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Transcript
Transport Layer for Mobile Ad Hoc Networks (MANETs) Cyrus Minwalla Maan Musleh COSC 6590 1 Overview What is TCP? TCP Challenges in MANETs TCP Based Solutions Split-TCP ATCP Recap 2 What is TCP? Sub-topics: Transport Layer overview TCP Summary Solutions Recap 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 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 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 6 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. 7 TCP Congestion Window 8 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 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 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 11 Solution Topology 12 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 13 Solutions for TCP 14 Split-TCP and ATCP 15 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 16 Split-TCP Overview: Motivation for Split-TCP How does Split-TCP work? Advantages/Disadvantages Performance Evaluation: Throughput vs. TCP Channel Capture Effect Recap 17 Split-TCP in Solution Topology 18 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 19 Channel Capture Effect Definition: “The most data-intense connection dominates the multiple-access wireless channel” [1] Higher SNR Early Start 20 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) 21 Split-TCP Segmentation 22 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 23 Split-TCP in a MANET (2) Steps: Node 1 initiates TCP session Nodes 4 and 13 are chosen as proxies ondemand Upon rx, 4 buffers packet If packet lost at 15, request made to 13 to retransmit 1 unaware of link failure at 15 24 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. 25 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) 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 Regular TCP Throughput Split-TCP Throughput 29 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 30 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 31 ATCP Overview: What is ATCP? Motivation for ATCP ATCP Infrastructure How ATCP works Is ATCP Successful? Performance vs. TCP ATCP Recap 32 What is ATCP? Overview: Ad Hoc TCP Network Layer Feedback Mechanism TCP State Control End-to-end Semantics Dependent on routing protocols 33 ATCP in Solution Topology 34 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 35 ATCP infrastructure ATCP is a thin layer that is layered between TCP and IP TCP IP TCP ATCP IP Sender ATCP states: Normal, Disconnected, Congested, and Loss 36 How ATCP works (1) - lossy channel Disconnected * Normal Congested * TCP sender in persist state RTO about To expire OR 3 dup ACKs New ACK Loss * ATCP Retransmits 37 Segments in buffer How ATCP works (2) - Congestion Disconnected Receive ECN TCP Transmits a new packet Congested * TCP sender in persist state * Normal RTO about To expire OR 3 dup ACKs New ACK Loss * ATCP Retransmits 38 Segments in buffer How ATCP works (3) Receive “Dest Unreachabl” ICMP Receive ECN TCP Transmits a new packet Congested * TCP sender in persist state - Node mobility Disconnected * Normal RTO about To expire OR 3 dup ACKs Receive Dup ACK or packet from receiver New ACK Loss * ATCP Retransmits 39 Segments in buffer 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 40 Performance vs. TCP (File Transfer Time) 41 Performance vs. TCP (2) (Congestion Window Size) 42 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 43 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 44 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 45 The End Thank you for your patience 46 Questions/Comments? 47