





Network Working Group                                         J. Renwick
Request for Comments: 2067                                 NetStar, Inc.
Category: Standards Track                                   January 1997
Obsoletes: 1374


                             IP over HIPPI

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

   ANSI Standard X3.218-1993 (HIPPI-LE[3]) defines the encapsulation of
   IEEE 802.2 LLC PDUs and, by implication, IP on HIPPI.  ANSI X3.222-
   1993 (HIPPI-SC[4]) describes the operation of HIPPI physical
   switches.  The ANSI committee responsible for these standards chose
   to leave HIPPI networking issues largely outside the scope of their
   standards; this document describes the use of HIPPI switches as IP
   local area networks.

   This memo is a revision of RFC 1374, "IP and ARP on HIPPI", and is
   intended to replace it in the Standards Track.  RFC 1374 has been a
   Proposed Standard since November, 1992, with at least 10
   implementations of IP encapsulation and HIPPI switch discipline.  No
   major changes to it are required.  However, the ARP part of RFC 1374
   has not had sufficient implementation experience to be advanced to
   Draft Standard.  The present document contains all of RFC 1374 except
   for the description ARP, which has been moved into a separate
   document.

TABLE OF CONTENTS

   1  Introduction.............................................  2
   2  Scope....................................................  3
      2.1   Changes from RFC 1374..............................  3
      2.2   Terminology........................................  4
   3  Definitions..............................................  4
   4  Equipment................................................  5
   5  Protocol ................................................  7
      5.1   Packet Format......................................  7
      5.2   48 bit Universal LAN MAC addresses................. 11
      5.3   I-Field Format..................................... 12



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      5.4   Rules For Connections.............................. 13
      5.5   MTU................................................ 15
   6  Camp-on ................................................. 16
   7  Path MTU Discovery....................................... 17
   8  Channel Data Rate Discovery.............................. 17
   9  Performance.............................................. 18
   10 Sharing the Switch....................................... 20
   11 References............................................... 21
   12 Security Considerations.................................. 21
   13 Author's Address......................................... 21
   14 Appendix A -- HIPPI Basics............................... 22
   15 Appendix B -- How to Build a Practical HIPPI LAN......... 27

1  Introduction

   The ANSI High-Performance Parallel Interface (HIPPI) is a simplex
   data channel.  Configured in pairs, HIPPI can send and receive data
   simultaneously at nearly 800 megabits per second.  (HIPPI has an
   equally applicable 1600 megabit/second option.) Between 1987 and
   1991, the ANSI X3T9.3 HIPPI working group drafted four documents that
   bear on the use of HIPPI as a network interface.  They cover the
   physical and electrical specification (HIPPI-PH [1]), the framing of
   a stream of bytes (HIPPI-FP [2]), encapsulation of IEEE 802.2 LLC
   (HIPPI-LE [3]), and the behavior of a standard physical layer switch
   (HIPPI-SC [4]).  HIPPI-LE also implies the encapsulation of Internet
   Protocol[5].  The reader should be familiar with the ANSI HIPPI
   documents, copies of which are archived at the site "ftp.network.com"
   in the directory "hippi", and may be obtained via anonymous FTP.

   HIPPI switches can be used to connect a variety of computers and
   peripheral equipment for many purposes, but the working group stopped
   short of describing their use as Local Area Networks.  This memo
   takes up where the working group left off, using the guiding
   principle that except for length and hardware header, Internet
   datagrams sent on HIPPI should be identical to the same datagrams
   sent on a conventional network, and that any datagram sent on a
   conventional 802 network[6] should be valid on HIPPI.














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2  Scope

   This memo describes the HIPPI interface between a host and a
   crosspoint switch that complies with the HIPPI-SC draft standard.
   Issues that have no impact on host implementations are outside the
   scope of this memo.  Host implementations that comply with this memo
   are believed to be interoperable on a network composed of a single
   HIPPI-SC switch.  They are also interoperable on a simple point-to-
   point, two-way HIPPI connection with no switch between them.  They
   may be interoperable on more complex networks as well, depending on
   the internals of the switches and how they are interconnected;
   however, these details are implementation dependent and outside the
   scope of this memo.

   Within the scope of this memo are:

      1.  Packet format and header contents, including HIPPI-FP, HIPPI-
      LE, IEEE 802.2 LLC[7] and SNAP.

      2.  I-Field contents

      3.  Rules for the use of connections.

   Outside of the scope are

      1.  Address Resolution (ARP)

      2.  Network configuration and management

      3.  Host internal optimizations

      4.  The interface between a host and an outboard protocol
      processor.

2.1  Changes from RFC 1374

   RFC 1374 described the use of ARP on HIPPI, but because of
   insufficient implementation experience, the description of ARP has
   been separated from IP encapsulation and moved to an Informational
   memo.  It may be returned to the standards track in the future if
   interest and implementations warrant it.










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   RFC 1374's specification of IP over HIPPI has been changed in this
   document.  Certain packet format options, permitted in RFC 1374, are
   no longer allowed:

           1.  Optional short burst first;

           2.  D1 fill bytes;

           3.  Nonzero D2 offset.

   That is, the header format is no longer variable and is required to
   be that which is recommended by RFC 1374.

   With these changes, it is possible to send packets which conform to
   the ANSI standards but not to this memo.  Because there are no RFC
   1374 implementations in use that used these options, we believe that
   all existing RFC 1374 implementations are compliant with the
   requirements of this memo, and there should be no interoperability
   problems associated with these changes.

2.2  Terminology

   In this document the use of the word SHALL in capital letters
   indicates mandatory points of compliance.

3  Definitions

   Conventional

      Used with respect to networks, this refers to Ethernet, FDDI and
      802 LAN types, as distinct from HIPPI-SC LANs.

