rfc1374.txt

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   The length of a HIPPI packet, including trailing fill, shall be a
   multiple of eight octets as required by HIPPI-LE.

   +----------+-----------+---------------------+-----------   ------+
   |          |           |                     | IP . . .     0 - 7 |
   | HIPPI-FP | HIPPI-LE  | IEEE 802.2 LLC/SNAP |              octets|
   |(8 octets)|(24 octets)|     (8 octets)      | ARP . . .     fill |
   +----------+-----------+---------------------+-----------   ------+

                        HIPPI Packet Structure


      HIPPI-FP Header

         ULP-id (8 bits) shall contain 4.

         D1_Data_Set_Present (1 bit) shall be set.

         Start_D2_on_Burst_Boundary (1 bit) shall be zero.

         Reserved (11 bits) shall contain zero.

         D1_Area_Size (8 bits) should be sent as 3.  Destinations shall
         accept any value that HIPPI-FP defines as legal: from 3 to 127
         (32 bit HIPPI) or 3 to 255 (64 bit HIPPI).

         D2_Offset (3 bits) may be any value from 0 to 7.

         D2_Size (32 bits) Shall contain the number of octets in the



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RFC 1374                  IP and ARP on HIPPI               October 1992


         IEEE 802.2 LLC Type 1 PDU, or zero if no PDU is present.  It
         shall not exceed 65,288 (decimal).  This value includes the
         IEEE 802.2 LLC/SNAP header and the IP datagram.  It does not
         include trailing fill octets.  (See "MTU," below.)

         The first octet of the IEEE 802.2 LLC PDU (SSAP) shall be
         located at offset "n" of the packet, where

             n = 8 + (D1_Area_Size*8) + D2_Offset

         as specified in HIPPI-FP.

      HIPPI-LE Header

         FC (3 bits) shall contain zero unless otherwise defined by
         local administration.

         Double_Wide (1 bit) shall contain one if the Destination
         associated with the sending Source supports 64 bit HIPPI
         operation.  Otherwise it shall contain zero.

         Message_Type (4 bits) contains a code identifying the type of
         HIPPI-LE PDU.  Defined values (binary) are:

            0  Data PDU
            1  Address Resolution Request PDU (AR_Request)
            2  Address Resolution Response PDU (AR_Response)
            3  Self Address Resolution Request PDU (AR_S_Request)
            4  Self Address Resolution Response PDU (AR_S_Response)
            5-11 Reserved by the ANSI X3T9.3 committee
            12-15 Locally Assigned

         Destination_Switch_Address is a 24-bit field containing the
         Switch Address of the Destination if known, otherwise zero.  If
         the address comprises less than 24 bits, it shall be right
         justified (occupying the least significant bits) in the field.

         Destination_Address_Type (4 bits) and Source_Address_Type (4
         bits) contain codes identifying the type of addresses in the
         Destination_Switch_Address and Source_Switch_Address fields
         respectively.  Defined values (binary) are:

            0  Unspecified
            1  HIPPI-SC Source Route (24 bits)
            2  HIPPI-SC Address (12 bits)
            3-11 Reserved by the ANSI X3T9.3 committee
            12-15 Locally Assigned




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RFC 1374                  IP and ARP on HIPPI               October 1992


         Source_Switch_Address is a 24-bit field containing the Switch
         Address of the Source.  If the address comprises less than 24
         bits, it shall be right justified (occupying the least
         significant bits) in the field.

         Reserved (16 bits) shall contain zero.

         Destination_IEEE_Address (48 bits) shall contain the 48 bit
         Universal LAN MAC Address of the Destination if known,
         otherwise zero.

         LE_Locally_Administered (16 bits) shall contain zero unless
         otherwise defined by local administration.

         Source_IEEE_Address (48 bits) shall contain the 48 bit
         Universal LAN MAC Address of the Source if known, otherwise
         zero.

      IEEE 802.2 LLC

         The IEEE 802.2 LLC Header shall begin in the first octet of the
         HIPPI-FP D2_Area.

         SSAP (8 bits) shall contain 170 (decimal).

         DSAP (8 bits) shall contain 170 (decimal).

         CTL (8 bits) shall contain 3 (Unnumbered Information).

      SNAP

         Organization Code (24 bits) shall be zero.

         EtherType (16 bits) shall be set as defined in Assigned Numbers
         [8] (IP = 2048, ARP = 2054, RARP = 32,821).
















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RFC 1374                  IP and ARP on HIPPI               October 1992


      31    28        23  21          15        10     7         2   0
      +-----+---------+-+-+-----------+---------+-----+---------+-----+
    0 |      04       |1|0|       Reserved      |      03       |  0  |
      +---------------+-+-+---------------------+---------------+-----+
    1 |                             (n+8)                             |
      +-----+-+-------+-----------------------------------------------+
    2 |[LA] |W|M_Type |          Destination_Switch_Address           |
      +-----+-+-------+-----------------------------------------------+
    3 | D_A_T | S_A_T |             Source_Switch_Address             |
      +-------+-------+---------------+-------------------------------+
    4 |            Reserved           |  [Destination_IEEE_Address]   |
      +-------------------------------+                               |
    5 |                                                               |
      +-------------------------------+-------------------------------+
    6 |             [LA]              |     [Source_IEEE_Address]     |
      +-------------------------------+                               |
    7 |                                                               |
      +===============+===============+===============+===============+
    8 |       AA      |      AA       |       03      |       00      |
      +---------------+---------------+---------------+---------------+
    9 |       00      |      00       |         [EtherType]           |
      +---------------+---------------+---------------+---------------+
   10 |Message octet 0|Message octet 1|Message octet 2| . . .         |
      +---------------+---------------+---------------+---            |
      |                            .  .  .
                                                                      |
      |        -------+---------------+---------------+---------------+
      |         . . . |  octet (n-2)  |  octet (n-1)  |     FILL      |
      +---------------+---------------+---------------+---------------+
   N-1|      FILL     |     FILL      |     FILL      |     FILL      |
      +---------------+---------------+---------------+---------------+

