Module 3: Routing Protocols and Transport Layer in Ad-hoc Networks
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Issues in designing a routing and Transport Layer protocol for Ad hoc networks- proactive routing, reactive routing (on-demand), hybrid routing- Classification of Transport Layer solutions-TCP over Ad hoc wireless Networks
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Module 3: Routing Protocols and Transport Layer in Ad-hoc Networks
- 1. UNIT III: Routing Protocols And Transport Layer In Ad Hoc Wireless Networks: Routing Protocol: Issues in designing a routing protocol for Ad hoc networks, Classification: proactive routing, reactive routing (on-demand), hybrid routing, Transport Layer protocol for Ad hoc networks, Design Goals of a Transport Layer Protocol for Ad Hoc Wireless Networks, Classification of Transport Layer solutions,TCP over Ad hoc wireless, Network Security: Security in Ad Hoc Wireless Networks, Network Security Requirements. Issues in designing a routing protocol for Ad hoc networks, The major challenges that a routing protocol designed for ad hoc wireless networks faces are: Network topology is highly dynamic due to movement of nodes. hence, an ongoing session suffers frequent path breaks. Disruption occurs due to the movement of either intermediate nodes in the path or end nodes . Wired network routing protocols cannot be used in adhoc wireless networks because the nodes are here are not stationary and the convergence is very slow in wired networks. Mobility of nodes results in frequently changing network topologies Routing protocols for ad hoc wireless networks must be able to perform efficient and effective mobility management. Abundant bandwidth is available in wired networks due to the advent of fiber optics and due to the exploitation of wavelength division multiplexing (WDM) technologies. In a wireless network, the radio band is limited, and hence the data rates it can offer are much less than what a wired network can offer. This requires that the routing protocols use the bandwidth optimally by keeping the overhead as low as possible. The limited bandwidth availability also imposes a constraint on routing protocols in maintaining the topological information. The broadcast nature of the radio channel poses a unique challenge in ad hoc wireless networks. The wireless links have time-varying characteristics in terms of link capacity and link-error probability. This requires that the adhoc wireless network routing protocol interact with the MAC layer to find alternate routes through better-quality links. Transmissions in ad hoc wireless networks result in collisions of data and control packets. Therefore, it is required that ad hoc wireless network routing protocols find paths with less congestion. The hidden terminal problem refers to the collision of packets at a receiving node due to the simultaneous transmission of those nodes that are not within the direct transmission range of the receiver, but are within the transmission range of the receiver. Collision occurs when both nodes transmit packets at the same time without knowing about the transmission of each other. Classification:
- 2. Based on the Routing Information Update Mechanism Ad hoc wireless network routing protocols can be classified into 3 major categories based on the routing information update mechanism. They are: Proactive routing protocols : These are also known as table-driven routing protocols. Each mobile node maintains a separate routing table which contains the information of the routes to all the possible destination mobile nodes. Since the topology in the mobile ad-hoc network is dynamic, these routing tables are updated periodically as and when the network topology changes. It has a limitation that is doesn’t work well for the large networks as the entries in the routing table becomes too large since they need to maintain the route information to all possible nodes. Every node maintains the network topology information in the form of routing tables by periodically exchanging routing information. Routing information is generally flooded in the whole network. Whenever a node requires a path to a destination, it runs an appropriate path-finding algorithm on the topology information it maintains. 1.Destination Sequenced Distance Vector Routing Protocol (DSDV): It is a pro-active/table driven routing protocol. It actually extends the distance vector routing protocol of the wired networks as the name suggests. It is based on the Bellman- ford routing algorithm. Distance vector routing protocol was not suited for mobile ad-hoc networks due to count-to-infinity problem. Hence, as a solution Destination Sequenced Distance Vector Routing Protocol (DSDV) came into picture. Destination sequence number is added with every routing entry in the routing table maintained by each node. A node will include the new update in the table only if the entry consists of the new updated route to the destination with higher sequence number. 2. Global State Routing (GSR): It is a pro-active/table driven routing protocol. It actually extends the link state routing of the wired networks. It is based on the Dijkstra’s routing algorithm. Link state routing protocol was not suited for mobile ad-hoc networks because in it, each node floods the link state routing information directly into the whole network i.e. Global flooding which may lead to the congestion of control packets in the network.
