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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.


 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.


 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 to  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.


 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
  1. ——————————–

| | | | | | |U/L|I/G| octet 0

  1. ——————————–

| | | | | | | | | octet 1

  1. ——————————–

| | | | | | | |FAI| octet 2

  1. ——————————–

| | | | | | | | | octet 3

  1. ——————————–

| | | | | | | | | octet 4

  1. ——————————–

| | | | | | | | | octet 5

  1. ——————————–
 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
 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 1993


 The author would like to thank John Moy, Fred Baker, Steve Deering,
 and Rob Enns for their review and constructive comments.


 [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
 11820 Edgewater Ct.
 Raleigh, NC 27614

Pusateri [Page 4]

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