Convergence Issues in Inter-Domain Routing and Connectivity Modeling of Ad Hoc Networks
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2006:45 LICENTIATE T H E SI S Convergence Issues in Inter-Domain Routing and Connectivity Modeling of Ad hoc Networks Johan Nykvist Luleå University of Technology Department of Computer Science and Electrical Engineering Division of Systems and Interaction 2006:45|: 402-757|: -c -- 06 ⁄45 -- Convergence Issues in Inter-Domain Routing and Connectivity Modeling of Ad hoc Networks Johan Nykvist Division of Systems and Interaction Department of Computer Science and Electrical Engineering Lule˚a University of Technology Lule˚a, Sweden Supervisor: Dr. Kaustubh S. Phanse ii To Maria and Oskar iv Abstract This thesis addresses two topics in computer networking: inter-domain routing and mod- eling intermittently connected networks (ICNs). The standard inter-domain routing protocol used in the Internet today is the Bor- der Gateway Protocol version 4 (BGP). BGP applies send-rate constraints to routing messages in order to limit its convergence complexity. We implement a simulator to study how the send-rate constraints affect BGP’s convergence properties and compare the results with a theoretical convergence model. Routing policies are used to control BGP’s path selection, which can give rise to routing divergence. We evaluate a path-history algorithm that can detect routing diver- gence, and show that the algorithm gives false positives under send-rate constraints. We use our simulator to show how likely it is that false positives occur in certain network configurations. Two solutions to solve this problem are proposed. Unlike the Internet, ICNs are networks in which connected end-to-end paths rarely exist. Device mobility, devices shutting down to conserve energy, and environmental con- straints are common causes of intermittent connectivity. To develop routing algorithms for these networks, it is essential to first understand the patterns of connectivity between network devices. We propose a temporal connection model that captures connectivity patterns in ICNs. A tool for exploring topological and temporal properties of ICNs is developed and the connectivity behaviors of two real ICNs are analyzed. v vi Contents Thesis Introduction 1 1 Introduction.................................. 1 2 Inter-DomainRouting............................ 3 3 AdhocandIntermittentlyConnectedNetworks.............. 6 4 FutureResearch................................ 8 Paper A Simulating Convergence Properties of BGP 11 1 Introduction.................................. 13 2 RelatedWorkandMotivation........................ 15 3 TheSimulator................................. 16 4 ExperimentsandResults........................... 17 5 ConclusionsandFutureWork........................ 21 Paper B Timing Issues in Detection of Routing Policy Disputes 25 1 Introduction.................................. 27 2 Background.................................. 29 3 ImplicationsofSend-RateConstraints................... 36 4MakingSPVP2-SRSound.......................... 42 5 DiscussionandFutureWork......................... 48 Paper C Modeling Mobile Networks with Ad hoc Connectivity 51 1 Introduction.................................. 53 2 OpportunisticCollaborativeNetworks.................... 55 3 ConnectivityModeling............................ 57 4 TheNetworkExplorationTool........................ 61 5 DiscussionandFutureWork......................... 62 References 65 viii Acknowledgements First, I want to thank my supervisor Kaustubh Phanse for his warm support and a good friendship. I also want to thank Lenka Carr-Motyˇckov´a for her supervision during the first years of my studies, and Olof Hagsand at KTH for being my co-supervisor during the same time. Other people at the department have also been important to me. I am very thankful to Pierre Fransson and H˚akan Jonsson. Apart from being good friends, they have given me a great deal of moral support. Thanks to Tomas Johansson who has always supported me in my research. I also thank all the Ph.D. students in the former Division of Computer Science and Networking for all the joyful moments that we have shared. I am very grateful to my parents that have always helped me and supported me through life, and also to my beloved Maria for her understanding, warm support, and encouragements. Finally, I want to express my deepest gratitude to my son Oskar who brings me so much happiness. He shows me how truly amazing and valuable life can be. My work has been partly funded by the Winternet program, the EU M˚al-1 project C4- DTN, and Vinnova. I am very grateful for this support. ix x Thesis Introduction xii Thesis Introduction As our circle of knowledge expands, so does the circumference of darkness surrounding it. Albert Einstein 1 Introduction The general topic