Network Working Group
Request for Comments: 2470
Category: Standards Track
M. Crawford
T. Narten
S. Thomas
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 © 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)
  1. 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


Source Address: 48-bit IEEE address of source station

      DSAP: Destination Service Access Point (for LLC/SNAP
           format, shall always contain the value 0xAA)


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)


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)


           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



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,


   would be


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)
   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", RFC 2373, July 1998.
   [ACONF]   Thomson, S. and T. Narten, "IPv6 Stateless Address
             Autoconfiguration", RFC 2462, 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", RFC 2469, December 1998.
   [CONF]    Thomson, S. and T. Narten, "IPv6 Stateless Address
             Autoconfiguration", RFC 1971, August 1996.
   [DISC]    Narten, T., Nordmark, E. and W. Simpson, "Neighbor
             Discovery for IP Version 6 (IPv6)", RFC 2461, December
   [EUI64]  "64-Bit Global Identifier Format Tutorial", http:
   [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 Indicate
             Requirement Levels," BCP 14, RFC 2119, 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

11. Authors' Addresses

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

   Phone: +1 630 840 3461

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

   Phone: +1 919 254 7798

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

   Phone: +1 404 872 4745

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