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# Block Resampling

Table 7.1: Estimated Hurst parameters and their confidence intervals for the connection arrival time series of UNC 1 PM and UNC 1 AM, and their Poisson arrival fits.
Trace Estimated Parameters
UNC 1 PM Conn. Arrivals =0.685041 C.I.=[0.646250, 0.723831]
Poisson secs. =0.506069 C.I.=[0.467279, 0.544860]
UNC 1 AM Conn. Arrival =0.756533 C.I.=[0.717743, 0.795324]
Poisson secs. =0.502217 C.I.=[0.463427, 0.541008]

The basic assumption of Poisson Resampling is that connection inter-arrivals are independent and identically distributed according to an exponential distribution, which results in a Poisson arrival process. While the choice of exponential inter-arrivals is reasonable given the measurement data presented in Figures 7.1 to 7.4, the arrival process may not necessarily be independent. On the one hand, we can argue that common application protocols make use of more than one connection, creating correlations among some start times. For example, web browsers often open several connections for downloading a web page. On the other hand, we focus on traces of highly aggregated traffic, where a large number of hosts start hundreds or even thousands of connections every second. The high aggregation could diminish or even eliminate completely any correlation structure in the connection arrival processes.

The analysis of our traces reveals non-negligible correlations in the connection arrival process. Figures 7.14 and 7.16 examine the arrival process for the UNC 1 PM trace. Using a time series of 10-millisecond bin counts, Figure 7.14 compares the burstiness of the original arrival process (dashed line) and that of a Poisson arrival process with the same mean inter-arrival time (solid line). The original arrival process was far more variable. Its standard deviation was equal to 346.07, while the one for the Poisson process was equal to 79.21. In order to further study the connection arrival process across a range of time-scales, we rely on wavelet analysis. Figure 7.16 shows the wavelet spectra of the original connection arrivals and a Poisson process with the same mean inter-arrival time. The Poisson process exhibits the expected flat spectrum of short-range dependent processes [HVA02]. On the contrary, the spectrum for the original connection arrivals follows a line with a substantial positive slope, which is characteristic of long-range dependent processes. The results of the wavelet-based estimation [AV98] of the Hurst parameters of these processes are given in table 7.1. The Poisson process has a Hurst parameter very close to the expected 0.5, while the original arrival process has a Hurst parameter of 0.685. This is consistent with moderate long-range dependence. For comparison, typical estimates of the Hurst parameter for packet and byte arrival processes are between 0.75 and 0.95, i.e., typical packet and byte arrival processes exhibit significantly stronger long-range dependence than this connection arrival process.

We performed a similar analysis for the UNC 1 AM trace, and the results are shown in Figures 7.15 and 7.17. As in the previous case, the time series plot shows a connection arrival process that is significantly more bursty than that of a Poisson process with the same mean. Note however than in this case there is some degree of non-stationarity. We observe a significantly larger number of connections started in the first 5 minutes, and a significantly lower number started in the last 10 minutes. In this case we compute the mean inter-arrival rate required to construct the Poisson arrivals using the middle 40 minutes of the trace. We therefore handle the effect of trace boundaries by ignoring the first and the last few minutes of the arrival process. The wavelet spectra for these middle 40 minutes and a Poisson process with the same mean arrival rate are shown in Figure 7.17. As in the UNC 1 PM case, the original connection arrival process exhibits clear long-range dependence. The estimated Hurst parameter in Table 7.1 reveals a somewhat stronger long-range dependence for the UNC 1 AM trace (0.757 vs. 0.685).

In summary, the connection arrival processes we have examined are consistent with significant long-range dependence. Therefore, it is desirable to develop the resampling and load scaling methods that can reproduce this structure, to cover experiments where the manner in which connections arrive is relevant for the network phenomenon studied using synthetic traffic. One example of this type of scenario is the evaluation of a router mechanism where the arrival of new connections creates new state in the router. For such a mechanism, a more bursty arrival process creates a more stringent workload, just like burstier traffic was shown by [BC98] to be more demanding on web server performance.

