IPng Working Group                                         Matt Crawford
Internet Draft                                                  Fermilab
                                                          March 21, 1997

              Transmission of IPv6 Packets over FDDI Networks

Status of this Memo

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    Distribution of this memo is unlimited.

1.  Introduction

    This memo specifies the MTU and frame format for transmission of
    IPv6 packets on FDDI networks, including a method for MTU
    determination in the presence of 802.1d bridges to other media.  It
    also specifies the method of forming IPv6 link-local addresses on
    FDDI 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 an FDDI network.

2.  Maximum Transmission Unit

    FDDI permits a frame length of 4500 octets (9000 symbols), including
    at least 22 octets (44 symbols) of Data Link encapsulation when
    long-format addresses are used.  Subtracting 8 octets of LLC/SNAP

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    header, this would, in principle, allow the IPv6 [IPV6] packet in
    the Information field to be up to 4470 octets.  However, it is
    desirable to allow for the variable sizes and possible future
    extensions of the MAC header and frame status fields.  The default
    MTU size for IPv6 packets on an FDDI network is therefore 4352
    octets.  This size may be reduced by a Router Advertisement [DISC]
    containing an MTU option which specifies a smaller MTU, or by manual
    configuration of a smaller value on each node.  If a Router
    Advertisement is received with an MTU option specifying an MTU
    larger than the default or the manually configured value, that MTU
    option may be logged to system management but must be otherwise

    For purposes of this document, information received from DHCP is
    considered "manually configured".

3.  Frame Format

    FDDI provides both synchronous and asynchronous transmission, with
    the latter class further subdivided by the use of restricted and
    unrestricted tokens.  Only asynchronous transmission with
    unrestricted tokens is required for FDDI interoperability.
    Accordingly, IPv6 packets shall be sent in asynchronous frames using
    unrestricted tokens.  The robustness principle dictates that nodes
    should be able to receive synchronous frames and asynchronous frames
    sent using restricted tokens.

    IPv6 packets are transmitted in LLC/SNAP frames, using long-format
    (48 bit) addresses.  The data field contains the IPv6 header and
    payload and is followed by the FDDI Frame Check Sequence, Ending
    Delimiter, and Frame Status symbols.

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                      0                   1
                      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                                     |      FC       |
                     |          Destination          |
                     +-                             -+
                     |             FDDI              |
                     +-                             -+
                     |            Address            |
                     |            Source             |
                     +-                             -+
                     |             FDDI              |
                     +-                             -+
                     |            Address            |
                     |     DSAP      |     SSAP      |
                     |      CTL      |      OUI ...  |
                     +-+-+-+-+-+-+-+-+               +
                     |          ... OUI              |
                     |           Ethertype           |
                     |             IPv6              |
                     +-                             -+
                     |            header             |
                     +-                             -+
                     |             and               |
                     +-                             -+
                     /            payload ...        /

    (Each tic mark represents one bit.)

    FDDI Header Fields:

    FC          The Frame Code must be in the range 50 to 57
                hexadecimal, inclusive, with the three low order bits
                indicating the frame priority.  The Frame Code should be
                in the range 51 to 57 hexadecimal, inclusive, for
                reasons given in the next section.

    DSAP, SSAP  Both the DSAP and SSAP fields shall contain the value AA
                hexadecimal, indicating SNAP encapsulation.

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    CTL         The Control field shall be set to 03 hexadecimal,
                indicating Unnumbered Information.

    OUI         The Organizationally Unique Identifier shall be set to
                000000 hexadecimal.

    Ethertype   The ethernet protocol type ("ethertype") shall be set to
                the value 86DD hexadecimal.

4.  Interaction with Bridges

    802.1d MAC bridges which connect different media, for example
    Ethernet and FDDI, have become very widespread.  Some of them do
    IPv4 packet fragmentation and/or support IPv4 Path MTU discovery
    [PMTU], many others do not, or do so incorrectly.  Use of IPv6 in a
    bridged mixed-media environment should not depend on support from
    MAC bridges.

    For correct operation when mixed media are bridged together, the
    smallest MTU of all the media must be advertised by routers in an
    MTU option.  If there are no routers present, this MTU must be
    manually configured in each node which is connected to a medium with
    larger default MTU.  Multicast packets on such a bridged network
    must not be larger than the smallest MTU of any of the bridged
    media.  Often, the subnetwork topology will support larger unicast
    packets to be exchanged between certain pairs of nodes.  To take
    advantage of high-MTU paths when possible, nodes transmitting IPv6
    on FDDI should implement the following simple mechanism for "FDDI
    adjacency detection".

