IP Multicast over Token-Ring Local Area Networks
RFC 1469

Document Type RFC - Historic (June 1993; No errata)
Last updated 2013-03-02
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Network Working Group                                        T. Pusateri
Request for Comments: 1469                                    Consultant
                                                               June 1993

            IP Multicast over Token-Ring Local Area Networks

Status of this Memo

   This RFC specifies an IAB standards track protocol for the Internet
   community, and requests discussion and suggestions for improvements.
   Please refer to the current edition of the "IAB Official Protocol
   Standards" for the standardization state and status of this protocol.
   Distribution of this memo is unlimited.

Abstract

   This document specifies a method for the transmission of IP multicast
   datagrams over Token-Ring Local Area Networks.  Although an interim
   solution has emerged and is currently being used, it is the intention
   of this document to specify a more efficient means of transmission
   using an assigned Token-Ring functional address.

Introduction

   IP multicasting provides a means of transmitting IP datagrams to a
   group of hosts.  A group IP address is used as the destination
   address in the IP datagram as documented in STD 5, RFC 1112 [1].
   These group addresses, also referred to as Class D addresses, fall in
   the range from 224.0.0.0 to 239.255.255.255.  A standard method of
   mapping IP multicast addresses to media types such as ethernet and
   fddi exist in [1] and RFC 1188 [2].  This document attempts to define
   the mapping for an IP multicast address to the corresponding Token-
   Ring MAC address.

Background

   The Token-Ring Network Architecture Reference [3] provides several
   types of addressing mechanisms.  These include both individual
   (unicast) and group addresses (multicast).  A special subtype of
   group addresses are called functional addresses and are indicated by
   a bit in the destination MAC address.  They were designed for widely
   used functions such as ring monitoring, NETBIOS, Bridge, and Lan
   Manager frames.  There are a limited number of functional addresses,
   31 in all, and therefore several unrelated functions must share the
   same functional address.

Pusateri                                                        [Page 1]
RFC 1469           IP Multicast over Token-Ring LANs           June 1993

   It would be most desirable if Token-Ring could use the same mapping
   as ethernet and fddi for IP multicast to hardware multicast
   addressing.  However, current implementations of Token-Ring
   controller chips cannot support this. To see why, we must first
   examine the Destination MAC address format.

Destination Address Format

   The destination MAC address consists of six octets.  In the following
   diagram of a MAC address, the order of transmission of the octets is
   from top to bottom (octet 0 to octet 5), and the order of
   transmission of the bits within each octet is from right to left (bit
   0 to bit 7).  This is the so-called "canonical" bit order for IEEE
   802.2 addresses.  Addresses supplied to or received from token ring
   interfaces are usually laid out in memory with the bits of each octet
   in the opposite order from that illustrated, i.e., with bit 0 in the
   high-order (leftmost) position within the octet.

            7   6   5   4   3   2   1   0

          ---------------------------------
          |   |   |   |   |   |   |U/L|I/G|       octet 0
          ---------------------------------
          |   |   |   |   |   |   |   |   |       octet 1
          ---------------------------------
          |   |   |   |   |   |   |   |FAI|       octet 2
          ---------------------------------
          |   |   |   |   |   |   |   |   |       octet 3
          ---------------------------------
          |   |   |   |   |   |   |   |   |       octet 4
          ---------------------------------
          |   |   |   |   |   |   |   |   |       octet 5
          ---------------------------------

   The low order bit of the high order octet is called the I/G bit. It
   signifies whether the address is an individual address (0) or a group
   address (1). This is comparable to the multicast bit in the DIX
   Ethernet addressing format.

   Bit position 1 of the high order octet, called the U/L bit, specifies
   whether the address is universally administered (0) or locally
   administered (1). Universally administered addresses are those
   specified by a standards organization such as the IEEE.

   If the I/G bit is set to 1 and the U/L bit is 0, the address must be
   a universally administered group address. If the I/G bit is 1 and the
   U/L bit is a 1, the address may be either a local administered group
   address or a functional address. This distinction is determined by

Pusateri                                                        [Page 2]
RFC 1469           IP Multicast over Token-Ring LANs           June 1993

   the Functional Address Indicator (FAI) bit located in bit position 0
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