16-06-2012, 11:02 AM
Multicast Routing Issues in Ad Hoc Networks
Multicast Routing Issues in Ad Hoc Networks.pdf (Size: 245.59 KB / Downloads: 89)
Abstract
The advent of ubiquitous computing and the proliferation of portable computing devices have raised the
importance of mobile and wireless networking. At the same time, the popularity of group-oriented computing
has grown tremendously. However, little has been accomplished to-date in bringing together the technologies
for group-oriented communication and mobile networking. In particular, most modern wireless/mobile and
ad hoc networks do not provide support for multicast communication. A major challenge lies in adapting
multicast communication to environments where mobility is unlimited and outages/failures are frequent.
Introduction
It is anticipated that a typical internetwork of the future will consist of a wired backbone, and a collection of
xed-infrastructure mobile, and fully mobile (or ad hoc) networks as depicted in Figure 1. Mobile hosts will be
connected with the rest of the infrastructure through xed switches (e.g., base stations), and ad hoc networks
(AHNs) via satellites or xed terrestrial switches.
Mobile Networks and Mobility Patterns
The research issues raised by multicast routing in mobile environments are closely related to the type of mobile
network in use. In this paper, we discuss multicast routing in the context of:
Mobile networks with xed infrastructure
Ad hoc networks
Internetworks comprising wired, xed infrastructure, and ad hoc networks
Mobility patterns add another dimension to the problem of multicast routing in mobile networks. Table 1
illustrates sample scenarios for each combination of mobile infrastructure type (rows) and mobility patterns
(columns). Mobility patterns are an important factor in the design of mobile multicast routing protocols.
Issues in Multicast Routing in AHNs
AHNs refer to (for the most part, wireless) networks where all network components are mobile. In general,
there is no real distinction in an AHN between a host and a router since all network hosts can be endpoints as
well as forwarders of trac.
Most research in the area of routing for AHNs has concentrated on routing for unicast communication. Notable
examples include: the Monarch project [11], the TORA algorithm [16], and the MMWS project [18]; they are
reviewed in Section 9.1 below.
Multicast routing and packet forwarding in AHNs is a fairly unexplored area [18]. One recent exception is
the Shared-Tree Wireless Network Multicast (ST-WIM) work at UCLA [3] which aims to adapt xed-network
multicast approaches (PIM Sparse Mode) [6] to AHNs. Our view is that, since xed network multicast routing
is based on state in routers (either hard or soft), it is fundamentally unsuitable for an AHN environment with
unconstrained mobility.
Multicast Routing Issues in Ad Hoc Networks (Obraczka and Tsudik)
time and then re-appears. Now, if a re-broadcast of an old multicast packet is received by the host in question,
it will be re-broadcast since, technically, a new neighbor has been acquired. While this is clearly suboptimal,
little can be done to prevent it (barring having to maintain a snapshot of all neighbors for each cached multicast
packet ID).
Whenever a host receives a previously seen multicast packet and, in the meantime, some new neighbors have
been acquired, these neighbors must be identied in the packet when it is re-broadcast. If this is not done,
broadcast storms can occur. Consider the situation in Figure 2. At time T1, packet ID = x arrives. At time
T2, a copy of the same packet x arrives. In the mean time, host 1 acquired a new neighbor, host 6. If host 1
were to re-broadcast packet x as is, each of its other neighbors (hosts 2-5) who have also acquired host 6 as a
new neighbor, would re-broadcast the same packet.