Explicit Multicast (Xcast) Concepts and Options

Authors:

• R. Boivie, N. Feldman of IBM
• Y. Imai of WIDE / Fujitsu
• W. Livens of ESCAUX
• D. Ooms of OneSparrow?
• O. Paridaens of Alcatel

Complete Citation

R. Boivie, N. Feldman, Y. Imai, W. Livens, D. Ooms, O. Paridaens. Explicit Multicast (Xcast) Concepts and Options Internet Engineering Task Force "draft-ooms-xcast-basic-spec-13.txt", July 2007

Download the Draft

Abstract

While traditional IP multicast schemes are scalable for very large multicast groups, they have scalability issues with a very large number of distinct multicast groups. This document describes Xcast (Explicit Multi-unicast (Xcast)), a new multicast scheme with complementary scaling properties: Xcast supports a very large number of small multicast sessions. Xcast achieves this by explicitly encoding the list of destinations in the data packets, instead of using a multicast group address.

This document discusses Xcast concepts and options in several areas; it does not provide a complete technical specification.

Annotations

This paper discusses a system a system known as Explicit Multicast. The authors attribute this method of multicast as being one of the oldest, suggested as far back as 1985. It bears very strong similarity to Small Group Multicast, indeed it shares two authors with the article I reviewed two weeks ago on SGM.

Most of the basics are the same as SGM: each packet will contain a list of destinations, and at each router it must be examined to determine if different targets will require divergent routes. If so, the packet is duplicated and the appropriate destinations are removed or preserved. I did not two specific changes in XCast. First, the option is left to a router of splitting a packet into N unicast packets at any point, where N is the number of targets remaining, and distributing the addresses appropriately. This would be done if the router knows that further downstream routers do not support XCast. Secondly, XCast allows the possibility of compressed headers. This would be done in order to allow a larger number of destinations with less adverse affects on the packet size.

A methodology similar to both XCast and Small Group Multicast is being implemented for TailSync? , and as such I will be citing both as sources in that work. Because our group size is small, a small group multicasting method seems reasonable and simple. We may also try other multi-casting methods and compare them, but SMG/XCast seem the best at the moment.

-- DavidMoore - 31 Oct 2007