategy detailed previously. Evidently, the larger is the value of B, the faster is the network infrastructure overloaded. It should be noted that in the scenario of B = ∞, the network infrastructure is overloaded more rapidly than the other scenarios illustrated. More precisely, the system overloading happens when the network load is only around 40%. Furthermore, it is noticed that the PRR scheme appears to result in a smaller increment of the overall network workload and the packet retransmission rate than the RR scheme with B ≥ 4, as it limits the number of required retransmissions to 1, but the PRR policy actually results in a more rapidly overloaded system than the RR scheme, owing to the low resource utilisation, which is attributed to the pre-reservation
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(a) Network throughput versus offered network load ρ
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(b) Buffer size requirement versus offered network load ρ
Figure 5.13: Network throughput and buffer size requirement under uniform traffic pattern.
feature. This drawback of the PRR scheme is confirmed by the network throughput performance and the required buffer size plotted in Figure 5.13. However, also due to the pre-reservation feature, the PRR policy holds an important advantage over the RR scheme, that is, it guarantees the contention performance, since the maximum number of packet retransmissions required in PRR is only 1. This aspect of the PRR will be investigated in detail later.
Figure 5.13(a) illustrates the normalised network throughput under a wide range of offered network loads. The results show that the introduction of the packet retransmission mechanism has greatly enhanced the throughput performance, as it facilitates the data recovery in the optical network. From the graph, it is observed that the throughput improvement attained by the RR scheme largely depends on the number of retransmissions B. This is expected as increasing the number of retrans- missions B allows for a greater number of lost packets to be recovered, thus improving the network throughput. Nevertheless, the relationship between the offered network load ρ and the resultant throughput exhibits a non-linear pattern, due to the existence of traffic loss in the optical network, as indicated in Figure 5.15. It is further shown in Figure 5.13(a) that at low network loads, the PRR scheme outperforms the RR scheme, but beyond a load of 0.4, the PRR scheme yields less through- put improvement. This mainly depends on the fact that the PRR scheme reserves the transmission channels for the lost packets prior to the packet retransmissions, which ensures the contention per- formance, but the downside is that the PRR scheme has low resource utilisation when the packet retransmissions are frequent.
Another important observation is that in all scenarios tested, as the network load continues to grow, the throughput performance degrades significantly. Particularly, the larger is the value of B, the
earlier this degradation occurs. The reason behind this situation is the presence of frequent packet retransmissions, which greatly limits the network capacity due to significant network resources be- ing wasted by failed transmission attempts. It should be pointed out that at the point where the sys- tem overloading just starts to occur, a small increment in network throughput is observed. As stated previously in Section 5.2, each input transmission channel of the OPSs is associated with a virtual electronic buffer queue in the cluster switch. When the system overloading just starts to occur, the virtual queue stores the new arriving packets and then transmits them towards the optical network where these packets may be successfully scheduled on the output channels of the OPSs. In this case, the overloading issue is slightly alleviated and thus a performance improvement is achieved. However, as the network load is further increased, the buffer queue is quickly overwhelmed and new arrivals are being blocked from the optical network, and consequently, no substantial improve- ment on network throughput can be achieved. It is noticed that the network throughput levels off at about 0.52, which corresponds to a normalised throughput of ∼2.1 per switch port, as each switch port carries K( = 4) wavelength channels.
Aside from the impact on network throughput performance, the capacity requirement of the re- transmission buffer also increases dramatically as the network load increases, see Figure 5.13(b). Importantly, a further increase in buffer size is observed when increasing the number of allowable packet retransmissions B. Once again, it is because frequent packet retransmissions considerably increase the resultant traffic load in the network, and accordingly a larger buffer size is required to store the copies of the increased amount of traffic.
Moreover, to illustrate the above mentioned performance characteristics of the packet retransmis- sion schemes for the case of non-uniform traffic, another set of simulations is conducted under hot-spot traffic pattern. The measurements are presented in Figure 5.14. Clearly, the observations show a similar behaviour as the simulation results plotted in Figures 5.12 - 5.13. The difference is that although the average offered network load ρ is still low, the transmission channels at some switch ports are already overloaded, which is attributed to two main factors: (i) the existence of hotspots in the non-uniform traffic matrix and (ii) the increased resulting network workload in- curred by frequent packet retransmissions.
