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Subsections


Simulated Network

There are may ways in which an asymmetric satellite connection might be deployed as part of an NSPs network. This paper considers two architectures.

US and NZ proxies

The main elements of the first of these architectures is shown in figure 1. The diagram shows web clients located in NZ connecting to a NZ proxy. This proxy in turn connects to a US based proxy which, in its turn, connects to web servers based in the US. There are three TCP connections involved in fetching a web page. The first connects the web client to the NZ proxy, the second is between the two proxies and the third is from the US proxy to the US server. Multiplexing is implemented between the proxies. That is the data for different replies may be interleaved on a single TCP connection between the proxies. The overhead of multiplexing is assumed to be, on average, 20 bytes per HTTP reply segment received from an HTTP server. It is expected multiplexing will improve the efficiency of the international link. Because TCP does not maintain the boundaries between application requests the data from (possibly different) HTTP reply packets may be repackaged for more efficient TCP transmission. In most cases the TCP segments will be the maximum MSS size. Because the connections between the proxies persist indefinitely the effect of TCP slow start is greatly reduced over the satellite component of the network, which is where would otherwise have the greatest effect.

NZ only proxy

A second, simpler, case is also considered. In this second case the US proxy is replaced with a router. Only two TCP connections are involved with fetching a web page. The first, from the web client to the NZ proxy, is the same as in the US-proxy case. The second connection is from the NZ proxy to the US server providing the web page. In this case there is a TCP connection across the international link for each concurrent HTTP request. In the earlier case the number of TCP connections may be limited by the proxies.

Table 1: Main Network Parameters
Satellite Bandwidth 34.368Mbps (E3)
Satellite Delay 320ms[4]
Terrestrial Bandwidth 34.368Mbps (E3)
Terrestrial Delay 60ms
TCP buffer size  
Proxies 32767
Servers as measured
Maximum Segment Size  
Between proxies 1460
Elsewhere as measured
Delayed in US cloud as measured
Delays in NZ cloud not simulated

The main parameters of the network are shown in table 1. The values have been chosen to match real network parameters where possible. The bandwidth delay product for this network is given by:

$BDP = (D_{sat} + D_{ter}) * B)$
where: $D_{sat}$ is the satellite latency
  $D_{ter}$ is the terrestrial latency
  $B$ is the link bandwidth
so

$\begin{tabular}{ll}
$BDP$\ &= $( 0.320 + 0.060 ) * 34.368$\\
&= $ 13.1$\ megabits\\
\end{tabular}$

That is the TCP buffers need to be larger than 1.6Mb to allow this link to be filled by a single TCP connection. If smaller buffers are used the buffer will be filled before the first data sent has been acknowledged and the flow of data into the link will be suspended while the transmitter waits for an acknowledgment. Standard TCP limits the window size to 64kb[1]. The big window extension for RFC1323[5] extends this limit to $2^{32}$. Use of this option is not widespread and is outside the control of the NSP. If an implementation is limited to 32767 bytes (a common implementation maximum) then the maximum bit rate for a single TCP connection over this link is:

$\begin{tabular}{ll}
$MBR$\ &= $S / ( D_{sat} + D_{ter}$\ )\\
&= $32767 * 8 / ( 0.320 + 0.060 )$\\
&= $689832 bps$\\
\end{tabular}$
where: $D_{sat}$ is the satellite latency
  $D_{ter}$ is the terrestrial latency
  $S$ is the buffer size


This effect is seen in simulation of the link and is shown in figure 4. This graph shows that the bandwidth consumed by a single TCP connection as the offered load increases plateau at around 700kbps. This bandwidth limitation is expected to impact negatively on the performance of the international connection, especially where a US based proxy is used and the number of connections between the US and NZ proxies is small. Real asymmetric satellite architectures would be not be as simple as the one described in this paper. Most will need more than a single proxy at each end of the international link to support the required load. The NZ proxy would almost certainly include a cache that satisfies some of the HTTP requests locally. There are many routers not shown, some of which are central to the satellite feed. Most real networks include equal terrestrial capacity in both directions, the satellite link serving to boost the incoming bandwidth. The simpler architecture used in this paper makes the simulation easier and allows the difference between the ways of using a satellite link of this type to be examined without interference from the full range of factors that would impact the performance of a real system.
next up previous
Next: Simulated Workload Up: The effect of multiplexing Previous: INTRODUCTION
A.McGregor, M.Pearson, J.Cleary
November 1998