* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Transport Issues in Ad Hoc and Sensor Networks
Zero-configuration networking wikipedia , lookup
Distributed firewall wikipedia , lookup
Asynchronous Transfer Mode wikipedia , lookup
Multiprotocol Label Switching wikipedia , lookup
IEEE 802.1aq wikipedia , lookup
Wireless security wikipedia , lookup
Wake-on-LAN wikipedia , lookup
Piggybacking (Internet access) wikipedia , lookup
Computer network wikipedia , lookup
List of wireless community networks by region wikipedia , lookup
Airborne Networking wikipedia , lookup
Deep packet inspection wikipedia , lookup
Cracking of wireless networks wikipedia , lookup
UniPro protocol stack wikipedia , lookup
Internet protocol suite wikipedia , lookup
Recursive InterNetwork Architecture (RINA) wikipedia , lookup
Transport Issues in Ad Hoc and Sensor Networks Παναγιώτης Παναγιώτου Τμήμα Ηλεκτρολόγων Μηχανικών και Μηχανικών Υπολογιστών Πολυτεχνική Σχολή Πανεπιστήμιο Κύπρου Computer Networks (ECE654) Project Presentation 19 November 2004 Ad Hoc Networks Wireless networks that operate without any centralized structure used for directing communication. Similar to users of cellular phones communicating without the aid of a phone tower or laptop users communicating with one another without a wireless access point available. Ad Hoc Networks Nodes send and receive messages via end-toend communication and also act as forwarding agents for other mobile stations. The multi-hop network consists of autonomous,mobile nodes that act as relay stations for paths of communication. Because there are no centralized routers or base stations, the nodes must also be able to handle constant route changes and terminations Ad Hoc Networks Transport Layer Intended to provide a more reliable method of communication that is not offered by the network layer. Transport layer protocols were initially intended to offer services regardless of the underlying network. Receives data from the user and transmits it to the receiver in order and without error. Makes it possible for the users to send and receive data without concern of network congestion or loss of packets. Ability to dynamically adjust window size to provide better flow control. Common Transport Protocols UDP: User Datagram Protocol Connectionless protocol that does not offer guaranteed packet delivery. Common Transport Protocols Traditional TCP: Transmission Control Protocol Most commonly used transport protocol. End to end protocol that provides reliable, in-order transmission over the unreliable IP. Operates independently of the routing mechanisms implemented in the network layer. Traditional TCP was designed to be used on wired networks. As it is, does not work well in wireless environments. Traditional TCP Problem It assumes every packet loss to be due to congestion even though the loss could be due to a number of possibilities. Mechanisms: Slow Start Congestion Avoidance Congestion Control in TCP – Threshold := CongestionWindow/2 – CongestionWindow := 1 TCP vs. Ad Hoc Networks Bit Errors – TCP immediately reduces its transmission speed to avoid further congestion. – A reduction in transmission speed is the exact opposite of what needs to be done to remedy the problem. – The optimal way would be to immediately resend the packet to the receiver. TCP vs. Ad Hoc Networks Path Change – In ad hoc networks, the mobility of the nodes causes the probability of path change to increase. – TCP will timeout and incorrectly assume network congestion – The optimal way would be to stop transmitting until the network has found a new path and then resume sending at the rate determined by the new node(s) and the network layer. Important Issues in Ad Hoc Networks Variable link quality Multi-user interference Power consumption Topological changes Propagation path loss Link Quality Quality of transmitters and receivers Distance from the Mobile Station to the receiver The degradation of each link can have an enormous effect on throughput. Multi user interference Wired networks: there is not really any threat of multiple nodes receiving packets even if they were not meant for them. Ad hoc wireless networks: transmitters do not have a pinpoint location to send data to. they must broadcast data / request to send data to every node in range. Need to decipher whether or not the broadcasted message is for the node. – YES accept the message and read it. – NO ignore it and go on. These problems can be seen in both the hidden and exposed station problems. Power Consumption An important issue that all mobile stations have. Not consistent or stable source of power. Wired networks: transmitting power does not really need to change. An ad hoc wireless networks: power consumption changes per mobile station Transmitting power needs to be adjusted depending on that distance which can continuously change with the inconsistent topology of ad hocwireless networks. Power Consumption: Distance At t1 mobile station A needs power P1 to transmit to mobile station B successfully. At t2 Y mobile station A needs power P2 to transmit successfully. Mobile station B has either moved closer or farther away from mobile station A. Path Loss Attenuation undergone by an electromagnetic wave in transit between a transmitter and a receiver. Many of the transport protocols treat path loss as congestion andimplement traditional congestion control mechanisms to compensate. Ad hoc wireless networks: path loss does not necessarily mean congestion. The destination node has moved out of range of the sender or is now on a different path. Congestion Control in this case is