Internet DRAFT - draft-ietf-ipng-token-ring
draft-ietf-ipng-token-ring
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Internet Engineering Task Force Stephen Thomas
INTERNET DRAFT AT&T Tridom
<draft-ietf-ipng-token-ring-01.txt> January 22, 1996
A Method for the Transmission of IPv6 Packets over Token Ring 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
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Drafts as reference material or to cite them other than as a
"working draft" or "work in progress."
To learn the current status of any Internet-Draft, please check the
``1id-abstracts.txt'' listing contained in the Internet Drafts
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Rim).
Distribution of this memo is unlimited.
Introduction
This memo specifies the frame format for transmission of IPv6 [IPV6]
packets and the method of forming IPv6 link-local addresses on Token
Ring networks [802.5]. It also specifies the content of the
Source/Target Link-layer Address option used by the Router
Solicitation, Router Advertisement, Neighbor Solicitation, and
Neighbor Advertisement messages described in [DISC], when those
messages are transmitted on a Token Ring. The memo concludes with a
description of the mapping of IPv6 multicast addresses to Token Ring
functional addresses.
Acknowledgments
Several members of the IEEE 802.5 Working Group contributed their
knowledge and experience to the drafting of this specification,
including Jim, Andrew Draper, George Lin, John Messenger, Kirk
Preiss, and Trevor Warwick. The author would also like to thank many
members of the IPng working group for their advice and suggestions,
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including Ran Atkinson, Scott Bradner, Matt Crawford, Steve Deering,
Francis Dupont, Robert Elz, Thomnas Narten, and Matt Thomas.
IPv6 Encapsulation
IPv6 packets are transmitted in LLC/SNAP frames. The data field
contains the IPv6 header and payload. The following figure shows a
complete 802.5 frame containing an IPv6 datagram.
+-------+-------+-------+-------+
| SD | AC | FC | |
+-----------------------+ |
| Destination Address |
| +-----------------------+
| | Source |
+-------+ Address +-------+
| | DSAP |
+-------+-------+-------+-------+
| SSAP | CTL | OUI |
+-------+-------+-------+-------+
| OUI | EtherType | |
+-------+---------------+ |
| |
~ IPv6 header and payload... ~
| |
+-------------------------------+
| FCS |
+-------+-------+---------------+
| ED | FS |
+-------+-------+
In the presence of source route bridges, a routing information field
(RIF) may appear immediately after the source address. A RIF is
present in frames when the most significant bit of the source
address is set to one. (This is the bit whose position corresponds
to that of the Individual/Group bit in the Destination Address.)
Token Ring Header Fields
SD - Starting Delimiter
AC - Access Control
FC - Frame Control
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Destination Address - 48-bit IEEE address of destination station
Source Address - 48-bit IEEE address of source station
DSAP - Destination Service Access Point (for LLC/SNAP format, shall
always contain the value 0xAA)
SSAP - Source Service Access Point (for LLC/SNAP format, shall
always contain the value 0xAA)
CTL - Control Field (for Unnumbered Information, shall always
contain the value 0x03)
OUI - Organizationally Unique Identifier (for EtherType encoding,
shall always contain the value 0x000000)
EtherType - Protocol type of encapsulated payload (for IPv6, shall
always contain the value 0x86DD)
FCS - Frame Check Sequence
ED - Ending Delimiter
FS - Frame Status
Maximum Transmission Unit
IEEE 802.5 networks have a maximum frame size based on the maximum
time a node may hold the token. This time depends on many factors
including the data signaling rate and the number of nodes on the
ring. Because the maximum frame size varies, implementations must
rely on static configuration or router advertisements [DISC] to
determine actual MTU sizes. Common default values include
approximately 2000, 4000, and 8000 octets.
In the absence of any other information, an implementation should
use a default MTU of 1500 octets. This size offers compatibility
with all common 802.5 defaults, as well as with Ethernet LANs in an
environment using transparent bridging.
In an environment using source route bridging, the process of
discovering the MAC-level path to a neighbor can yield the MTU for
the path to that neighbor. The information is contained in the
largest frame (LF) subfield of the routing information field. This
field limits the size of the information field of frames to that
destination, and that information field includes both the LLC
[802.2] header and the IPv6 datagram. Since, for IPv6, the LLC
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header is always 8 octets in length, the IPv6 MTU can be found by
subtracting 8 from the maximum frame size defined by the LF
subfield. If an implementation uses this information to determine
MTU sizes, it must maintain separate MTU values for each neighbor.
A detailed list of the LF values and the resulting maximum frame
size can be found in [802.1D]. To illustrate the calculation of IPv6
MTU, the following table lists several common values. Note that some
of the 802.1D LF values would result in an IP MTU less than 576
bytes. This size is less than the IPv6 minimum, and communication
across paths with those MTUs is generally not possible using IPv6.
