





Network Working Group                                           D. Katz
Request for Comments: 1390                          cisco Systems, Inc.
STD: 36                                                    January 1993


             Transmission of IP and ARP over FDDI Networks

Status of this Memo

   This RFC specifies an IAB standards track protocol for the Internet
   community, and requests discussion and suggestions for improvements.
   Please refer to the current edition of the "IAB Official Protocol
   Standards" for the standardization state and status of this protocol.
   Distribution of this memo is unlimited.

Abstract

   This memo defines a method of encapsulating the Internet Protocol
   (IP) datagrams and Address Resolution Protocol (ARP) requests and
   replies on Fiber Distributed Data Interface (FDDI) Networks.

   This RFC is the product of the IP over FDDI Working Group of the
   Internet Engineering Task Force (IETF).

Acknowledgments

   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.  The author would also like to acknowledge the
   contributions of the IP Over FDDI Working Group of the IETF, members
   of ANSI ASC X3T9.5, and others in the FDDI community.

Conventions

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

      "Must," "Shall," 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.






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RFC 1390                      IP Over FDDI                  January 1993


Introduction

   The goal of this specification is to allow compatible and
   interoperable implementations for transmitting IP datagrams [1] and
   ARP requests and replies [2].

   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
   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   | F   |
        +-------------+ D S |
        |  FDDI PHY   | D M |
        +-------------+ I T |
        |  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.

   It is the explicit intent of this memo to allow the interoperability
   of IP and ARP between stations on FDDI networks and stations on
   Ethernet networks via translational bridges.

   The FDDI standards define both single and dual MAC stations.  This
   document describes the use of IP and ARP on single MAC stations
   (single-attach or dual-attach) only.





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RFC 1390                      IP Over FDDI                  January 1993


Packet Format

   IP datagrams and ARP requests and replies sent on FDDI networks shall
   be encapsulated within the 802.2 LLC and Sub-Network Access Protocol
   (SNAP) [12] 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 [13]).

   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 [13] (IP = 2048, ARP = 2054).


      ...--------+--------+--------+
                 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 48-bit FDDI addresses
   shall 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



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RFC 1390                      IP Over FDDI                  January 1993


   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 [13].  The
   hardware type code assigned for Ethernet networks is 1 [13].
   Unfortunately, differing values between Ethernet and IEEE 802
   networks cause interoperability problems in bridged environments.  In
   order to not preclude interoperability with Ethernets in a bridged
   environment, ARP packets shall be transmitted with a hardware type
   code of 1.  ARP packets shall be accepted if received with a hardware
   type code of 1.

   The protocol type code for IP is 2048 [13].

   The hardware address length is 6.

   The protocol address length (for IP) is 4.

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

   In order to not preclude interoperability in a bridged environment,
   the hardware addresses in ARP packets (ar$sha, ar$tha) must be
   carried in "canonical" bit order, with the Group bit positioned as
   the low order bit of the first octet.  As FDDI addresses are normally
   expressed with the Group bit in the high order bit position, the
   addresses must be bit-reversed within each octet.

   Although outside the scope of this document, it is recommended that
   MAC addresses be represented in canonical order in all Network Layer
   protocols carried within the information field of an FDDI frame.

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 [14]).






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RFC 1390                      IP Over FDDI                  January 1993


Multicast Support

   A method of supporting IP multicasting is specified in [15].  This
   method shall be used in FDDI networks if IP multicasting is to be
   supported.  The use of this method may require the ability to copy
   frames addressed to any one of an arbitrary number of multicast
   (group) addresses.

   An IP multicast address is mapped to an FDDI group address by placing
   the low order 23 bits of the IP address into the low order 23 bits of
   the FDDI group address 01-00-5E-00-00-00 (in "canonical" order).
   [See 13, page 29.]

   For example, the IP multicast address:

            224.255.0.2

   maps to the FDDI group address:

            01-00-5E-7F-00-02

   in which the multicast (group) bit is the low order bit of the first
   octet (canonical order).  When bit-reversed for transmission in the
   destination MAC address field of an FDDI frame (native order), it
   becomes:

            80-00-7A-FE-00-40

   that is, with the multicast (group) bit as the high order bit of the
   first octet, that being the first bit transmitted on the medium.

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.  Hosts directly connected to FDDI networks shall not
   use trailers.

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" [16].







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RFC 1390                      IP Over FDDI                  January 1993


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 leaves roughly 4470 octets
      for data after the LLC/SNAP header is taken into account.

      However, in order to allow future extensions to the MAC header and
      frame status fields, it is desirable to reserve additional space
      for MAC overhead.

      Therefore, the MTU of FDDI networks shall be 4352 octets.  This
      provides for 4096 octets of data and 256 octets of headers at the
      network layer and above.  Implementations must not send packets
      larger than the MTU.

      Gateway implementations must be prepared to accept packets as
      large as the MTU and fragment them when necessary.  Gateway
      implementations should be able to accept packets as large as can
      be carried within a maximum size FDDI frame and fragment them.

      Host implementations should be prepared to accept packets as large
      as the MTU; however, hosts must not send datagrams longer than 576
      octets unless they have explicit knowledge that the destination is
      prepared to accept them.  Host implementations may accept packets
      as large as can be carried within a maximum size FDDI frame.  A
      host may communicate its size preference in TCP-based applications
      via the TCP Maximum Segment Size option [17].

      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 [17] for
      further information.

      There is no minimum packet size restriction on FDDI networks.

      In order to not preclude interoperability with Ethernet in a
      bridged environment, FDDI implementations must be prepared to
      receive (and ignore) trailing pad octets.

