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Versions: 00 01                                                         
IPng Working Group                                 Stephen Thomas
Internet Draft                                         TransNexus
                                                September 7, 1997

      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

     Internet Drafts are draft documents valid for a maximum of
     six months. Internet Drafts may be updated, replaced, or
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     To learn the current status of any Internet-Draft, please
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     Coast), or munnari.oz.au (Pacific Rim).

     This Internet Draft expires December 15, 1997.

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, Neighbor Solicitation and Neighbor
     Advertisement messages when those messages are transmitted on
     a Token Ring network.

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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

     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 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

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     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.   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

                |  SD   |  AC   |  FC   |       |
                +-----------------------+       |
                |      Destination Address      |
                |       +-----------------------+
                |       |     Source            |
                +-------+    Address    +-------+
                |                       | DSAP  |
                | SSAP  |  CTL  |      OUI      |
                |  OUI  |   EtherType   |       |
                +-------+---------------+       |
                |                               |
                ~  IPv6 header and payload...   ~
                |                               |
                |              FCS              |
                |  ED   |  FS   |

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     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)

          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:

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                            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 token [CONF] for a Token Ring interface is 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

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     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 token 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 token. 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 token, as
     defined above, to the prefix FE80::/64.

  10 bits            54 bits                  64 bits
|1111111010|         (zeros)       |      Interface Token       |

6.   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.

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                 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 as
               the interface token.

     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.

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     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.

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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

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, Matt Crawford, Steve Deering, Francis Dupont, Robert
     Elz, Thomas Narten, 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

     [AARCH]   R. Hinden, S. Deering, "IP Version 6 Addressing
               Architecture", draft-ietf-ipngwg-addr-arch-v2-01.txt.

     [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.

     [CONF]    S. Thomson, T. Narten, "IPv6 Stateless Address
               Autoconfiguration", RFC 1971.

     [DISC]    T. Narten, E. Nordmark, W. A. Simpson, "Neighbor
               Discovery for IP Version 6 (IPv6)", RFC 1970.

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     [EUI64]  "64-Bit Global Identifier Format Tutorial", http:

     [IPV6]    S. Deering, R. Hinden, "Internet Protocol, Version
               6 (IPv6) Specification", RFC 1883.

     [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.  Author's Address

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

     Email: stephen.thomas@transnexus.com

     Phone: +1 404 872 4745
     Fax:   +1 404 872 9515

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