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Introduction to Mobile Ad Hoc Networking
Mobile ad hoc networks (also known as MANETs [27]) are networks dynamically formed and maintained by a number of nodes that self-organize their wireless connectivity, without using any prior infrastructure or central control. As the topology changes due to the mobility of some nodes or to any other reason, the nodes dynamically reconfigure themselves and continue to provide the maximum network connectivity, which may or may not encompass Internet access (i.e. a MANET can also be a stand-alone network). Basically, the MANET philosophy is, in mobile wireless environments, similar to what the original Internet paradigm was in static wired environments. That is to say: any node can join the network, as nodes there are more or less equals, each one of them simply contributing as much as it can to extend the network’s reach. Similarily to the origins of the Internet itself, MANETs have emerged from a military context, in the 1960s – at the time, they were called packet radio networks. But with the boom of mobile devices and wireless communications, mobile ad hoc networking has seen its scope extended to commercial environments as well. MANETs essentially aim at achieving robust and efficient mobile wireless networking by incorporating routing functionality into mobile nodes. Ad hoc mobility has been the subject of numerous research and standardization efforts during the last few years, in academia as well as in the industry. It is foreseen as an important Internet component in the near future.
However, the challenges of ad hoc networking are somewhat different from those of wired networking, and thus, many classical Internet solutions successfully used on wired networks fail in a mobile ad hoc environment. Basically, these differences come from two aspects: (i) the use of broadcast wireless interfaces instead of traditional wired or point-to-point wireless interfaces, and (ii) the possible native mobility of a majority (if not all) of the nodes in the network, instead of assuming essentially static nodes. In the following, we will give a more detailed overview of the new challenges encountered with ad hoc networking.
Challenges with Wireless Links using Broadcast Radio Interfaces
The use of broadcast wireless interfaces challenges one of the basis of the Internet architecture: the division of the network into a well defined hierarchy of subnets. Indeed, Internet’s foundation relies on there being a tie between a physical link shared by hosts and managed by a router, on one hand, and a particular range of IP addresses on the other hand. All the devices on a link (say, devices with an interface on the same Ethernet LAN for example) are supposed to share the same range of IP addresses, then called a subnet. However, the use of broadcast radio interfaces (the most popular wireless solution so far, such as IEEE 802.11 [38]) makes the notion of shared link become very vague. The most common link properties fail in most cases, as a MANET is very often not contained in a single radio range. For instance, a single transmission is not able to reach all the nodes in the subnet (hosts or routers), and a shared physical segment cannot be accurately identified, as a MANET spans over several radio hops. In particular, this multi-hop aspect distinguishes MANETs from the traditional use of wireless links to access points infrastructures. MANETs’ wireless multi-hop nature also invalidates the traditional strict division of Internet nodes between (a) routers, that actively participate in network building and maintenance, and (b) hosts, that just use the network. Depending on the topology, which changes often due to mobility, any node in the MANET may be required to act as forwarder to complete a multi-hop broadcast, for instance. In this respect, nodes in a MANET are neither routers nor hosts, but rather more or less both. And therefore, nodes are all equals a priori.
Challenges with Mobility beyond Simple Edge Mobility
Another basis of the Internet infrastructure is the tie between a node’s identity and its topological locationor in other words, IP addressing. The Internet was essentially designed for static nodes, and therefore it made sense to condense the location and the identity of a node into a single piece of information: its IP address. When combined with the organization of the network into a precise hierarchy of subnets and their well known associated range of IP addresses, this gives a very simple and efficient approach to networking, that is also scalable to large, heterogeneous networks – the basis of the Internet. Node mobility is an issue in such an organization. Indeed, if a node changes location, it must also change its ID, which is a problem for most applications. Some solutions have been developed to solve this problem in case of limited edge mobility, and for a limited fraction of mobile nodes in the network. Approaches like MobileIP [39] and NEMO [40] are such solutions (see Chapter 4). However, in a mobile ad hoc environment, these techniques fail as Internet connectivity may be indirect. Some nodes in the MANET may indeed be several hops away from the nodes providing Internet access, and the basic mechanisms behind these solutions do not handle multi-hops. Actually, in this case, the initial step will fail most of the time: the mobile node will not be configured as it should, with a correct care-of address. Enhancements such as Hierarchical Mobile-IP [41] or the Nested NEMO mechanism described in [40] (see Chapter 4) may be able to cope to some extent with a mostly static multi-hop topology. But none of these solutions can handle (i) a fast changing topology, (ii) an arbitrary number of nodes, and (iii) an arbitrary MANET diameter.
Moreover, philosophically, if a bigger proportion of the nodes in the Internet are mobile, the concept behind Mobile IP is indeed questionnable in terms of scalability, as it may then produce prohibitive amounts of overhead with on one hand IP-in-IP encapsulation, and on the other hand extremely sub-optimal routing.
Mobile Ad Hoc Networks: a Harsher Environment
In general, MANETs induce a harsher environment compared to what it used to be so far, with wire-line networks for instance. Mobile ad hoc networking imposes scarcer wireless bandwidth, while at the same time featuring greater topology change rates due to mobility, and lower transmission quality. MANET nodes experience more complex interference issues due to the use of wireless broadcast links and its incurred hidden node problems. Many nodes have inherent energy constraints, running on battery for instance. Security is looser due to the use of wireless links and to the nodes’ mobility. These facts point at a difficult task: having to fit much more signalling into much less available bandwidth, with nodes that have limited processing capabilities. Moreover, full-fledged schemes for routing and address configuration are needed to handle the mobile multi-hop mix between fixed Internet nodes and MANET nodes. The following chapters will describe some of these schemes.
