A Comparison of Mechanisms for Improving TCP Performance over Wireless Links

1. A Comparison of Mechanisms for Improving TCP Performance over Wireless Links By: Hari B., Venkata P. et. al. Presented by: Nitin Bahadur Advanced Computer Networks

2. How I plan to keep you Awake • Review of TCP Congestion Control and Wireless issues • Discussion of techniques presented in the paper • Evaluation of some techniques • What is a Handoff ??? • New approaches proposed in recent years Advanced Computer Networks

3. Assumptions Assumes packet losses are due to congestion Assumes an underlying wired network TCP Congestion Control • Fast Retransmit • if three duplicate acks before timeout, retransmit • Fast Recovery • no slow start after retransmit • go directly to half the last successful congestion win. ( Cwin = Cwin/2 ) • Coarse grained Timeouts Advanced Computer Networks

4. Implications to Wireless Networks • Wireless losses are different from congestion losses • weak signal, corruption, incomplete packet, lost bits • TCP treats both losses similarly • reduces congestion window size • degrades performance for wireless • Coarse grained timeouts are bad for lossy wireless networks • slower retransmissions • consistent small window size • reduced bandwidth !!!! Advanced Computer Networks

5. Solutions • Approaches presented in the paper • Split Connection • End 2 End • Link Layer TCP aware • Other recent ones Advanced Computer Networks

6. SACK Receiver sends ack for up to 3 sets of non-contiguous data received 0 1 2 34 5 6 7 8 9 0 - 9 Base Station 0-2 4-6 8-9 Cumulative Ack. + Seq. no. of packet causing the ack. 0 3 0 1 2 3 1, 3 Sender can determine which packets were lost before timeout occurs or 3 duplicate acks are received SMART Smart assumes no packet reordering on wired link Advanced Computer Networks

7. Disadvantages • Loss of end-end TCP model • Limited buffering available at base station • Timeouts on wired TCP due to retransmissions on wireless TCP • Problems in handoff as it now involves 2 TCP connections Split Connection Schemes • Divide TCP connection into 2 connections…..isolate wired network from wireless network • Use SACK or SMART for performance enhancement TCP II TCP I Wired N/w Wireless Link Advanced Computer Networks

8. End -End Schemes w/ SMART or SACK • Using SMART/SACK, sender can detect multiple losses faster • Faster and efficient retransmit scheme • No need for 3 duplicate acks or coarse timeout • End -End model is maintained Flaws • Still considers wireless losses as due to congestion • Does invoke congestion control….small congestion window Advanced Computer Networks

9. E2E w/ Explicit Loss Notification • Pkt. Loss on wireless link -> Ack. w/ ELN bit set • Sender retransmits on receiving first (not third) duplicate ack w/ ELN bit set • Power and time saving !!!!! • Sender does not invoke congestion control in such cases large congestion window……even at high rate wireless losses Open Issues How to distinguish b/w congestion and wireless losses ? Scheme does not detect multiple losses….add SACK/SMART Advanced Computer Networks

10. 0 3 1,3 Drawbacks Layer Violation !!!!! Bursty losses/slow wireless links lead to TCP sender timeouts while agent is trying to retransmit LL-SMART-TCP-Aware Scheme • Maintain cache of un-acked packets at Home Agent • Use a LL retransmission scheme with finer granularity timeout • Use SMART for efficient retransmissions • Suppress duplicate ack from reaching sender 0 1 2 3 Base Station Advanced Computer Networks

11. Effectiveness of LL and E2E schemes Advanced Computer Networks

12. Losses due to handoff….During establishing of new route/new • cell registration • Rerouting through BS or direct routing to MH ??? X Sender Handoff Issues • Mobile hosts (MH) and cell • Handoff takes place when MH changes Base Station Advanced Computer Networks

13. Conclusion The paper presented a taxonomy and comparison of various approaches But all approaches have drawbacks…….so none have become a standard today. The results presented do not consider losses arising from congestion…..so are not practical. How I wish the figures were animated for better understanding !! Advanced Computer Networks

14. EBSN Explicit Bad State Notification (EBSN) • Base Station sends EBSN message to sender if packets cannot be transmitted successfully • Sender changes Timeout based on current RTT • Timeout is reset to original on receipt of new ack. • Eliminates unnecessary timeouts 0 1 2 0 Advanced Computer Networks

15. Multiple Acks Proposal • Base Station sends a Partial Ack to sender • Base station reliably sends packets to mobile client • Sender does not retransmit/invoke congestion control on timeout, just discards the Partial Ack • Receiver sends Complete Ack to sender • Similar to ELN……but results in excess traffic towards sender Advanced Computer Networks

16. Flaws Receiver cannot distinguish between congestion and transmission losses…..performance degradation Delayed Duplicate Acks (Dupacks) • TCP - unaware technique, good for encrypted data • Base Station uses a LL retransmission scheme • This scheme uses LL acks…not TCP duplicate acks !! • TCP receiver delays 3rd & other Dupacks • High Priority to LL acks & retransmitted pkts Advanced Computer Networks

17. Other Proposed Schemes • Explicit Loss Notification to Receiver (ELNR) • Explicit Delayed Dupack Activation Notification (EDDAN) • Wireless Explicit Congestion Notification (WECN) • Forward Explicit Congestion Notification (FECN) • Extended Link Failure Notification (ELFN) • Appropriate Byte Counting • Loss Predictors Advanced Computer Networks