   Destination

      The HIPPI implementation that receives data from a HIPPI Source.

   Node

      An entity consisting of one HIPPI Source/Destination pair that is
      connected by parallel or serial HIPPI to a HIPPI-SC switch and
      that transmits and receives IP datagrams.  A node may be an
      Internet host, bridge, router or gateway.  This memo uses the term
      node in place of the usual "host" to indicate that a host might be
      connected to the HIPPI LAN not directly, but through an external
      adaptor that does some of the protocol processing for the host.






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   Serial HIPPI

      An implementation of HIPPI in serial fashion on coaxial cable or
      optical fiber, informally standardized by implementor's agreement
      in the Spring of 1991.

   Switch Address

      A value used as the address of a node on a HIPPI-SC network.  It
      is transmitted in the I-field.  HIPPI-SC switches may map Switch
      Addresses to physical port numbers.

   Source

      The HIPPI implementation that generates data to send to a HIPPI
      Destination.

   Universal LAN Address (ULA)

      A 48 bit globally unique address, administered by the IEEE,
      assigned to each node on an Ethernet, FDDI, 802 network or HIPPI-
      SC LAN.

4  Equipment

   A HIPPI network can be composed of nodes with HIPPI interfaces, HIPPI
   cables or serial links, HIPPI-SC switches, gateways to other
   networks.

   Each HIPPI interconnection between a node and a switch SHALL consist
   of a pair of HIPPI links, one in each direction.

   If a link between a node and the switch is capable of the 1600
   Megabit/second data rate option (i.e. Cable B installed for 64 bit
   wide operation) in either direction, the node's HIPPI-PH
   implementation SHALL also be capable of 32 bit operation (Cable B
   data suppressed) and SHALL be able to select or deselect the 1600Mb/s
   data rate option at the establishment of each new connection.













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   The following figure shows a sample HIPPI switch configuration.

                                                      +-----+
                                                      | H 4 |
      |                                               +--+--+
      |                   +----+    +----+    +----+     |
      |                   | H1 |    | H2 |    | H3 |   +-++
      |   +--+            +-++-+    +-++-+    +-++-+   |PP|
      +---+H5|              ||        ||        ||     ++++
      |   +--+              ||        ||        ||      ||
      |                 +---++--------++--------++------++----+
      |                 |                                     |
      |   +----+        |              HIPPI-SC               |
      +---+ G1 +--------+                                     |
      |   |    +--------+               Switch                |
      |   +----+        |                                     |
      |                 +---++--------++--------++------++----+
      |   +--+              ||        ||        ||      ||
      +---+H6|              ||                         ++++
      |   +--+            +-++-+                       |PP|
      |                   |    |                       +-++
      |                   | G2 |                         |
      |                   |    |                      +--+--+
      |                   +--+-+                      | H 7 |
      |                      |                        +-----+
                             |
           -----+------------+-------+-----------+-------------+------
                |                    |           |             |
                |                    |           |             |
             +--+--+              +--+--+     +--+--+       +--+--+
             | H 8 |              | H 9 |     | H10 |       | H11 |
             +-----+              +-----+     +-----+       +-----+

      Legend:  ---+---+---+--  =  802 network, Ethernet or FDDI
                           ||  =  Paired HIPPI link
                            H  =  Host computer
                           PP  =  Outboard Protocol Processor
                            G  =  Gateway

                       A possible HIPPI configuration











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   A single HIPPI-SC switch has a "non-blocking" characteristic, which
   means there is always a path available from any Source to any
   Destination.  If the network consists of more than one switch, the
   path from a Source to a Destination may include a HIPPI link between
   switches.  If this link is used by more than one Source/Destination
   pair, a "blocking" network is created: one Source may be blocked from
   access to a Destination because another Source is using the link it
   shares.  Strategies for establishing connections may be more
   complicated on blocking networks than on non-blocking ones.

   This memo does not take blocking issues into account, assuming that
   the HIPPI LAN consists of one HIPPI-SC switch or, if the network is
   more complex than that, it presents no additional problems that a
   node must be aware of.

5  Protocol

5.1  Packet Format

   The HIPPI packet format for Internet datagrams SHALL conform to the
   HIPPI-FP and HIPPI-LE draft standards, with further restrictions as
   imposed by this memo.  Because this memo is more restrictive than the
   ANSI standards, it is possible to send encapsulated IP datagrams that
   conform to the ANSI standards, but are illegal according to this
   memo.  Destinations may either accept or ignore such datagrams.

   To summarize the additional restrictions on ANSI standards found
   here:

           Any short burst must be the last burst of the packet.
           Leading short bursts are not permitted.

           Nonzero values for the HIPPI-FP D2_Offset field are not
           permitted.

           The D1_AreaSize SHALL be 3 (64-bit words).  No D1 Fill is
           permitted.

   Note: Although this document is for IP over HIPPI, the encapsulation
   described below accommodates ARP as well.

   The HIPPI-FP D1_Area SHALL contain the HIPPI-LE header.  The HIPPI-FP
   D2_Area, when present, SHALL contain one IEEE 802.2 Type 1 LLC
   Unnumbered Information (UI) PDU.  Support of IEEE 802.2 XID, TEST and
   Type 2 PDUs is not required on HIPPI, and Destinations that receive
   these PDUs may either ignore them or respond correctly according to
   IEEE 802.2 requirements.




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

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

                          HIPPI Packet Structure

        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) SHALL be sent as 3.

        D2_Offset (3 bits) SHALL be zero.

        D2_Size (32 bits) Shall contain the number of bytes in the
        IEEE 802.2 LLC Type 1 PDU, or zero if no PDU is present.  It
        SHALL NOT exceed 65,288.  This value includes the IEEE 802.2
        LLC/SNAP header and the IP datagram.  It does not include
        trailing fill bytes.  (See "MTU", below.)

