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Network Working Group                                        T. PusateriRequest for Comments: 1469                                    Consultant                                                               June 1993IP Multicast over Token-Ring Local Area NetworksStatus 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] andRFC 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 byPusateri                                                        [Page 2]

RFC 1469           IP Multicast over Token-Ring LANs           June 1993   the Functional Address Indicator (FAI) bit located in bit position 0   of octet 2.  If the FAI bit is 0, the address is considered a   functional address.  And if the FAI bit is 1, this indicates a   locally administered group address.   Different functional addresses are made by setting one of the   remaining 31 bits in the address field. These bits include the 7   remaining bits in octet 2 as well as the 8 bits in octets 3, 4, and   5. It is not possible to create more functional addresses by setting   more than one of these bits at a time.   Three methods exist for mapping between an IP multicast address and a   hardware address. These include:      1.   The all rings broadcast address      2.   The assigned functional address      3.   The existing IEEE assigned IP Multicast group addresses   In order to insure interoperability, all systems supporting IP   multicasting on each physical ring must agree on the hardware address   to be used. Therefore, the method used should be configurable on a   given interface.  Bridges may provide a means to translate between   different methods for each physical ring that is being bridged.   Method (3) is recommended but due to hardware limitations of Token-   Ring controller chips, may not be possible. In this case, Method (2)   is preferred over Method (1).  For backward compatibility, systems   that support (2) MUST also support (1). And systems that support (3)   MUST also support (2) and therefore (1).  In the absence of   configuration information, the default should be to use the assigned   functional address (2).IP Multicast Functional Address   Because there is a shortage of Token-Ring functional addresses, all   IP multicast addresses have been mapped to a single Token-Ring   functional address. In canonical form, this address is 03-00-00-20-   00-00.  In non-canonical form, it is C0-00-00-04-00-00.  It should be   noted that since there are only 31 possible functional addresses,   there may be other protocols that are assigned this functional   address as well.  Therefore, just because a frame is sent to the   functional address 03-00-00-20-00-00 does not mean that it is an IP   multicast frame.Pusateri                                                        [Page 3]

RFC 1469           IP Multicast over Token-Ring LANs           June 1993Acknowledgments   The author would like to thank John Moy, Fred Baker, Steve Deering,   and Rob Enns for their review and constructive comments.References   [1] Deering, S., "Host Extensions for IP Multicasting", STD 5,RFC 1112, Stanford University, August 1989.   [2] Katz, D., "A Proposed Standard for the Transmission of IP       Datagrams over FDDI Networks",RFC 1188, Merit/NSFNET,       October 1990.   [3] IBM Token-Ring Network, Architecture Reference, Publication SC30-       3374-02, Third Edition, (September, 1989).Security Considerations   Security issues are not discussed in this memo.Author's  Address   Thomas J. Pusateri   Consultant   11820 Edgewater Ct.   Raleigh, NC 27614   EMail: pusateri@cs.duke.eduPusateri                                                        [Page 4]