- 3. Hence, as a solution Global State Routing Routing Protocol (GSR) came into the picture. Global state routing doesn’t flood the link state routing packets globally into the network. In GSR, each of the mobile node maintains one list and three tables namely, adjacency list, topology table, next hop table and distance table. Reactive routing protocols: These are also known as on-demand routing protocol. In this type of routing, the route is discovered only when it is required/needed. The process of route discovery occurs by flooding the route request packets throughout the mobile network. It consists of two major phases namely, route discovery and route maintenance. Do not maintain the network topology information. It Obtains the necessary path when it is required, by using a connection establishment process. Dynamic Source Routing protocol (DSR): It is a reactive/on-demand routing protocol. In this type of routing, the route is discovered only when it is required/needed. The process of route discovery occurs by flooding the route request packets throughout the mobile network. It consists of two phases: Route Discovery: This phase determines the most optimal path for the transmission of data packets between the source and the destination mobile nodes. Route Maintenance: This phase performs the maintenance work of the route as the topology in the mobile ad- hoc network is dynamic in nature and hence, there are many cases of link breakage resulting in the network failure between the mobile nodes. Ad-Hoc On Demand Vector Routing protocol (AODV): It is a reactive/on-demand routing protocol. It is an extension of dynamic source routing protocol (DSR) and it helps to remove the disadvantage of dynamic source routing protocol. In DSR, after route discovery, when the source mobile node sends the data packet to the destination mobile node, it also contains the complete path in its header. Hence, as the network size increases, the length of the complete path also increases and the data packet’s header size also increases which makes the whole network slow. Hence, Ad-Hoc On Demand Vector Routing protocol came as solution to it. The main difference lies in the way of storing the path, AODV stores the path in the routing table whereas DSR stores it in the data packet’s header itself. It also operates in two phases in the similar fashion: Route discovery and Route maintenance. Hybrid routing protocols: It basically combines the advantages of both, reactive and pro-active routing protocols. These protocols are adaptive in nature and adapts according to the zone and position of the source and destination mobile nodes. One of the most popular hybrid routing protocol is Zone Routing Protocol (ZRP). The whole network is divided into different zones and then the position of source and destination mobile node is observed. If the source and destination mobile nodes are present in the same zone, then proactive routing is used for the transmission of the data packets between them. And if the source and destination mobile nodes are present in different zones, then reactive routing is used for the transmission of the data packets between them. Combine the best features of the above two categories.
- 4. Nodes within a certain distance from the node concerned, or within a particular geographical region, are said to be within the routing zone of the given node. For routing within this zone, a table-driven approach is used. For nodes that are located beyond this zone, an on-demand approach is used. Transport Layer protocol for Ad hoc networks • The objectives of a transport layer protocol include setting up of: • End-to-end connection • End-to-end delivery of data packets • Flow control • Congestion control • Transport layer protocols • User Datagram Protocol (UDP): It is simplest Transport Layer communication protocol available of the TCP/IP protocol suite. It involves minimum amount of communication mechanism. Unreliable and connection- less transport layer protocols. It send short packets of data, called datagrams. • Transmission Control Protocol (TCP): It is one of the main protocols in TCP/IP networks. Whereas the IP protocol deals only with packets, TCP enables two hosts to establish a connection and exchange streams of data. TCP guarantees delivery of data and the delivery in the same order in which they were sent. It is reliable, byte-stream-based, and connection-oriented transport layer protocols. • These traditional wired transport layer protocols are not suitable for ad hoc wireless networks. Issues : Issues while designing a transport layer protocol for ad hoc wireless networks: Induced traffic refers to the traffic at any given link due to the relay traffic through neighboring links. • Induced throughput unfairness refers to the throughput unfairness at the transport layer due to the throughput/delay unfairness existing at the lower layers such as the network and MAC layers. • Separation of congestion control, reliability, and flow control could improve the performance of the transport layer. • Power and bandwidth constraints affects the performance of a transport layer protocol. • Misinterpretation of congestion occurs in ad hoc wireless networks. • Completely decoupled transport layer needs to adapt to the changing network environment. • Dynamic topology affects the performance of a transport layer. Design Goals of a Transport Layer Protocol for Ad Hoc Wireless Networks The following are the important goals to be met while designing a transport layer protocol for ad hoc wireless networks: • The protocol should maximize the throughput per connection. • It should provide throughput fairness across contending flows. • The protocol should incur minimum connection setup and connection maintenance overheads. It should minimize the resource requirements for setting up and maintaining the connection in order to make the protocol scalable in large networks. • The transport layer protocol should have mechanisms for congestion control
