





Network Working Group                                            D. Katz
Request for Comments:  1103                                 Merit/NSFNET
                                                               June 1989

              A Proposed Standard for the Transmission of
                    IP Datagrams over FDDI Networks


Status of this Memo

   This RFC specifies a method of encapsulating the Internet Protocol
   (IP) [1] datagrams and Address Resolution Protocol (ARP) [2] requests
   and replies on Fiber Distributed Data Interface (FDDI) Networks.
   This RFC specifies a proposed protocol standard for the Internet
   community.  Comments are welcome.  Distribution of this memo is
   unlimited.

Acknowledgment

   This memo draws heavily in both concept and text from RFC 1042 [3],
   written by Jon Postel and Joyce K. Reynolds of USC/Information
   Sciences Institute.

Conventions

   The following language conventions are used in the items of
   specification in this document:

      "Must" or "Mandatory"--the item is an absolute requirement of the
      specification.

      "Should" or "Recommended"--the item should generally be followed
      for all but exceptional circumstances.

      "May" or "Optional"--the item is truly optional and may be
      followed or ignored according to the needs of the implementor.

Introduction

   The goal of this specification is to allow compatible and
   interoperable implementations for transmitting IP datagrams and ARP
   requests and replies.

   The Fiber Distributed Data Interface (FDDI) specifications define a
   family of standards for Local Area Networks (LANs) that provides the
   Physical Layer and Media Access Control Sublayer of the Data Link
   Layer as defined by the ISO Open System Interconnection Reference
   Model (ISO/OSI).  Documents are in various stages of progression



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   toward International Standardization for Media Access Control (MAC)
   [4], Physical Layer Protocol (PHY) [5], Physical Layer Medium
   Dependent (PMD) [6], and Station Management (SMT) [7].  The family of
   FDDI standards corresponds to the IEEE 802 MAC layer standards [8, 9,
   10].

   The remainder of the Data Link Service is provided by the IEEE 802.2
   Logical Link Control (LLC) service [11].  The resulting stack of
   services appears as follows:

           +-------------+
           |   IP/ARP    |
           +-------------+
           |  802.2 LLC  |
           +-------------+
           |  FDDI MAC   |
           +-------------+
           |  FDDI PHY   |
           +-------------+
           |  FDDI PMD   |
           +-------------+

   This memo describes the use of IP and ARP in this environment.  At
   this time, it is not necessary that the use of IP and ARP be
   consistent between FDDI and IEEE 802 networks, but it is the intent
   of this memo not to preclude Data Link Layer interoperability at such
   time as the standards define it.

Packet Format

   IP datagrams and ARP requests and replies sent on FDDI networks must
   be encapsulated within the 802.2 LLC and Sub-Network Access Protocol
   (SNAP) data link layers and the FDDI MAC and physical layers.  The
   SNAP must be used with an Organization Code indicating that the SNAP
   header contains the EtherType code (as listed in Assigned Numbers
   [12]).

   802.2 LLC Type 1 communication (which must be implemented by all
   conforming 802.2 stations) is used exclusively.  All frames must be
   transmitted in standard 802.2 LLC Type 1 Unnumbered Information
   format, with the DSAP and the SSAP fields of the 802.2 header set to
   the assigned global SAP value for SNAP [11].  The 24-bit Organization
   Code in the SNAP must be zero, and the remaining 16 bits are the
   EtherType from Assigned Numbers [12] (IP = 2048, ARP = 2054).







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     ...--------+--------+--------+
                MAC Header        |                           FDDI MAC
     ...--------+--------+--------+

     +--------+--------+--------+
     | DSAP=K1| SSAP=K1| Control|                            802.2 LLC
     +--------+--------+--------+

     +--------+--------+---------+--------+--------+
     |Protocol Id or Org Code =K2|    EtherType    |        802.2 SNAP
     +--------+--------+---------+--------+--------+

     The total length of the LLC Header and the SNAP header is 8
     octets.

     The K1 value is 170 (decimal).

     The K2 value is 0 (zero).

     The control value is 3 (Unnumbered Information).

Address Resolution

   The mapping of 32-bit Internet addresses to 16-bit or 48-bit FDDI
   addresses must be done via the dynamic discovery procedure of the
   Address Resolution Protocol (ARP) [2].

   Internet addresses are assigned arbitrarily on Internet networks.
   Each host's implementation must know its own Internet address and
   respond to Address Resolution requests appropriately.  It must also
   use ARP to translate Internet addresses to FDDI addresses when
   needed.

   The ARP protocol has several fields that parameterize its use in any
   specific context [2].  These fields are:

         hrd   16 - bits     The Hardware Type Code
         pro   16 - bits     The Protocol Type Code
         hln    8 - bits     Octets in each hardware address
         pln    8 - bits     Octets in each protocol address
         op    16 - bits     Operation Code

   The hardware type code assigned for IEEE 802 networks is 6 [12].
   FDDI networks, although not IEEE 802 networks per se, are
   semantically equivalent and use the same type code.

   The protocol type code for IP is 2048 [12].




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   The hardware address length is 2 for 16-bit FDDI addresses, or 6 for
   48-bit FDDI addresses.

   The protocol address length (for IP) is 4.

   The operation code is 1 for request and 2 for reply.

Broadcast Address

   The broadcast Internet address (the address on that network with a
   host part of all binary ones) must be mapped to the broadcast FDDI
   address (of all binary ones) (see [13]).

Trailer Formats

   Some versions of Unix 4.x bsd use a different encapsulation method in
   order to get better network performance with the VAX virtual memory
   architecture.  Consenting systems on the same FDDI network may use
   this format between themselves.  Details of the trailer encapsulation
   method may be found in [14].  However, all hosts must be able to
   communicate using the standard (non-trailer) method.

Byte Order

   As described in Appendix B of the Internet Protocol specification
   [1], the IP datagram is transmitted over FDDI networks as a series of
   8-bit bytes.  This byte transmission order has been called "big-
   endian" [15].

MAC Layer Details

   Packet Size

      The FDDI MAC specification [4] defines a maximum frame size of
      9000 symbols (4500 octets) for all frame fields, including four
      symbols (two octets) of preamble.  This gives the following MAC
      layer overhead:














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                Field                    Size in Octets

                Preamble                     2
                Start Delimiter              1
                Frame Control                1
                Destination Address          6 (2)
                Source Address               6 (2)
                FCS                          4
                End Delimiter/Frame Status   2

                Total                        22 (14)
                Remaining for Data           4478 (4486)

      Subtracting the 8 byte LLC/SNAP header, this gives a maximum
      packet size (MTU) of 4470 (4478) octets.  For compatibility
      purposes, the maximum packet size used with IP datagrams or ARP
      requests and replies must be consistent on a particular network.

      The overhead calculations (above) assume a standard Frame Status
      field consisting of three symbols.  Additional Implementor Defined
      frame status information, although permitted by the FDDI MAC
      specification, must not be used with IP datagrams because it
      affects the maximum packet size.

      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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   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: Dave_Katz@um.cc.umich.edu








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