Techniques for Making IP Multicast Simple and Scalable

This paper describes Root Administered Multicast Addressing (RAMA), a protocol for wide-area IP multicast that scales to a large number of simultaneous groups with topologically distant members. RAMA solves the widearea “rendezvous” problem by making the root of the distribution tree explicit in the multicast group identifier. This is done by extending the 4-byte IPv4 multicast group address format to an 8-byte address format, (R;G); where R is the unicast address of the root of the multicast distribution tree and G is a group identifier, unique with respect to R and administered by it. Data distribution occurs via a single shared bi-directional tree, allowing scalable operation for multiple senders. RAMA generalizes two recent protocols, Simple Multicast [18] and EXPRESS Multicast [10], into a common protocol that has the desirable features of both. RAMA has several advantages over existing designs for wide-area IP multicast: (i) it is a single protocol that works both within a domain and across domains, (ii) it provides efficient support for multiple-sender as well as single-sender applications, (iii) since group identifiers are allocated and administered locally with respect to the root, there is no need for globally coordinated Internetwide multicast address allocation. Hence, RAMA does not require a companion heavy-weight multicast address allocation infrastructure. The extended address format also provides a larger number of multicast addresses, mitigating the address availability problem. In this paper, we motivate and describe the RAMA protocol architecture and the engineering issues in developing and deploying it. Extended addressing in RAMA requires changes to end hosts, and we outline the design and implementation of RAMA using IP options for a BSD-based end host operating system. Unfortunately, IP options are not handled efficiently by today’s routers which forward such packets in a slow path, typically in software. To address this issue, we also discuss variants of RAMA that do not require the extended address format.