Reliable multicast: when many must absolutely positively receive it

C. Metz
1998 IEEE Internet Computing  
IP-based networking scales from the small corporate intranet to the global Internet, enabling end users to send mail, retrieve files, and browse the World Wide Web. These time-tested and familiar applications are designed to operate over a logical unicast (point-to-point) connection consisting of exactly two hosts, typically a client and a server. However, it may sometimes be useful to have multiple hosts participate in the same communications session using a "shared channel" (similar to the
more » ... (similar to the broadcast service on shared-media LANs such as Ethernet). This has led to the development of a suite of applications based on the notion of group communications, or multicasting. Multicasting enables a group of hosts to communicate and share the information simultaneously. IP Multicast technology allows the underlying IP network to transport a copy of the sending host's information to all members of the group, which can range in size from a few hosts to several thousand. Members of a multicast group are identified by a single group address rather than by their individual host addresses. As Internet pioneer Steve Deering pointed out in 1988, multicasting is much more efficient than unicasting for applications that share the same data, because it minimizes "the transmission overhead on the sender and the network, and reduces the time it takes for all destinations to receive the information . . . ." 1 The first well-documented public demonstration of IP Multicast was an audiocast of a 1992 Internet Engineering Task Force meeting. 2 Since then the Internet, vendor, and research communities have worked to advance the state of IP Multicast applications and technology. There are a growing number of commercially available and deployable multicast applications supporting distance learning, group collaboration, and multiparty videoconferencing. In 1996, a consortium of vendors, applications developers, and other interested parties formed the IP Multicast Initiative to promote the awareness, understanding, and implementation of IP Multicast applications and technologies. Challenges Facing IP Multicast Because IP Multicast uses connectionless IP networking, there is no guarantee that the packets will be reliably and sequentially delivered to all members of the group. Invoking TCP at the endpoints to provide reliability in a multicast session is not an option since TCP supports only unicast connections. Yet some multicast application candidates-such as distributed software updates, electronic media delivery, and database replication-require a high, if not absolute, degree of reliability to function in the real world. Therefore, the challenges in introducing reliability to the IP Multicast environment are to ensure that the data is received by all group members, that a simple and fast recovery mechanism for data loss is present, and that the manner in which it is done is both scalable and efficient. Much research has focused on these challenges and, more generally, addressed the topic of Reliable IP Multicast (RM). The IETF and the Reliable Multicast Research Group of the Internet Research Task Force (IRTF) have been involved in coordinating the research, development, and standardization of reliable multicast. More recently several vendors have announced and are delivering solutions that support reliable multicast for specific applications. Given the ubiquitous reach of the Internet and the growing requirement to transport and deliver meaningful information to large enduser populations, this is an area that will continue to get a lot of attention. IP Multicast Basics The basic components of the IP Multicast model are s a group membership protocol, s a multicast routing protocol, and s a group address. A sender addresses multicast packets to a group address-not the individual address of each receiving host. The range of possible multicast group addresses is contained in the Class D address space (224.0.0.0-239.255.255.255). Through the Internet Group Management Protocol (IGMP), the receiving host informs a directly attached multicast-capable router of its desire to join (or leave) a specific multicast group. Routers employ a multicast routing protocol, such as the
doi:10.1109/4236.707685 fatcat:4pb43wfpunaelmbl6brakwnlqi