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 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)





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   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)

   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 byte of the
   HIPPI-FP D2_Area.

   SSAP (8 bits) SHALL contain 170 ('AA'h).

   DSAP (8 bits) SHALL contain 170 ('AA'h).

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

SNAP

   Organization Code (24 bits) SHALL be zero.




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   EtherType (16 bits) SHALL be set as defined in Assigned Numbers [8]:
   IP = 2048 ('0800'h), ARP = 2054 ('0806'h), RARP = 32,821 ('8035'h).

      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 byte 0 |Message byte 1 |Message byte 2 | . . .         |
      +---------------+---------------+---------------+---            |
      |                            .  .  .
                                                                      |
      |        -------+---------------+---------------+---------------+
      |         . . . |  byte (n-2)   |  byte (n-1)   |     FILL      |
      +---------------+---------------+---------------+---------------+
   N-1|      FILL     |     FILL      |     FILL      |     FILL      |
      +---------------+---------------+---------------+---------------+

















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                            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 message)
              (n) is the number of bytes in the IP message.
              [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] bytes complete the HIPPI packet to an even
              number of 32 bit words.  The number of fill bytes
              is not counted in the data length.

IEEE 802.2 Data

   The IEEE 802.2 Data SHALL begin in the byte following the EtherType
   field.  Fill bytes SHALL be used following the Data as necessary to
   make the number of bytes 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 bytes 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 bytes on Cable B come logically before
   those on Cable A.  Within each byte, the most significant bit is the
   highest numbered signal.

5.2  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.












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   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 byte 0 |L|G|  ULA byte  1  |
     +---------------+---------------+---------------+---------------+
     |  ULA byte  2  |  ULA byte  3  |  ULA byte  4  |  ULA byte  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 may be 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.

5.3  I-Field format

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

         Locally Administered (bit 31) SHALL be zero.

         Reserved (bits 30, 29) should be zero.  Destinations SHALL
         accept any value for these bits.

         Double wide (bit 28) SHALL be set when Source Cable B is
         connected and the Source wants a 64 bit connection.  It SHALL
         be zero otherwise.

         Direction (bit 27) should be sent as zero, however
         Destinations SHALL accept either zero or one and interpret
         the Routing Control field accordingly, per HIPPI-SC.

         Path Selection (bits 26, 25) SHALL be 00, 01, or 11 (binary)
         at the Source's option.  00 (source route mode) indicates
         that the I-field bits 23-00 contain a 24 bit source route; 01
         or 11 (logical address mode) indicate that bits 23-00 contain
         12 bit Source and Destination Addresses.  The value 11 is



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         meaningful when more than one route exists from a Source to a
         Destination; it allows the switch to choose the route.  Use
         of 01 forces the switch always to use the same route for the
         same Source/Destination pair.

         Camp-on (bit 24) may be 1 or 0; however, a Source SHALL NOT
         make consecutive requests without Camp-on to the same
         Destination while the requests are being rejected.  The
         purpose of this restriction is to prevent a node from
         circumventing the fair share arbitration mechanism of the
         switch by repeating requests at a very high rate.

         If logical address mode is used:

            Source Address (bits 23-12) is not used.

            Destination Address (bits 11-0) SHALL contain the Switch
            Address of the Destination.

        If source route mode is used:

            Routing control (bits 23-00) SHALL contain the route to
            the Destination.

5.4  Rules For Connections

   The following rules for connection management by Source and
   Destination are intended to insure frequent, fair share access to
   Destinations for which multiple Sources are contending.  If possible,
   nodes should transfer data at full HIPPI speeds and hold connections
   no longer than necessary.

   A source may hold a connection for as long as it takes to send 68
   HIPPI bursts at what ever speed the two connected nodes can achieve
   together.  The number of packets sent in one connection is not
   limited, except that the number of bursts over all the packets should
   not exceed 68.  This is not a recommendation to send as many packets
   as possible per connection; one packet per connection is acceptable.
   The purpose of this limit is to give each Source an fair share of a
   common Destination's bandwidth.  Without a limit, if there is a
   Destination that is constantly in demand by multiple Sources, the
   Source that sends the most data per connection wins the greatest
   share of bandwidth.

   The limit of 68 bursts is not absolute.  An implementation may check
   the burst count after transmission of a packet and end the connection
   if it is greater than or equal to some threshold.  If this is done,
   the threshold should be less than 68 depending on the typical packet



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   size, to ensure that the 68 burst limit is not normally exceeded.
   For instance, a Source sending 64K packets would send two per
   connection (130 bursts) if it checked for 68 at the end of each
   packet.  In this situation the Source is required to check for a
   value small enough that it will not send a second packet in the same
   connection.

   Destinations SHALL accept all packets that arrive during a
   connection, and may discard those that exceed its buffering capacity.
   A Destination SHALL NOT abort a connection (deassert CONNECT) simply
   because too many bursts were received; however a Destination may
   abort a connection whose duration has exceeded a time period of the
   Destination's choosing, as long as the Source is allowed ample time
   to transmit its quota of bursts.

   The rules admonish the node to do certain things as fast as it can,
   however there is no absolute measure of compliance.  Nodes that
   cannot transfer data at full HIPPI speeds can still interoperate but
   the faster the implementation, the better the performance of the
   network will be.

   Assuming that bursts flow at the maximum rate, the most important
   factor in network throughput is the connection switching time,
   measured from the deassertion of REQUEST by the Source at the end of
   one connection to its first assertion of BURST after the
   establishment of the new connection.

   Implementations should keep this time as short as possible.  For a
   guideline, assuming parallel HIPPI and a single HIPPI-SC switch, ten
   microseconds permits nearly full HIPPI throughput with full-sized
   packets, and at 60 microseconds the available throughput is reduced
   by about 10%.  (See "Performance", below.)