not useful and wastes bandwidth and power (this is not needed). TCP based modifications Wired TCP: TCP-Reno TCP-Vegas TCP-Tahoe TCP-Sack Ad Hoc TCP: TCP-BuS TCP-F TCP-BuS Proposed by Dongkyun Kim, C.K. Toh, and Yanghee Choi. Main differentiating property is the introduction of buffering capabilities in the mobile nodes. Also does feedback information for detecting route disconnection. Modifications 1. Explicit notifications for route failures and route reestablishment ERDN (explicit route disconnection notification) Stop Transmission ERSN (explicit route successful notification) Start Transmission Modifications Use of extended timeout values Packets are still sent by the sender and the node that sent the ERDN message buffers these packets. Problem: the time it takes for a new path to be discovered might be smaller than the time the sender waits before thinking that the packet has been lost. The timeout value for the buffered packets is doubled. Modifications 3. Selective retransmission of lost packets. 4. Avoidance of unnecessary requests for fast transmission. 5. Reliable transmission of control messages. 2 timers: ERDN_RET_TIMER and ERSN_RET_TIMER TCP-F Created by Chandran, Raghunathan, Venkatesan, and Prakash Uses feedback to overcome the limitations of TCP Goal: "source informed of route failure so that is does not unnecessarily invoke congestion control and can refrain from sending any further packets until the route is restored." TCP-F When an intermediate node in the path between the sender and the receiver encounters a route disruption it propagates a route failure notification (RFN) packet to the source and records the occurrence. Each node that receives the RFN invalidates the particular route and prevents incoming packets intended for the destination from passing though that route. If one of the intermediate nodes knows an alternate route, the RFN can be discarded and communication can begin on the new route. If not,the RFN gets propagated all the way back to the sender. Snooze State When sender receives a RFM, it goes into a "snooze" state: 1. Ceases sending all packets. 2. Marks all timers as invalid. 3. Freezes the packet window, the retransmit timers, and window size. 4. Starts a route failure timer. It remains in a snooze state until it receives notification of a reestablished route by a RRN (route reestablishment notification packet). If a node discovers a working route,it propagates the message back towards the sender. All further RRNs for the same source-destination combination are then discarded. ATCP Ad hoc Transmission Control Protocol Acts as an intermediary between the network layer and standard TCP It is important for applications to be able to use standard TCP without any modifications. ATCP is a thin layer between the network layer and TCP. Packet Loss When ATCP detects packet loss due to a route failure or path change, it does not notify TCP of the packet loss. Instead, it places TCP into persist mode by changing the receiver’s advertised window size to 0. This causes TCP to temporarily stop sending without attempting congestion control. Once ATCP has detected that the network layer has reestablished a path, it will notify TCP to begin sending again by setting the receiver’s window to the previous size. High Bit Error Rate Same technique for high bit error rate. When ATCP detects that a packet has been corrupted, it places TCP in persist mode and retransmits the packet itself without notifying TCP of the packet loss. Advantages Current method of congestion control (TCP) is available without any modifications. When ATCP detects packet loss due to congestion, it simply steps aside and lets TCP handle it as it would in a wired network. It supports the use of current TCP implementations, which will allow better interoperability with pre-existing applications. Programmers will not need to learn a new protocol to use for ad hoc network applications. Data Flow ATP (1) Application controlled Transport Protocol The application should be able to decide the QoS offered by the transport layer. ATP also allows for the priority levels for different applications to be determined dynamically, allowing for more flexibility in multiple application environments. ATP is based on a good idea that applications should be able todynamically set their priority. Example: parametic needs to look up patient data and communicate with another doctor at the same time. Disadvantages It omits much of the functionality required by an ideal transport layer protocol. Reliable delivery is left to the user application, which can query ATP to see if an ACK has arrived but must determine itself when to retransmit. It completely overlooks congestion control, packet errors, and path loss. These issues are left to the user application for implementation. ENIC ENhanced Inter-layer Communication and control The OSI model layers should have better communication with the other layers. In order to avoid the hidden station and exposed station problem in wireless networks, the ENIC protocol utilizes Request-To-Send (RTS) and Clear-To-Send (CTS) packets. Hidden and Exposed Station A Δ B Γ Ε Ζ Η ENIC Ability to freeze the TCP state when a route has failed. The network layers in ENIC will monitor the connections between the sender and receiver and will notify the transport layer of a failure with an Explicit Route State Notification (ERSN). At this point, the protocol will save the current state of its variables (retransmission time, congestion window, etc.) until the network layer has determined a new route. The transport layer will periodically probe