Improving the Performance of TCP in the Presence of Packet Reordering

Packet reordering adversely affects the behavior and performance of the Transmission Control Protocol (TCP), the predominant transport protocol on today's Internet. This shortcoming is becoming a problem of increasing importance, as packet reordering is on the rise on some Internet paths. The issue at the heart of TCP's poor performance in the presence of packet reordering is its inability to distinguish well between packet loss and packet reordering. In particular, the fast retransmission algorithm, which was added to TCP to improve its performance in the presence of packet loss, often misinterprets reordered packets as lost packets. This is problematic, as TCP uses packet loss as an indication of congestion. Thus, in addition to retransmitting the packet, a TCP sender performing a fast-retransmission also reduces its sending rate by cutting its congestion window in half. The spurious triggering of these actions when reordering is present needlessly degrades end-to-end throughput. We have designed and implemented a backwards-compatible extension to TCP that helps to disambiguate packet reordering from packet loss, greatly reducing the number of spurious fast retransmissions even when excessive reordering occurs. Our approach, which modifies only the receiver, involves withholding for a short, adaptively-determined period of time the duplicate acknowledgments produced by out-of-order packets, rather than sending them back to the sender immediately. Doing so provides the receiver with more time to distinguish between loss and reordering before either setting off a fast re-transmission or resuming normal data flow. Experimental results for connections using our modified receiver show significant improvements in TCP performance in the presence of reordered packets, achieving, in cases of heavy reordering, more than five times the transfer throughput of connections using an unmodified receiver. As a result, we believe the introduction of our scheme augurs well for the future of TCP over paths that reorder packets. Thesis Supervisor: Hari Balakrishnan Thesis Supervisor: Robert Morris Title: Assistant Professor Title: Assistant Professor