Introduction

migrate between wireless access points that have direct access to the wired Internet and wireless ad hoc networks that are isolated and do not have any such access to the infrastructure.

The Internet Protocol (IP) [4] and other associated protocols [5] have served the world very well even during the explosive growth that has taken place over the years of their existence. To support possibly billions of Internet-accessible nodes (e.g. cell phones, automobiles and PDAs), a protocol change on the network layer to IP version 6 (IPv6) [6] is under way. IPv6 will enable cost-efficient availability of permanent IP addresses for all these devices, and many more not yet imagined.

Manageable and scalable support for routers that are themselves mobile is useful for maintaining the robustness provided by dynamic IP routing and for providing support for general network configurations that are not easily handled otherwise.

These mobile routers can provide access to arbitrary network topologies with no specific restrictions on the depth of forwarding paths or on their connectivity to fixed and/or mobile parts of the routing fabric.

Problems identified in router mobility (packet routing with mobile routers in mobile networks) can be considered as special cases of routing by nodes in ad hoc networks, which are being standardized by the Mobile Ad hoc Networking (Manet) working group [7]. Currently, the Manetworking group have released AODV [8], Dynamic Source Routing (DSR) [9], Optimized Link State Routing (OLSR) [10] and Topology Broadcast Based on Reverse-Path Forwarding (TBRPF) [11] as experimental RFCs to be used as base routing protocols in with ad hoc net-works. Although these are all IPv4-based protocols, they are all easily extended to support IPv6. For the purpose of this paper, we describe how such reactive and proactive protocols can be considered, under the assumption that the IPv4 address fields are simply expanded to be long enough for IPv6 addresses. For the partic-ular case of AODV, we rely on the already existing specification for the AODV for IPv6 (AODV6) [12] for detailed experimentation with the operation of our proposals with IPv6-based ad hoc networks. A comparison between AODV and AODV6 shows that the protocols are not quite identical, but the major change is that the fields in the message header have been rearranged for better alignment.

Such changes have no effect on address autoconfiguration procedures or gateway discovery. We expect that our recommendations in this paper will be immediately useful for these other base routing protocols just mentioned, once their IPv6 ver-sions are specified.

Dynamic routing protocol solutions for ad hoc networks (including those men-tioned above) meet requirements such as

• multihop forwarding capability;

• loop freedom for all routing paths;

• elimination of the counting to the infinity problem, avoiding nonconverging metric-based routing scenarios in distance vector protocols [13];

• low processing and memory requirements and

3.1. Introduction 37

• self-starting operation without the need for user intervention.

These requirements are relevant when a dynamic routing protocol must operate in arbitrary, rapidly changing network topologies. Routing protocols considered withinManetalso fit related applicability requirements. Our solution for Internet connectivity works with the variousManetprotocols, but there are certain differ-ences that depend on the choice of base protocol. As a simple example, the choice of protocol will affect the names and message formats of various routing control signals.

Whenever any node of a mobile ad hoc network (called a Manet node) comes into contact with a node that has connectivity to the global Internet, cooperation between these two nodes can provide global connectivity for every other node in the ad hoc network. The cooperative node that has global connectivity is called the Internet Gateway and is treated by every node within the ad hoc network (i.e.

every Manet node) as a default router. This conforms well with standard treatment for default routers, especially if the model for the ad hoc network routing protocol determines next hops for the various ad hoc host and network destinations. This much can work even if the gateway node does not run any other routing protocol except the ad hoc routing protocol (which is needed so that it can answer requests for its own address). In our discussion, since the gateway node also runs the base ad hoc network routing protocol, it is also considered to be a Manet node.

If, in addition, nodes in an IPv6 ad hoc network need to receive packets from the global Internet as well as transmit them, then the gateway has to take steps so that it will be a forwarding node along the path for packets transmitted from within the Internet toward a Manet node as destination. This means that the gateway node might have to provide reachability information for the addresses of every other Manet node. In the Internet, reachability information is given by way of routing protocols such as Open Shortest Path First Protocol for IPv6 (OSPFv6) [14] Rout-ing Information Protocol (RIP) for IPv6 (RIPng) [15] or Border Gateway Protocol for IPv6 (BGP) [16]; the gateway node has to be assigned a routing prefix, and it has to have the ability to forward packets toward any node whose address has that routing prefix as the leading bits of its address. For all Manet nodes that have addresses conforming to the routing prefix(es) advertised by the gateway, this will work fine. Our method enables Manet nodes to autoconfigure addresses that con-form to IPv6 infrastructure routing requirements.

However, some Manet nodes must receive packets that are sent to addresses that do not conform to the set of prefixes advertised by the gateway. The gateway node should not advertise reachability for those topologically incorrect addresses.

Otherwise, host routes for those particular nodes would have to be injected into the distributed database consisting of the routing tables for the Internet infrastructure routers, an approach that is known to be unscalable, unmanageable and difficult to secure. Instead, the Manet nodes acquire topologically correct addresses that conform to the advertised prefix and use the acquired addresses to enable reception of the packets delivered to the topologically incorrect address.

IPv6 Global

Internet Gateway Prefix

node node

node

node Internet

network Ad hoc Reachability

Fig. 3.1:Internet reachability from an ad hoc network.

Such persistent (‘always-on’) IP-address reachability is a very desirable prop-erty for Internet connectivity. Previous developments [17, 18] provide to wireless devices the same level of possibilities for Internet connectivity as wired computers, including such persistent connectivity wherever the physical links are available.

This enables mobility regardless of the wireless link layer, and with all transport and application protocols. These solutions do not require any modifications to the network-layer routing infrastructure, in order to maintain the layered end-to-end communication model, backward compatibility and robustness of the Internet.

Mobile IPv6 [18] provides this persistent reachability for IPv6 mobile nodes, by hiding the movement of a host away from its home domain. This combination of mobile node reachability and router mobility in IPv6, illustrated in Figure 3.1, is a likely requirement for future mobile Internet devices. If, as expected, the future Internet is mostly populated with mobile devices, scalability will be a key consid-eration for mobility management. For many small sensor devices, computational complexity has to be kept at a minimum to conserve battery power and avoid pro-cessing delays. Manual configuration is unacceptable for any such high-volume devices. Furthermore, protocol simplicity is also a requirement, since a complex solution would be difficult to develop on all the different network platforms that are going to be prevalent within the wireless Internet. Mobile IPv6 affords impor-tant advantages for making such wireless attachments, especially regarding router advertisement and address autoconfiguration.

We propose the following steps for managing ad hoc connectivity to the Inter-net:

1. If no address is currently configured, acquire a canonical site-local address.

2. If an advertisement is received, configure a globally valid IPv6 address