Transport Issues in Ad Hoc and Sensor Networks

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 communicatingwithout the aid of a phone tower or laptop users communicatingwith one another without a wireless access point available.

Ad Hoc Networks • Nodes send and receive messages via end-toendcommunication and also act as forwarding agents for othermobile stations. • The multi-hop network consists of autonomous,mobile nodes that act as relay stations for paths ofcommunication. • Because there are no centralized routers or basestations, the nodes must also be able to handle constant routechanges and terminations

Ad Hoc Networks

Transport Layer • Intended toprovide a more reliable method of communication that is notoffered by the network layer. • Transport layer protocols wereinitially intended to offer services regardless of the underlyingnetwork. • Receives data from the user andtransmits it to the receiver in order and without error. • Makes it possible for the users to send and receivedata without concern of network congestion or loss ofpackets. • Ability to dynamicallyadjust window size to provide better flow control.

Common Transport Protocols UDP: • User Datagram Protocol • Connectionless protocol that does not offer guaranteed packetdelivery.

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 tocongestion even though the loss could be due to a number ofpossibilities. • 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 transmissionspeed to avoid further congestion. • A reduction in transmission speed is the exact opposite ofwhat needs to be done to remedy the problem. • The optimal waywould be to immediately resend thepacket to the receiver.

TCP vs. Ad Hoc Networks • Path Change • In adhoc networks, the mobility of the nodes causes the probability ofpath change to increase. • TCP willtimeout and incorrectly assume network congestion • The optimal way would be to stop transmitting until the network has found a new path and thenresume sending at the rate determined by the new node(s) and thenetwork layer.

Important Issues in Ad Hoc Networks • Variable link quality • Multi-user interference • Power consumption • Topologicalchanges • Propagation path loss

Link Quality • Quality of transmitters andreceivers • Distance from the Mobile Station to the receiver • The degradation of each link canhave an enormous effect on throughput.

Multi user interference • Wired networks: there is not really any threat ofmultiple nodes receiving packets even if they were not meant forthem. • Ad hoc wireless networks: transmitters do not have apinpoint location to send data to.  they must broadcastdata / request to send data to every node in range. • Need to decipherwhether or not the broadcasted message is for the node. • YES  accept themessage and read it. • NO  ignore it and go on. • These problems can be seen in boththe hidden and exposed station problems.

Power Consumption • An important issue that all mobile stationshave. • Not consistent or stable source of power. • Wirednetworks: transmitting power does not really need to change. • An ad hoc wireless networks: power consumptionchanges per mobile station • Transmitting power needs to beadjusted depending on that distance which cancontinuously change with the inconsistent topology of ad hocwireless networks.

Power Consumption: Distance • At t1 mobile station Aneeds power P1 to transmit to mobile station B successfully. • At t2 Y mobile station A needs power P2 to transmitsuccessfully.  Mobile station B has eithermoved closer or farther away from mobile station A.

Path Loss • Attenuation undergone by anelectromagnetic wave in transit between a transmitter and areceiver. • Many of the transport protocols treat path loss as congestion andimplement traditional congestion control mechanisms to compensate. • Ad hoc wireless networks: path loss does notnecessarily mean congestion. • The destinationnode has moved out of range of the sender or is now on a differentpath.  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, andYanghee Choi. • Main differentiating property is theintroduction of bufferingcapabilities in the mobile nodes. • Also does feedbackinformation for detecting route disconnection.

Modifications • 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 timeoutvalues • Packets are still sent by the sender and the nodethat sent the ERDN message buffers these packets. • Problem: the time it takes for a new path to be discovered might besmaller than the time the sender waits before thinking that thepacket has been lost.  The timeout value for the buffered packets is doubled.

Modifications • Selective retransmission oflost packets. • Avoidance of unnecessaryrequests for fast transmission. • Reliable transmission of control messages. • 2 timers: ERDN_RET_TIMER andERSN_RET_TIMER

TCP-F • Created by Chandran, Raghunathan,Venkatesan, and Prakash • Uses feedback to overcome thelimitations of TCP • Goal:"source informedof route failure so that is does notunnecessarily invoke congestioncontrol and can refrain from sending any further packets until theroute is restored."

TCP-F • When an intermediate node in the path between thesender and the receiver encounters a route disruption it propagatesa route failure notification (RFN) packet to the source and recordsthe occurrence. • Each node that receives the RFN invalidates theparticular route and prevents incoming packets intended for thedestination from passing though that route. • If one of theintermediate nodes knows an alternate route, the RFN can bediscarded and communication can begin on the new route. • If not,the RFN gets propagated all the way back to the sender.

