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rfc:rfc1374

Network Working Group J. Renwick Request for Comments: 1374 A. Nicholson

                                                   Cray Research, Inc.
                                                          October 1992
                        IP and ARP on HIPPI

Status of this Memo

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

Abstract

 The ANSI X3T9.3 committee has drafted a proposal for the
 encapsulation of IEEE 802.2 LLC PDUs and, by implication, IP on
 HIPPI.  Another X3T9.3 draft describes the operation of HIPPI
 physical switches.  X3T9.3 chose to leave HIPPI networking issues
 largely outside the scope of their standards; this document discusses
 methods of using of ANSI standard HIPPI hardware and protocols in the
 context of the Internet, including the use of HIPPI switches as LANs
 and interoperation with other networks.  

Table of Contents

    Introduction                                                   2
    Scope                                                          2
    Definitions                                                    3
    Equipment                                                      4
    Protocol                                                       6
       Packet Format                                               6
       48 bit Universal LAN MAC addresses                         10
       I-Field Format                                             11
       Rules For Connections                                      13
       MTU                                                        15
    Camp-on                                                       16
    Address Resolution                                            16
       ARP and RARP Message Format                                17
       ARP Procedure                                              21
       ARP Implementation Methods                                 22

Renwick & Nicholson [Page 1] RFC 1374 IP and ARP on HIPPI October 1992

       ARP Example                                                23
       Discovery of One's Own Switch Address                      25
    Path MTU Discovery                                            27
    Channel Data Rate Discovery                                   27
    Performance                                                   29
    Sharing the Switch                                            31
    Appendix A -- HIPPI Basics                                    31
    Appendix B -- How to Build a Practical HIPPI LAN              37
    References                                                    41
    Security Considerations                                       42
    Authors' Addresses                                            42

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 octets (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
 "nsco.network.com" in the directory "hippi," and may be obtained via
 anonymous FTP until they become published standards.
 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.

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

Renwick & Nicholson [Page 2] RFC 1374 IP and ARP on HIPPI October 1992

 may as well be interoperable on more complex networks, 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.  To the extent that a gateway acts as a host on a
 HIPPI-SC LAN, its behavior is within 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], SNAP and ARP
 2.  I-Field contents
 3.  HIPPI switch address resolution, including self discovery
 4.  Rules for the use of connections.
 Outside of the scope are
 1.  Vendor dependent solutions for multicast or third party ARP
 2.  Network configuration and management
 3.  Host internal optimizations
 4.  The interface between a host and an outboard protocol processor.

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 ARP and 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 & Nicholson [Page 3] RFC 1374 IP and ARP on HIPPI October 1992

 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.

Equipment

 A HIPPI network can be composed of nodes with HIPPI interfaces, HIPPI
 cables or serial links, HIPPI-SC switches, gateways to other networks
 and, possibly, proprietary equipment that multicasts or responds to
 ARP requests on behalf of the real nodes.
 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.
 The following figure shows a sample HIPPI switch configuration.

Renwick & Nicholson [Page 4] RFC 1374 IP and ARP on HIPPI October 1992

                                                 +-----+
 |                                               | H 4 |
 |                                               +--+--+
 |                   +----+    +----+    +----+     |
 |                   | H1 |    | H2 |    | H3 |   +-++
 |   +--+            +-++-+    +-++-+    +-++-+   |PP|
 +---+H5|              ||        ||        ||     ++++
 |   +--+              ||        ||        ||      ||
 |                 +---++--------++--------++------++----+
 |                 |                                     |    +---+
 |   +----+        |              HIPPI-SC               +----+ARP|
 +---+ 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
                     ARP  =  ARP Agent
                  A possible HIPPI configuration
 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

Renwick & Nicholson [Page 5] RFC 1374 IP and ARP on HIPPI October 1992

 complicated on blocking networks than on non-blocking ones.
 This memo ignores blocking issues, 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.

Protocol

 Packet Format
 The HIPPI packet format for Internet datagrams shall conform to the
 HIPPI-FP and HIPPI-LE draft standards.  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.
 The length of a HIPPI packet, including trailing fill, shall be a
 multiple of eight octets as required by HIPPI-LE.
 +----------+-----------+---------------------+-----------   ------+
 |          |           |                     | IP . . .     0 - 7 |
 | HIPPI-FP | HIPPI-LE  | IEEE 802.2 LLC/SNAP |              octets|
 |(8 octets)|(24 octets)|     (8 octets)      | ARP . . .     fill |
 +----------+-----------+---------------------+-----------   ------+
                      HIPPI Packet Structure
    HIPPI-FP Header
       ULP-id (8 bits) shall contain 4.
       D1_Data_Set_Present (1 bit) shall be set.
       Start_D2_on_Burst_Boundary (1 bit) shall be zero.
       Reserved (11 bits) shall contain zero.
       D1_Area_Size (8 bits) should be sent as 3.  Destinations shall
       accept any value that HIPPI-FP defines as legal: from 3 to 127
       (32 bit HIPPI) or 3 to 255 (64 bit HIPPI).
       D2_Offset (3 bits) may be any value from 0 to 7.
       D2_Size (32 bits) Shall contain the number of octets in the

