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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.


 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


 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

Renwick Standards Track [Page 1] RFC 2067 IP over HIPPI January 1997

    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 ""
 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.

Renwick Standards Track [Page 2] RFC 2067 IP over HIPPI January 1997

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

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.

Renwick Standards Track [Page 3] RFC 2067 IP over HIPPI January 1997

 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

    Used with respect to networks, this refers to Ethernet, FDDI and
    802 LAN types, as distinct from HIPPI-SC LANs.
    The HIPPI implementation that receives data from a HIPPI Source.
    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.

Renwick Standards Track [Page 4] RFC 2067 IP over HIPPI January 1997

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

Renwick Standards Track [Page 5] RFC 2067 IP over HIPPI January 1997

 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

Renwick Standards Track [Page 6] RFC 2067 IP over HIPPI January 1997

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

Renwick Standards Track [Page 7] RFC 2067 IP over HIPPI January 1997

 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.)


 FC (3 bits) SHALL contain zero unless otherwise defined by local
 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)

Renwick Standards Track [Page 8] RFC 2067 IP over HIPPI January 1997

 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
 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

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).


 Organization Code (24 bits) SHALL be zero.

Renwick Standards Track [Page 9] RFC 2067 IP over HIPPI January 1997

 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      |

Renwick Standards Track [Page 10] RFC 2067 IP over HIPPI January 1997

                          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

Renwick Standards Track [Page 11] RFC 2067 IP over HIPPI January 1997

 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
      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

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

Renwick Standards Track [Page 12] RFC 2067 IP over HIPPI January 1997

       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

Renwick Standards Track [Page 13] RFC 2067 IP over HIPPI January 1997

 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
 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.

Renwick Standards Track [Page 14] RFC 2067 IP over HIPPI January 1997

 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

Renwick Standards Track [Page 15] RFC 2067 IP over HIPPI January 1997

 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
 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.

Renwick Standards Track [Page 16] RFC 2067 IP over HIPPI January 1997

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.
                    |        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     |

Renwick Standards Track [Page 17] RFC 2067 IP over HIPPI January 1997

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

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.

Renwick Standards Track [Page 18] RFC 2067 IP over HIPPI January 1997

 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:

Renwick Standards Track [Page 19] RFC 2067 IP over HIPPI January 1997

                   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

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.

Renwick Standards Track [Page 20] RFC 2067 IP over HIPPI January 1997

11 References

 [1]  ANSI X3.183-1991, High-Performance Parallel Interface -
      Mechanical, Electrical and Signalling Protocol Specification
 [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-
 [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
 Mailing List:

Renwick Standards Track [Page 21] RFC 2067 IP over HIPPI January 1997

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
 +----------+                                        +----------+
 |          |                                        |          |
 |          +--------+                      +--------+          |
 |          | 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.

Renwick Standards Track [Page 22] RFC 2067 IP over HIPPI January 1997

HIPPI Signal Hierarchy

 HIPPI has the following hardware signals:
    Source to Destination
       INTERCONNECT B (64 bit only)
       CLOCK (25 MHz)
       DATA (32 or 64 signals)
       PARITY (4 or 8 signals)
    Destination to Source
       INTERCONNECT B (64 bit only)
 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
 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

Renwick Standards Track [Page 23] RFC 2067 IP over HIPPI January 1997

                | 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.

Renwick Standards Track [Page 24] RFC 2067 IP over HIPPI January 1997

 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--------------+       +-+       +-+       +-+     +--------
                    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.

Renwick Standards Track [Page 25] RFC 2067 IP over HIPPI January 1997

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.

Renwick Standards Track [Page 26] RFC 2067 IP over HIPPI January 1997

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.

Renwick Standards Track [Page 27] RFC 2067 IP over HIPPI January 1997

 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
 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

Renwick Standards Track [Page 28] RFC 2067 IP over HIPPI January 1997

 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
 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.

Renwick Standards Track [Page 30]

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