  Although delay-based congestion control protocols such as FAST promise to
deliver better performance than traditional TCP Reno, they have not yet been
widely incorporated to the Internet. Several factors have contributed to their
lack of deployment. Probably, the main contributing factor is that they are not
able to compete fairly against loss-based congestion control protocols. In
fact, the transmission rate in equilibrium of delay-based approaches is always
less than their fair share when they share the network with traditional
TCP-Reno derivatives, that employ packet losses as their congestion signal.
There are also other performance impairments caused by the sensitivity to
errors in the measurement of the congestion signal (queuing delay) that reduce
the efficiency and the intra-protocol fairness of the algorithms. In this paper
we report, analyze and discuss some recent proposals in the literature to
improve the dynamic behavior of delay-based congestion control algorithms, and
FAST in particular. Coexistence of sources reacting differently to congestion,
identifying congestion appearance in the reverse path and the persistent
congestion problem are the issues specifically addressed.
