A Comparison of Mechanisms for Improving TCP Performance over Wireless Links

1. A Comparison of Mechanisms for Improving TCP Performance over Wireless Links Course : CS898T Instructor : Dr.Chang - Swapna Sunkara

2. Presentation Overview • Motivation • Protocol Proposals for TCP Enhancement • Implementation Details • Performance Analysis • Conclusion

3. Motivation • Transmission Control Protocol (TCP) is known to be a reliable transport protocol . • Packet losses in the Wireless Access Networks are mistaken for congestion losses. • Handoffs and bit-errors cause significant packet losses and variable delays. • Invoking congestion control and avoidance algorithms to deal with these losses yield lower performance in wireless environments

4. Proposed Protocols • The three ideas are proposed in three different classes of schemes . • Link-Layer Protocol (TCP-aware link layer) • End-to-End Protocol • Split-Connection protocol

5. Link-Layer Protocol • Hides link-related and noncongestion related losses from the TCP sender. • Uses local retransmissions and Forward error correction (FEC). • Simple way to achieve this is to have the base station attempt to retransmit the packets lost due to the wireless connection. • Sender may face the problem of receiving several duplicate acknowledgements from the receiver and also retransmit, resulting in redundant retransmits. • Suppressing duplicate acknowledgements from the receiver, which is known as the snoop scheme.

6. End-to-End Protocol • Uses either selective acknowledgements or explicit loss notification. • Selective acknowledgements (SACK's) technique allows the sender to recover from multiple packet losses without a timeout. • SACK's can be used with SMART ACKs. • Explicit loss notification (ELN) attempts to have the sender distinguish between losses caused by congestion and noncongestion.

7. Split-Connection Protocol • Attempts the isolation of source, the sender from the wireless link by terminating the TCP connection at the base station. • Both wireless and the wired links are considered as two separate connections. • The base station and the destination host connections are separated. • Second connection can use either negative or selective acknowledgments, rather than just using the regular TCP to improve its performance over the wireless link.

8. Implementation Details • The various protocols are evaluated, implemented and tested in a wireless testbed • End-to-End throughput and goodput are used as metrics of performance for the wired and wireless links. • They have great impacts on the effort, generality and the performance of the links. • The throughput is the number of bytes per transfer time. • The goodput for any link is defined as the ratio of the actual transfer size to the total number of bytes transmitted over that link.

9. Performance Analysis • For all LL protocols, LAN performance is almost the same. • • Simple link-layer reliable protocols could adversely • impact TCP performance. • TCP Reno over wireless links results in low throughput due to the large number of timeouts. • Both ELN and SACK will result in good performance. • Split-connection protocols result in good throughput but the performance does not exceed that of an LL protocol.

10. Conclusion • The reliable link-layer protocol with some knowledge of TCP semantics results in very good performance. • Splitting the end-to-end connection at the base station is not a requirement for good throughput. • Selective acknowledgements and explicit loss notifications result in significant performance improvements.

11. ?? Questions ??

12. Thank you