PUBLISHED IN: COMPUTER COMMUNICATIONS, VOLUME 21, NUMBER 18, 1998, PP. 1645–1654 1 IP Multicast Group Management for Point-to-Point Local Distribution George Xylomenos and George C. Polyzos {xgeorge,polyzos}@cs.ucsd.edu Computer Systems Laboratory, Department of Computer Science and Engineering, University of California, San Diego, La Jolla, CA 92093-0114, USA Abstract—We examine the applicability of existing IP multicast mechanisms for Point-to-Point links such as wired and wireless telephone lines. We identify problems such as overhead due to IGMP leave latency and unnecessary probing of hosts, both im- portant issues for power constrained mobile hosts and low band- width wireless links. We propose alternative mechanisms that pre- serve the IP multicasting model but employ join/leave messages to track group membership. We describe the implementation re- quirements of our mechanisms and compare them to existing ones with respect to performance, mobile power efficiency, interoper- ability, robustness and implementation complexity, demonstrating that the join/leave approach is uniformly superior for this environ- ment. I. I NTRODUCTION The traditional modes of communication in computer net- works are unicast and broadcast, where messages are sent to one and all hosts in a network, respectively. Multicast, where messages are sent to an arbitrary set of hosts referred to by a single identifier, can be viewed either as an intermediate case, or as a generalized mode encompassing both unicast and broad- cast as special cases. Multicast provides the ability to address logical sets of hosts as single entities, thus easing the imple- mentation of distributed and replicated services. Compared to broadcast, multicast economizes on host and network resources by only delivering data to required recipients rather than to all hosts. If multiple unicasts were used to achieve multicast de- livery semantics, the sender would need to track a potentially huge set of intended recipients, while duplicate data transmis- sions would occur wherever paths to separate recipients shared network links. On the Internet, the Internet Protocol (IP) joins local area networks that employ heterogeneous technologies into a sin- gle wide area internetwork, providing a common network layer service interface to end-to-end layers. IP originally only sup- ported unicast and broadcast, which are either natively sup- ported or easily implemented on top of each other on any type of network link. Although multicast is harder to implement, IP extensions that support it have been developed, leading to the deployment of the Mbone, a wide area testbed [6] used for experimentation with multicast enabled applications. Au- dio and video conferencing and distribution applications were among the early adopters of the MBone, due to multicast’s po- tential for bandwidth savings for multipoint communications. This research was supported in part by a National Semiconductor Corporation Graduate Fellowship and the UC MICRO program. On the other hand, the single logical address concept is useful for resource discovery and automatic host configuration, which are becoming more important as host mobility is introduced in IP [9]. All servers that provide the same type of service can share a well-known multicast address so that they can be eas- ily located. Finally, group communication based applications, such as dynamic distributed routing, can substitute multicast for broadcast or multiple unicasts for efficiency. The IP multicast extensions basically consist of the native multicast mechanisms available in broadcast based LANs, plus additional mechanisms for wide area multicast distribution over backbone Point-to-Point (PtP) links. Recently however, the use of local PtP links has been increasing steadily on the Internet. Many hosts are connected to their Internet Service Provider (ISP) via a telephone link, that may be either analog or dig- ital, wireline or wireless. The Asynchronous Transfer Mode (ATM) LANs are an example of a PtP link based LAN. Finally, the mobility extensions for IP [9] support local multicast dis- tribution using virtual PtP links, or tunnels. All these local PtP links, whether physical or virtual, are sufficiently different from shared medium LANs to warrant closer examination of the im- plications of transplanting the existing IP multicast mechanisms to them. In this paper, we separate models from mechanisms in or- der to identify the problems that emerge when existing mech- anisms are used with local PtP links, and propose alternative optimized mechanisms that remain compatible with the IP mul- ticast model. The main problems that we try to solve are over- head due to IGMP leave latency and unnecessary continuous probing of hosts, issues especially important for mobile hosts and wireless links. In Section II we describe the IP multicast model and its supporting mechanisms, as well as their potential problems when used with PtP local distribution. In Section III we examine alternative approaches to these problems and iden- tify a join/leave mechanism as the most promising one. In Sec- tion IV we show how our mechanisms can be implemented by modifying existing multicast implementations. In Section V we evaluate the proposed mechanisms by first examining the performance of existing and proposed mechanisms, and then discussing how they compare with respect to mobile power ef- ficiency, interoperability, robustness and implementation com- plexity. We present our conclusions in Section VI.