RFC2470 - Transmission of IPv6 Packets over Token Ring Networks

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　　Network Working Group M. Crawford
Request for Comments: 2470 Fermilab
Category: Standards Track T. Narten
IBM
S. Thomas
TransNexus
December 1998

Transmission of IPv6 Packets over Token Ring Networks

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (1998). All Rights Reserved.

1. Introduction

This memo specifies the MTU and frame format for transmission of IPv6
packets on Token Ring networks. It also specifies the method of
forming IPv6 link-local addresses on Token Ring networks and the
content of the Source/Target Link-layer Address option used the
Router Solicitation, Router Advertisement, Redirect, Neighbor
Solicitation and Neighbor Advertisement messages when those messages
are transmitted on a Token Ring network.

Implementors should be careful to note that Token Ring adaptors
assume addresses are in non-canonical rather than canonical format,
requiring that special care be taken to insure that addresses are
processed correctly. See [CANON] for more details.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [KWORD].

2. 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 manual 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 [LLC]
header and the IPv6 datagram. Since, for IPv6, the LLC 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 [BRIDGE]. 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 1280 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), but only if not
greater than value from any router advertisements

2. Router advertisements, but only if not greater than any manual
configuration (including DHCP)

3. Manual configuration (including DHCP)

4. Default of 1500

3. Frame Format

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 |
+-------+-------+

Token Ring Header Fields

SD: Starting Delimiter

AC: Access Control

FC: Frame Control

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

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.)

The RIF is a variable-length field that (when present) contains a
two-octet Routing Control (RC) header, followed by zero or more two-
octet Route Designator fields:

0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Routing Control: |Bcast| Length |D| LF |rsvd |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Route Designator 1: | Segment 1 |Bridge1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ... ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Route Designator N: | Segment N |BridgeN|
(0 <= N <= 7) +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Route Designator Fields:

Bcast: Broadcast Indicator, Defined values:

10x: All Routes Explorer
11x: Spanning Tree Explorer
0xx: Specifically Routed Frame

Length: Total length of RIF field in octets

D: Direction of source route. A value of 0 means that
the left-to-right sequence of Route Designators
provides the path from the sender to recipient. A
value of 0 indicates the sequence goes from
recipient to sender.

LF: Largest Frame

rsvd: Reserved

On transmission, the Route Designator fields give the sequence of
(bridge, LAN segment) numbers the packet is to traverse. It is the
responsibility of the sender to provide this sequence for
Specifically Routed Frames, i.e., unicast IP datagrams.

4. Stateless Autoconfiguration

The Interface Identifier [AARCH] for a Token Ring interface is based
on the EUI-64 identifier [EUI64] derived from the interface's built-
in 48-bit IEEE 802 address. The OUI of the Token Ring address (the
first three octets) becomes the company_id of the EUI-64 (the first
three octets). The fourth and fifth octets of the EUI are set to the
fixed value FFFE hexadecimal. The last three octets of the Token Ring
address become the last three octets of the EUI-64.

The Interface Identifier is then formed from the EUI-64 by
complementing the "Universal/Local" (U/L) bit, which is the next-to-
lowest order bit of the first octet of the EUI-64. Complementing
this bit will generally change a 0 value to a 1, since an interface's
built-in address is expected to be from a universally administered
address space and hence have a globally unique value. A universally
administered IEEE 802 address or an EUI-64 is signified by a 0 in the
U/L bit position, while a globally unique IPv6 Interface Identifier
is signified by a 1 in the corresponding position. For further
discussion on this point, see [AARCH].

For example, the Interface Identifier for a Token Ring interface
whose built-in address is, in hexadecimal and in canonical bit order,

34-56-78-9A-BC-DE

would be

36-56-78-FF-FE-9A-BC-DE.

A different MAC address set manually or by software should not be
used to derive the Interface Identifier. If such a MAC address must
be used, its global uniqueness property should be reflected in the
value of the U/L bit.

An IPv6 address prefix used for stateless autoconfiguration of a
Token Ring interface must have a length of 64 bits.

5. Link-Local Address

The IPv6 link-local address [AARCH] for a Token Ring interface is
formed by appending the Interface Identifer, as defined above, to the
prefix FE80::/64.

