Modeling TCP-vegas under on/off traffic

Abstract

There has been a significant amount of research toward modeling variants of the Transmission Control Protocol (TCP) in order to understand the impact of this protocol on file transmission times and network utilization. Analytical models have emerged as a way to reduce the time required for evaluation when compared with more traditional evaluations performed using event driven simulators such as ns. In addition, when designed carefully, analytical models help researchers make design decisions about novel TCP mechanisms.A wide variety of techniques have been applied to the problem of TCP modeling with a fair amount of success. These techniques range over renewal theory [10,17], Markov chains [11], and fluid models [1]. However, until the recent seminal work of papers using fix-point methods [3,4,5,6,7,8,14,15,18], no modeling technique was able to mimic both the structure of a TCP source and the interaction a source has with the network. The fix-point methods take the novel approach of separating the modeling of network behavior from the modeling of the behavior within a TCP source, and then allowing the two to tune each other via feedback. The fix-point framework allows general network topologies to be analyzed, and issues such as the interpretation of end-to-end loss rates in multiple bottleneck networks are addressed in [3,7,8,15,18].In this paper, we generalize the framework based on a fixed point method introduced by Casetti and Meo in [4,5] in order to allows us to model TCP-Vegas connections. The framework of Casetti and Meo, which uses a Markov chain to model the TCP source, has a few advantages over other fixed point methods: (1) it can model explicit details of TCP, making it possible to distinguish different flavors of TCP; (2) it allows modeling on-off traffic sources; (3) it gives the fraction of time that TCP spends in each state, from which we can evaluate the effectiveness of each mechanism of the protocol.A majority of existing analytical models focus on TCP-Reno, the most widely deployed variant of TCP, and there has been little research on analytical models of TCP-Vegas, a more recently proposed variant. Analytical models of TCP-Vegas have been difficult to develop because TCP-Vegas uses observed delay to detect an incipient stage of congestion and try to adjust the sending rate before packets are lost. Prior studies on measurement and simulation of the performance of TCPVegas suggest that in many situations it is able to provide users higher throughput and lower loss rates than TCP-Reno.Hence, it is an important task to model the performance of TCP-Vegas in order to understand how this protocol performs in a network shared with other variants of TCP.Because TCP-Vegas uses observed delay, as well as loss events, to adjust the congestion window size, while TCP-Reno uses only loss events, the extension made in this work to the framework of Casetti and Meo for modeling TCP-Reno is nontrivial. We need to make two major changes. First, analyzing the network model to determine the loss rate and the...

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