   All HIPPI electrical signaling SHALL comply with HIPPI-PH.  In every
   case, the following rules go beyond what HIPPI-PH requires.

   Rules for the Source

   1.  Do not assert REQUEST until a packet is ready to send.

   2.  Transmit bursts as quickly as READYs permit.  Except for
       the required HIPPI Source Wait states, there should be no
       delay in the assertion of BURST whenever the Source's READY
       counter is nonzero.

   3.  Make a best effort to ensure that connection durations do
       not exceed 68 bursts.




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   4.  Deassert REQUEST immediately when no packet is available
       for immediate transmission or the last packet of the
       connection has been sent.

   Rules for the Destination

   1.   Reject all connections if unable to receive packets.
        This frees the requesting Source to connect to other
        Destinations with a minimum of delay.  Inability to receive
        packets is not a transient condition, but is the state of the
        Destination when its network interface is not initialized.

   2.  A HIPPI node should be prepared to efficiently accept
       connections and process incoming data packets.  While this
       may be best achieved by not asserting connect unless 68
       bursts worth of buffers is available, it may be possible to
       meet this requirement with fewer buffers.  This may be due to
       a priori agreement between nodes on packet sizes, the speed
       of the interface to move buffers, or other implementation
       dependent considerations.

   3.  Accept a connection immediately when buffers are
       available.  The Destination should never delay the acceptance
       of a connection unnecessarily.

   4.  Once initialized, a Destination may reject connection
       requests only for one of the following reasons:

     1.  The I-field was received with incorrect parity.

     2.  The I-field contents are invalid, e.g. the "W" bit set when the
         Destination does not support the 1600 megabit data rate option,
         the "Locally Administered" bit is set, the Source is not
         permitted to send to this Destination, etc.

     Transient conditions within the Destination, such as temporary
     buffer shortages, must never cause rejected connections.

   5.  Ignore aborted connection sequences.  Sources may time
       out and abandon attempts to connect; therefore aborted
       connection sequences are normal events.

5.5  MTU

   Maximum Transmission Unit (MTU) is defined as the length of the IP
   packet, including IP header, but not including any overhead below IP.
   Conventional LANs have MTU sizes determined by physical layer
   specification.  MTUs may be required simply because the chosen medium



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   won't work with larger packets, or they may serve to limit the amount
   of time a node must wait for an opportunity to send a packet.

   HIPPI has no inherent limit on packet size.  The HIPPI-FP header
   contains a 32 bit D2_Size field that, while it may limit packets to
   about 4 gigabytes, imposes no practical limit for networking
   purposes.  Even so, a HIPPI-SC switch used as a LAN needs an MTU so
   that Destination buffer sizes can be determined.

   The MTU for HIPPI-SC LANs is 65280 bytes.

   This value was selected because it allows the IP packet to fit in one
   64K byte buffer with up to 256 bytes of overhead.  The overhead is 40
   bytes at the present time; there are 216 bytes of room for expansion.

         HIPPI-FP Header                  8 bytes
         HIPPI-LE Header                 24 bytes
         IEEE 802.2 LLC/SNAP Headers      8 bytes
         Maximum IP packet size (MTU) 65280 bytes
                                      ------------
                           Total      65320 bytes (64K - 216)

6  Camp-on

   When several Sources contend for a single Destination, the Camp-on
   feature allows the HIPPI-SC switch to arbitrate and ensure that all
   Sources have fair access.  (HIPPI-SC does not specify the method of
   arbitration.)  Without Camp-on, the contending Sources would simply
   have to retry the connection repeatedly until it was accepted, and
   the fastest Source would usually win.  To guarantee fair share
   arbitration, Sources are prohibited from making repeated requests to
   the same Destination without Camp-on in such a way as to defeat the
   arbitration.

   There is another important reason to use Camp-on: when a connection
   without Camp-on is rejected, the Source cannot determine whether the
   rejection came from the requested Destination or from the switch.
   The Source also cannot tell the reason for the rejection, which could
   be either that the Destination was off line or not cabled, or the I-
   field was erroneous or had incorrect parity.  Sources should not
   treat a rejection of a request without Camp-on as an error.  Camp-on
   prevents rejection due to the temporary busy case; with one
   exception, rejection of a Camp-on request indicates an error
   condition, and an error event can be recorded.  The exception occurs
   when a 64 bit connection is attempted to a Destination that does not
   have Cable B connected, resulting in a reject.  This case is covered
   in "Channel Data Rate Discovery", below.




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7  Path MTU Discovery

   RFC 1191 [9] describes the method of determining MTU restrictions on
   an arbitrary network path between two hosts.  HIPPI nodes may use
   this method without modification to discover restrictions on paths
   between HIPPI-SC LANs and other networks.  Gateways between HIPPI-SC
   LANs and other types of networks should implement RFC 1191.

8  Channel Data Rate Discovery

   HIPPI exists in two data rate options (800 megabit/second and 1600
   megabit/second).  The higher data rate is achieved by making the
   HIPPI 64 bits parallel instead of 32, using an extra cable containing
   32 additional data bits and four parity bits.  HIPPI-SC switches can
   be designed to attach to both.  Source and Destination HIPPI
   implementations can be designed to operate at either rate, selectable
   at the time a connection is established.  The "W" bit (bit 28) of the
   I-field controls the width of the connection through the switch.
   Sources with both cables A and B attached to the switch may set the
   "W" bit to request a 1600 megabit/second connection.  If the
   requested destination also has both cables attached, the switch can
   connect Source to Destination on both cables.  If the requested
   Destination has only Cable A, the switch rejects the request.
   Sixty-four bit Sources can connect to 32 bit Destinations by
   requesting with the "W" bit clear and not using Cable B.  Sixty-four
   bit Destinations must examine the "W" bit in the received I-field and
   use or ignore Cable B accordingly.  Note that both INTERCONNECT
   signals stay active while a 64 bit HIPPI is used in 32 bit mode.

