rfc1374.txt

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   In the following discussion, the terms "requester" and "target" are
   used to identify the node requesting address resolution and the node
   whose address it wishes to discover, respectively.  In third party
   ARP (see "ARP Implementation Methods," below) the source of a reply
   is an ARP agent node, not the target node.

   ARP and RARP Message Format

      The HIPPI ARP/RARP protocol uses the same packet format as ARP for
      Ethernet.  ARP packets shall be transmitted with a hardware type
      code of 1 (as for Ethernet).  Furthermore, ARP packets shall be
      accepted if received with hardware type codes of either 1 or 6
      (IEEE 802 networks).

      ar$hrd (16 bits) shall contain 1.

      ar$pro (16 bits) shall contain the IP protocol code 2048
      (decimal).

      ar$hln (8 bits) shall contain 6.

      ar$pln (8 bits) shall contain 4.

      ar$op  (16 bits) shall contain 1 for requests, 2 for responses.

      ar$sha (48 bits) in requests shall contain the requester's ULA.
      In replies it shall contain the target node's ULA.

      ar$spa (32 bits) in requests shall contain the requester's IP
      address if known, otherwise zero.  In replies it shall contain the
      target node's IP address.

      ar$tha (48 bits) in requests shall contain the target's ULA if
      known, otherwise zero.  In replies it shall contain the
      requester's ULA.

      ar$tpa (32 bits) in requests shall contain the target's IP address
      if known, otherwise zero.  In replies it shall contain the



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RFC 1374                  IP and ARP on HIPPI               October 1992


      requester's IP address.

      The format of the six octets of the ULA shall be the same as
      required in the HIPPI-LE header (see "48 bit Universal LAN MAC
      Addresses" above), except for the alignment of the Source ULA with
      respect to the 32 bit HIPPI word, which is different between ARP
      and HIPPI-LE.  No bit reversal is necessary as is required with
      FDDI [11].

      31    28        23  21          15        10     7         2   0
      +-----+---------+-+-+-----------+---------+-----+---------+-----+
    0 |      04       |1|0|         000         |      03       |  0  |
      +---------------+-+-+---------------------+---------------+-----+
    1 |                              36                               |
      +-----+-+-------+-----------------------+-----------------------+
    2 |[LA] |W|   1   |          000          |   Target Switch Addr  |
      +-----+-+-------+-----------------------+-----------------------+
    3 |   2   |   2   |          000          |Requester's Switch Addr|
      +---------------+---------------+-------+-----------------------+
    4 |             00 00             |                               |
      +-------------------------------+                               |
    5 |                           Target ULA                          |
      +-------------------------------+-------------------------------+
    6 |             [LA]              |                               |
      +-------------------------------+                               |
    7 |                        Requester's ULA                        |
      +===============+===============+===============+===============+
    8 |       AA      |      AA       |       03      |       00      |
      +---------------+---------------+---------------+---------------+
    9 |       00      |      00       |        EtherType (2054)       |
      +---------------+---------------+-------------------------------+
   10 |            hrd (1)            |           pro (2048)          |
      +---------------+---------------+-------------------------------+
   11 |    hln (6)    |    pln (4)    |            op (1)             |
      +---------------+---------------+-------------------------------+
   12 |                 Requester's ULA octets 0 - 3                  |
      +-------------------------------+-------------------------------+
   13 | Requester's ULA octets 4 - 5  | Requester's IP Address upper  |
      +-------------------------------+-------------------------------+
   14 | Requester's IP Address lower  |    Target ULA octets 0 - 1    |
      +-------------------------------+-------------------------------+
   15 |                   Target ULA octets 2 - 5                     |
      +---------------------------------------------------------------+
   16 |                       Target IP Address                       |
      +---------------------------------------------------------------+

                HIPPI ARP/RARP Request (logical address mode)




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RFC 1374                  IP and ARP on HIPPI               October 1992


   All ARP requests shall be sent with the I-field bit 28 set to zero,
   i.e. requesting a 32 bit connection.

   Unless another convention is locally defined for ARP requests, the
   I-field Path Selection bits may be set to binary 01 or 11 (logical
   address mode), and Destination Address field set to the HIPPI-SC
   address reserved for traffic conventionally directed to the IEEE
   802.1[12] broadcast address (which HIPPI-SC defines as FE0, hex).

   Reply packets shall be sent with I-field Path Selection and Routing
   Control fields set according to the Source_Address_Type and
   Source_Switch_Address fields in the request.

   In the HIPPI-LE header of ARP/RARP requests and replies the following
   fields shall be set:

   Double-Wide should be 1 if the HIPPI Destination at the sending node
   can accept 64 bit HIPPI connections.

   Message_Type shall contain an address resolution type code as defined
   in HIPPI-LE.  It shall be set appropriately to the value of the ARP
   operation code (ar$op) in piggybacked ARP messages:

         +-----------------------+-----------------------+
         |       ARP ar$op       | HIPPI-LE Message_Type |
         +=======================+=======================+
         |ARP Request (1)        |AR_Request (1)         |
         |ARP Reply (2)          |AR_Response (2)        |
         +-----------------------+-----------------------+
         |Reverse ARP Request (3)|AR_Request (1)         |
         |Reverse ARP Reply (4)  |AR_Response (2)        |
         +-----------------------+-----------------------+

   There is no ARP message corresponding to HIPPI-LE self address
   discovery; these packets are sent without ULP data.

   Destination_Switch_Address in requests shall be the Switch Address of
   the target node if known, otherwise zero.  In replies it shall be the
   requesting node's Switch Address

   Destination_Address_Type shall be 1 if the Destination_Switch_Address
   is a source route, 2 if it is a 12 bit address.

   Source_Address_Type shall be 1 if the Source_Switch_Address is a
   source route, 2 if it is a 12 bit address.

