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Split TCP: Improving TCP Performance in Mobile Ad-hoc Networks

This overview discusses the challenges of using TCP in MANETs and explores the Split TCP solution, which improves TCP performance in wireless networks by breaking the connection into segments with proxies, reducing bandwidth consumption and channel capture effect.

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Split TCP: Improving TCP Performance in Mobile Ad-hoc Networks

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  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.

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