Understanding TCP Incast Throughput Collapse in Datacenter Networks

This paper is another examination of solutions to and the cause of the incast problem. They analyze the incast phonemon with and without the changes proposed in the CMU paper (much lower minRTO/disabling delayed ACKs). Unlike the CMU paper, the evaluation here primarily uses a constant data per source scenario rather than a constant data per sink scenario. They find that this fixed-source-size workload yields more complex goodput behavior than the other scenario. And in contrast the CMU findings, the authors find that delayed ACKs, in fact, do help against incast for large numbers of senders because they induce increased variance in the RTT estimate and TCP window size.

The authors produced a simple numerical model of the throughput collapse behavior assuming that it is determined by the number of retransmission timeout events and the interpacket wait time. From this model and refinements, the authors produce an explanation for the throughput collapse behavior: first, throughput collapses as the number of retransmission timeouts increases with the number of senders, then the width of period in which retransmission timeouts are triggered (per congestion event) increases, so recovery senders become desynchronized, but eventually the width of that distribution becomes so wide that senders about to experience a timeout interfere with senders just recovering from a timeout.

It is disappointing that the qualitative observations which would seem to explain the throughput collapse are not incorporated into the model, nor is an good empirical way to test for their presence beyond fitting a more complicated model attempted or suggested.