TCP fast recovery strategies: analysis and improvements

摘要

To match an ideal Internet gateway which rigorously enforces fair sharing among competing TCP connections, an ideal TCP sender should possess two properties while obeying congestion avoidance and control principles. First, the TCP sender which under-uses network resources should avoid retransmission time-outs. When experiencing network congestion, a TCP connection should not time-out unless it has already reduced its congestion window to one packet but still cannot survive. Second, the TCP sender which over-uses network resources should lower its bandwidth. The congestion window for a connection should decrease each time a lost packet is detected because an ideal gateway will drop packets, during congestion, with a probability proportional to the bandwidth of the connection. Following these guidelines, we propose network-sensitive Reno (Net Reno), a set of optimizations that can be added to a traditional Reno TCP sender. Using the TCP's self-clocking property and the packet conservation rule, Net Reno improves Reno and its variants (New-Reno and SACK), in reducing TCP retransmission time-outs (RTOs) and in being conservative in network usage during the fast recovery phase. We have shown that over 85% of RTOs are due to small congestion windows that prevent fast retransmission and recovery algorithms from being effective. This implies that sophisticated recovery schemes such as SACK will have limited benefits for these loads. Net Reno overcomes this problem with a small window optimization. Net Reno can recover any number of packet losses without time-outs as long as the network keeps at least one packet alive for the connection.