An Investigation into Transport Protocols and Data Transport ...

9.2. Preferential Flow Handling Using DiffServ 225
under 90% AF allocation still show serious problems with the cwnd dynamic
which consequently results in reduced goodput performance.
The main difference between ScalableTCP and HS-TCP flows are that the
occurrence of the slow start like events for HS-TCP are much lower, making
the effects of the congestion moderation much less noticeable. As such,
the HS-TCP cwnd dynamics are much cleaner and nearer to the theoretical
traces than those of Scalable TCP. However, having congestion moderation
turned off has less overall effect on the cwnd dynamics when AF is allocated
900Mbit/sec - regardless of the TCP algorithm used.
The inference is that even without the implications of congestion moderation,
SACK processing is still a dominant factor which causes cwnd dynamic
deviations from theory that result in the observed decreased goodput performance.
When the AF allocation is 700Mbit/sec, it was observed that the
dominant factor is congestion moderation - as the removal of the function
call completely resolves the problem of deviations of the cwnd dynamic from
theory. But when the AF allocation is 900Mbit/sec, the flux of SACKs is so
high that removing the congestion moderation code only partially resolves
the problems of cwnd dynamics entering low values.
9.2.5 Active Queue Management
In this section, the negative effects of SACK processing are mitigated using
an active queue management solution. The objective was to lower the rate
of SACK events after multiple drops and therefore allow the sending host to
process the SACK Blocks appropriately.
Neither Standard TCP nor HSTCP experienced as many slow-starts as
ScalableTCP. Therefore, an AQM solution was implemented for ScalableTCP