   The following table summarizes the possible combinations, the
   switch's action for each, and the width of the resulting connection.

                                     Destination
                      +-------------------+-------------------+
                      |        32         |        64         |
           +----+-----+-------------------+-------------------+
           |    | W=0 |     Accept 32     |     Accept 32     |
           | 32 +-----+-------------------+-------------------+
           |    | W=1 |        N/A        |        N/A        |
   Source  +----+-----+-------------------+-------------------+
           |    | W=0 |     Accept 32     |     Accept 32     |
           | 64 +-----+-------------------+-------------------+
           |    | W=1 |      Reject       |     Accept 64     |
           +----+-----+-------------------+-------------------+







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HIPPI Connection Combinations

   If the path between a 64 bit Source and a 64 bit Destination includes
   more than one switch, and the route between switches uses a link that
   is only 32 bits wide, the switch rejects 64 bit connection requests
   as if the Destination did not have 64 bit capability.

   In a mixed LAN of 32 bit and 64 bit HIPPIs, a 64 bit Source needs to
   know the data rates available at each Destination and on the path to
   it.  This can be known a priori by manual configuration, or it can be
   discovered dynamically.  The only reliable method of discovery is
   simply to attempt a 64 bit connection with Camp-on.  As long as 64
   bit connections succeed, the Source knows the Destination and path
   are double width.  If a 64 bit connection is rejected, the Source
   tries to connect for 32 bits.  If the 32 bit connection succeeds, the
   Source assumes that the Destination or path is not capable of double
   width operation, and uses only 32 bit requests after that.  If the 32
   bit request is rejected, the Source assumes that the Destination or
   path is down and makes no determination of its capability.

   The Double_Wide bit in the HIPPI-LE header, if nonzero, gives the
   node that receives it a hint that the 64 bit connection attempt may
   be worthwhile when sending on the return path.

   Note that Camp-on must be used at least in the 64 bit attempt,
   because it removes some ambiguity from the meaning of rejects.  If
   the request is made with the "W" bit and no Camp-on, a reject could
   mean either that the Destination has no Cable B or that it is simply
   busy, and no conclusion can be drawn as to its status for 64 bit
   connections.

9  Performance

   The HIPPI connection rules are designed to permit best utilization of
   the available HIPPI throughput under the constraint that each
   Destination must be made available frequently to receive packets from
   different Sources.  This discipline asks both Sources and
   Destinations to minimize connection setup overhead to deliver high
   performance.  Low connection setup times are easily achieved by
   hardware implementations, but overhead may be too high if software is
   required to execute between the initial request of a connection and
   the beginning of data transfer.  Hardware implementations in which
   connection setup and data transfer proceed from a single software
   action are very desirable.

   HIPPI connections are controlled by HIPPI Sources; a Destination,
   being unable to initiate a disconnect without the possibility of data
   loss, is a slave to the Source once it has accepted a connection.



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   Optimizations of connection strategy are therefore the province of
   the HIPPI Source, and several optimizations are permitted.

   If the rate of available message traffic is less than the available
   HIPPI throughput and Destinations are seldom busy when a connection
   is requested, connection optimizations do not pay off and the
   simplest strategy of waiting indefinitely for each connection to be
   made and sending messages strictly in the order queued cannot be
   improved upon.  However if some nodes are slow, or network
   applications can send or receive messages at a higher aggregate rate
   than the available HIPPI bandwidth, Sources may frequently encounter
   a busy Destination.  In these cases, certain host output queuing
   strategies may enhance channel utilization.  Sources may maintain
   separate output queues for different HIPPI Destinations, and abandon
   one Destination in favor of another if a connection attempt without
   Camp-on is rejected or a connection request with Camp-on is not
   accepted within a predetermined interval.  Such a strategy results in
   aborted connection sequences (defined in HIPPI-PH:  REQUEST is
   deasserted before any data is sent).  Destinations must treat these
   as normal events, perhaps counting them but otherwise ignoring them.

   Two components of connection setup time are out of the control of
   both Source and Destination.  One is the time required for the switch
   to connect Source to Destination, currently less than four
   microseconds in the largest commercially available (32 port) switch.
   The second component is the round trip propagation time of the
   REQUEST and CONNECT signals, negligible on a standard 25 meter copper
   HIPPI cable, but contributing a total of about 10 microseconds per
   kilometer on fiber optic links.  HIPPI-SC LANs spanning more than a
   few kilometers will have reduced throughput.  Limited span networks
   with buffered gateways or bridges between them may perform better
   than long serial HIPPI links.

   A Source is required to drop its connection after the transmission of
   68 HIPPI bursts.  This number was chosen to allow the transmission of
   one maximum sized packet or a reasonable number of smaller sized
   packets.  The following table lists some possibilities, with
   calculated maximum burst and throughput rates in millions (10**6) of
   bytes per second:












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                     Maximum HIPPI Throughput Rates

        Number  Number  Hold  Burst  ------Max throughput MB/sec-------
   User   of      of    Time  Rate    Connection Setup Overhead (usec)
   Data Packets Bursts (usec) MB/sec  10    30    60    90   120   150
   ---- ------- ------ ------ ------ ----  ----  ----  ----  ----  ----
   63K     1      64    654    98.7  97.2  94.4  90.4  86.8  83.4  80.3
   32K     2      66    665    98.6  97.1  94.3  90.4  86.8  83.5  80.4
   16K     4      68    667    98.3  96.8  94.1  90.2  86.6  83.3  80.2
    8K     7      63    587    97.8  96.1  93.0  88.7  84.8  81.2  77.8
    4K    13      65    551    96.7  95.0  91.7  87.2  83.1  79.4  76.0
    2K    22      66    476    94.6  92.7  89.0  84.0  79.6  75.6  72.0
    1K    34      68    384    90.8  88.5  84.2  78.5  73.5  75.8  65.3

