  BitTorrent has recently introduced LEDBAT, a novel application-layer
congestion control protocol for data exchange. The protocol design starts from
the assumption that network bottlenecks are at the access of the network, and
that thus user traffic competes creating self-inducing congestion. To relieve
from this phenomenon, LEDBAT is designed to quickly infer that self-induced
congestion is approaching (by detecting relative changes of the one-way delay
in the transmission path), and to react by reducing the sending rate prior that
congestion occurs. Prior work has however shown LEDBAT to be affected by a
latecomer advantage, where newly arriving connections can starve already
existing flows. In this work, we propose modifications to the congestion window
update mechanism of the LEDBAT protocol that aim at solving this issue,
guaranteeing thus intra-protocol fairness and efficiency. Closed-form
expressions for the stationary throughput and queue occupancy are provided via
a fluid model, whose accuracy is confirmed by means of ns2 packet level
simulations. Our results show that the proposed change can effective solve the
latecomer issue, without affecting the other original LEDBAT goals at the same
time.
