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 ?? ?? Questions ??

12  Thank you