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Network Working Group M.
Request for Comments: 2470
Category: Standards Track T.
S.
December 1998
Transmission of IPv6 Packets over Token Ring
Status of this
This document specifies an Internet standards track protocol for
Internet community, and requests discussion and suggestions
improvements. Please refer to the current edition of the "
Official Protocol Standards" (STD 1) for the standardization
and status of this protocol. Distribution of this memo is unlimited
Copyright
Copyright (C) The Internet Society (1998). All Rights Reserved
1.
This memo specifies the MTU and frame format for transmission of IPv
packets on Token Ring networks. It also specifies the method
forming IPv6 link-local addresses on Token Ring networks and
content of the Source/Target Link-layer Address option used
Router Solicitation, Router Advertisement, Redirect,
Solicitation and Neighbor Advertisement messages when those
are transmitted on a Token Ring network
Implementors should be careful to note that Token Ring
assume addresses are in non-canonical rather than canonical format
requiring that special care be taken to insure that addresses
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
document are to be interpreted as described in [KWORD].
2. Maximum Transmission
IEEE 802.5 networks have a maximum frame size based on the
time a node may hold the token. This time depends on many
including the data signaling rate and the number of nodes on
ring. Because the maximum frame size varies, implementations
Crawford, et. al. Standards Track [Page 1]
RFC 2470 IPv6 over Token Ring December 1998
rely on manual configuration or router advertisements [DISC]
determine actual MTU sizes. Common default values
approximately 2000, 4000, and 8000 octets
In the absence of any other information, an implementation should
a default MTU of 1500 octets. This size offers compatibility with
common 802.5 defaults, as well as with Ethernet LANs in
environment using transparent bridging
In an environment using source route bridging, the process
discovering the MAC-level path to a neighbor can yield the MTU
the path to that neighbor. The information is contained in
largest frame (LF) subfield of the routing information field.
field limits the size of the information field of frames to
destination, and that information field includes both the LLC [LLC
header and the IPv6 datagram. Since, for IPv6, the LLC header
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
implementation uses this information to determine MTU sizes, it
maintain separate MTU values for each neighbor
A detailed list of the LF values and the resulting maximum frame
can be found in [BRIDGE]. To illustrate the calculation of IPv6 MTU
the following table lists several common values. Note that some
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
paths with those MTUs is generally not possible using IPv6.
LF (base) LF (extension) MAC MTU IP
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,
implementation should choose from those sources according to
following priorities
1. Largest Frame values from source route
(only for specific, unicast destinations), but only if
greater than value from any router
2. Router advertisements, but only if not greater than any
configuration (including DHCP
Crawford, et. al. Standards Track [Page 2]
RFC 2470 IPv6 over Token Ring December 1998
3. Manual configuration (including DHCP
4. Default of 1500
3. Frame
IPv6 packets are transmitted in LLC/SNAP frames. The data
contains the IPv6 header and payload. The following figure shows
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
SD: Starting
AC: Access
FC: Frame
Destination Address: 48-bit IEEE address of
Source Address: 48-bit IEEE address of source
DSAP: Destination Service Access Point (for LLC/
format, shall always contain the value 0xAA
Crawford, et. al. Standards Track [Page 3]
RFC 2470 IPv6 over Token Ring December 1998
SSAP: Source Service Access Point (for LLC/SNAP format
shall always contain the value 0xAA
CTL: Control Field (for Unnumbered Information,
always contain the value 0x03)
OUI: Organizationally Unique Identifier (for
encoding, shall always contain the value 0x000000)
EtherType: Protocol type of encapsulated payload (
IPv6, shall always contain the value 0x86DD
FCS: Frame Check
ED: Ending
FS: Frame
In the presence of source route bridges, a routing information
(RIF) may appear immediately after the source address. A RIF
present in frames when the most significant bit of the source
is set to one. (This is the bit whose position corresponds to that
the Individual/Group bit in the Destination Address.)
