An Investigation into Transport Protocols and Data Transport ...

6.2. Discussion and Deployment Considerations of New-TCP Algorithms129
the congestion points [GM, BC]. Another problem related to this is the
prevalence of asymmetric links on the Internet [BPK99] which means that
the actual calculation of the bandwidth delay product could be wrong and
hence also affect the calculation of the appropriate window size. Indeed, it is
paramount that TCP Vegas and FAST maintain some indication of the RTT
through the injection of packets in order to successfully operate, otherwise
persistent congestion may occur [LWA98, JWL + 03].
However, the same problem of avoiding the induction of packet loss on
a network also prevents algorithms like Vegas and FAST from being able to
compete with other loss-based algorithms on the same link. As loss based
algorithms only decrease their sending rates in response to loss, in order for
FAST to compete comparatively it will have to induce loss. As TCP Vegas
is incapable of inducing packet loss, it is unable to compete with aggressive
loss based flows [KRB05].
6.2.2 Buffering and Packet Bursts
[KRB05] state that the losses experienced by a TCP flow under drop tail
conditions are related to the algorithm’s increase parameter. This is important
as it implies that the aggressiveness of any algorithm is limited by the
provision of network buffers available.
ScalableTCP as shown in Figure 6.2 has a continuously high increase
value as a result of its multiplicative increase parameter. H-TCP, on the
other hand, experiences only periods of very high α. It should be noted
however, that the increase is only temporary: with its time based increase
factor, the extra buffering after congestion is reduced due to the reversion of
H-TCP back to α = 1.