- 5. and flow control in the network. • It should be able to provide both reliable and unreliable connections as per the requirements of the application layer. • The protocol should be able to adapt to the dynamics of the network such as the rapid change in topology and changes in the nature of wireless links from uni-directional to bidirectional or vice versa. • One of the important resources, the available bandwidth, must be used efficiently. • The protocol should be aware of resource constraints such as battery power and buffer sizes and make efficient use of them. • The transport layer protocol should make use of information from the lower layers in the protocol stack for improving the network throughput. • It should have a well-defined cross-layer interaction framework for effective, scalable, and protocol-independent interaction with lower layers. • The protocol should maintain end-to-end semantics. Classification of Transport Layer solutions The top-level classification divides the protocols as extensions of TCP for ad hoc wireless networks and other transport layer protocols which are not based on TCP. The solutions for TCP over ad hoc wireless networks can further be classified into split approaches and end-to-end approaches. TCP over Ad hoc wireless The transmission control protocol (TCP) is the most predominant transport layer protocol in the Internet today. It transports more than 90% percent of the traffic on the Internet. Its reliability, end-to-end congestion control mechanism, bytestream transport mechanism, and, above all, its elegant and simple design have not only contributed to the success of the Internet, but also have made TCP an influencing protocol in the design of many of the other protocols and applications. Its adaptability to the congestion in the network has been an important feature leading to graceful degradation of the services offered by the network at times of extreme congestion. TCP in its traditional form was designed and optimized only for wired networks. Extensions of TCP that provide improved performance across wired and single-hop wireless networks were discussed in Chapter 4. Since TCP is
- 6. widely used today and the efficient integration of an ad hoc wireless network with the Internet is paramount wherever possible, it is essential to have mechanisms that can improve TCP’s performance in ad hoc wireless networks. This would enable the seamless operation of application-level protocols such as FTP, SMTP, and HTTP across the integrated ad hoc wireless networks and the Internet. This section discusses the issues and challenges that TCP experiences when used in ad hoc wireless networks as well as some of the existing solutions for overcoming them. TCP NOT PERFORM WELL IN AD-HOC WIRELESS NETWORKS DUE TO FOLLOWING REASONS • Misinterpretation of packet loss • Frequent path breaks • Effect of path length • Misinterpretation of congestion window • Asymmetric link behavior • Uni-directional path: TCP ACK requires RTS-CTS-Data-ACK exchange • Multipath routing • Network partitioning and remerging • The use of sliding-window-based transmission Split-TCP • It provides a unique solution to the channel fairness problem by splitting the transport layer objectives into congestion control and end-to-end reliability. • Splits a long TCP connection into a set of short concatenated TCP connections with a number of selected intermediate nodes as terminating points of these short connections. • Advantages • Improved throughput • Improved throughput fairness • Reduced impact of mobility • Disadvantages • It requires modifications to TCP protocol. • The end-to-end connection handling of traditional TCP is disturbed. • The failure of substitution nodes can lead to throughput degradation. TCP with Explicit Link Failure Notification (TCP-ELFN) • Handle explicit link failure notification • Use TCP probe packets for detecting the route reestablishment.
- 7. • The ELFN is originated by the node detecting a path break upon detection of a link failure to the TCP sender. • Advantages: • improves the TCP performance by decoupling the path break information from the congestion information by the use of ELFN. • Less dependent on the routing protocol and requires only link failure notification • Disadvantages • When the network is partitioned, the path failure may last longer • The congestion window after a new route is obtained may not reflect the achievable transmission rate acceptable to the network and TCP receiver. Feedback-based TCP (TCP Feedback – TCP-F) • Requires the support of a reliable link layer and a routing protocol that can provide feedback to the TCP sender about the path breaks. • The routing protocol is expected to repair the broken path within a reasonable time period. • Advantages: Simple, permits the TCP congestion control mechanism to respond to congestion • Disadvantages: • If a route to the sender is not available at the failure point (FP), then additional control packets may need to be generated for routing the route failure notification (RFN) packet. • Requires modification to the existing TCP. • The congestion window after a new route is obtained may not reflect the achievable transmission rate acceptable to the network and the TCP-F receiver. TCP Over Ad Hoc Wireless Network TCP with buffering capability and sequence information (TCP-BuS) It Uses feedback information from an intermediate node on detection of a path break. Use localized query (LQ) and REPLY to find a partial path Upon detection of a path break, an upstream intermediate node originates an explicit route disconnection notification (ERDN) message.
