rfc1103.txt

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      Gateway implementations must be prepared to accept full-length
      packets and fragment them when necessary.

      Host implementations should be prepared to accept full-length
      packets; however, hosts must not send datagrams longer than 576
      octets unless they have explicit knowledge that the destination is
      prepared to accept them.  A host may communicate its size
      preference in TCP-based applications via the TCP Maximum Segment
      Size option [16].

      Datagrams on FDDI networks may be longer than the general Internet
      default maximum packet size of 576 octets.  Hosts connected to an
      FDDI network should keep this in mind when sending datagrams to
      hosts that are not on the same local network.  It may be
      appropriate to send smaller datagrams to avoid unnecessary
      fragmentation at intermediate gateways.  Please see [16] for
      further information.

      There is no minimum packet size restriction on FDDI networks.

Other MAC Layer Issues

   The FDDI MAC specification does not require that 16-bit and 48-bit
   address stations be able to interwork fully.  It does, however,



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   require that 16-bit stations have full 48-bit functionality, and that
   both types of stations be able to receive frames sent to either size
   broadcast address.  For use with IP and ARP, all communicating
   stations on a LAN must use a consistent address size.
   Implementations must discard any IP or ARP packets received with an
   unimplemented or inactive address size.  16-bit and 48-bit
   implementations may coexist on the same FDDI network; however, if
   they wish to interwork they must be considered separate IP networks
   and linked with an IP router capable of supporting 16-and 48-bit
   addresses simultaneously.

   Group (multicast) addresses are defined by the FDDI MAC specification
   but are not necessarily supported by existing hardware.  Therefore,
   this feature must not be used by IP and ARP.

   The FDDI MAC specification defines two classes of frames,
   Asynchronous and Synchronous.  Asynchronous frames are further
   controlled by a priority mechanism and two classes of token,
   Restricted and Unrestricted.  Only the use of Unrestricted tokens and
   Asynchronous frames are required by the standard for FDDI
   interoperability.  The priority mechanism is currently implemented
   locally by the transmitting station and the Priority field in
   Asynchronous frames is ignored by other stations.  This field will
   likely be interpreted by Transparent Bridges once they are defined.
   There is no default value for priority called out in the MAC
   standard.

   Therefore, all IP and ARP frames must be transmitted as Asynchronous
   frames using Unrestricted tokens, and the Priority value is a matter
   of local convention.  Implementations should make the priority a
   tunable parameter for future use.  It is recommended that
   implementations provide for the reception of IP and ARP packets in
   Synchronous frames.

   After packet transmission, FDDI provides Frame Copied (C) and Address
   Recognized (A) indicators.  There are four possible combinations of
   the indicators with the following semantics:

            (C)      (A)
            Reset    Reset   The frame was not received by any station.
            Reset    Set     The addressed station is congested.
            Set      Reset   Reserved.
            Set      Set     The addressed station received the frame.

   Implementations may use these indicators to provide some amount of
   error detection and correction:

      If the Frame Copied bit is reset but the Address Recognized bit is



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      set, receiver congestion has occurred.  It is recommended, though
      not mandatory, that hosts retransmit the offending packet a small
      number of times (4) or until congestion no longer occurs.

      If the both the Address Recognized indicator and the Frame Copied
      indicator are reset, an implementation has three options: (1)
      ignore the error and throw the packet away, (2) return an ICMP
      destination unreachable message to the source, or (3) delete the
      ARP entry which was used to send this packet and send a new ARP
      request to the destination address.  The latter option is the
      preferred approach since it will allow graceful recovery from
      first hop bridge and router failures and changed hardware
      addresses.

      As of this writing there is a proposal within ANSI to set the
      Frame Copied indicator and reset the Address Recognized indicator
      when a frame is forwarded by a Transparent Bridge.  For future
      compatibility, implementations should interpret this combination
      of indicators as if the frame were successfully delivered to the
      destination (i.e., do nothing).

IEEE 802.2 Details

   While not necessary for supporting IP and ARP, all implementations
   must support IEEE 802.2 standard Class I service in order to be
   compliant with 802.2.  This requires supporting Unnumbered
   Information (UI) Commands, eXchange IDentification (XID) Commands and
   Responses, and TEST link (TEST) Commands and Responses.

   When an XID or TEST command is received, a response must be returned
   with Destination and Source addresses, and DSAP and SSAP, swapped.

