Transport Layer for Mobile Ad Hoc Networks (MANETs)

1. Transport Layer for Mobile Ad Hoc Networks (MANETs) Cyrus Minwalla Maan Musleh COSC 6590

2. Overview • What is TCP? • TCP Challenges in MANETs • TCP Based Solutions • Split-TCP • ATCP • Recap

3. What is TCP? • Sub-topics: • Transport Layer overview • TCP Summary • Solutions • Recap

4. Transport Layer • In the OSI model, the transport layer is responsible for: • Reliable end-to-end connection • End-to-end delivery • Flow control • Congestion control • In-order packet delivery

5. 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

6. 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 tx successful, congestion window doubled. Continues until Cmax is reached • After Cw ≥ Cmax, Cw = Cw + 1 • If timeout before ACK, TCP assumes congestion

7. 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.

8. TCP Congestion Window

9. Why does TCP struggle in MANETs? • Dynamic network topology • Nodes in constant motion • Network Topology undergoes periodic changes • Multi-hop paths • Variable path lengths per node • Longer path = higher failure rate

10. 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

11. 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

12. Solution Topology

13. 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

14. Solutions for TCP

15. Split-TCP and ATCP

16. TCP Recap • Works well in wired • Fails in wireless due to frequent connection breaks: • Mobile nodes being rerouted • Packets lost due to lossy channel • Multi-hop paths more prone to failure • Present solutions tackle subset of problems • Two solutions: Split-TCP and ATCP

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

18. Split-TCP in Solution Topology

19. 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

20. Channel Capture Effect • Definition: • “The most data-intense connection dominates the multiple-access wireless channel” [1] • Higher SNR • Early Start

21. 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)

22. Split-TCP Segmentation

23. 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

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

25. Split-TCP in a MANET (3) • Sender is unaware of transient link failure. Congestion window not reduced • Packet retransmissions only incorporate part of link --> Bandwidth reduced • 4 may act as proxy for 12 as well, channel capture eliminated.

26. Is Split-TCP successful? • Pros: • Increased throughput • Increased fairness • Restricted channel capture effect • Cons: • Modified end-to-end connection • Proxy movement adversely affects protocol performance • Congestion at individual nodes (if only proxy between partitions)

27. 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%

28. Performance vs. TCP:Throughput Comparison

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

30. Split-TCP Recap • 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

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

32. ATCP Overview: • What is ATCP? • Motivation for ATCP • ATCP Infrastructure • How ATCP works • Is ATCP Successful? • Performance vs. TCP • ATCP Recap

33. What is ATCP? • Overview: • Ad Hoc TCP • Network Layer Feedback Mechanism • TCP State Control • End-to-end Semantics • Dependent on routing protocols

34. ATCP in Solution Topology

35. Motivation for ATCP • Issues addressed by ATCP: • Packet loss due to high BER or collision • Route changes • Network partitions • Out-of-Order Packets • Congestion • CWND

36. ATCP infrastructure • ATCP is a thin layer that is layered between TCP and IP • Sender ATCP states: Normal, Disconnected, Congested, and Loss TCP TCP ATCP IP IP

37. How ATCP works (1) - lossy channel Disconnected * Normal New ACK RTO about To expire OR 3 dup ACKs Loss * Congested * TCP sender in persist state ATCP Retransmits Segments in buffer

38. How ATCP works (2) - Congestion Disconnected * Normal Receive ECN TCP Transmits a new packet New ACK RTO about To expire OR 3 dup ACKs Loss * Congested * TCP sender in persist state ATCP Retransmits Segments in buffer

39. How ATCP works (3) - Node mobility Disconnected * Receive Dup ACK or packet from receiver Receive “Dest Unreachabl” ICMP Normal Receive ECN TCP Transmits a new packet New ACK RTO about To expire OR 3 dup ACKs Loss * Congested * TCP sender in persist state ATCP Retransmits Segments in buffer

40. Is ATCP Successful? • Pros: • Maintenance of end-to-end TCP semantics • Compatibility with traditional TCP • Invisibility to TCP • Cons: • Dependency on the network layer protocol to detect route changes and partitions • Addition of a thin ATCP layer to TCP

41. Performance vs. TCP (File Transfer Time)

42. Performance vs. TCP (2)(Congestion Window Size)

43. ATCP Recap • Introduces a thin layer between IP and TCP • Maintain End-to-End Semantics • Does not interfere with TCP functions • Depends on the Network Layer to detect route changes and partitions

44. Final Recap • TCP does not perform well in MANETs • The presented solutions fix various aspects of TCP. • Currently there is no comprehensive solution that fixes all the problems • Applications are requirement specific

45. 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 • [5] Improving TCP Performance over Wireless Networks; Kenan Xu, Queen’s University2003

46. The End Thank you for your patience

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