1. 1 Ad-hoc Transport Layer Protocol (ATCP) EECS 4215

2. 2 Overview What is TCP? TCP Challenges in MANETs ATCP

3. 3 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

4. 4 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

5. 5 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

6. 6 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.

7. 7 TCP Congestion Window

8. 8 Why does TCP struggle in MANETs? 1.Dynamic network topology Node mobility Dynamic channel quality 2.Multi-hop paths Variable path lengths per node Longer path = higher failure rate

9. 9 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

10. 10 Solutions for TCP in MANETs Various solutions Most solutions tackle only a subset of problems Often fixing one part of TCP may break another part

11. 11 Solution Topology

12. 12 Why focus on TCP based solutions and not new transport protocols? 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

13. 13 TCP Summary Works well in wired networks Fails in wireless networks due to frequent link breaks: Node mobility Packets lost due to lossy channels Multi-hop paths more prone to failure Most solutions tackle only a subset of problems

14. 14 ATCP Overview: What is ATCP? Motivation for ATCP ATCP Infrastructure How ATCP works Is ATCP Successful? Performance vs. TCP ATCP Recap

15. 15 What is ATCP? Overview: Ad-hoc TCP Network layer feedback mechanism TCP state control End-to-end semantics Dependent on routing protocols

16. 16 ATCP in Solution Topology

17. 17 Motivation for ATCP Issues addressed by ATCP: Packet loss due to high BER or collision Route changes due to node mobility Network partitions Out-of-order packets Congestion Congestion window size adjustment

18. 18 ATCP infrastructure  ATCP is a thin layer that is layered between TCP and IP  Sender’s ATCP states: Normal, Disconnected, Congested, and Loss TCP IP TCP ATCP IP

19. 19 How ATCP works – Lossy Channels Disconnected * Congested Normal Loss * * TCP sender in persist mode RTO about to expire OR 3 dup ACKs New ACK ATCP retransmits segments in buffer + ATCP transmits unacknowledged segments from TCP’s buffer, and maintains its own timers for these segments. + When a new ACK comes, ATCP forwards it to TCP and returns to normal state.

20. 20 How ATCP works - Congestion Disconnected * Congested Normal Loss * Receive ECN TCP transmits a new packet * TCP sender in persist mode RTO about to expire OR 3 dup ACKs New ACK ATCP retransmits segments in buffer + Congestion: ECN flag set in ACK and data packets + ATCP does not interfere. + After TCP transmits a new segment, ATCP returns to normal state.

21. How ATCP works – Disconnected Network generates ICMP message “Destination Unreachable”. ATCP: disconnected mode TCP: persist mode generates probe packets Receiver responds with a data packet or ACK. ATCP and TCP return to normal state. 21

22. 22 How ATCP works – Disconnected Disconnected * Congested Normal Loss * Receive “ Destination Unreachable ” ICMP Receive ECN TCP transmits a new packet Receive dup ACK or packet from receiver * TCP sender in persist mode RTO about to expire OR 3 dup ACKs New ACK ATCP retransmits segments in buffer

23. ATCP Diagram 23

24. 24 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

25.  BER = 10 -5  1 MB File Transfer  20 measurements  90% confidence interval 25 Performance: Loss Case + 5 Pentium PCs + 32-Kbps channels + Simulated hop-by- hop delay

26. 26 ATCP vs. TCP: Congestion Window

27. 200ms period every 5s (80 congestions) 1200s to 3000s transfer time (TCP) 460s to 500s transfer time (ATCP) Performance: BER and Congestion 27

28.  Partitioned for 1 min every 5 minutes Performance: BER and Partition 28

29. 29 ATCP vs. TCP: File Transfer Time 1MB file transfer

30. 30 ATCP Summary 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

31. 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 [5] Improving TCP Performance over Wireless Networks; Kenan Xu, Queen ’ s University 2003