   These calculations are based 259 40 ns clock periods to transmit a
   full burst and 23 clock periods for a short burst.  (HIPPI-PH
   specifies three clock periods of overhead per burst.) A packet of "n"
   kilobytes of user data consists of "n" full bursts and one short
   burst equal in length to the number of bytes in the HIPPI, LLC, IP
   and TCP headers.  "Hold Time" is the minimum connection duration
   needed to send the packets.  "Burst Rate" is the effective transfer
   rate for the duration of the connection, not counting connection
   switching time.  Throughput rates are in megabytes/second, accounting
   for connection switching times of 10, 30, 60, 90, 120 and 150
   microseconds.  These calculations ignore any limit on the rate at
   which a Source or Destination can process small packets; such limits
   may further reduce the available throughput if small packets are
   used.

10 Sharing the Switch

   Network interconnection is only one potential application of HIPPI
   and HIPPI-SC switches.  While network applications need very frequent
   transient connections, other applications may favor longer term or
   even permanent connections between Source and Destination.  Since the
   switch can serve each Source or Destination with hardware paths
   totally separate from every other, it is quite feasible to use the
   same switch to support LAN interconnects and computer/peripheral
   applications simultaneously.

   Switch sharing is no problem when unlike applications do not share a
   HIPPI cable on any path.  However if a host must use a single input
   or output cable for network as well as other kinds of traffic, or if
   a link between switches must be shared, care must be taken to ensure
   that all applications are compatible with the connection discipline
   described in this memo.  Applications that hold connections too long
   on links shared with network traffic may cause loss of network
   packets or serious degradation of network service.



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11 References

   [1]  ANSI X3.183-1991, High-Performance Parallel Interface -
        Mechanical, Electrical and Signalling Protocol Specification
        (HIPPI-PH).

   [2]  ANSI X3.210-1992, High-Performance Parallel Interface - Framing
        Protocol (HIPPI-FP).

   [3]  ANSI X3.218-1993, High-Performance Parallel Interface -
        Encapsulation of IEEE 802.2 (IEEE Std 802.2) Logical Link
        Control Protocol Data Units (802.2 Link Encapsulation) (HIPPI-
        LE).

   [4]  ANSI X3.222-1993, High-Performance Parallel Interface - Physical
        Switch Control (HIPPI-SC).

   [5]  Postel, J., "Internet Protocol", STD 5, RFC 791, USC/Information
        Sciences Institute, September 1981.

   [6]  IEEE, "IEEE Standards for Local Area Networks: Logical Link
        Control", IEEE, New York, New York, 1985.

   [7]  IEEE, "IEEE Standards for Local Area Networks: Logical Link
        Control", IEEE, New York, New York, 1985.

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

   [9]  Mogul, J.C., and Deering, S.E., "Path MTU discovery", RFC 1191,
        Stanford University, November, 1990.

12 Security Considerations

   Security issues are not discussed in this memo.

13 Author's Address

   John K. Renwick
   NetStar, Inc.
   10250 Valley View Road
   Minneapolis, MN USA 55344

   Phone: (612) 996-6847
   EMail: jkr@NetStar.com

   Mailing List: hippi-ext@think.com




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14 Appendix A -- HIPPI Basics

   This section is included as an aid to readers who are not completely
   familiar with the HIPPI standards.

   HIPPI-PH describes a parallel copper data channel between a Source
   and a Destination.  HIPPI transmits data in one direction only, so
   that two sets are required for bidirectional flow.  The following
   figure shows a simple point-to-point link between two computer
   systems:

   +----------+                                        +----------+
   |          |                                        |          |
   |          +--------+                      +--------+          |
   |          | HIPPI  |        Cable         | HIPPI  |          |
   |          |        +--------------------->|        |          |
   |          | Source |                      | Dest.  |          |
   |  System  +--------+                      +--------+  System  |
   |    X     +--------+                      +--------+    Y     |
   |          | HIPPI  |        Cable         | HIPPI  |          |
   |          |        |<---------------------+        |          |
   |          | Dest.  |                      | Source |          |
   |          +--------+                      +--------+          |
   |          |                                        |          |
   +----------+                                        +----------+

A Simple HIPPI Duplex Link

   Parallel copper cables may be up to 25 meters in length.

   In this document, all HIPPI connections are assumed to be paired
   HIPPI channels.

   HIPPI-PH has a single optional feature: it can use a single cable in
   each direction for a 32 bit parallel channel with a maximum data rate
   of 800 megabit/second, or two cables for 64 bits and 1600
   megabit/second.  Cable A carries bits 0-31 and is used in both modes;
   Cable B carries bits 32-63 and is use only with the 1600
   megabit/second data rate option.












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HIPPI Signal Hierarchy

   HIPPI has the following hardware signals:

      Source to Destination

         INTERCONNECT A
         INTERCONNECT B (64 bit only)
         CLOCK (25 MHz)
         REQUEST
         PACKET
         BURST
         DATA (32 or 64 signals)
         PARITY (4 or 8 signals)

      Destination to Source

         INTERCONNECT A
         INTERCONNECT B (64 bit only)
         CONNECT
         READY

   The INTERCONNECT lines carry DC voltages that indicate that the cable
   is connected and that the remote interface has power.  INTERCONNECT
   is not used for signaling.

   The CLOCK signal is a continuous 25 MHz (40 ns period) square wave.
   All Source-to-Destination signals are synchronized to the clock.

