Reliable Transport Layers in Wireless Networks

1. Reliable Transport Layers in Wireless Networks Mark Perillo Electrical and Computer Engineering

2. Review of TCP • Originally designed for wired networks • Assumptions: • Lost packets are caused by network congestion • Fairly stable RTT • Constant bandwidth • Lengthy connections

3. Characteristics of Wireless Environment • High bit error rates • Errors occur in bursts • High latency when FEC used • Short duration sessions • When adaptive coding is used, bandwidth may vary dramatically

4. Problems With TCP in Wireless Environment • Congestion is often incorrectly assumed to be the cause of lost packets • TCP sender may attempt to resend and then begin a slow start • Throughput limited to inefficient point • Connections short enough so that three-way handshake is relatively large overhead

5. Approaches to Solutions • Link-layerprovide information / reliability at TCP-aware lower layer • End-to-endmake nodes aware of dropped packets, use mechanisms to differentiate between causes • Split-connectionhide link failure from source node

6. Link-layer Protocols • Typical mechanisms used in a wired environment (not TCP-aware) Forward error correction (FEC) Retransmissions (ARQ) • Problems in a wireless environment Timer interactions, fast retransmission interactions, large round-trip variations • Conclusion: A TCP-aware link layer could be very beneficial

7. Snoop Protocol • TCP aware link-layer protocol • Snoop module at base station monitors/caches messages and performs local retransmissions when necessary • Hides low-quality link, temporary disconnections from sender • Full compatibility with existing TCP

8. Snoop_data() Algorithm

9. Snoop_ack() Algorithm

10. End-to-end Solutions • Can be purely end-to-end or end-to-end with help from lower layers • Purely end-to-end mechanisms desirable since no changes need to made to existing network infrastructure • Mechanisms typically used: Explicit loss notification (ELN) - gives information about the cause of the lost packet via one-bit flag in TCP header Selective Acknowledgements (SACKs)

11. Internet Control Message Protocol (ICMP) • Detects corrupted TCP segment at link level • Although message is corrupted, it can be used to signal non-congestion-related loss to the transport layer (TCP header option) • Sending TCP session maintains "corruption experienced" state for two RTT intervals • During "corruption experienced" state, lost packets are retransmitted without halving the congestion window size

12. Wireless TCP (WTCP) • Purely end-to-end protocol • Rate-based transmission control • Receiver uses inter-packet delay to make decisions regarding rate control • Predicts cause of packet loss based on history of packet losses during normal state • Fast startup scheme • Reliability – SACKs, CACKs

13. Split-Connection Solutions

14. Indirect TCP • Split-connection protocol • Used for connection between mobile host and fixed host (with intermediate router) • Wired link and wireless link deal with lost packets differently • Higher level acknowledgements might be necessary in some applications • Complicated handoff procedure

15. Advantages/Disadvantages of Split-connection Solutions + Separates flow control and congestion control between wireless link and wired link + Results in good bandwidth at sender - End-to-end semantics of TCP violated - TCP header needs to be processed multiple times along route - Slow handoff procedures

16. Summary • Existing transport protocols such as TCP not well-suited for wireless networks • Solutions are of general type link-layer, end-to-end, and split-connection • Most experimental results show that TCP-aware link-level solutions give the best results in terms of overall network throughput