INTERNET DRAFT D. Ooms Alcatel July, 2000 Expires January, 2001 Taxonomy of xcast/sgm proposals Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Abstract Recently several multicast mechanisms were proposed that scale better with the number of multicast sessions than traditional multicast does (DCM [BLAZ], SGM [BOIV], Somecast [HELD], MDO6 [IMAI], CLM [OOMS], ERM [SHAN], REUNITE [STOI]). These proposals are also known as Explict Multicast (xcast; explicit list of destinations) or Small Group Multicast (sgm; the main application being few-to-few communication). To stimulate and streamline the discussion this draft is an attempt to make a taxonomy of these mechanisms. 1. Introduction Recently several multicast mechanisms were proposed that scale better with the number of multicast sessions than traditional multicast does (DCM [BLAZ], SGM [BOIV], Somecast [HELD], MDO6 [IMAI], CLM [OOMS], ERM [SHAN], REUNITE [STOI]). These proposals are also known as Explict Multicast (xcast; explicit list of destinations) or Small Ooms Expires January 2001 [Page 1] Internet Draft draft-ooms-xcast-taxonomy-00.txt July 2000 Group Multicast (sgm; the main application being few-to-few communication). To stimulate and streamline the discussion this draft is an attempt to make a taxonomy of these mechanisms. In the introduction of [OOMS] it is described how data delivery to multiple destinations is a trade-off between bandwidth consumption, state&signaling per session and header processing per packet. This draft discusses how the various mechanisms make a specific trade-off in the "plane of conservation of misery", and the consequences of each approach are investigated. 2. Definitions and Abbreviations In this draft we will distinguish following entities in a point-to- multipoint delivery tree: - sender = source = root - receiver = destination = leaf - branching router (for a certain stream): router in which a packet of the stream is forwarded on more than one interface - non-branching router (for a certain stream): router in which a packet of the stream is forwarded on one interface - hosts = sender + receivers - routers = branching routers + non-branching routers Throughout the document, the following abbreviations will be used: FO = fan-out = number of branches a tree has in a certain router I-D = inter-domain I-A = inter-area MC = multicast (current and new schemes) OND = Original Number of Destinations = number of destinations of the session RND = Remaining Number of Destinations = number of destinations in some branch of the tree tbl = table Ooms Expires January 2001 [Page 2] Internet Draft draft-ooms-xcast-taxonomy-00.txt July 2000 UC = unicast 3. State&Signaling per session vs Processing per packet One of the most important characteristics is the trade-off between state&signaling per session and processing per packet. Table 1 gives an overview of the Forwarding State used to deliver a packet to multiple destinations for the various methods. The number between brackets indicates the number of look-ups per router. -------------------------------------------------------------- Forwarding State & Number of Look-ups -+---------------------+-----------------+-------------------- |sender/packet |non-branching |branching | |router |router =+=====================+=================+==================== 1|MC address |MC entry (1) |MC entry (1) -+---------------------+-----------------+-------------------- 2|list of destinations |UC tbl (RND) |UC tbl (RND) -+---------------------+-----------------+-------------------- 3|next branching router|UC tbl (1) |UC tbl + | | |next branch tbl (FO) -+---------------------+-----------------+-------------------- 4|tree |UC tbl (1) |UC tbl (FO) -+---------------------+-----------------+-------------------- 5|tractable list |UC tbl (2) |UC tbl (1