10/1/2015 9:14 PM1 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 C w : Initial value of C w = 1 (packet) If trx 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

5 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.

6 TCP Congestion Window

7 Why does TCP perform badly in MANETs? 1.Dynamic network topology Node mobility Network partition 2.Multi-hop paths Variable path lengths Longer path = higher failure rate

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

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 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%

27 Performance vs. TCP: Throughput Comparison

28 Performance vs. TCP: Channel Capture Effect Regular TCP Throughput Split-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, [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 [5] Improving TCP Performance over Wireless Networks; Kenan Xu, Queen’s University 2003.