the network layer for a new route by using a Route Recovery Timer (RRT). ENIC handles route change and path loss without a serious reduction in throughput. In the case ofnetwork congestion, ENIC will use standard TCP slow-start algorithm. ATP (2) Ad hoc Transport Protocol Communication between layers needed for startup rate estimation, congestion control, and path loss/failure. This is a common theme among the ad hoc protocols, so most lower layers should support this communication. The intermediate nodes are required to piggyback data rates onto packets to the nodes. This allows the sender to determine the amount of congestion in the network. Sensor Networks Transport Protocols PSFQ: Pump Slowly and Fetch Quickly ESRT Event-to-Sink Reliable Transport for Wireless Sensor Networks CODA: Congestion Detection and Avoidance in Sensor Networks Sensor Networks Expectations for transport protocol for sensor networks Reliability, congestion control Constraints Resource constraints – power, storage, computation complexity, data rates These constraints are application specific. Sensor Networks General notion for sensor networks Low data Rate Power limited Storage limited High data Rate Not power limited Not storage limited User Sink PSFQ Pump Slowly and Fetch Quickly A Reliable Transport Protocol for Wireless Sensor Networks Multi-Hop Packet Forwarding 1 2 1 3 4 1 1 2 2 2 3 3 3 When No Link Loss – Multi-Hop Forwarding takes place Recovering from Errors 1 3 2 1 1 4 1 2 lost 3 3 3 Recover 2 Recover 2 Recover 2 Error Recovery Control Messages are wasted How PSFQ Recovers from Errors “Store and Forward” 1 3 2 1 2 1 2 lost 4 1 3 Recover 2 2 2 3 2 3 No wastage of the Error Recovery control messages PSFQ Pump Schedule 1 2 1 Tmin Tmax t 1 Tmin Tmax 1 If not duplicate and in-order and TTL not 0 Cache and Schedule for Forwarding at time t (Tmin<t<Tmax) “Fetch Quickly” Operation 1 2 1 1 2 2 lost Tr 3 Recover 2 2 Tr Tmin Tmax 2 “Proactive Fetch” 1 2 last-1 last Tproc last CODA Congestion Detection and Avoidance in Sensor Networks Energy efficient congestion control scheme Three mechanisms are involved – Congestion Detection – Open-loop hop-by-hop backpressure – Closed-loop multi-source regulation Congestion Detection Accurate and efficient congestion detection is important – Buffer queue length or Buffer occupancy – not a good measure of the congestion. – Channel loading – sample channel at appropriate time to detect congestion. – Report rate/Fidelity measurement – slow, observed over a longer period Open-Loop Hop-by-Hop Backpressure 1 2 3 4 5 6 Congestion detected Closed Loop Multi-Source Regulation 1 2 1,2,3 Regulate bit is set ACK 4,5,6 7,8 ACK Congestio n detected ESRT Event-to-Sink Reliable Transport for Wireless Sensor Networks ESRT Event-to-sink reliability S Self-configuration Energy awareness [low power consumption requirement!] Congestion Control Variation in complexity at source and sink. [computation complexity] ESRT’s Definition of Reliability Reliability is measured in terms of the number of packets received. Or reporting frequency (number of packets / decision interval). Observed reliability: number of received data packets in decision interval at the sink. Desired reliability: number of packets required for reliable event detection. Normalized reliability is observed reliability / desired reliability. Algorithm for ESRT If congestion and low reliability: decrease reporting frequency aggressively. (exponential decrease) If congestion and high reliability: decrease reporting to relieve congestion. No compromise on reliability (multiplicative increase) If no congestion and low reliability: increase reporting frequency aggressively (multiplicative increase) If no congestion and high reliability: decrease reporting slowing (half the slope) References 1. 2. 3. 4. Tanenbaum, A. Computer Networks, Third Edition. Prentice Hall John Calagaz, Wade Chatam, Brian Eoff and John A. Hamilton, Jr. On the Current State of Transport Layer Protocols in Mobile Ad hoc Networks. Liu, J., Singh, S. ATCP: TCP for mobile ad hoc networks. Selected Areas in Communications, IEEE Journal on, Volume: 19 Issue: 7, (July 2001). Dongkyun K., Toh, C.K., and Choi, Y. TCP-BuS: improving TCP performance in wireless ad hoc networks. In Communications, 2000 IEEE International Conference on, Volume: 3, (18-22 June 2000), 1707 -1713. References 5. 6. 7. Sun, D., and Man, H. Performance comparison of transport control protocols over mobile ad hoc networks. In Personal, Indoor and Mobile Radio Communications 12th IEEE International Symposium on, Volume: 2, (30 Sept.-3 Oct. 2001), G-83 -G-87. Chandran, K., Sudarshan, R., Venkatesan, S., Prakash, R. A Feedback-Based Scheme for Improving TCP Performance in Ad Hoc Wireless Networks. In IEEE Personal Communications, (February 2001). Holland, G., Vaidya, N. Analysis of TCP performance over mobile ad hoc networks, Wireless Networks, Volume 8 Issue 2/3 March 2002. References 8. Liu, J. and Singh, S., ATP: Application controlled Transport Protocol for Mobile Ad Hoc Networks. Wireless Communications and Networking Conference, 1999. IEEE, (21-24 Sept. 1999) 1318 -1322 vol.3. 9. Sundaresan, K., Anantharaman, V., Hsieh, H., and Sivakumar, R. ATP: a reliable transport protocol for adhoc networks. Proceedings of the 4th ACM international symposium on Mobile ad hoc networking & computing. (June 2003). 10. Sun, D., and Man, H. ENIC - an improved reliable transport scheme for mobile ad hoc networks. Global Telecommunications Conference, 2001. GLOBECOM '01. IEEE, Volume: 5, (25-29 Nov. 2001). End of presentation Τέλος Παρουσίασης Άλλες ερωτήσεις;