Poisson Resampling cannot reproduce this observed long-range dependence in the connection arrival process since its inter-arrivals times come from independently sampling an exponential distribution. For this reason, we propose a second resampling technique that can reproduce the long range dependence in the connection arrival process. The starting point is the intuition that dependencies between connection start times are far more likely to occur within relatively small periods. For example, web browsing results in new connections started according to the sequence of web page downloads and the way the browser opens new connections to the servers in which these pages are found. This results in brief bursts of connections whose start times are correlated. We use this intuition to develop a resampling method wherein the resampled objects are not individual connections, but groups of connections started during the same period, which we call blocks. The key idea of our Block Resampling method is that sampling blocks of connections rather than individual connections preserves the relative offsets of connection start times within blocks, and therefore the dependency structure7.6Our method is derived from the Moving Block Bootstrap method [ET93].

Figure 7.18: Block resamplings of UNC 1 PM: impact of different block lengths on the wavelet spectrum of the connection arrival time series.

Figure 7.19: Block resamplings of UNC 1 AM: impact of different block lengths on the wavelet spectrum of the connection arrival time series.

Block Resampling proceeds in the following manner: Given a trace , we divide it in blocks of duration , so that the first block groups together connections started in the interval , the second block groups together connections started in the interval , and so on. The block resampled trace is obtained by concatenating randomly sampled blocks, and adjusting the start time of connections in each block by the time offset of the new location of this block. For example, if the random resampling puts block as the first block of , the start times of the i-th connection vector in this block is set to . Similarly, if is placed in the fourth location of , the start times of the i-th connection in this block are set to . More formally, when the -th block in the original trace becomes the -th block in the block resampling, the start time of the i-th connection vector in is set to

Block Resampling chooses blocks for with replacement, making it possible to create new traces that are longer than the original from which the blocks are obtained.

As pointed out by Efron and Tibshirani [ET93], choosing the block duration can be a difficult problem. In our case, we found a clear trade-off between block duration and how well long-range dependence was preserved in the resampled trace. The shorter the block duration, the larger the number of distinct trace resamplings that can be performed from the same trace . This number is equal to for resampled traces with the same duration of the original trace. However, if the duration of the blocks is too small, the process of connection arrivals in the resampled trace exhibits a dependency structure that does not resemble the one in the original trace.

Figure 7.18 explores the impact of block duration on the long-range dependence of the connection arrival process in the resampled trace. The top left plot shows the wavelet spectra of the connection arrivals for UNC 1 PM and for 5 block resamplings where the block duration was 1 second. There is a clear and consistent flat region after octave 8, which shows that blocks of 1 second are too short to preserve the long-range dependence of the original connection arrival process. As the block duration is increased in subsequent plots, we observe an increasingly better match between the arrivals in the block resamplings and the arrivals in the original trace. Blocks with a duration of 30 seconds or 1 minute provide the best trade off between blocks that are large enough to ensure realistic long-range dependence in the connection arrival process, and blocks that are short enough to provide a large number of distinct resamplings. The same sensitivity analysis was performed for the UNC 1 AM trace and the results are shown in Figure 7.19. Block durations of 30 seconds or 1 minute are also shown to perform well.

Figure 7.20: Block resamplings of UNC 1 PM: average offered load vs. number of connection vectors (left) and corresponding histograms of average offered loads (right) in 3,000 resamplings.

Figure 7.21: Wavelet spectra of several random subsamplings of the connection vectors in UNC 1 PM (left) and 1 AM (right)

Table 7.2: Estimated Hurst parameters and their confidence intervals for five subsamplings obtained from the connection arrival time series of UNC 1 PM and UNC 1 AM
Trace Estimated Parameters
UNC 1 PM Conn. Arrivals =0.727540 C.I.=[0.701687, 0.753393]
Subsample 90% Conn. =0.724175 C.I.=[0.698322, 0.750028]
Subsample 80% Conn. =0.724046 C.I.=[0.698193, 0.749899]
Subsample 70% Conn. =0.718502 C.I.=[0.692649, 0.744354]
Subsample 60% Conn. =0.701378 C.I.=[0.675525, 0.727230]
Subsample 50% Conn. =0.701020 C.I.=[0.675167, 0.726872]
UNC 1 AM Conn. Arrivals =0.746591 C.I.=[0.720738, 0.772444]
Subsample 90% Conn. =0.738659 C.I.=[0.712806, 0.764512]
Subsample 80% Conn. =0.725030 C.I.=[0.699177, 0.750882]
Subsample 70% Conn. =0.715679 C.I.=[0.689827, 0.741532]
Subsample 60% Conn. =0.696723 C.I.=[0.670870, 0.722576]
Subsample 50% Conn. =0.691139 C.I.=[0.665287, 0.716992]

