SMART Retransmission: Performance with Overload and Random Losses

Feedback flow control, in conjunction with limited buffering in the network, inevitably leads to packet loss. Effective congestion control requires not only effective flow control but also a good retransmission strategy. We present a new retransmission strategy called SMART that combines the best features of the traditional go-back-n and selective-retransmit strategies. We show, first, that go-back-n retransmission with static window flow control leads to congestion collapse when the nominal load exceeds the link capacity. Second, we can avert congestion collapse by replacing go-back-n with SMART retransmission, even with static window flow control. Third, SMART retransmission, when combined with Packet-Pair rate-based flow control, performs extremely well, both when losses are due to buffer overflows and when losses are random.