   When responding to an XID or a TEST command, the value of the Final
   bit in the response must be copied from the value of the Poll bit in
   the command.

   The XID command or response has an LLC control field value of 175
   (decimal) if the Poll/Final bit is off or 191 (decimal) if the
   Poll/Final bit is on.

   The TEST command or response has an LLC control field value of 227
   (decimal) if the Poll/Final bit is off or 243 (decimal) if the
   Poll/Final bit is on.

   Command frames are identified by having the high order bit of the
   SSAP address reset to zero.  Response frames have the high order bit
   of the SSAP address set to one.




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RFC 1103            IP Datagrams over FDDI Networks            June 1989


   XID response frames must include an 802.2 XID Information field of
   129.1.0 indicating Class I (connectionless) service.

   TEST response frames must echo the information field received in the
   corresponding TEST command frame.

Appendix on Numbers

   The IEEE specifies numbers in bit transmission order, or bit-wise
   little-endian order.  The Internet protocols are documented in byte-
   wise big-endian order.  This may cause some confusion about the
   proper values to use for numbers.  Here are the conversions for some
   numbers of interest.

       Number        IEEE    IEEE        Internet    Internet
                     HEX     Binary      Binary      Decimal

       UI Op Code    C0      11000000    00000011    3
       SAP for SNAP  55      01010101    10101010    170
       XID           F5      11110101    10101111    175
       XID           FD      11111101    10111111    191
       TEST          C7      11000111    11100011    227
       TEST          CF      11001111    11110011    243
       Info          818000                          129.1.0

References

  [1]  Postel, J., "Internet Protocol", RFC-791, USC/Information
       Sciences Institute, September 1981.

  [2]  Plummer, D., "An Ethernet Address Resolution Protocol - or -
       Converting Network Protocol Addresses to 48.bit Ethernet Address
       for Transmission on Ethernet Hardware", RFC-826, MIT, November
       1982.

  [3]  Postel J., and J. Reynolds, "A Standard for the Transmission of
       IP Datagrams over IEEE 802 Networks", RFC1042, USC/Information
       Sciences Institute, February, 1988.

  [4]  ISO, "Fiber Distributed Data Interface (FDDI) - Media Access
       Control", ISO 9314-2, 1988.  See also ANSI X3.139-1987.

  [5]  ISO, "Fiber Distributed Data Interface (FDDI) - Token Ring
       Physical Layer Protocol", ISO 9314-1, 1988.  See also ANSI
       X3.148-1988.

  [6]  ISO, "Fiber Distributed Data Interface (FDDI) - Physical Layer
       Medium Dependent", ISO DIS 9314-3, 1988.  See also ANSI X3.166-



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       198x.

  [7]  ANSI, "FDDI Station Management", ANSI X3T9.5/84-49 Rev 4.0, 1988.

  [8]  IEEE, "IEEE Standards for Local Area Networks: Carrier Sense
       Multiple Access with Collision Detection (CSMA/CD) Access Method
       and Physical Layer Specifications", IEEE, New York, New York,
       1985.

  [9]  IEEE, "IEEE Standards for Local Area Networks: Token-Passing Bus
       Access Method and Physical Layer Specification", IEEE, New York,
       New York, 1985.

  [10] IEEE, "IEEE Standards for Local Area Networks: Token Ring Access
       Method and Physical Layer Specifications", IEEE, New York, New
       York, 1985.

  [11] IEEE, "IEEE Standards for Local Area Networks: Logical Link
       Control", IEEE, New York, New York, 1985.

  [12] Reynolds, J.K., and J. Postel, "Assigned Numbers", RFC-1010,
       USC/Information Sciences Institute, May 1987.

  [13] Braden, R., and J. Postel, "Requirements for Internet Gateways",
       RFC-1009, USC/Information Sciences Institute, June 1987.

  [14] Leffler, S., and M. Karels, "Trailer Encapsulations", RFC-893,
       University of California at Berkeley, April 1984.

  [15] Cohen, D., "On Holy Wars and a Plea for Peace", Computer, IEEE,
       October 1981.

  [16] Postel, J., "The TCP Maximum Segment Size Option and Related
       Topics", RFC-879, USC/Information Sciences Institute, November
       1983.

Author's Address

   Dave Katz Merit/NSFNET 1075 Beal Ann Arbor, MI 48109-2112

   Phone: 1-800-66-MERIT

   Email: [e-mail]








Katz                                                            [Page 9]