- 8. Advantages Performance improvement and avoidance of fast retransmission Use on-demand routing protocol Disadvantages Increased dependency on the routing protocol and the buffering at the intermediate nodes The failure of intermediate nodes may lead to loss of packets. The dependency of TCP-BuS on the routing protocol many degrade its performance. Ad Hoc TCP (ATCP) • uses a network layer feedback mechanism to make the TCP sender aware of the status of the network path • Based on the feedback information received from the intermediate nodes, the TCP sender changes its state to the persist state, congestion control state, or the retransmit state. • When an intermediate node finds that the network is partitioned, then the TCP sender state is changed to the persist state. • The ATCP layer makes use of the explicit congestion notification (ECN) for maintenance for the states. • Advantages • Maintain the end-to-end semantics of TCP • Compatible with traditional TCP • Provides a feasible and efficient solution to improve throughput of TCP • Disadvantages • The dependency on the network layer protocol to detect the route changes and partitions • The addition of a thin ATCP layer to the TCP/IP protocol changes the interface functions currently being used.
- 9. Other transport layer protocols: Application Controlled Transport Protocol (ACTP) • A light-weight transport layer protocol and not an extension to TCP. • ACTP assigns the responsibility of ensuring reliability to the application layer. • ACTP stands in between TCP and UDP where TCP experiences low performance with high reliability and UDP provides better performance with high packet loss in ad hoc wireless networks. • Advantages • Provides the freedom of choosing the required reliability level to the application layer. • Scalable for large networks • There is no congestion window • Disadvantages • It is not compatible with TCP. • Could lead to heavy congestion Ad Hoc Transport Protocol Ad hoc transport protocol (ATP) is specifically designed for ad hoc wireless networks and is not a variant of TCP. The major aspects by which ATP defers from TCP are (i) coordination among multiple layers, (ii) rate based transmissions, (iii) decoupling congestion control and reliability, and (iv) assisted congestion control. Similar to other TCP variants proposed for ad hoc wireless networks, ATP uses services from network and MAC layers for improving its performance. ATP uses information from lower layers for (i) estimation of the initial transmission rate, (ii) detection, avoidance, and control of congestion, and (iii) detection of path breaks. Network Security Security in Ad Hoc Wireless Networks: As mentioned , due to the unique characteristics of ad hoc wireless networks, such networks are highly vulnerable to security attacks compared to wired networks or infrastructure-based wireless networks. The following sections discuss the various security requirements in ad hoc wireless networks, the different types of attacks possible in such networks, and some of the solutions proposed for ensuring network security. Network Security Requirements A security protocol for ad hoc wireless networks should satisfy the following requirements. The requirements listed below should in fact be met by security protocols for other types of networks also. • Confidentiality: The data sent by the sender (source node) must be comprehensible only to the intended receiver (destination node). Though an intruder might get hold of the data being sent, he/she must not be able to derive any useful information out of the data. One of the popular techniques used for ensuring confidentiality is data encryption. • Integrity: The data sent by the source node should reach the destination
- 10. node as it was sent: unaltered. In other words, it should not be possible for any malicious node in the network to tamper with the data during transmission. • Availability: The network should remain operational all the time. It must be robust enough to tolerate link failures and also be capable of surviving various attacks mounted on it. It should be able to provide the guaranteed services whenever an authorized user requires them. • Non-repudiation: Non-repudiation is a mechanism to guarantee that the sender of a message cannot later deny having sent the message and that the recipient cannot deny having received the message. Digital signatures, which function as unique identifiers for each user, much like a written signature, are used commonly for this purpose.