An Investigation into Transport Protocols and Data Transport ...

9.1. Transfer Tests Across Dedicated Private Wide Area Networks 213
9.1.8 Summary
The main conclusion from these sets of experiments is that there are implementation
problems associated with the deployment of New-TCP algorithms.
The problem stems not directly from the dynamics of the cwnd update algorithms,
but with the actual TCP protocol when used at high speeds.
For typical trans-Atlantic link such as DataTAG, as much as 10,000 packets
worth of segments must be kept in memory to facilitate TCP’s sliding
window. Under delayed acknowledgments, this could result in flux of 5,000
packets per RTT.
The processing burden of having to scoreboard all these acks is increased
with SACK information that is supposed to aid faster recovery upon congestion
detection under multiple packet losses per window. It was demonstrated
that a high flux of SACK Blocks can cause processing lock-out, and therefore
a stall in the TCP ack clocking, and more importantly, a serious decrease in
the goodput performance of the TCP stack.
Another concern is that Linux TCP receivers do not strictly adhere to
delayed acking, and as result may generate many ‘stretched ack’s’ which also
break the ack clock and may skew the calculation of RTT/RTO and cwnd
to the number of packets in flight.
The result of this is under these ‘dubious’ circumstances, the TCP sender
will recalculate the cwnd to be the number of packets in flight (as per [Pos81b,
Jac88]) which will result in deviations from the theoretical cwnd dynamics
and hence reduced goodput performance.
The interaction of many New-TCP flows were also compared and contrasted.
Due to the reduced bandwidth delay product of having many flows,
the effects of the above mentioned implementations are often mitigated.