Congestion Control for High Bandwidth-Delay Product Networks

Summary

This paper addresses the problems resulting from increase in per-flow bandwidth and high latency links, and proposes a solution to this problem.

As more high-bandwidth links and low-latency satellite links get added to the internet, TCP becomes more and more inefficient, no matter what queueing scheme is used. The authors put forth a new protocol called Explicit Control Protocol (XCP), which they've found to outperform TCP in both high-bandwidth, low-latency situations as well as conventional situations. The authors also argue that XCP achieves fair bandwidth allocation, short queues, high utilization and near-zero packet drops. One major feature of XCP is that the controllers for fairness and efficiency are decoupled, allowing each controller to optimize for the one area.

XCP does not use a binary feedback mechanism to signal congestion. Instead, it adds a feedback field in the header, which holds a number that indicates how much the sender should slow down, based on the bottleneck link through the network. XCP routers need to compute the feedback so as to optimize efficiency and min-max fairness.

The authors run several simulations to test XCP performance against TCP Reno. It uses the following queueing protocols: random early discard, random early marking, adaptive virtual queue, and core stateless fair queueing. Their simulations confirm that XCP has high utilization and fairness. It has short queues and near-zero packet drops. Over all the queueing protocols, XCP always performed at least as well as TCP.

Criticism and Questions

I thought this paper was interesting. It's one of the few papers that tries to rebuild a protocol from scratch, which definitely has its own problems. Generally, those problems surround the practicality of the protocol. I liked how the authors stated their assumptions upfront, but also addressed the practicality of deploying a protocol using a new packet format, and provided suggestions on how to do so. One thing I am still wondering about is what the impact would be on the sources to use and understand the XCP packet format.