Using Measurements to Validate Simulation Models of TCP/IP over High Speed ATM Wide Area Networks

Overview of TCP/IP over ATM

1 Importance of TCP/IP over ATM

Probably the most succesful idea in data networking over the past twenty years has been the concept of internetworking [13]. It is a method for interconnecting networks, regardless of the particular networking technology (Ethernet, ATM, HIPPI, Frame Relay, FDDI), used by the individual systems. What makes internetworking possible is the development of protocols like TCP/IP. The TCP/IP protocol suite is the internetworking protocol used on the Internet, a global collection of networks connecting millions of computers and users, and incorporating a large variety of different network technologies [13]. It allows computers of all sizes, from many different computer vendors, running totally different operating systems, to communicate with each other [17].
ATM technology is the emerging standard adopted by telecommunications and computer vendors for high speed networks. The fast-cell switching technology employed by ATM helps to provide scalable (in size and speed) networks [5]. Further, ATM technology provides bandwidth on demand and enables the integration of real-time and data traffic over the same physical medium for wide area networks.
Although TCP/IP and ATM often have been viewed as competitors, their complementary strengths and limitations form a natural alliance that combines the best aspects of both technologies [14]. In the near future, a large portion of the traffic carried by the ATM networks will be generated by applications written to run over a TCP/IP protocol stack [11]. In fact, many of the existing ATM networks employ TCP/IP over ATM technology.

2 Previous Studies of TCP/IP over ATM

Numerous simulation and experimental studies have been performed in order to predict the performance of TCP/IP over ATM under congestion and buffer overflow conditions that arise from bandwidth mismatches or multiple sources contending for the same link [2, 5, 8, 11, 15, 16, 19]. The simulation study in [16] examines the performance of TCP over ATM under conditions of network congestion. The results show that the TCP/IP over ATM performance is poor when there is congestion caused by small switch buffers and large TCP segment and window sizes. This performance degradation is caused by a loss-rate multiplier effect caused by the switch dropping cells from multiple packets. In [15], the performance of TCP connections over ATM networks without ATM-level congestion control is investigated. Simulation results of congested networks show that the effective throughput of TCP over ATM can be quite low when cells are dropped at the congested ATM switch. To improve the performance, a mechanism called early packet discard (EPD) which brings throughput performance to its optimal level is proposed.
The work in [11] considers some undesirable interactions between the congestion control scheme used in TCP and the policing mechanisms used in ATM networks that can significantly degrade the throughput of TCP traffic. It is shown that in the presence of policing, once a TCP connection has increased its window size beyond the sustainable cell rate (SCR) times the round-trip time and if the bottleneck capacity exceeds the SCR, the buffer at the site providing the policing mechanism fills up quickly and most of the packets in the TCP window are dropped. This causes the average throughput to be significantly lower than SCR value. In order to improve the performance, the use of smarter policing or cell-level traffic shaping schemes is suggested. The work in [2,12] describes performance measurements taken from the MAGIC gigabit testbed relating to the performance of TCP in wide area ATM networks. Results show that the TCP rate control mechanism alone is inadequate for congestion avoidance and control in wide area gigabit networks. It is also illustrated that TCP augmented by cell-level pacing allows the full link capacity to be utilized. Cell level pacing is necessary because the TCP rate control mechanism does not control traffic burstiness sufficiently to avoid congestion-induced cell losses in wide area networks. These studies separately present simulation and measurement results. However, comparison of performance predictions from measurement techniques and simulation models of TCP/IP-ATM networks is needed to refine our understanding of the network operation. Validation of models against measurements is also required so simulation models can be used with confidence to capture the effects of network control, and to accurately characterize the performance of ATM WANs. However, in order to achieve that goal, the precision level of the models (which is directly proportional to the model execution time) needs to be determined because of the computational complexity of the simulation.

The ACTS ATM Internetwork (AAI)

KU AAI TCP over ATM Papers

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