Snooze State • When senderreceives a RFM, it goes into a "snooze" state: • Ceasessending all packets. • Marks all timers as invalid. • Freezes the packet window, the retransmit timers, andwindow size. • Starts a route failure timer. It remains in a snooze state until it receives notification of areestablished route by a RRN (routereestablishment notification packet). If a node discovers a working route,it propagates the message back towards the sender. All furtherRRNs for the same source-destination combination are thendiscarded.

ATCP Ad hoc Transmission Control Protocol • Acts as anintermediary between thenetwork layer and standard TCP • It is important for applications to beable to use standard TCP without any modifications. • ATCP is athin layer between the network layer and TCP.

Packet Loss • When ATCP detects packet loss due to a route failure or pathchange, it does not notify TCP of the packet loss. • Instead, it placesTCP into persist mode by changing the receiver’s advertisedwindow size to 0. • This causes TCP to temporarily stop sendingwithout attempting congestion control. • Once ATCP has detectedthat the network layer has reestablished a path, it will notify TCPto begin sending again by setting the receiver’s window to theprevious size.

High Bit Error Rate • Same technique for high biterror rate. • When ATCP detects that a packet has been corrupted, itplaces TCP in persist mode and retransmits the packet itselfwithout notifying TCP of the packet loss.

Advantages • Currentmethod of congestion control (TCP) is available without anymodifications. • When ATCP detects packet loss due to congestion,it simply steps aside and lets TCP handle it as it would in a wirednetwork. • It supports the use of current TCP implementations, which willallow better interoperability with pre-existing applications. • Programmers will not need to learn a new protocol to use for adhoc network applications.

Data Flow

ATP (1) Application controlled Transport Protocol • The application should be able todecide the QoS offered by the transport layer. • ATP also allows for thepriority levels for different applications to be determineddynamically, allowing for more flexibility in multiple applicationenvironments. • ATP is based on a good idea that applications should be able todynamically set their priority. • Example: parametic needs to look up patientdata and communicate with another doctor at the same time.

Disadvantages • It omitsmuch of the functionality required by an ideal transport layerprotocol. • Reliable delivery is left to the user application, whichcan query ATP to see if an ACK has arrived but must determineitself when to retransmit. • It completely overlookscongestion 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 modellayers should have better communication with the other layers. • In order to avoidthe hidden station and exposed station problem in wirelessnetworks, the ENIC protocol utilizes Request-To-Send (RTS) andClear-To-Send (CTS) packets.

Hidden and Exposed Station A B Γ Δ Ε Ζ Η

ENIC • Ability to freeze theTCP state when a route has failed. • The network layers in ENICwill monitor the connections between the sender and receiver andwill notify the transport layer of a failure with an Explicit RouteState Notification (ERSN). • At this point, the protocol will save thecurrent state of its variables (retransmission time, congestionwindow, etc.) until the network layer has determined a new route. • The transport layer will periodically probe the network layer for anew route by using a Route Recovery Timer (RRT).  ENIC handles route change and pathloss without a serious reduction in throughput. • In the case ofnetwork congestion, ENIC will use standard TCP slow-startalgorithm.

ATP (2) Ad hoc Transport Protocol • Communication between layers needed for startuprate estimation, congestion control, and path loss/failure. • Thisis a common theme among the ad hoc protocols, so most lowerlayers should support this communication. • Theintermediate nodes are required to piggyback data rates ontopackets to the nodes. This allows the sender to determine theamount 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.

Low data Rate High data Rate Power limited Not power limited Storage limited Not storage limited User Sink Sensor Networks General notion for sensor networks

PSFQ Pump Slowly and Fetch Quickly • A Reliable Transport Protocol for Wireless Sensor Networks

1 2 3 4 1 1 1 2 2 2 3 3 3 Multi-Hop Packet Forwarding When No Link Loss – Multi-Hop Forwarding takes place

1 3 4 2 1 1 1 2 lost 3 3 Recover 2 3 Recover 2 Recover 2 Recovering from Errors Error Recovery Control Messages are wasted

1 3 4 2 1 1 2 2 lost 1 3 Recover 2 2 2 2 3 3 How PSFQ Recovers from Errors“Store and Forward” No wastage of the Error Recovery control messages

1 2 1 t Tmin 1 Tmax Tmin 1 Tmax PSFQ Pump Schedule If not duplicate and in-order and TTL not 0 Cache and Schedule for Forwarding at time t (Tmin<t<Tmax)

1 2 1 1 2 2 lost 3 Tr Tr Recover 2 2 Tmin 2 Tmax “Fetch Quickly” Operation

1 2 last-1 last Tproc last “Proactive Fetch”

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

1 3 2 4 Congestion detected 5 6 Open-Loop Hop-by-Hop Backpressure

1 2 Regulate bit is set 1,2,3 ACK 4,5,6 Congestion detected 7,8 ACK Closed Loop Multi-Source Regulation

Transport Issues in Ad Hoc and Sensor Networks