acm sigcomm

"Tail loss" in TCP connections is a situation that occurs when packets at the end of a burst are lost. These losses cannot be recovered through TCP’s fast retransmit mechanism, thus must be recovered through retransmission timeouts (RTOs). Recent work [11] found that tail loss is surprisingly common and can have a significant impact on flow completion times. There are two current proposals for addressing tail loss in TCP to reduce flow completion delays: tail loss probe (TLP), which was introduced in [11] and is currently an Internet Draft[9], and RTO restart (RTOR), which also currently exists as an Internet Draft [14]. In this paper by Rajiullah et al., the authors seek to empirically compare the effectiveness of these two approaches. They also evaluate a combined approach, which they call TLPR. In laboratory experiments with losses that are controlled to explicitly occur in the tail of a burst, and in experiments in which losses arise purely due to congestion, the authors find that the combined approach almost always performs better than either TLP or RTOR alone. Since any modifications to TCP can have widespread impact, the reviewers of this paper pressed the authors to provide the most clear explanations possible in all phases of their work, and to be concrete and quantitative in their comparisons. Interestingly, it was the reviewers of this paper that suggested using a combination of TLP and RTOR which, as the paper shows, results in the most effective approach. There are still questions regarding how realistic the evaluations are and how the mechanisms will perform in the wild, but it is likely that some form of these proposed modifications to TCP will eventually be standardized and that this work will be instrumental in that process. Public review written by