   The REQUEST and CONNECT lines are used to establish logical
   connections.  A connection is always initiated by a Source as it
   asserts REQUEST.  At the same time it puts 32 bits of data on DATA
   lines 0-31, called the I-field.  The Destination samples the DATA
   lines and can complete a connection by asserting CONNECT.  Packets
   can be transmitted only while both REQUEST and CONNECT are asserted.

   A Destination can also reject a connection by asserting CONNECT for
   only a short interval between 4 and 16 HIPPI clock periods (160-640
   nanoseconds).  The Source knows a connection has been accepted when
   CONNECT is asserted for more than 16 clocks or it receives a READY
   pulse.

   Either Source or Destination can terminate a connection by
   deasserting REQUEST or CONNECT, respectively.  Normally connections
   are terminated by the Source after its last Packet has been sent.  A
   Destination cannot terminate a connection without potential loss of
   data.




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                  +------+-------------------------+------+
                  | Idle |        Connected        | Idle | . . .
                  +------+-------------------------+------+
                        /                           \
                       /                             \
                      /                               \
                     /                                 \
                    /                                   \
                   +-------+ +-------+ +-------+ +-------+
                   |I-field| |Packet | |Packet | |Packet |
                   +-------+ +-------+ +-------+ +-------+
                            /         \
                           /           \
                          /             \
                         /               \
                        /                 \
                       /                   \
                      /                     \
                     +-----+ +-----+   +-----+
                     |Burst| |Burst|...|Burst|
                     +-----+ +-----+   +-----+

                    HIPPI Logical Framing Hierarchy

   The Source asserts PACKET for the duration of a Packet transmission,
   deasserting it to indicate the end of a Packet.  A sequence of Bursts
   comprise a Packet.  To send a burst, a Source asserts the BURST
   signal for 256 clock periods, during which it places 256 words of
   data on the DATA lines.  The first or last Burst of a Packet may be
   less than 256 clock periods, allowing the transmission of any
   integral number of 32 or 64 bit words in a Packet.




















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   The READY signal is a pulse four or more clock periods long.  Each
   pulse signals the Source that the Destination can receive one Burst.
   The Destination need not wait for a burst before sending another
   READY if it has burst buffers available; up to 63 unanswered READYs
   may be sent, allowing HIPPI to operate at full speed over distances
   of many kilometers.  If a Source must wait for flow control, it
   inserts idle periods between Bursts.

                +------------------------------------------------+
      REQUEST---+                                                +----
                      +--------------------------------------------+
      CONNECT---------+                                            +--
                         +---------------------------------------+
      PACKET-------------+                                       +----

                       +-+   +-+   +-+   +-+   +-+   +-+   +-+   +-+
      READY------------+ +---+ +---+ +---+ +---+ +---+ +---+ +---+ +--

                         +-------+ +-------+ +-------+ +-----+
      BURST--------------+       +-+       +-+       +-+     +--------

      DATA------I-field----DATA------DATA------DATA-----DATA----------

                      HIPPI Signal Timing Diagram

Serial HIPPI

   There is no ANSI standard for HIPPI other than the parallel copper
   cable version.  However an implementors' agreement exists, specifying
   a serial protocol to extend HIPPI signals on optical fiber or coaxial
   copper cable.  Serial links may be used interchangeably with parallel
   links to overcome HIPPI distance limitations; they are transparent to
   the Source and Destination, except for the possibility of longer
   propagation delays.

















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I-Field and Switch Control

   The REQUEST, CONNECT and I-field features of HIPPI-PH were designed
   for the control of switches as described in HIPPI-SC.  A switch is a
   hub with a number of input and output HIPPI ports.  HIPPI Sources are
   cabled to switch input ports, and switch output ports are cabled to
   HIPPI Destinations.  When a HIPPI Source requests a connection, the
   switch interprets the I-field to select an output port and
   electrically connects the HIPPI Source to the HIPPI Destination on
   that port.  Once connected, the switch does not interact with the
   HIPPIs in any way until REQUEST or CONNECT is deasserted, at which
   time it breaks the physical connection and deasserts its output
   signals to both sides.  Some existing switch implementations can
   switch connections in less than one microsecond.  There is the
   potential for as many simultaneous connections, each transferring
   data at HIPPI speeds, as there are input or output ports on the
   switch.  A switch offers much greater total throughput capacity than
   broadcast or ring media.

      31    28  26    23                      11                     0
      +-+---+-+-+---+-+-----------------------+-----------------------+
      |L|   |W|D|PS |C|    Source Address     |  Destination Address  |
      +-+---+-+-+---+-+-----------------------+-----------------------+

                  HIPPI-SC I-field Format (Logical Address Mode)

           L  = Locally defined (1 => entire I-field is locally defined)
           W  = Width (1 => 64 bit connection)
           D  = Direction (1 => swap Source and Destination Address)
           PS = Path Selection (01 => Logical Address Mode)
           C  = Camp-on (1 => wait until Destination is free)

   HIPPI-SC defines I-field formats for two different addressing modes.
   The first, called Source Routing, encodes a string of port numbers in
   the lower 24 bits.  This string specifies a route over a number of
   switches.  A Destination's address may differ from one Source to
   another if multiple switches are used.

   The second format, called Logical Address Mode, defines two 12 bit
   fields, Source Address and Destination Address.  A Destination's 12
   bit Switch Address is the same for all Sources.  Switches commonly
   have address lookup tables to map 12 bit logical addresses to
   physical ports.  This mode is used for networking.








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Control fields in the I-field are:

   L  The "Locally Defined" bit, when set, indicates that the I-field
      is not in the standard format.  The meaning of bits 30-0 are
      locally defined.