From Figures 5.12 - 5.14, it is observed that in all scenarios under study, once system overload- ing occurs, the throughput performance degrades rapidly and the required buffer capacity increases drastically. This is an undesirable situation in datacentre networking. A common way of overcom- ing this issue is to limit the retransmission buffer and the number of allowable retransmissions so
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(a) Resulting network workload
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(b) Packet retransmission rate
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(c) Network throughput
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(d) Buffer size requirement
Figure 5.14: (a) resulting network workload, (b) packet retransmission rate, (c) network throughput and (d) buffer size requirement are plotted as a function of the offered network load ρ under non- uniform traffic pattern.
as to avoid the network capacity being wasted by unsuccessful retransmissions. Based on the buffer size analysis illustrated in Figures 5.13(b) and 5.14(d), the retransmission buffer length is limited to 25KB, which allows the storage of 40 packets. Nonetheless, limiting the buffer size leads to high packet loss, especially under heavy load conditions, due to traffic overflowing from the queue. This effect will be illustrated in the performance analysis regarding the packet loss rate and the commu- nication latency in the following set of experiments. The results are taken for both the uniform and non-uniform traffic patterns.
Figures 5.15 and 5.16 show the performance characteristics with respect to packet loss rate and mean end-to-end communication delay under both uniform and hot-spot traffic patterns. The mea- surements indicate that in the RR scheme, as the number of allowable retransmissions B increases,
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(a) Blocking probability
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(b) Overall communication latency
Figure 5.15: Network performance versus offered network load ρ under uniform traffic pattern.
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(a) Blocking probability
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(b) Overall communication latency
Figure 5.16: Network performance versus offered network load ρ under hot-spot traffic pattern.
the contention performance improves considerably. Of course, there is a trade-off to be considered as frequent packet retransmissions introduce additional input queueing delay and retransmission delay. Evidently, when B increases to a sufficiently high value, say B = 24, there is no major differ- ence in performance between the scenarios of B = 24 and B = ∞. Note that in the case of B = ∞, although the number of allowable packet retransmissions is unlimited, traffic loss still occurs under very high load conditions due to retransmission buffer overflow, as a result of the retransmission buffer size being limited. It has also been found that PRR exhibits a very similar performance pattern as RR with large B. The observations show that the PRR scheme generally achieves signifi- cantly better loss performance than the RR strategy, but it leads to a higher communication latency. This is attributed to the fact that PRR guarantees the successful retransmission of the lost packets,
but to achieve this, it may need to store the retransmitted packets in the virtual buffer queue for a longer time so as to avoid input contentions between the retransmitted packets at switch inputs. The figures further illustrate that as the network load ρ increases, the packet loss rate and the com- munication latency first rise smoothly, but at a certain point the performance impairment becomes more evident, because the optical network has reached its capacity limitation as a consequence of frequent packet retransmissions. At this point, the packet loss, arising from traffic overflowing from the limited buffer queue, starts to dominant the overall contention performance. It should be pointed out that, in Figure 5.15, before reaching this particular point, the performance degradation appears to slow down. This is attributed to the effective network load being reduced, since part of the of- fered network load is blocked from the optical network due to limited buffer depth, and thus the contention problem is slightly improved, yielding a small reduction in the average packet loss rate. Beyond this point, there are no major differences in loss performance between different scenarios, but a more significant increase of the communication delay is observed.
The above performance analysis suggests that the packet retransmission mechanism can substan- tially enhance the contention and throughput performance of the proposed flexible optical network, with good latency performance under low traffic loads, especially the PRR scheme. Nevertheless, at high traffic loads, the network performance degrades drastically, arising from the system over- loading issue due to frequent packet retransmissions. Thus, the retransmission mechanism is only suitable when the amount of traffic that needs retransmission is low. For that reason, the packet re- transmission scheme is utilised in conjunction with optical buffering which has proven to be highly effective in lowering packet loss, and is capable of carrying the majority of the contention traffic with proper dimensioning. This hybrid congestion control scheme will be the subject of investiga- tion in the following section.