   Other MAC Layer Issues

      The FDDI MAC specification does not require that 16-bit and 48-



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RFC 1390                      IP Over FDDI                  January 1993


      bit address stations be able to interwork fully.  It does,
      however, 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.  In order to avoid
      interoperability problems, only 48-bit addresses shall be used
      with IP and ARP.

      The FDDI MAC specification defines two classes of LLC 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.

      All IP and ARP frames shall be transmitted as Asynchronous LLC
      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, as well as Restricted Asynchronous
      frames.

      After packet transmission, FDDI provides Frame Copied (C) and
      Address Recognized (A) indicators.  The use of these indicators is
      a local implementation decision.  Implementations may choose to
      perform link-level retransmission, ARP cache entry invalidation,
      etc., based on the values of these indicators and other
      information.  The semantics of these indicators, especially in the
      presence of bridges, are not well defined as of this writing.
      Implementors are urged to follow the work of ANSI ASC X3T9.5 in
      regard to this issue in order to avoid interoperability problems.

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.  Described below is the minimum functionality
   necessary for a conformant station.  Some of the functions are not
   related directly to the support of the SNAP SAP (e.g., responding to
   XID and TEST commands directed to the null or global SAP addresses),
   but are part of a general LLC implementation.  Implementors should
   consult IEEE Std. 802.2 [11] for details.

   802.2 Class I LLC requires the support of Unnumbered Information (UI)
   Commands, eXchange IDentification (XID) Commands and Responses, and
   TEST link (TEST) Commands and Responses.  Stations need not be able
   to transmit XID and TEST commands, but must be able to transmit
   responses.



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RFC 1390                      IP Over FDDI                  January 1993


   Encodings

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

      The UI command has an LLC control field value of 3.

      The XID command/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/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.

   Elements of Procedure

      UI responses and UI commands with the Poll bit set shall be
      ignored.  UI commands having other than the SNAP SAP in the DSAP
      or SSAP fields shall not be processed as IP or ARP packets.

      When an XID or TEST command is received, an appropriate response
      must be returned.  XID and TEST commands must be responded to only
      if the DSAP is the SNAP SAP (170 decimal), the Null SAP (0
      decimal), or the Global SAP (255 decimal).  XID and TEST commands
      received with other DSAP values must not be responded to unless
      the station supports the addressed service.  Responses to XID and
      TEST frames shall be constructed as follows:

         Destination MAC:  Copied from Source MAC of the command
         Source MAC:  Set to the address of the MAC receiving the
                command
         DSAP:  Copied from SSAP of the command
         SSAP:  Set to 171 decimal (SNAP SAP + Response bit) if the
                DSAP in the command was the SNAP SAP or the Global SAP;
                set to 1 decimal (Null SAP + Response bit) if the DSAP
                in the command was the Null SAP

      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.

      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.



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RFC 1390                      IP Over FDDI                  January 1993


Appendix on Numbers

   The IEEE specifies numbers as bit strings with the least significant
   bit first, or bit-wise little-endian order.  The Internet protocols
   are documented in bit-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        Internet    Internet
                        Binary      Binary      Decimal

       UI               11000000    00000011    3
       SAP for SNAP     01010101    10101010    170
       Global SAP       11111111    11111111    255
       Null SAP         00000000    00000000    0
       XID              11110101    10101111    175
       XID Poll/Final   11111101    10111111    191
       XID Info                                 129.1.0
       TEST             11000111    11100011    227
       TEST Poll/Final  11001111    11110011    243

Differences between this document and RFC 1188

   The following is a summary of the differences between RFC 1188 and
   this document:

      A reference to a future dual-MAC document has been removed.

      A statement of explicit intent to support FDDI/Ethernet
      interoperability has been added.

      The acceptance of ARP frames bearing hardware type code 6 (IEEE
      802) has been removed.

      The references have been updated.

      The author's address has been updated.

References

   [1] Postel, J., "Internet Protocol", STD 5, 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.




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RFC 1390                      IP Over FDDI                  January 1993


   [3] Postel, J., and J. Reynolds, "A Standard for the Transmission of
       IP Datagrams over IEEE 802 Networks", RFC 1042, USC/Information
       Sciences Institute, February 1988.

   [4] ISO, "Fiber Distributed Data Interface (FDDI) - Media Access
       Control", ISO 9314-2, 1989.  See also ANSI X3.139-1987.

   [5] ISO, "Fiber Distributed Data Interface (FDDI) - Token Ring
       Physical Layer Protocol", ISO 9314-1, 1989.  See also ANSI
       X3.148-1988.

   [6] ISO, "Fiber Distributed Data Interface (FDDI) - Physical Layer
       Medium Dependent", ISO DIS 9314-3, 1989.  See also ANSI X3.166-
       199x.

   [7] ANSI, "FDDI Station Management", ANSI X3T9.5/84-49 Rev 7.1, 1992.

   [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] IEEE, "Draft Standard P802.1A--Overview and Architecture", 1989.

  [13] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC 1340,
       USC/Information Sciences Institute, July 1992.

  [14] Braden, R., and J. Postel, "Requirements for Internet Gateways",
       STD 4, RFC 1009, USC/Information Sciences Institute, June 1987.

  [15] Deering, S., "Host Extensions for IP Multicasting", STD 5, RFC
       1112, Stanford University, August 1989.

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





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RFC 1390                      IP Over FDDI                  January 1993


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

Security Considerations

   Security issues are not discussed in this memo.

Author's Address

   Dave Katz
   cisco Systems, Inc.
   1525 O'Brien Dr.
   Menlo Park, CA  94025

   Phone: (415) 688-8284
   EMail: dkatz@cisco.com


































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