An Investigation into Transport Protocols and Data Transport ...

9.1. Transfer Tests Across Dedicated Private Wide Area Networks 211
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Figure 9.17: cwnd and SACK Block trace of H-TCP with 400Mbit/sec CBR Background
Traffic Loads on DataTAG.
recovery should occur completely within one RTT.
However, the use of SACKs also imposes an extra processing burden for
the TCP sender. This is shown in Figure 9.17 2 which shows a Web100 trace
of H-TCP on DataTAG with 400Mbit/sec background CBR traffic.
The
zoomed plot on the right shows a magnified section of the same experiment.
Each measurement point for each variable is indicated with a marker. It is
apparent that shortly after the initial congestion notification that there are
periods where no measurements were taken. This is demonstrated with the
lack of measurement markers and the large increases of the number of SACK
Blocks as soon as measurement continues.
This can be explained with the fact that Web100 is composed of two
parts; a kernel part which actually increments all the relevant variables, such
as the number of SACK Blocks; and a user-land library which polls the
/proc structure to retrieve the TCP measurements. Therefore, the periods
of silence shown in Figure 9.17 are due to the lack of servicing by the system
of the user-land application. Meanwhile, the counters are still incremented
2 Graph courtesy of Baruch Evans of Hamilton Institute.