" On the accuracy of TCP performance models "

Hans-Peter Schwefel (a), Manfred Jobmann (b), Daniel Höllisch (b), Daniel Heyman (c)

(a) Siemens AG, 81359 München, Germany
(b) Technische Universität München, 80290 München, Germany
(c) AT&T Labs, Middletown, NJ 07748, USA


Abstract:

Despite the fact that most of todays' Internet traffic is transmitted via the TCP protocol, the performance behavior of networks with TCP traffic is still not well understood. Recent research activities have lead to a number of performance models for TCP traffic, but the degree of accuracy of these models in realistic scenarios is still questionable.

This paper provides a comparison of the results (in terms of average throughput per connection) of three different `analytic' TCP models: I. the throughput formula in [Padhye et al. 98], II. the modified Engset model of [Heyman et al. 97], and III. the analytic TCP queueing model of [Schwefel 01] that is a packet based extension of (II).

Results for all three models are computed for a scenario of N identical TCP sources that transmit data in individual TCP connections of stochastically varying size. The results for the average throughput per connection in the analytic models are compared with simulations of detailed TCP behavior. All of the analytic models are expected to show deficiencies in certain scenarios, since they neglect highly inuential parameters of the actual real simulation model: The approach of Model (I) and (II) only indirectly considers queueing in bottleneck routers, and in certain scenarios those models are not able to adequately describe the impact of buffer space, neither qualitatively nor quantitatively.

Furthermore, (II) is insensitive to the actual distribution of the connection sizes. As a consequence, their prediction would also be insensitive of so-called long-range dependent properties in the traffic that are caused by heavy-tailed connection size distributions. The simulation results show that such properties cannot be neglected for certain network topologies: LRD properties can even have counter-intuitive impact on the average goodput, namely the goodput can be higher for small buffer sizes.

Keywords:
Transmission Control Protocol (TCP), Throughput Models, ON/OFF TraÆc, Queueing Models, Long-Range Dependence, TCP simulations