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

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

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

4. 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. 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. 6 How does TCP work? Establishes an end-to-end connection: Acknowledgement based packet delivery Assigns a congestion window C w : Initial value of C w = 1 (packet) If tx successful, congestion window doubled. Continues until C max is reached After C w ≥ C max, C w = C w + 1 If timeout before ACK, TCP assumes congestion

7. 7 How does TCP work? (2) TCP response to congestion is drastic: A random backoff timer disables all transmissions for duration of timer C w is set to 1 C max is set to C max / 2 Congestion window can become quite small for successive packet losses. Throughput falls dramatically as a result.

8. 8 TCP Congestion Window

9. 9 Why does TCP struggle in MANETs? 1.Dynamic network topology Nodes in constant motion Network Topology undergoes periodic changes 2.Multi-hop paths Variable path lengths per node Longer path = higher failure rate

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

11. 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. 12 Solution Topology

13. 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. 14 Solutions for TCP

15. 15 Split-TCP and ATCP

16. 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. 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. 18 Split-TCP in Solution Topology

19. 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. 20 Channel Capture Effect Definition: “The most data-intense connection dominates the multiple-access wireless channel” [1] Higher SNR Early Start

21. 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. 22 Split-TCP Segmentation

23. 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. 24 Split-TCP in a MANET (2) 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

25. 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. 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. 27 Performance Evaluation Test bench Specifics: ns-2 Simulator 50 mobile nodes initially equidistant 1 km 2 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. 28 Performance vs. TCP: Throughput Comparison

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

30. 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. 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. 32 ATCP Overview: What is ATCP? Motivation for ATCP ATCP Infrastructure How ATCP works Is ATCP Successful? Performance vs. TCP ATCP Recap

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

34. 34 ATCP in Solution Topology

35. 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. 36 ATCP infrastructure  ATCP is a thin layer that is layered between TCP and IP  Sender ATCP states: Normal, Disconnected, Congested, and Loss TCP IP TCP ATCP IP

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

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

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

40. 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. 41 Performance vs. TCP (File Transfer Time)

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

43. 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. 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. 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 University 2003

46. 46 The End Thank you for your patience

47. 47 Questions/Comments?