Paper Review : TCP Vegas : End to End Congestion Avoidance on a Global Internet

 

Reviewer : Seh Leng Lim

 

This paper describes an implementation of TCP, based on modifications to the Reno implementation of TCP, that is refered to as TCP Vegas.

 

The main contribution of the paper is its proposal for improving TCP, which includes a new timeout mechanism, a novel approach to congestion avoidance that tries to control the number of extra buffers the connection occupies in the network, and a modified slow-start mechanism. TCP Vegas has very promising experimental results from both simulation and the Internet, whereby TCP Vegas achieves between 37 and 71% better throughput with one-fifth to one-half losses, as compared to TCP Reno. TCP Vegas is just as fair as TCP Reno, it does not suffer from stability problems and does not adversely affect latency.

 

The key main ideas expounded are:

†††† (a) A new retransmission mechanism which retransmits relevant segment when the difference between the current time and the time recorded for sending is greater than the timeout value

(b)    A congestion avoidance mechanism which limits the number of buffers per connection

(c)    A modified slow-start mechanism which allows exponential growth every other RTT, thereby enabling detection and avoidance of congestion during slow-start

 

I think that the paper has a significant contribution (rating of 4) to the study of congestion avoidance mechanisms in the computer networks. The paper does have convincing experimental results to show that TCP Vegas does improve performance over TCP Reno on the Internet. At the same time, the authors have also attempted to perform simulations to study issues such as how Vegas connections affect Reno connections, and what happens when all connections are running over Vegas.So far, the simulation results have been promising.

The authors mentioned two alternative approaches such as having an intelligent buffer manager in the router to guarantee real-time connections as well as selective ACKs to decrease the number of unnecessarily retransmitted packets. However, the authors have brushed them both away as having no influence on TCP Vegas. For the first, the authors opined that real-time connections can request (and pay ) for a minimally acceptable bandwidth, and then use TCP Vegas to achieve end-to-end congestion control. For the second, the authors declare that selective acks have very little effect on TCP Vegasí retransmissions as there were only 6 KB per MB of unnecessarily retransmitted packets.

 

Researchers and builders who build TCP/IP stacks, routers or even internet applications will have a better appreciations of TCP Vegas and how it improves network connectivity.