   Source_Switch_Address in requests shall be the Switch Address of the
   requesting node if known, otherwise zero.  In replies it shall be the



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RFC 1374                  IP and ARP on HIPPI               October 1992


   target node's Switch Address.

   Destination_IEEE_Address shall be the same as the ar$tha field in the
   ARP message.

   Source_IEEE_Address shall be the same as the ar$sha field in the ARP
   message.

      31    28        23  21          15        10     7         2   0
      +-----+---------+-+-+-----------+---------+-----+---------+-----+
    0 |      04       |1|0|         000         |      03       |  0  |
      +---------------+-+-+---------------------+---------------+-----+
    1 |                              36                               |
      +-----+-+-------+-----------------------+-----------------------+
    2 |[LA] |W|   2   |          000          |Requester's Switch Addr|
      +-----+-+-------+-----------------------+-----------------------+
    3 |   2   |   2   |          000          | Target Switch Address |
      +---------------+---------------+-------+-----------------------+
    4 |             00 00             |                               |
      +-------------------------------+                               |
    5 |                        Requester's ULA                        |
      +-------------------------------+-------------------------------+
    6 |             [LA]              |                               |
      +-------------------------------+                               |
    7 |                           Target ULA                          |
      +===============+===============+===============+===============+
    8 |       AA      |      AA       |       03      |       00      |
      +---------------+---------------+---------------+---------------+
    9 |       00      |      00       |        EtherType (2054)       |
      +---------------+---------------+-------------------------------+
   10 |            hrd (1)            |           pro (2048)          |
      +---------------+---------------+-------------------------------+
   11 |    hln (6)    |    pln (4)    |            op (2)             |
      +---------------+---------------+-------------------------------+
   12 |                    Target ULA octets 0 - 3                    |
      +-------------------------------+-------------------------------+
   13 |    Target ULA octets 4 - 5    |    Target IP Address upper    |
      +-------------------------------+-------------------------------+
   14 |    Target IP Address lower    | Requester's ULA octets 0 - 1  |
      +-------------------------------+-------------------------------+
   15 |                 Requester's ULA octets 2 - 5                  |
      +---------------------------------------------------------------+
   16 |                    Requester's IP Address                     |
      +---------------------------------------------------------------+

                     HIPPI ARP/RARP Reply (logical address mode)





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RFC 1374                  IP and ARP on HIPPI               October 1992


ARP procedure

   The combined HIPPI-LE/ARP packet contains six addresses, three each
   for the requester and the target:

      Requester's IP Address          (ARP)
      Requester's ULA                 (ARP and HIPPI-LE)
      Requester's Switch Address      (HIPPI-LE)

      Target's IP Address             (ARP)
      Target's ULA                    (ARP and HIPPI-LE)
      Target's Switch Address         (HIPPI-LE)

   Internet ARP concerns the IP Address and ULA; HIPPI-LE address
   resolution concerns the ULA and Switch Address.  Thus the ULA appears
   in both parts of the packet.

   Successful ARP results in tables in each node that map remote nodes'
   IP addresses to ULAs and ULAs to Switch Addresses, so that when an
   application requests a connection to a remote node by its IP address,
   both the remote ULA and Switch Address can be determined, a correct
   HIPPI-LE header can be built, and a connection to the node can be
   established using the correct Switch Address in the I-field.  Any
   recipient of an ARP request or reply may use information in the
   packet to augment its tables, even if it is neither the target node
   nor the requester.

   Note that the use of ULAs with HIPPI is not required.  In both the
   HIPPI-LE header and the Internet ARP message, the fields that contain
   ULAs should be set to zero when the ULA is not known.  Address
   resolution consists of two separate protocols, HIPPI-LE address
   resolution and Internet ARP, neither of which can function
   independently without ULAs.  However HIPPI Switch Address resolution
   can work without ULAs if the two protocols are piggybacked and
   treated as one operation in which Internet addresses are mapped
   directly to switch addresses.  With the exception of the optional
   self-address resolution request, which has no analogous Internet
   protocol, HIPPI-LE address resolution and Internet ARP messages
   should be sent together as a single HIPPI packet.

   If ULAs are used, the HIPPI-LE address resolution request can be sent
   without a piggybacked 802.2 LLC PDU, so it is possible to map ULAs to
   HIPPI Switch Addresses without using ARP.  Nodes shall accept both
   piggybacked and non-piggybacked forms of HIPPI-LE address resolution
   messages.

   The recipient of an address resolution request, having first updated
   its address mapping tables with any new information it can find in



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RFC 1374                  IP and ARP on HIPPI               October 1992


   the request, checks to see if it is the target node.  If it is, it
   generates a reply by filling in the unknown target address fields
   according to the HIPPI-LE message type and the ARP operation code,
   and swapping the four pairs of source/target address fields.  Then it
   connects to the requesting node with the Source Switch Address from
   the request, and sends the reply packet.

   A node is the target of an address resolution request if the request
   contains one of the following:

   1.  the node's ULA in the Destination_IEEE_Address field of a HIPPI-
       LE AR_Request message

   2.  the node's IP address in the target protocol address field
       (ar$tpa) of a piggybacked Internet ARP message

   If two target fields are known but are not mapped together in the
   recipient's address mapping tables, it may do one of three things:

   1.  treat the request as having two targets, and send correct replies
       for both to the requester.

   2.  assume its own tables are invalid and ignore the request.

   3.  assume one of the "known" target fields is correct and respond as
       if the other had been unknown.

   The best choice depends on which fields conflict and the nature of
   the implementation.  Choice 3 is probably best for ordinary nodes,
   but third party ARP agents may have reason to use one of the other
   two.  Future experience may shed light on this.

ARP Implementation Methods