Internet Connectivity Basics

willing to forward packets destined to other nodes. The network is typically a graph with multiple hops between some end points owing to the limited transmission ranges of the individual wireless nodes.

Ad hoc mobile communication with the global Internet is problematic if only host-based routing is used. Each node, which is allowed to have an arbitrary IP ad-dress in an ad hoc network, would require a host route propagated to every router of the fixed network; clearly, this is an unscalable approach. A solution that hides the state space explosion caused by host routes is needed. Since we wish to de-sign a protocol that can support ubiquitous mobility and connectivity support for all possible nodes and routers, including those moving to an ad hoc network from the fixed network, this IP address should be any globally routable (unicast) ad-dress. In this paper, we do not consider multicast addresses, which usually require specialized support from the routing protocol.

Ad hoc networks therefore should support efficient Internet connectivity, in-cluding mobility management. Our observation is that Mobile IP, considered as an access protocol, reduces the need for host route dissemination. By providing access with a topologically correct address and maintaining the address mapping of the mobile host/router only in binding caches of correspondent nodes, Mobile IPv6 reduces host route state maintenance to a small number of nodes.

When an IPv6 mobile node has become part of an ad hoc network, it may need to obtain a default route that it can use to transmit packets to destinations within the IPv6 Internet. A node in the fixed network finds a default router by means of router discovery. In IPv6, this is accomplished by way of Router Advertisement messages as specified in the Neighbor Discovery Protocol (NDP) [19]. Such advertisements include all the information that IPv6 nodes need to establish viable communica-tions to the Internet by using the link carrying the advertisement. However, NDP cannot be used unchanged in mobile ad hoc networks since the messages used for next-hop default router discovery, the Router Advertisements and Router Solicita-tions, are used with link-local scope addresses and can only reach nodes one hop away (i.e. neighbors).

To simplify the process enabling a node to establish connectivity through an Internet Gateway in a multihop ad hoc network, we make some assumptions cor-responding to some natural constraints:

• Gateways are routers located between the ad hoc network and the Internet, to provide Internet connectivity. No special assumptions are made, except that the gateways follow the protocol specifications in this chapter.

• The gateway advertises a topologically correct global routing prefix, so that packets transmitted anywhere in the Internet with the destination address belonging to that routing prefix can be routed toward the gateway. A node in an ad hoc network is allowed (but not required!) to have a preexisting arbitrary global IP address.

• A node that needs bidirectional connectivity to the Internet has to support a

3.2. Internet Connectivity Basics 41

discovery mechanism to find a globally reachable network prefix to be used in the ad hoc network.

• A manet node can use Stateless Address Autoconfiguration [20] for acquir-ing a topologically correct (routable) address within the ad hoc network. All IPv6 nodes are already required to support this procedure.

• Data forwarding within the ad hoc network, for packets initiated inside the ad hoc network, should use nonintrusive forwarding mechanisms that do not require any changes to nodes or routers (or routing protocols) in the fixed network.

• Any IPv6 node sending a packet to a Manet node does not have to have any knowledge that the node is in an ad hoc network.

Furthermore, a node that wishes to receive packets delivered to its persistent IPv6 address (if it has one) has to perform the following operations:

• learn from the gateway how to configure a topologically correct address and

• use this address as a care-of address with Mobile IPv6 so that this opera-tion substitutes the router discovery part of Mobile IPv6 operaopera-tion; the lo-cation update operations of Mobile IPv6 (using Binding Updates) remain unchanged.

In an ad hoc network, for a node to start communicating with other nodes in the Internet, the node must discover an Internet Gateway to obtain a globally routable prefix. This can happen as part of the boot-time procedures or it can happen as the mobile node migrates into an ad hoc network. The node makes use of the discovered prefix information to configure its network interfaces with globally routable IPv6 addresses. This discovery also provides IPv6 addresses of gateways and enables setting up default routes to the Internet through them, analogous to the way that IPv6 nodes on fixed networks configure default routers and prefix information from Router Advertisements. Gateways can unicast reply messages that include their own global prefix and IPv6 address. It is usually also important for gateways to discover routes toward such requesting nodes, since typically such nodes need bidirectional communications.

If a Manet node solicits prefix information, any intermediate node that has the requested information may supply it to the mobile node. As is typical with route discovery operations, such a requesting node may receive reply messages from multiple intermediate notes, each of which satisfies its request. The issues in selecting a particular default route are much the same as with IPv6 [19, 21] there are not yet well understood policies available to help with the decision process. For a Manet node, the gateway that is closest (as measured by hop count) would often be selected.

With the routing protocol in an ad hoc network running on the gateway, inter-mediate nodes can resolve the host route for the gateway by the exchange of routing

protocol messages. Subsequently, intermediate nodes can forward any packet to the gateway, which then can receive the packet and route it to its final destination. Care should be taken so that each intermediate node does not independently attempt to discover a route to the final destination address; this would result in unnecessary traffic overhead. Whether or not the final destination node exists inside the ad hoc network, intermediate nodes that do not have a host route for the destination would themselves forward the packet along their default route to the Internet. Although intermediate nodes may themselves obtain a default route by searching for the des-tination or for the gateways, these actions are obviously redundant. If intermediate nodes are involved with forwarding to default routes, we require those nodes to acquire their default route to the Internet at the same time the source node does, by means that are a very natural extension to the typical route discovery operation.