10 bits 54 bits 64 bits
+----------+-----------------------+----------------------------+
|1111111010| (zeros) | Interface Identifier |
+----------+-----------------------+----------------------------+

6. Address Mapping -- Unicast

The procedure for mapping unicast 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.

0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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 Token Ring IEEE 802
address, in canonical bit order. This is the address the
interface currently responds to, and may be different from
the built-in address used to derive the Interface
Identifier.

When source routing bridges are used, the source route for
the path to a destination can be extracted from the RIF
field of received Neighbor Advertisement messages. Note that
the RIF field of received packets can be reversed into a
source route suitable for transmitting return traffic by
toggling the value of the 'D' bit and insuring that the
Bcast field is set to indicate a Specifically Routed Frame.

7. 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.

MAC Addr (canonical) IPv6 Multicast Addresses

03-00-80-00-00-00 All-Nodes (FF01::1 and FF02::1) and
solicited node (FF02:0:0:0:0:1:FFXX: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

In a bridged token ring network, all multicast packets SHOULD be sent
with a RIF header specifying the use of the Spanning Tree Explorer.

Note: it is believed that some (very) old bridge implementations do
not properly support the Spanning Tree Explorer mechanism. In such
environments, multicast traffic sent through bridges must use a RIF
with the All Routes Explorer. Consequently, an implementation MAY
wish to allow the sending of IP multicast traffic using an All Routes
Explorer. However, such an ability must be configurable by a system
administrator and the default setting of the switch MUST be to use
the Spanning Tree Explorer.

8. Security Considerations

Token Ring, like most broadcast LAN technologies, has inherent
security vulnerabilities. For example, any sender can claim the
identity of another and forge traffic. It is the responsibility of
higher layers to take appropriate steps in those environments where
such vulnerabilities are unacceptable.

9. 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,
including Ran Atkinson, Scott Bradner, Steve Deering, Francis Dupont,
Robert Elz, and Matt Thomas. A special thanks is due Steve Wise, who
gave the most relevant advice of all by actually trying to implement
this specification while it was in progress.

10. References

[802.5] 8802-5 : 1995 (ISO/IEC) [ANSI/IEEE 802.5, 1995
Edition] Information technology--Telecommunications and
information exchange between systems--Local and
metropolitan area networks--Specific requirements-- Part 5:
Token ring access method and physical layer specification.

[AARCH] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC2373, July 1998.

[ACONF] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC2462, December 1998.

[BRIDGE] 10038: 1993 (ISO/IEC) [ANSI/IEEE Std 802.1D, 1993 Edition]
Information technology--Telecommunications and information
exchange between systems--Local area networks--Media access
control (MAC) bridges.

[CANON] Narten, T. and C. Burton, "A Caution on Canonical Bit Order
Of Link-Layer Addresses", RFC2469, December 1998.

[CONF] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC1971, August 1996.

[DISC] Narten, T., Nordmark, E. and W. Simpson, "Neighbor
Discovery for IP Version 6 (IPv6)", RFC2461, December
1998.

[EUI64] "64-Bit Global Identifier Format Tutorial", http:
//standards.ieee.org/db/oui/tutorials/EUI64.html.

[IPV6] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC2460, December 1998.

[KWORD] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels," BCP 14, RFC2119, March 1997.

[LLC] 8802-2 : 1994 (ISO/IEC) [ANSI/IEEE 802.2, 1994 Edition]
Information technology--Telecommunications and information
exchange between systems--Local and Metropolitan area
networks--Specific requirements-- Part 2: Logical link
control.

11. Authors' Addresses

Matt Crawford
Fermilab MS 368
PO Box 500
Batavia, IL 60510 USA

Phone: +1 630 840 3461
EMail: crawdad@fnal.gov

Thomas Narten
IBM Corporation
P.O. Box 12195
Research Triangle Park, NC 27709-2195 USA

Phone: +1 919 254 7798
EMail: narten@raleigh.ibm.com

Stephen Thomas
TransNexus
430 Tenth Street NW Suite N204
Atlanta, GA 30318 USA

Phone: +1 404 872 4745
EMail: stephen.thomas@transnexus.com

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