Mobile Multicast

Abstract

The submission of datagrams from a node to a group of receivers must be seen as a powerful extension of the Internet routing layer (Deering, 1989). Multicast group communication forms an integral building block of a wide variety of applications, ranging from public content distribution and streaming over voice and video conferencing, collaborative environments and gaming up to the self-organization of distributed systems. Its support by network layer multicast will be needed, whenever globally distributed, scalable, serverless or instantaneous communication is required. As broadband media delivery more and more emerges to be a typical mass scenario, scalability and bandwidth efficiency of multicast routing continuously gains relevance. Internet multicasting will be of particular importance to mobile environments, where users commonly share frequency bands of limited capacity. The rapidly increasing mobile reception of 'infotainment' streams may soon require a wide deployment of mobile multicast services. The fundamental approach to deal with mobility in the next generation Internet is stated in the Mobile IPv6 RFC (Johnson, Perkins & Arkko, 2004). Multicast has only roughly been treated therein, but raises quite distinctive aspects within a mobility aware Internet infrastructure. On the one hand multicast routing itself supports dynamic route configuration, as members may join and leave ongoing group communication over time. On the other hand multicast group membership management and routing procedures are intricate and too slow to function smoothly for mobile users. In addition multicast imposes a special focus on source addresses. Applications commonly identify contributing streams through source addresses, which must not change during sessions, and routing paths in most protocols are chosen from destination to source. Mobile multicast has been a concern for about ten years (Xylomenos & Polyzos, 1997) and led to innumerous proposals for solutions. Intricate multicast routing procedures, though, are not easily extensible to comply with mobility requirements. Any client subscribed to a group while in motion, requires delivery branches to pursue its new location; any mobile source requests the entire delivery tree to adapt to its changing positions. Significant effort has been already invested in protocol designs for mobile multicast receivers. Only limited work has been dedicated to multicast source mobility, which poses the more delicate problem (Romdhani, et al., 2004a). In multimedia conference scenarios each member commonly operates as receiver and as sender for multicast based group communication. In addition, real-time communication such as voice or video over IP places severe temporal requirement on mobility protocols: Seamless handover scenarios need to limit disruptions or delay to less than 100 ms. Jitter disturbances are not to exceed 50 ms. Note that 100 ms is about the duration of a spoken syllable in real-time audio. While multicast routing itself has been proposed to support mobility in the Internet (Helmy, 2000), consensus on an efficient, robust and widely deployable scheme of multicast for mobile hosts is still lacking (Schmidt & Wählisch, 2005a). In this review we will summarize the state of the art in current work on multicast to Mobile IPv6 networks. The principle conceptual problems are discussed and analyzed. Propositions for improvement and possible directions to further proceed towards a mobile multicast are presented.