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P36
BEREC recognises that packet loss and packet retransmission has a negative impact on the throughput of each TCP connection and hence the IAS speed.
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P40
• The duration of the test in this example is 10 seconds, however a longer fixed duration could be selected;
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P41
• To mitigate the effect of TCP slow-start, an initial retrieval of a slowstartX.dat file is made to maximise subsequent throughput;
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P42
• Once the transfer of slowstartX.dat file is finished for each connection, the real test commences (using same TCP socket) without delay;
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P43
• All .dat files referred to above contain random data, which cannot be compressed and
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P44
• The test is stopped after a total of 10 seconds, and the valid measured upload/download volume is based on the total transferred volumes in Measurement Data#1, Measurement Data#2 & Measurement Data#3.
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P45
It is recommended that the HTTP transfers are made using chunked transfer encoding to enable the sending side to stop the transfer at the appropriate time.
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P46
The diagram below shows the HTTP transfers in more detail in a message sequence chart format.
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P47
The following sections discuss the ways to calculate the actual data transferred under this methodology.
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3.1.2Calculating speed based on TCP payload
P48
Calculating the TCP payload is relatively straightforward as compared to calculating the IP Payload (see section 3.1.3). For a given HTTP connection, both the client and the server are mutually aware of the data volume transferred. This data volume will vary for each connection due to the recommendation that the measurement test duration is fixed.
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P49
Note that the amount of data transferred will also include the HTTP headers, so it is recommended that in cases where the exact size of these headers is not known, that a fixed 500 byte value is added to the total file size as an approximation.
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P50
The error introduced by this approximation is considered to be negligible except in cases where the test is run very briefly or on extremely slow links.
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3.1.3Calculating speed based on IP packet payload
P51
Calculating speed based on IP packet payload is more complex due to the fact that most platforms don’t allow clients to access this information directly, so it must be calculated based on assumptions, and the results of this calculation is an approximation.
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