                           HIPPI Packet Format

      Words 0-1:  HIPPI-FP Header
      Words 2-7:  D1 Area (HIPPI-LE Header)
      Words 8-9:  D2 Area (IEEE 802.2 LLC/SNAP)
      Words 10-(N-1):  D2 Area (IP or ARP message)
      (n) is the number of octets in the IP or ARP message.
      +====+ denotes the boundary between D1 and D2 areas.
      [LA] fields are zero unless used otherwise locally.
      Abbreviations:  "W"      = Double_Wide field;
                      "M_Type" = Message_Type field;
                      "D_A_T"  = Destination_Address_Type;
                      "S_A_T"  = Source_Address_Type;
      [FILL] octets complete the HIPPI packet to an even
      number of 32 bit words.  The number of fill octets
      is not counted in the data length.



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RFC 1374                  IP and ARP on HIPPI               October 1992


   IEEE 802.2 Data

      The IEEE 802.2 Data shall follow the EtherType field immediately.
      Fill octets shall be used following the Data as necessary to make
      the number of octets in the packet a multiple of 8.  In accordance
      with HIPPI-FP, the amount of this fill is not included in the
      D2_Size value in the HIPPI-FP Header.

      The order of the octets in the data stream is from higher numbered
      to lower numbered data signal (left to right) within the HIPPI
      word, as specified in HIPPI-FP Clause 7, "Word and byte formats."
      With the 1600 megabit/second data rate option (64 bit) bits 32
      through 63 are on Cable B, so that the four octets on Cable B come
      logically before those on Cable A.  Within each octet, the most
      significant bit is the highest numbered signal.

48 bit Universal LAN MAC Addresses

   IEEE Standard 802.1A specifies the Universal LAN MAC Address.  The
   globally unique part of the 48 bit space is administered by the IEEE.
   Each node on a HIPPI-SC LAN should be assigned a ULA.  Multiple ULAs
   may be used if a node contains more than one IEEE 802.2 LLC protocol
   entity.

   The format of the address within its 48 bit HIPPI-LE fields follows
   IEEE 802.1A canonical bit order and HIPPI-FP bit and byte order:

   31              23              15               7              0
   +-------------------------------+---------------+---------------+
   |      (not used for ULA)       |ULA octet 0|L|G|  ULA octet 1  |
   +---------------+---------------+---------------+---------------+
   |  ULA octet 2  |  ULA octet 3  |  ULA octet 4  |  ULA octet 5  |
   +---------------+---------------+---------------+---------------+

                    Universal LAN MAC Address Format

      L (U/L bit) = 1 for Locally administered addresses, 0 for
      Universal.
      G (I/G bit) = 1 for Group addresses, 0 for Individual.

   The use of ULAs is optional, but encouraged.  Although ULAs are not
   used by HIPPI-SC switches, they are helpful for HIPPI Switch Address
   resolution, and for distinguishing between multiple logical entities
   that may exist within one node.  They may also be used by gateway
   devices that replace HIPPI hardware headers with the MAC headers of
   other LANs.  Carrying the ULAs in the HIPPI header may simplify these
   devices, and it may also help if HIPPI is used as an interface to
   some future HIPPI based LAN that uses ULAs for addressing.



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RFC 1374                  IP and ARP on HIPPI               October 1992


Recommended HIPPI-FP Options

   HIPPI-FP allows some flexibility in the construction of a HIPPI
   packet, including placement of short bursts, optional fill and offset
   octets between the D1 and D2 areas and fill following the D2 data.
   For efficiency, Sources should limit the use of these options:

   1.  Send the short burst as the last burst of the packet rather than
       the first.

   2.  Do not place fill octets between the HIPPI-LE header and the
       start of the D2_Area.

   3.  Use no more than seven octets after the D2 Data, as needed to
       make the total packet length a multiple of 8 octets.

One HIPPI-FP option is forbidden: setting the Start_D2_on_Burst_Boundary
flag to one.  This places no limitation on the formation of packets into
a series of bursts; a Source may segment the packet in any legal manner
according to HIPPI-FP, including forcing the D2_Area to start on a burst
boundary.  The purpose of the Start_D2_on_Burst_Boundary flag is to help
preserve the segmentation of the packet for some device-control
protocols that use the first burst boundary to separate command and data
areas within the packet.  Requiring this flag to be clear means that
when a packet arrives at the Destination its burst boundaries might not
be exactly as the Source sent them.  This may occur if a HIPPI packet
passes over some other medium in the route between HIPPI LANs.

Notwithstanding these recommendations, each Destination shall accept any
well-formed HIPPI packet within the definitions in HIPPI-FP.

Note that neither HIPPI-FP nor HIPPI-LE limits the number of fill bytes
placed between the end of the IP packet and the end of the HIPPI-PH
packet.  Some source implementations may add fill sufficient to overflow
a destination input buffer.  To avoid interpreting valid packets as
errors, destinations should ignore overflow conditions and verify that
at least the number of bytes indicated by the IP header actually
arrived.

I-Field format

   The I-field bits, as defined in HIPPI-SC, shall be set as follows: