2 What ’ s the Problem ? 2 . 1 Basics TCP uses the following algorithm to adjust its congestion window

We are working on a modification to TCP’s window increase/decrease algorithm that would allow TCP to run with high congestion windows with realistic packet drop rates. TCP’s sending rate is roughly packets per round-trip time, for the packet loss rate on the path. This is a direct consequence of TCP’s halving its congestion window in response to loss, and increasing the congestion window by one packet per round-trip time otherwise. TCP’s response function, with its average congestion window of packets, places an upper bound on achievable congestion windows, given some underlying packet drop rate from corruption and/or congestion. For example, if the packet corruption rate is , this limits the average congestion window to . (Note that, for 1500-byte packets, a packet corruption rate of results from a bit corruption rate of ! " ) For example, for a TCP connection with 1500-byte packets and a 100 ms round-trip time, filling a 10 Gbps pipe would require a congestion window of $# %'& % % % packets, and a packet drop rate of at most one drop every () & & & packets (because (* + ! ). This is at most one drop per ,/. seconds (because ,0 ( 12 435 6 ! ). This is past the limits of achieveable fiber error rates. In contrast, to fill a 100Mb pipe, the loss rate must not exceed 1 in 500,000 packets, or equivalently, one per ,7 8. seconds (because ,9 :( '1; 43< = ! ). It is easy to design congestion control schemes that achieve higher sending rates at a given loss rate. However, the challenge is to do so while retaining the TCP-compatibility (or TCP-friendliness) properties of the congestion control algorithm; any new congestion control algorithm will have to coexist with existing TCP implementations, and the challenge is to enable the modified congestion control algorithms to achieve high speed while, at the same time, not unfairly stealing bandwidth from unmodified TCPs. Some might argue that fairness does not matter, and that future networks will use QoS mechanisms and per-flow scheduling (or other forms of router-enforced fairness). In contrast, we expect that, while QoS mechanisms and a range of scheduling mechanisms will likely have an important place in future networks, best-effort traffic and FIFO scheduling will continue to have their place also, due to their fundamental simplicity and “good enough” performance.