  Last year, the official BitTorrent client switched to LEDBAT, a new
congestion control algorithm targeting a lower-than Best Effort transport
service. In this paper, we study this new protocol through packet-level
simulations, with a special focus on a performance comparison with other
lower-than Best Effort protocols such as TCP-LP and TCP-NICE: our aim is indeed
to quantify and relatively weight the level of Low-priority provided by such
protocols.
  Our results show that LEDBAT transport generally achieves the lowest possible
level of priority, with the default configurations of TCP-NICE and TCP-LP
representing increasing levels of aggressiveness. In addition, we perform a
careful sensitivity analysis of LEDBAT performance, by tuning its main
parameters in both inter-protocol (against TCP) and intra-protocol (against
LEDBAT itself) scenarios. In the inter-protocol case, although in case of
misconfiguration LEDBAT competes as aggressively as TCP, however we show that
it is not possible to achieve an arbitrary level of low-priority by merely
tuning its parameters. In the intra-protocol case, we show that coexistence of
legacy flows with slightly dissimilar settings, or experiencing different
network conditions, can result in significant unfairness.
