next up previous
Next: Conclusions Up: The effect of multiplexing Previous: Simulator Design

Subsections


Results

Figure 4: Link usage against load for a single TCP connection
\begin{figure}
\begin{center}
\begin{tabular}{c}
\psfig {figure=load2NZ-1x.eps,width=7cm,height=7cm}\\
\end{tabular}
\end{center}
\end{figure}

The results of the simulations are presented in the following graphs. Each point on a graph represents a simulation run. The points have been joined with (straight) lines. As discussed in section 2 the bandwidth delay product of the network combined with the 32Kb maximum window size limits the throughput that can be obtained by a single TCP connection. A simulation of a single TCP connection over the asymmetric satellite link is shown in figure 4. The curve is asymptotic to about 700kbps. This corresponds closely to the calculated maximum of 690kbps suggesting that, for this case at least, the asymmetric nature of the link behaves in the same way as a symmetric link with the same total delay.

Figure 5:
\begin{figure}
\begin{center}
\begin{tabular}{c}
\psfig {figure=load2NZ.eps,w...
...at.eps,width=15cm,height=10.5cm}\\
\end{tabular}
\end{center}
\end{figure}

Link Utilisation

In figure 5(a) the load to NZ is plotted against the total NZ bound traffic presented to the US proxy or router. The presented load already includes the TCP headers required to deliver it to the US proxy or router from the HTTP server. Although each TCP connection is limited to 700kbps there are many concurrent TCP connections in the no-US-proxy case (see figure 7(a)). This allows the link to saturate under high loads. The slope of the line through most of the graph is about $1.04$ indicating that there are very few retransmissions occurring. The graph does not tail off until the link is within 0.5% of being saturated.3 Figure 5(b) shows that page latencies increase dramaticly at this time. If there are a large number of concurrent connections between the caches the US-proxy case performs in a very similar way to the no-US-proxy case. The lines for 50 and 70 connections have a slightly smaller slope than the no-US-proxy case indicating a small efficiency gain through repackaging the load on the more heavily used TCP connections. Examination of figure 7(a) shows that there are upto 30 concurrent HTTP requests being carried over a single inter-cache TCP connection. The efficiency gain is small and is probably not a significant saving. For smaller numbers of connections (35 and below) the link does not reach saturation. Instead the TCP connections reach their saturation point and they limit the flow of packets to the international link.

Latency

Perhaps the most interesting result of the study is shown in figure 5(b). The graph shows the average time required to fetch a set of sample pages that were present in all simulations. The result for the US-proxy case, with a large number of connections, is around 25% lower (1s per HTTP request) than the no-US-proxy case. This results from the reuse of the international TCP connections saving most of the cost of slow start. The saving for an HTML page with multiple components may be even greater. A closer examination of the start of the curves (see figure 7(b)) shows that for very low loads the gain is less. This is the case because the initial slow start on the international TCP connections is not amortized over as many HTTP requests and consequently it has a larger effect on the total latency. For smaller numbers of connections the latency rises rapidly as the TCP throughput limit is approached. Comparison of figures 5(b) and 5 indicates that this begins to occur when the TCP connections reach about 75% of their capacity. To achieve the best HTTP latency performance more connections are required than are needed to saturate the international link.

Figure 6: Buffer Usage
\begin{figure}
\begin{center}
\begin{tabular}{cc}
\psfig {figure=buf-to-NZ-m....
...S-p.eps,width=9cm,height=10.5cm}\\
\end{tabular}
\end{center}
\end{figure}

Figure 6 shows the buffer space needed in the routers which feed each end of the international link. Figures [*](a) and (c) show the mean usage while figures  [*](b) and (d) show the peak usage. Note that the graphs have different scales. The peak usage is more erratic than the mean because of subtle interactions between connections. In the no-US-proxy case the buffer space required to avoid packet loss becomes very large as the link to NZ saturates. This is also true of the US-proxy case if the number of connections is large enough to allow the link to saturate. If there are too few TCP connections to carry the load the mean buffer usage reduces as the TCP connections throttle their use of the link. Buffer (or link) usage is never heavy in the NZ to US direction in the simulation.4

Figure 7:
\begin{figure}
\begin{center}
\begin{tabular}{cc}
\psfig {figure=cxn.eps,widt...
...ps,width=9cm,height=10.5cm}\\
\\
\end{tabular}
\end{center}
\end{figure}

Figure 7(a) shows the number of connections between the US proxy and servers for the US-proxy case. In the no-US-proxy case it shows the number of connections from the NZ proxy to US servers. In the latter case this increases rapidly when the international link is saturated because the HTTP requests take a long time to complete (see figure 5(b)). In general the no-US-proxy case uses more connections than the US-proxy case because the` connections take longer to complete. The single inter-proxy TCP connection curve flattens at around 18Mbps because there is insufficient capacity in the NZ to US direction to carry the requests. This is also apparent in figure 7(c). The typical relationship between inbound and outbound traffic can be seen in figure 7(d). When there are sufficient TCP connections to carry the load this shows an inbound to outbound ratio of about 1:19. The difference between the no-US-proxy case and the US-proxy case in figures 7(c) and (d) indicates the saving made by repackaging HTTP requests into a smaller number of larger TCP packets. This has a more significant effect than in the US to NZ direction because HTTP requests are smaller than HTTP replies. The effect is probably not useful in current practice because NZ to US links are not normally saturated. This is because of the requirement to purchase symmetric terrestrial connections. In the longer term the saving may be valuable if the asymmetry introduced by unidirectional satellite links causes the NZ to US links to saturate.

Footnotes

... saturated.3
Note that this graph shows presented load against load carried not presented load against useful data transmitted.
... simulation.4
A real US/NZ link would be more heavily used in the US direction because of requests on NZ servers from US clients. These are not simulated here. We assume that sufficient NZ/US capacity exists to carry the client requests to the US.

next up previous
Next: Conclusions Up: The effect of multiplexing Previous: Simulator Design
A.McGregor, M.Pearson, J.Cleary
November 1998