The RIF is a variable-length field that (when present) contains
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
11x: Spanning Tree
0xx: Specifically Routed
Crawford, et. al. Standards Track [Page 4]
RFC 2470 IPv6 over Token Ring December 1998
Length: Total length of RIF field in
D: Direction of source route. A value of 0 means
the left-to-right sequence of Route
provides the path from the sender to recipient.
value of 0 indicates the sequence goes
recipient to sender
LF: Largest
rsvd:
On transmission, the Route Designator fields give the sequence
(bridge, LAN segment) numbers the packet is to traverse. It is
responsibility of the sender to provide this sequence
Specifically Routed Frames, i.e., unicast IP datagrams
4. Stateless
The Interface Identifier [AARCH] for a Token Ring interface is
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 (
first three octets) becomes the company_id of the EUI-64 (the
three octets). The fourth and fifth octets of the EUI are set to
fixed value FFFE hexadecimal. The last three octets of the Token
address become the last three octets of the EUI-64.
The Interface Identifier is then formed from the EUI-64
complementing the "Universal/Local" (U/L) bit, which is the next-to
lowest order bit of the first octet of the EUI-64.
this bit will generally change a 0 value to a 1, since an interface'
built-in address is expected to be from a universally
address space and hence have a globally unique value. A
administered IEEE 802 address or an EUI-64 is signified by a 0 in
U/L bit position, while a globally unique IPv6 Interface
is signified by a 1 in the corresponding position. For
discussion on this point, see [AARCH].
For example, the Interface Identifier for a Token Ring
whose built-in address is, in hexadecimal and in canonical bit order
34-56-78-9A-BC-
would
36-56-78-FF-FE-9A-BC-DE
Crawford, et. al. Standards Track [Page 5]
RFC 2470 IPv6 over Token Ring December 1998
A different MAC address set manually or by software should not
used to derive the Interface Identifier. If such a MAC address
be used, its global uniqueness property should be reflected in
value of the U/L bit
An IPv6 address prefix used for stateless autoconfiguration of
Token Ring interface must have a length of 64 bits
5. Link-Local
The IPv6 link-local address [AARCH] for a Token Ring interface
formed by appending the Interface Identifer, as defined above, to
prefix FE80::/64.
10 bits 54 bits 64
+----------+-----------------------+----------------------------+
|1111111010| (zeros) | Interface Identifier |
+----------+-----------------------+----------------------------+
6. Address Mapping --
The procedure for mapping unicast IPv6 addresses into Token
link-layer addresses is described in [DISC]. The Source/Target Link
layer Address option has the following form when the link layer
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).
Crawford, et. al. Standards Track [Page 6]
RFC 2470 IPv6 over Token Ring December 1998
Token Ring Address: The 48 bit Token Ring IEEE 802
address, in canonical bit order. This is the address
interface currently responds to, and may be different
the built-in address used to derive the
Identifier
When source routing bridges are used, the source route
the path to a destination can be extracted from the
field of received Neighbor Advertisement messages. Note
the RIF field of received packets can be reversed into
source route suitable for transmitting return traffic
toggling the value of the 'D' bit and insuring that
Bcast field is set to indicate a Specifically Routed Frame
7. Address Mapping --
All IPv6 packets with multicast destination addresses are
to Token Ring functional addresses. The following table shows
specific mapping between the IPv6 addresses and Token Ring
addresses (in canonical form). Note that protocols other than IPv
may use these same functional addresses, so all Token Ring
destined to these functional addresses are not guaranteed to be IPv
datagrams
MAC Addr (canonical) IPv6 Multicast
03-00-80-00-00-00 All-Nodes (FF01::1 and FF02::1)
solicited node (FF02:0:0:0:0:1:FFXX:XXXX
03-00-40-00-00-00 All-Routers addresses (FF0X::2)
03-00-00-80-00-00 any other multicast address with
least significant bits = 000
03-00-00-40-00-00 any other multicast address with
least significant bits = 001
03-00-00-20-00-00 any other multicast address with
least significant bits = 010
03-00-00-10-00-00 any other multicast address with
least significant bits = 011
03-00-00-08-00-00 any other multicast address with
least significant bits = 100
Crawford, et. al. Standards Track [Page 7]
RFC 2470 IPv6 over Token Ring December 1998
03-00-00-04-00-00 any other multicast address with
least significant bits = 101
03-00-00-02-00-00 any other multicast address with
least significant bits = 110
03-00-00-01-00-00 any other multicast address with
least significant bits = 111
In a bridged token ring network, all multicast packets SHOULD be
with a RIF header specifying the use of the Spanning Tree Explorer
Note: it is believed that some (very) old bridge implementations
not properly support the Spanning Tree Explorer mechanism. In
environments, multicast traffic sent through bridges must use a
with the All Routes Explorer. Consequently, an implementation
wish to allow the sending of IP multicast traffic using an All
Explorer. However, such an ability must be configurable by a
administrator and the default setting of the switch MUST be to
the Spanning Tree Explorer
8. Security
Token Ring, like most broadcast LAN technologies, has
security vulnerabilities. For example, any sender can claim
identity of another and forge traffic. It is the responsibility
higher layers to take appropriate steps in those environments
such vulnerabilities are unacceptable
9.