    A node which implements FDDI adjacency detection and has it enabled
    on an FDDI interface must set a non-zero LLC priority in all
    Neighbor Advertisement, Neighbor Solicitation and, if applicable,
    Router Advertisement frames transmitted on that interface.  (In IEEE
    802 language, the user_priority parameter of the M_UNITDATA.request
    primitive must not be zero.)  If FDDI adjacency detection has been
    disabled on an FDDI interface, the priority field of those frames
    must be zero.

    Note that an IPv6 frame which originated on an Ethernet, or
    traversed an Ethernet, before being translated by an 802.1d bridge
    and delivered to a node's FDDI interface will have zero in the
    priority field, as required by [BRIDGE].  (There's a fine point
    here: a conforming bridge may provide a management-settable Outbound
    User Priority parameter for each port.  However, the author is
    unaware of any product that provides this optional capability and,
    in any case, the default value for the parameter is zero.)

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    If a node N1 receives, in an FDDI frame with a non-zero LLC
    priority, a valid Router Advertisement, Neighbor Advertisement, or
    Neighbor Solicitation from a node N2, then N1 may send unicast IPv6
    packets to N2 with sizes up to the default IPv6 FDDI MTU (4352
    octets), regardless of any smaller MTU configured manually or
    received in a Router Advertisement MTU option.  N2 may be the IPv6
    destination or the next hop router to the destination.

    Nodes implementing FDDI adjacency detection must provide a
    configuration option to disable the mechanism.  This option may be
    used when a smaller MTU is desired for reasons other than mixed-
    media bridging.  By default, FDDI adjacency detection should be

    The only contemplated use of the LLC priority field of the FC octet
    is to aid in per-destination MTU determination.  It would be
    sufficient for that purpose to require only that Router
    Advertisements, Neighbor Advertisements, and Neighbor Solicitations
    sent on FDDI always have non-zero priority.  However, it may be
    simpler or more useful to transmit all IPv6 packets on FDDI with
    non-zero priority.

5.  Stateless Autoconfiguration

    The interface token [CONF] for an FDDI interface is the EUI-64
    identifier [EUI64] derived from the interface's built-in 48-bit IEEE
    802 address.  The OUI of the Ethernet 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 Ethernet
    address become the last three octets of the EUI-64.

    For example, the interface token for an Ethernet 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.

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

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6.  Link-Local Addresses

    The IPv6 link-local address [AARCH] for an FDDI interface is formed
    by appending the interface token, as defined above, to the prefix

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

7.  Address Mapping -- Unicast

    The procedure for mapping IPv6 addresses into FDDI link-layer
    addresses is described in [DISC].  The Source/Target Link-layer
    Address option has the following form when the link layer is FDDI.

                      0                   1
                      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                     |     Type      |    Length     |
                     |                               |
                     +-            FDDI             -+
                     |                               |
                     +-           Address           -+
                     |                               |

    Option fields:

    Type        1 for Source Link-layer address.
                2 for Target Link-layer address.

    Length      1 (in units of 8 octets).

    FDDI Address
                The 48 bit FDDI 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 address token.

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8.  Address Mapping -- Multicast

    An IPv6 packet with a multicast destination address DST, consisting
    of the sixteen octets DST[1] through DST[16], is transmitted to the
    FDDI multicast address whose first two octets are the value 3333
    hexadecimal and whose last four octets are the last four octets of

                     |0 0 1 1 0 0 1 1|0 0 1 1 0 0 1 1|
                     |   DST[13]     |   DST[14]     |
                     |   DST[15]     |   DST[16]     |

9.  Security Considerations

    Security considerations are not addressed in this memo.

10.  Acknowledgments

    Erik Nordmark and Matt Thomas contributed to the method for
    interaction with bridges.

11.  References

    [AARCH] R. Hinden, S. Deering "IP Version 6 Addressing
            Architecture", RFC 1884.

    [BRIDGE]ISO/IEC 10038 : 1993 [ANSI/IEEE Std 802.1D] 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.

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

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

    [PMTU]  J. Mogul, S. Deering "Path MTU Discovery", RFC 1191.

12.  Author's Address

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

    Phone: +1 630 840-3461

    EMail: crawdad@fnal.gov

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