Internet DRAFT - draft-pusateri-tokenring-lan
HTTP/1.1 200 OK
Date: Tue, 09 Apr 2002 10:58:06 GMT
Server: Apache/1.3.20 (Unix)
Last-Modified: Sun, 15 Nov 1992 04:39:00 GMT
INTERNET DRAFT Consultant
IP Multicast over Token-Ring Local Area Networks
Status of this Memo
This document is an Internet Draft. 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. Internet Drafts may be updated, replaced, or obsoleted by
other documents at any time. It is not appropriate to use Internet
Drafts as reference material or to cite them other than as a
``working draft'' or ``work in progress.''
Please check the 1id-abstracts.txt listing contained in the
internet-drafts Shadow Directories on nic.ddn.mil, nnsc.nsf.net,
nic.nordu.net, ftp.nisc.sri.com, or munnari.oz.au to learn the
current status of any Internet Draft.
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 RFC 1112 . These
group addresses, also referred to as Class D addresses, fall in the
range from 126.96.36.199 to 188.8.131.52. A standard method of
mapping IP multicast addresses to media types such as ethernet and
fddi exist in  and RFC 1188 . This document attempts to define
the mapping for an IP multicast address to the corresponding Token-
Ring MAC address.
Pusateri [Page 1]
RFC DRAFT November 1992
The Token-Ring Network Architecture Reference  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.
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. They are shown
with the high order octet (octet 0) from top to bottom as they are
transmitted but with the low order bit on the right (canonical
order). Each bit within an octet must be reversed before
transmission on the media. This format should be familiar to most
members of the Internet community.
| | | | | | |U/L|I/G| octet 0
7 6 5 4 3 2 1 0
| | | | | | | | | octet 1
| | | | | | | |FAI| octet 2
| | | | | | | | | octet 5
Pusateri [Page 2]
RFC DRAFT November 1992
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.
If the I/G bit is set to 1 for a group address, 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
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.
There exists three methods 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 multi-
casting 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).
Pusateri [Page 3]
RFC DRAFT November 1992
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 func-
tional 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.
The author would like to thank John Moy, Fred Baker, and Rob Enns for
their review and constructive comments.
Thomas J. Pusateri
11820 Edgewater Ct.
Raleigh, NC 27614
 S. Deering, Host Extensions for IP Multicasting, Internet Working
Group Request for Comments 1112. Network Information Center, SRI
International, Menlo Park, California, (Aug, 1989).
 D. Katz, A Proposed Standard for the Transmission of IP Datagrams
over FDDI Networks, Internet Working Group Request for Comments
1188. Network Information Center, SRI International, Menlo Park,
California, (Oct, 1990).
 IBM Token-Ring Network, Architecture Reference, Publication SC30-
3374-02, Third Edition, (September, 1989).
Pusateri [Page 4]