rfc1103.txt

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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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RFC 1103            IP Datagrams over FDDI Networks            June 1989


   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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RFC 1103            IP Datagrams over FDDI Networks            June 1989


     ...--------+--------+--------+
                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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RFC 1103            IP Datagrams over FDDI Networks            June 1989


   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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RFC 1103            IP Datagrams over FDDI Networks            June 1989


                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.