LF (base) LF (extension) MAC MTU IP MTU
001 000 1470 1462
010 000 2052 2044
011 000 4399 4391
100 000 8130 8122
101 000 11407 11399
110 000 17749 17741
111 000 41600 41592
When presented with conflicting MTU values from several sources, an
implementation should choose from those sources according to the
following priorities:
1. Largest Frame values from source route bridging (only for
specific, unicast destinations)
2. Router advertisements
3. Static configuration
4. Default of 1500
Stateless Autoconfiguration and Link-local Addresses
The address token [CONF] for a Token Ring interface is the built-in
48-bit IEEE 802 address associated with that interface, in canonical
bit order. (Note: multiple interfaces on a system may share the same
IEEE 802 address.) A different MAC address set manually or by
software should not be used as the address token.
An IPv6 address prefix used for stateless autoconfiguration of a
Token Ring interface must be 80 bits in length.
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The IPv6 Link-local address [AARCH] for a Token Ring interface is
formed by appending the interface's IEEE 802 address to the 80-bit
prefix FE80::.
+-------+-------+-------+-------+
| FE 80 00 00 |
+-------+-------+-------+-------+
| 00 00 00 00 |
+-------+-------+-------+-------+
| 00 00 | |
+-------+-------+ |
| Token Ring Address |
+-------+-------+-------+-------+
Address Mapping - Unicast
The procedure for mapping IPv6 addresses into Token Ring link layer
addresses is described in [DISC]. The Source/Target Link Layer
Address option has the following form when the link layer is Token
Ring.
+-------+-------+-------+-------+
| Type |Length | |
+-------+-------+ |
| Token Ring Address |
+-------+-------+-------+-------+
Option Fields:
Type 1 for Source Link Layer Address
2 for Target Link Layer Address
Length 1 (in units of 8 octets)
Token Ring Address
The 48-bit IEEE 802 address, in canonical bit order.
Address Mapping - Multicast
All IPv6 packets with multicast destination addresses are
transmitted to Token Ring functional addresses. The following table
shows the specific mapping between the IPv6 addresses and Token Ring
functional addresses (in canonical form). Note that protocols other
than IPv6 may use these same functional addresses, so all Token Ring
frames destined to these functional addresses are not guaranteed to
be IPv6 datagrams.
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MAC Func Addr (canonical) IPv6 Multicast Addresses
03 00 80 00 00 00 all nodes (FF0X::1) and solicited
node (FF02::1:XXXX:XXXX) addresses
03 00 40 00 00 00 all routers addresses (FF0X::2)
03 00 00 80 00 00 any other multicast address with three
least significant bits = 000
03 00 00 40 00 00 any other multicast address with three
least significant bits = 001
03 00 00 20 00 00 any other multicast address with three
least significant bits = 010
03 00 00 10 00 00 any other multicast address with three
least significant bits = 011
03 00 00 08 00 00 any other multicast address with three
least significant bits = 100
03 00 00 04 00 00 any other multicast address with three
least significant bits = 101
03 00 00 02 00 00 any other multicast address with three
least significant bits = 110
03 00 00 01 00 00 any other multicast address with three
least significant bits = 111
Security Considerations
Security considerations are not addressed in this memo.
References
[802.1D] IEEE Standards for Local and Metropolitan Area Networks:
Media Access Control (MAC) Bridges. ANSI/IEEE Std 802.1D, 1993
Edition.
[802.2] IEEE Standards for Local Area Networks: Logical Link
Control. ANSI/IEEE Std 802.2-1985.
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[802.5] IEEE Standards for Local and Metropolitan Area Networks:
Token Ring Access Method and Physical Layer Specifications.
IEEE Std 802.5-1995.
[AARCH] R. Hinden, S. Deering, IP Version 6 Addressing Architecture.
RFC 1884.
[CONF] S. Thomson, IPv6 Stateless Address Autoconfiguration.
Currently draft-ietf-addrconf-ipv6-auto-07.txt.
[DISC] T. Narten, E. Nordmark, W. A. Simpson, Neighbor Discovery
for IP Version 6 (IPv6). Currently draft-ietf-ipngwg-
discovery-03.txt.
[IPV6] S. Deering, R. Hinden, Internet Protocol, Version 6 (IPv6)
Specification. RFC 1883.
Author's Address
Stephen Thomas
AT&T Tridom Phone: (770) 514-3522
840 Franklin Court Fax: (770) 514-3491
Marietta, GA 30067 USA Email: stephen.thomas@tridom.com
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