Overview:

Several factors affect the overall performance of TCP in a gigabit wireless LAN. In this project we seek to investigate these factors with a view to proposing a better transport protocol for such a gigabit wireless LAN. The project is a part of the overall Yamacraw initiative to develop a working prototype of a 1Gbps wireless LAN.

The project will have three dimensions. We will examine TCP performance in gigabit networks in general. We will also investigate the behavior of TCP on a MAC layer with bandwidth guarantees. Lastly, we will look at the TCP performance in a rapidly varying physical channel, characteristic of the Wireless LAN environment.

Simulations will be performed and observations characterized with a view to proposing a better transport protocol for the environment under consideration. We will also implement our results in a test-bed to validate simulation results.

Results / Status:

Results

In the first phase of the project, we investigate the performance of TCP over networks that provide Quality of Service (QoS) guarantees with a view to modifying the congestion control mechanisms of TCP to take advantage of bandwidth guarantees provided by the network.

We have simulated the behavior of TCP in networks with guaranteed bandwidth and observed that since TCP is unaware of the underlying bandwidth reservations provided by the network, it fails to efficiently utilize the reserved bandwidth provided by the network.

Figure 1 shows the topology used for our simulation. There are 6 flows, three of which have 4, 2 and 1 Mbps reservations respectively while the remaining three are best-effort flows.

Figure 1: Network Topology

We propose GTCP, which includes modifications to default TCP's startup and congestion control mechanisms to leverage bandwidth reservations provided by the network. Specifically, TCP is made aware of the reserved bandwidth at startup. Also TCP's fast recovery algorithm is modified to improve performance especially in networks with high loss rates.

The modifications implemented in GTCP significantly improve the achieved throughput, compared to default TCP's performance. This is as shown below in Figure 2a below:

Figure 2a

Figure 2b shows that GTCP achieves the expected instantaneous compliant throughput for each of the reserved flows.

Figure 2b

We also investigated how GTCP scales with increasing network capacity, round-trip time, number of flows with reservation and the amount of reservation and our results show that GTCP indeed scales well.

Status

At the moment, we are working on implementing the TCP with the new changes (GTCP) on the Yamacraw Wireless LAN prototype (as shown in Figure 3a), which has a QoS-enabled, TDMA-TDD based Medium Access Control (MAC). This essentially involves interfacing (Figure 3b) and network programming and aims to demonstrate the gains of GTCP over default-TCP in a QoS enabled MAC.

Figure 3a

Figure 3b

Furthermore, we are investigating the effects of lost acknowledgements on the reverse path, different data paths for marked/unmarked packets and losses for marked packets (controlled load service).

Publications & Presentations:

• Y. Zhu, O. Oladeji and R. Sivakumar,
  "Enhancing TCP for Networks with Guaranteed Bandwidth Services,"
  IEEE Global Communications Conference (GLOBECOM), San Francisco, CA, USA, December 2003.

Software Downloads:

People:

• Yujie Zhu (Student)
• Kyu-Han Kim (Alumnus)
• Raghupathy Sivakumar (Professor)

References & Related Work:

TCP in Gigabit Networks

• S. Pink, "TCP/IP on Gigabit Networks," in "High Performance Networks - Frontiers and Experiences," A. Tantawy, Ed., Kluwer Academic Publishers, 1994.
• J. Chase, A. Gallatin and K. Yocum, "End System Optimizations for High-Speed TCP," IEEE Communications Magazine, vol. 39, no. 4, pp. 68-74, April 2001.

TCP over Wireless Networks / LANs

• G. Xylomenos, G. Polyzos, P. Mahonen and M. Saaranen, "TCP Performance Issues over Wireless Links," IEEE Communications Magazine, vol. 39, no. 4, pp. 52-58, April 2001.
• G. Xylomenos and G. Polyzos, "TCP and UDP Performace over a Wireless LAN," Proceedings of IEEE INFOCOM, New York, NY USA, March 1999.
• A. DeSimone, M.-C. Chuah and O.-C. Yue, "Throughput Performannce of Transport-Layer Protocols over Wireless LANs," Proceedings of IEEE Globecom, Houston, TX USA, December 1993.

TCP over Networks with Guaranteed Bandwidth

• W. Feng, D. Kandlur, D. Saha and K. Shin, "Understanding and Improving TCP Performance Over Networks with Minimum Rate Guarantees," IEEE/ACM Transactions on Networking, April 1999.
• Ikjun Yeom and A. L. Narasimha Reddy, "Realizing Throughput Guarantees in Differentiated Services Networks," TAMU-ECE-9806, 1998.
• P. Pieda, N. Seddigh and B. Nandy, "The Dynamics of TCP and UDP Interaction in P-QoS Differentiated Services Networks," The 3rd Canadian Conference on Broadband Research, November 1999.