of this thesis is modeling of and finding forwarding paths in computer networks [29]. The thesis is divided into two parts. The first part reviews the background and sets the stage for the scientific research reported in this thesis and summarizes the three included papers. The second part comprises the papers. The first two papers deal with inter-domain routing issues in the Internet. The third paper deals with modeling of connectivity in intermittently connected networks. The remainder of this section gives a general description of data forwarding and rout- ing in computer networks. Section 2 explains how routing is organized into intra-domain and inter-domain routing in the Internet, and introduces the standard inter-domain rout- ing protocol in use today. Section 3 gives an overview of ad hoc and intermittently connected networks. Finally, Section 4 presents some directions for future work. 1.1 Data Forwarding and Routing In many computer communication networks, hosts, such as desktop computers and servers, are not able to communicate directly. Instead, they rely on intermediating de- vices that forward their data over several hops. Typically, the forwarding paths are not obvious. There can be several paths between any two network devices, and one path can be better suited than another, since communication links and forwarding devices can have various capacities in terms of, for example, bandwidth and propagation delay. Paths with high bandwidth are suitable for large bulk data transfers, whereas paths with short end-to-end delays and little, or no congestion are suitable for real-time traffic. It is beneficial in general to select the shortest paths possible (i.e., the paths with the fewest number of hops), since this minimizes the overall network load. 1 2 Thesis Introduction The act of establishing forwarding paths between network entities is called routing. Routing in a network is usually based on information about the network’s topology and metrics, such as hop count, bandwidth, and propagation delay. However, network topologies and metrics can change over time. Feasible paths can disappear and new ones evolve when devices are connected to and disconnected from a network. Thus, routing needs to be a continuous activity. Topology changes are in general more frequent in wireless and mobile networks than in fixed networks with wired links, for example, due to fading wireless link channels, battery power outage in the devices, and mobility. 1.2 Routers and Routing Protocols Intermediate devices that perform routing and forward packets between hosts are called routers. Routers communicate using a routing protocol to gather network topology and metric information about a network. A number of routing protocols exist, which differ in operation and features. Two examples of routing protocols are the Routing Information Protocol (RIP) [35] and the Open Shortest Path First (OSPF) protocol [36]. Many, but not all, protocols define criteria for path selection. Most routing protocols are distributed. This means that routing information and the workload of path calculation is shared by several routers. There are two major classes of distributed routing protocols: Distance-Vector (DV) routing protocols and Link-State (LS) routing protocols. DV routing protocols use a distributed variant of the Bellman-Ford algorithm [14] to calculate forwarding paths. A DV protocol does not create a complete map of the network topology; a DV router only knows its distance to each destination in the network, and to which peer it should forward data for a given destination. A commonly used distance metric is the hop count. Peering DV routers exchange their distances to all destinations. A router decreases its distance to a destination if the sum of a peer’s distance to the destination and the distance from the itself to that peer is less than its current distance. The router forwards data to the peer that renders the shortest distance to the destination. RIP is an example of a DV routing protocol. LS routing protocols distribute a graph that represents the entire network topology to all routers in a network. The routers uses the graph to individually calculate for- warding paths from themselves to all destinations in the network. The path calculation is commonly done using Dijkstra’s shortest-path algorithm [14]. This approach requires that all routers use the same path selection criterion, since the use of different criteria can give rise to routing loops. OSPF is an example of a LS routing protocol. The prime benefits of using distributed protocols are scalability and robustness. Dis- tributed routing protocols are also less sensitive to failures of routers and links. No single point of failure can bring down the entire network as in the case of having a single device that performs routing for the entire network.