   W  The Width bit, when set, requests a 64 bit connection through
      the switch.  It is meaningless if Cable B is not installed at
      the Source.  If W is set and either the Source or the requested
      Destination has no Cable B to the switch, the switch rejects
      the connection.  Otherwise the switch connects both Cable A and
      Cable B if W is set, or Cable A only if W is clear.  This
      feature is useful if both Source and Destination
      implementations can selectively disable or enable Cable B on
      each new connection.

   D  The Direction bit, when set, reverses the sense of the Source
      Address and Destination Address fields.  In other words, D=1
      means that the Source Address is in bits 0-11 and the
      Destination Address is in bits 12-23.  This bit was defined to
      give devices a simple way to route return messages.  It is not
      useful for LAN operations.

   PS The Path Selection field determines whether the I-field
      contains Source Route or Address information, and in Logical
      Address mode, whether the switch may select from multiple
      possible routes to the destination.  The value "01" selects
      Logical Address mode and fixed routes.

   C  The Camp-on bit requests the switch not to reject the
      connection if the selected Destination is busy (connected to
      another Source) but wait and make the connection when the
      Destination is free.

15 Appendix B -- How to Build a Practical HIPPI LAN

   "IP on HIPPI" describes the network host's view of a HIPPI local area
   network without providing much information on the architecture of the
   network itself.  Here we describe a network constructed from
   available HIPPI components, having the following characteristics:

   1.  A tree structure with a central HIPPI-SC compliant hub and
   optional satellite switches

   2.  Each satellite is connected to the hub by just one bidirectional
   HIPPI link.





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   3.  Serial HIPPI or transparent fiber optic HIPPI extender devices
   may be used in any link.

   4.  Some satellites may be a particular switch product which is not
   HIPPI-SC compliant.

   5.  Host systems are attached either directly to the hub or to
   satellites, by single bidirectional links in which both HIPPI cables
   go to the same numbered switch port.

Switch Address Management

   Switch addresses use a flat address space.  The 12-bit address is
   subdivided into 6 bits of switch number and 6 bits of port number.

   11                       5                     0
      +-----------------------+-----------------------+
      |     Switch Number     |      Port Number      |
      +-----------------------+-----------------------+

Logical Address Construction

   Switches may be numbered arbitrarily.  A given host's address
   consists of the number of the switch it is directly attached to and
   the physical port number on that switch to which its input channel is
   attached.

   In the singly-connected tree structure, there is exactly one path
   between any pair of hosts.  Since each satellite must be connected
   directly to the hub, the maximum length of this path is three hops,
   and the minimum length is one.  Each HIPPI-SC compliant switch is
   programmed to map each of the host switch addresses to the
   appropriate output port: either the port to which the host is
   directly attached or a port that is linked to another switch in the
   path to it.

Special Treatment of Nonstandard Switches

   There is one commercially available switch that was designed
   before the drafting of HIPPI-SC and is not fully compliant.  It is
   in common use, so it is worth making some special provisions to
   allow its use in a HIPPI LAN.  This switch supports only the
   Source Route mode of addressing with a four bit right shift that
   can be disabled by a hardware switch on each input port.
   Addresses cannot be mapped.  The switch does not support the "W",
   "D", or "PS" fields of the I-field; it ignores their contents.
   Use of this switch as a satellite will require a slight deviation
   from normal I-field usage by the hosts that are directly attached



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   to it.  Hosts attached to standard switches are not affected.

   For a destination connected to a non compliant satellite, the
   satellite uses only the least significant four bits of the I-field
   as the address.  Since the address contains the destination's
   physical port number in the least significant bits, its port will
   be selected.  Nonstandard switches should be set to disable I-
   field shifting at the input from the hub, so that the destination
   host will see its correct switch address in the I-field when
   performing self-address discovery.  I-field shifting must be
   enabled on the satellite for each input port to which a host is
   attached.

   Hosts attached to nonstandard satellites must deviate from the
   normal I-field usage when connecting to hosts on another switch.
   It is suggested that all host implementations have this capability
   as long as the nonstandard switches remain in use.  The host must
   know, by some manual configuration method, that it is connected to
   a nonstandard switch, and it must have its "link port" number;
   that is, the number of the port on the satellite that is connected
   to the hub.

   The normal I-field format for a 32-bit connection, per the
   document, is this:

   31        26    23                      11                     0
   +---------+---+-+-----------------------+-----------------------+
   |0 0 0 0 0|x 1|C|        Unused         |  Destination Address  |
   +---------+---+-+-----------------------+-----------------------+

   The special I-field format is:

   31        26  24                15                     4 3     0
   +---------+---+-+---------------+-----------------------+-------+
   |0 0 0 0 0|x 1|C|    Unused     |  Destination Address  | Link  |
   +---------+---+-+---------------+-----------------------+-------+

   This I-field is altered by shifting the lower 24 bits left by four
   and adding the link port number.  Camp-on is optional, and the PS
   field is set to 01 or 11 (the host's option) as if the switch
   supported logical address mode.  All other I-field bits are set to
   zero.  When the host requests a connection with this I-field, the
   switch selects a connection through the link port to the hub, and
   shifts the lower 24 bits of the I-field right by four bits.  The link
   port number is discarded and the I-field passed through to the hub is
   a proper HIPPI-SC I-field selecting logical address mode.





Renwick                     Standards Track                    [Page 29]

RFC 2067                     IP over HIPPI                  January 1997


   A host on a nonstandard satellite may use the special I-field format
   for all connection requests.  If connecting to another host on the
   same satellite, this will cause the connection to take an
   unnecessarily long path through the hub and back.  If an optimization
   is desired, the host can be given additional information to allow it
   to use the standard I-field format when connecting to another host on
   the same switch.  This information could consist of a list of the
   other hosts on the same switch, or the details of address formation,
   along with the switch number of the local satellite, which would
   allow the host to analyze the switch address to determine whether or
   not the destination is on the local switch.  This optimization is
   fairly complicated and may not always be worthwhile.







































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