Several members of the IEEE 802.5 Working Group contributed
knowledge and experience to the drafting of this specification
including Jim, Andrew Draper, George Lin, John Messenger,
Preiss, and Trevor Warwick. The author would also like to thank
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,
gave the most relevant advice of all by actually trying to
this specification while it was in progress
Crawford, et. al. Standards Track [Page 8]
RFC 2470 IPv6 over Token Ring December 1998
10.
[802.5] 8802-5 : 1995 (ISO/IEC) [ANSI/IEEE 802.5, 1995
Edition] Information technology--Telecommunications
information exchange between systems--Local
metropolitan area networks--Specific requirements-- Part 5:
Token ring access method and physical layer specification
[AARCH] Hinden, R. and S. Deering, "IP Version 6
Architecture", RFC 2373, July 1998.
[ACONF] Thomson, S. and T. Narten, "IPv6 Stateless
Autoconfiguration", RFC 2462, December 1998.
[BRIDGE] 10038: 1993 (ISO/IEC) [ANSI/IEEE Std 802.1D, 1993 Edition
Information technology--Telecommunications and
exchange between systems--Local area networks--Media
control (MAC) bridges
[CANON] Narten, T. and C. Burton, "A Caution on Canonical Bit
Of Link-Layer Addresses", RFC 2469, December 1998.
[CONF] Thomson, S. and T. Narten, "IPv6 Stateless
Autoconfiguration", RFC 1971, August 1996.
[DISC] Narten, T., Nordmark, E. and W. Simpson, "
Discovery for IP Version 6 (IPv6)", RFC 2461,
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", RFC 2460, December 1998.
[KWORD] Bradner, S., "Key words for use in RFCs to
Requirement Levels," BCP 14, RFC 2119, March 1997.
[LLC] 8802-2 : 1994 (ISO/IEC) [ANSI/IEEE 802.2, 1994 Edition
Information technology--Telecommunications and
exchange between systems--Local and Metropolitan
networks--Specific requirements-- Part 2: Logical
control
Crawford, et. al. Standards Track [Page 9]
RFC 2470 IPv6 over Token Ring December 1998
11. Authors'
Matt
Fermilab MS 368
PO Box 500
Batavia, IL 60510
Phone: +1 630 840 3461
EMail: crawdad@fnal.
Thomas
IBM
P.O. Box 12195
Research Triangle Park, NC 27709-2195
Phone: +1 919 254 7798
EMail: narten@raleigh.ibm.
Stephen
430 Tenth Street NW Suite N204
Atlanta, GA 30318
Phone: +1 404 872 4745
EMail: stephen.thomas@transnexus.
Crawford, et. al. Standards Track [Page 10]
RFC 2470 IPv6 over Token Ring December 1998
Full Copyright
Copyright (C) The Internet Society (1998). All Rights Reserved
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BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE
Crawford, et. al. Standards Track [Page 11]
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