Exploring Multi-Path Communication in Hybrid Mobile Ad Hoc Networks

Introduction

Ambient computing requires the seamless integration of heterogeneous networks. Resources in an environment may be available through independent networks using different technologies, and users must be able to access them regardless of communication heterogeneity. For instance, a group of collocated resources (e.g., a printer and a projector) may be connected through a Bluetooth network while another remote resource is only accessible through WiFi (e.g., a file server). Fortunately, having multiple network interfaces embedded into a single mobile user device can greatly simplify this task. Current cell phones featuring heterogeneous network interfaces (e.g., WiFi, Bluetooth and cellular 3G) can provide users with access not only to resources on networks directly connected to the device but also by forming ad hoc networks and accessing resources on remote networks through other mobile devices.

We call these mobile networks that use heterogeneous wireless technologies Hybrid Mobile Ad hoc NETworks (HMANETs). They are typically formed by independently managed networks connected to each other by multi-homed devices (also referred to as bridges). All nodes in such networks are potentially mobile (as opposed to wireless mesh networks (Akyildiz, Wang, & Wang, 2005)) and any two nodes may be directly connected by multiple different links (in opposition to traditional mobile ad hoc networks (MANETs) where any two nodes share at most one direct connection). More importantly, those connections use different technologies and present heterogeneous properties such as delay, throughput, security, energy consumption and cost. Figure 1 shows the differences between MANETs and HMANETs.

Figure 1.

A MANET and an HMANET

One consequence of the network interface diversity is that there are potentially multiple paths, with different properties, between the source and destination nodes of a message. Applications can take advantage of this path redundancy by using multiple paths for a single communication. Nodes can send the same message through different paths to tolerate unpredictable connection failures, which is particularly important in networks that present unstable structures. Nodes can also send parts of a message through different links to enhance communication privacy and resistance against eavesdropping. Messages can be divided into shares in such a way that an attacker has to control a certain number of shares to reconstruct the whole message.

However interesting the multi-path approach might be, the implementation of point-to-point multi-path communication on an HMANET is complex. Our goal is to enable a node embedding multiple interfaces to establish a session with another node equipped with multiple interfaces, regardless of the IP addresses they use to communicate; messages that belong to the same session may have different source and destination IP addresses. The Internet Protocol suite, however, does not natively support such communication through multiple paths. It is not possible, for instance, to create a single unicast TCP session that spans multiple source and destination addresses. Even when using connection-less UDP, application-layer code is necessary to determine that packets containing different source and destination IP addresses are part of the same session. Multicast can be used to send the same message to several destination addresses through multiple paths, but it does not support sending from different source IP addresses or sending different parts of the same message to multiple destination addresses. Additionally, the topology of a HMANET changes frequently and multi-path communication must adapt dynamically to the properties of available connections.

Such characteristics call for a middleware approach for multi-path routing. By managing multi-path routes on a layer above the network, the middleware can profit from the routing protocols already implemented and running locally on each network and create an overlay network responsible for routing packets among hybrid networks. Multi-path routing on this overlay network is performed regardless of the IP addresses used as the message source or destination since the middleware can more easily manage sessions comprising different overlay connections between nodes on hybrid networks.