The Block Resampling method described so far makes it possible to construct a resampled of arbitrary duration but it cannot be used to adjust its offered load. In order to perform this task, we can rely on thinning, when the offered load of is above our intended offered load, and on thickening, when the offered load of is below our intended offered load. Block thinning involves randomly removing connections from . Theoretical work by Hohn and Veitch [HV03]has shown that the thinning of a long-range dependent process does not change its long-range dependence structure. Our own experimentation confirms this result. Figure 7.21 shows the wavelet spectra of thinned versions of the connection arrivals in the UNC 1 PM trace (left) and the UNC 1 AM trace (right). The overall energy level decreases as the fraction of connections removed from each block increases. However, the spectra maintain their shapes, which demonstrates that the degree of the long-range dependence remains unchanged. The estimated Hurst parameters for these two traces is presented in Table 7.2. The values reveal only a moderate decrease in the Hurst parameter even when half of the connections are dropped.

Block thickening consists of combining more than one block in each of the disjoint intervals of , i.e., to fusion'' one or more blocks from to form a single block in . This makes the offered load a multiple of the original load. For example, to double the load, the connection vectors of two randomly chosen blocks will be placed in the first interval, those from another pair of randomly chosen blocks will be placed in the second interval, and so on. The new start times of the connection vectors in the resampled trace are computed using Equation 7.2, but being careful to use the right for each connection vector.

To achieve offered loads that are not a multiple of the original load, we can combine basic thickening and thinning using a two-step process. The first step is to create a preliminary version of by combining as many blocks as possible without exceeding the target load. The second step is to complete'' this trace by combining it with another block-resampled trace that has been thinned in such a manner that the combined load of the two resampled traces matches the intended load. For example, in order to create a with 2.5 times the load of , a first thickened trace is created by combining two blocks in each position. This trace is then combined with second trace that has been thinned to half of the offered load of . From our analysis in Figure 7.20, we can see that is not necessarily equal to twice the offered load of . For this reason is actually thinned to exactly the offered load needed to complement , and not just to half of the original offered load This careful thinning makes the scaling match the intended load in a highly precise manner. We can therefore achieve any intended load with the Block Resampling method, so it is as flexible as Poisson Resampling. In accordance with our earlier analysis, accurate thinning cannot rely on any correlation between the number of connections and the offered load, so it must be driven by Equation 7.1, just like byte-driven Poisson Resampling. Therefore, our final resampling technique is Byte-driven Block Resampling.

Figures 7.22 and 7.23 show the result of several testbed experiments where Byte-driven Block Resampling is used to create new traces. The results demonstrate that traces resampled using this method achieve a very good approximation of the target offered loads. As in the case of Byte-driven Poisson Resampling, the achieved loads are slightly higher than target ones due to the packetization overhead, which is not taken into account in the resampling.

One interesting question is whether the effort to preserve the scaling of the connection arrival process has any effect on the generated traffic aggregate. To understand this question, we can compare the process of packet (or byte) arrivals from block resamplings and from Poisson Resampling, since the former fully preserves connection arrival long-range dependence and the latter fully eliminates it. Figure 7.24 shows the wavelet spectra of the packet arrivals in UNC 1 PM and those in two testbed experiments where byte-driven block resamplings of UNC 1 PM were replayed. Figure 7.25 shows the same wavelet spectrum of the packet arrivals in UNC 1 PM, and also the spectra from three testbed experiments where byte-driven Poisson resamplings of UNC 1 PM were replayed. Both resampling methods achieve equally good approximations of the packet scaling found in the original trace. In other words, according to this type of analysis, the simpler Poisson Resampling method performs as well as the more elaborate Block Resampling method. This is a confirmation, using a closed-loop traffic generation approach, of the results by Hohn et al. in [HVA02], which were obtained using (open-loop) semi-experiments. This is not to say that long-range dependence in the arrival of connections (e.g., arrival of flow state or cache misses to a router) can be safely ignored, since other metrics and experimental results may be more sensitive to this characteristic of the synthetic traffic.

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Doctoral Dissertation: Generation and Validation of Empirically-Derived TCP Application Workloads