Table of contents :
I Introduction to Internet Routing
1 Internet History
1.1 The ARPANET and Packet Switching
1.2 From NSFNET to the Internet
1.3 Internet Standards
2 Routing in the Internet
2.1 Internet Protocols Architecture
2.2 IP Addressing
2.3 Hosts and Routers, Usual Internet Terminology
2.4 IP Packets
2.5 The Role of Routing
2.6 Autonomous Systems
2.7 Routing Techniques
2.7.1 Hop-by-Hop Distribution
2.7.2 Distance Vector Routing
2.7.3 Link State Routing
3 Internet Routing Cornerstones: OSPF and BGP
3.1 BGP
3.1.1 Peer-to-Peer Sessions
3.1.2 BGP Messages, Paths and Attributes
3.1.3 Policy Routing
3.2 OSPF
3.2.1 Reliable Flooding, Sequence Numbers and Aging
3.2.2 Overhead Optimizations with Interface Types
3.2.3 Convergence Acceleration with the Database Exchange Mechanism
3.2.4 Scaling with Hierarchical Routing with Areas
3.2.5 OSPF Applicability
II Routing Challenges with Mobility in the Internet
4 Edge Mobility
4.1 Introduction to Edge Mobility
4.2 Mobile IP
4.2.1 Performance Issues with Mobile IP
4.3 NEMO
4.3.1 Performance Issues with NEMO
5 Ad Hoc Mobility
5.1 Introduction to Mobile Ad Hoc Networking
5.2 Challenges with Wireless Links using Broadcast Radio Interfaces
5.3 Challenges with Mobility beyond Simple Edge Mobility
5.4 Mobile Ad Hoc Networks: a Harsher Environment
III Internet Routing and Mobility Solutions for Ad Hoc Networks
6 Ad Hoc Routing
6.1 Introduction to Ad Hoc Routing
6.2 OLSR: Link State Routing Optimized for MANETs
6.2.1 MPR Techniques
6.2.2 Unified Formats
6.2.3 Additional Features
6.3 wOSPF: Router Ad Hoc Mobility in the Internet
6.3.1 Introduction to OSPF on MANETs
6.3.2 OSPF’s Issues on Ad Hoc Networks
6.3.3 Overview of the OSPF Wireless Interface Type
6.3.4 Shortcomings of the wOSPF Approach
6.4 MANEMO: Network Ad Hoc Mobility in the Internet
6.4.1 Nested NEMO Suboptimal Routing and Encapsulations Issues
6.4.2 NEMO Route Optimization with Mobile Ad Hoc Networking
6.4.3 NEMO Tunneling Optimization using MANET Routing
7 Optimizing Control Traffic in Mobile Ad Hoc Networks
7.1 Flooding Optimization Techniques
7.1.1 Introduction to Flooding
7.1.2 The MPR Technique
7.1.3 The Gateway Mechanism
7.1.4 The CDS Technique
7.1.5 Performance Evaluation via Mathematical Modelling
7.1.6 Performance Evaluation via Simulation
7.2 Partial Topology Optimization Techniques
7.2.1 Introduction to Partial Topology
7.2.2 On-Demand Topology Information
7.2.3 Link State Optimization
7.2.4 Partial Topology Optimizations for Overlapping Relays OSPF
7.3 Optimizing Reliability for Link State Information
7.3.1 Introduction to Reliable Link State Synchronization
7.3.2 ManagingWired, Ad Hoc Heterogeneity
7.3.3 Definition of Link State Database Signatures
7.3.4 Signature Exchanges
7.3.5 Database Exchange
7.3.6 Performance Evaluation of the Database Signature ExchangeMechanism
7.3.7 Applicability of the Database Signature ExchangeMechanism
8 Scalability Solutions for Massive Ad Hoc Topologies
8.1 Hierarchical Routing with Trees
8.1.1 Introduction to Hierarchical Networking
8.1.2 OLSR Tree Formation and Maintenance
8.1.3 Tree Options
8.1.4 Hierarchical Routing with OLSR Trees
8.2 Fish Eye Enhancement
8.2.1 Introduction to Link State Routing coupled with Fish Eye
8.2.2 Modeling Ad Hoc Networks
8.2.3 OSPF and OLSR Scalability
8.2.4 Scaling Properties of OSPF and OLSR Enhanced with Fish Eye Strategy
8.2.5 Comparison with Previous Results on Ad Hoc Network Capacity
9 Integration Solutions for Ad Hoc Networks in the Internet
9.1 The Security Problem in Ad Hoc Networks
9.1.1 Incorrect Traffic Generation
9.1.2 Incorrect Traffic Relaying
9.1.3 Secure Integration of Ad Hoc Networking in the Internet
9.2 Address Autoconfiguration in Ad Hoc Networks
9.2.1 A Light-weight AutoconfigurationMechanism for OLSR
9.2.2 Local Beaconing
9.2.3 Temporary Address Assignment
9.2.4 Permanent Address Assignment
9.3 Duplicate Address Detection
9.3.1 Introduction to Duplicate Address Detection
9.3.2 Performing Duplicate Address Detection in an OLSR Network
9.3.3 Shortcomings of the Passive Approach
9.3.4 Resolving Duplicate Address Conflicts
10 Conclusions on Ad Hoc Networking
