TCP in Mobile Ad-hoc Networks ─ Split TCP

1. TCP in Mobile Ad-hoc Networks ─ Split TCP CSE 6590

2. Overview • What is TCP? • TCP challenges in MANETs • TCP-based solutions • Split-TCP • ATCP

3. TCP: A Brief Review • TCP: Transmission Control Protocol • Specified in 1974 (TCP Tahoe) • Data stream  TCP packets • Reliable end-to-end connection • In-order packet delivery • Flow and congestion control

4. How does TCP work? • Establishes an end-to-end connection: • Acknowledgement based packet delivery • Assigns a congestion window Cw: • Initial value of Cw = 1 (packet) • If trx successful, congestion window doubled. Continues until Cmax is reached • After Cw ≥ Cmax, Cw = Cw + 1 • If timeout before ACK, TCP assumes congestion

5. How does TCP work? (2) • TCP response to congestion is drastic: • A random backoff timer disables all transmissions for duration of timer • Cw is set to 1 • Cmax is set to Cmax / 2 • Congestion window can become quite small for successive packet losses. • Throughput falls dramatically as a result.

6. TCP Congestion Window

7. Why does TCP perform badly in MANETs? • Dynamic network topology • Node mobility • Network partition • Multi-hop paths • Variable path lengths • Longer path = higher failure rate

8. Why does TCP struggle in MANETs? (2) • Lost packets due to high BER (Bit Error Rate): • BER in wired: 10-8 – 10-10 • BER in wireless: 10-3 – 10-5

9. Solutions for TCP in MANETs • Various solutions present • Most solutions generally tackle a subset of the problem • Often, fixing one part of TCP breaks another part • Competing interests exist in the standards laid out by OSI

10. Solution Topology

11. Why focus on TCP-based solutions? • We want to choose solutions which maintain close connection to TCP • Upper layers in the OSI model affected by choice of transport layer protocol • Modifications may affect interactions with the Internet • Alternative methods only useful for isolated networks

12. Solutions for TCP

13. Split-TCP and ATCP

14. TCP Summary • Works well in wired • Fails in wireless networks due to frequent connection breaks: • Mobile nodes move • Packets lost due to lossy channels • Multi-hop paths more prone to failure • Present solutions tackle subset of problems • Two solutions: Split-TCP and ATCP

15. Split-TCP Overview • Motivation for Split-TCP • How does Split-TCP work? • Advantages/Disadvantages • Performance Evaluation: • Throughput vs. TCP • Channel Capture Effect • Summary

16. Split-TCP in Solution Topology

17. Motivation for Split-TCP • Issues addressed by Split-TCP: • Throughput degradation with increasing path length • Channel capture effect (802.11) • Mobility issues with regular TCP

18. Channel Capture Effect • Definition: • “The most data-intense connection dominates the multiple-access wireless channel” [1] • Higher SNR • Early start • Example: 2 simultaneous heavy-load TCP flows located close to each other.

19. How does Split-TCP work? • Connection between sender and receiver broken into segments • A proxy controls each segment • Regular TCP is used within segments • Global end-to-end connection with periodic ACKs (for multiple packets)

20. Split-TCP Segmentation

21. Split-TCP in a MANET: Proxy Functionality • Proxies: • Intercept and buffer TCP packets • Transmit packet, wait for LACK • Send local ACK (LACK) to previous proxy • Packets cleared upon reception of LACK • Increase fairness by maintaining equal connection length

22. Steps: Node 1 initiates TCP session Nodes 4 and 13 are chosen as proxies on-demand Upon trx, 4 buffers packets If a packet lost at 15, request made to 13 to retransmit 1 unaware of link failure at 15 Split-TCP in a MANET (2)

23. Split-TCP in a MANET (3) • Sender is unaware of transient link failure. Congestion window not reduced. • Packet retransmissions only incorporate part of a path  bandwidth usage is reduced. • Channel capture effect is alleviated (see next slide).

24. Channel capture alleviated

25. Is Split-TCP successful? • Pros: • Increased throughput • Increased fairness • Restricted channel capture effect • Cons: • Modified end-to-end connection • Proxy movement/failure adversely affects protocol performance • Congestion at proxy nodes if another fails

26. Performance Evaluation • Test bench Specifics: • ns-2 Simulator • 50 mobile nodes initially equidistant • 1 km2 Area • Nodes maintain constant velocity: • Arbitrary direction • Random changes at periodic intervals • Optimal segment length: 3 ≤ n ≤ 5 nodes • Measured improvement: Throughput increases by 5% to 30%

27. Performance vs. TCP:Throughput Comparison

28. Performance vs. TCP:Channel Capture Effect Split-TCP Throughput Regular TCP Throughput

29. Split-TCP: Summary • Break link into segments with proxies • Use proxies to buffer packets at segments • Employ TCP locally in segments • Reduce bandwidth consumption and channel capture effect

30. Issues Not Addressed • Does not maintain end-to-end semantics • Periodic ACK failures means major retransmissions • Packet loss due to high BER • Out-of-order packets • Proxy link failure affects performance

31. References • [1] Split-TCP for Mobile Ad Hoc Networks; Kopparty et al. • [2] ATCP: TCP for Mobile Ad Hoc Networks; Jian Liu, Suresh Singh, IEEE Journal, 2001. • [3] A Feedback-Based Scheme for Improving TCP Performance in Ad Hoc Wireless Networks; Kartik Chandran et al. • [4] Ad Hoc Wireless Networks: Architectures and Protocols; C. Siva Ram Murthy and B. S. Manoj; section 9.5.7. • [5] Improving TCP Performance over Wireless Networks; Kenan Xu, Queen’s University 2003.