Renwick & Nicholson [Page 6] RFC 1374 IP and ARP on HIPPI October 1992

       IEEE 802.2 LLC Type 1 PDU, or zero if no PDU is present.  It
       shall not exceed 65,288 (decimal).  This value includes the
       IEEE 802.2 LLC/SNAP header and the IP datagram.  It does not
       include trailing fill octets.  (See "MTU," below.)
       The first octet of the IEEE 802.2 LLC PDU (SSAP) shall be
       located at offset "n" of the packet, where
           n = 8 + (D1_Area_Size*8) + D2_Offset
       as specified in HIPPI-FP.
    HIPPI-LE Header
       FC (3 bits) shall contain zero unless otherwise defined by
       local administration.
       Double_Wide (1 bit) shall contain one if the Destination
       associated with the sending Source supports 64 bit HIPPI
       operation.  Otherwise it shall contain zero.
       Message_Type (4 bits) contains a code identifying the type of
       HIPPI-LE PDU.  Defined values (binary) are:
          0  Data PDU
          1  Address Resolution Request PDU (AR_Request)
          2  Address Resolution Response PDU (AR_Response)
          3  Self Address Resolution Request PDU (AR_S_Request)
          4  Self Address Resolution Response PDU (AR_S_Response)
          5-11 Reserved by the ANSI X3T9.3 committee
          12-15 Locally Assigned
       Destination_Switch_Address is a 24-bit field containing the
       Switch Address of the Destination if known, otherwise zero.  If
       the address comprises less than 24 bits, it shall be right
       justified (occupying the least significant bits) in the field.
       Destination_Address_Type (4 bits) and Source_Address_Type (4
       bits) contain codes identifying the type of addresses in the
       Destination_Switch_Address and Source_Switch_Address fields
       respectively.  Defined values (binary) are:
          0  Unspecified
          1  HIPPI-SC Source Route (24 bits)
          2  HIPPI-SC Address (12 bits)
          3-11 Reserved by the ANSI X3T9.3 committee
          12-15 Locally Assigned

Renwick & Nicholson [Page 7] RFC 1374 IP and ARP on HIPPI October 1992

       Source_Switch_Address is a 24-bit field containing the Switch
       Address of the Source.  If the address comprises less than 24
       bits, it shall be right justified (occupying the least
       significant bits) in the field.
       Reserved (16 bits) shall contain zero.
       Destination_IEEE_Address (48 bits) shall contain the 48 bit
       Universal LAN MAC Address of the Destination if known,
       otherwise zero.
       LE_Locally_Administered (16 bits) shall contain zero unless
       otherwise defined by local administration.
       Source_IEEE_Address (48 bits) shall contain the 48 bit
       Universal LAN MAC Address of the Source if known, otherwise
       zero.
    IEEE 802.2 LLC
       The IEEE 802.2 LLC Header shall begin in the first octet of the
       HIPPI-FP D2_Area.
       SSAP (8 bits) shall contain 170 (decimal).
       DSAP (8 bits) shall contain 170 (decimal).
       CTL (8 bits) shall contain 3 (Unnumbered Information).
    SNAP
       Organization Code (24 bits) shall be zero.
       EtherType (16 bits) shall be set as defined in Assigned Numbers
       [8] (IP = 2048, ARP = 2054, RARP = 32,821).

Renwick & Nicholson [Page 8] RFC 1374 IP and ARP on HIPPI October 1992

    31    28        23  21          15        10     7         2   0
    +-----+---------+-+-+-----------+---------+-----+---------+-----+
  0 |      04       |1|0|       Reserved      |      03       |  0  |
    +---------------+-+-+---------------------+---------------+-----+
  1 |                             (n+8)                             |
    +-----+-+-------+-----------------------------------------------+
  2 |[LA] |W|M_Type |          Destination_Switch_Address           |
    +-----+-+-------+-----------------------------------------------+
  3 | D_A_T | S_A_T |             Source_Switch_Address             |
    +-------+-------+---------------+-------------------------------+
  4 |            Reserved           |  [Destination_IEEE_Address]   |
    +-------------------------------+                               |
  5 |                                                               |
    +-------------------------------+-------------------------------+
  6 |             [LA]              |     [Source_IEEE_Address]     |
    +-------------------------------+                               |
  7 |                                                               |
    +===============+===============+===============+===============+
  8 |       AA      |      AA       |       03      |       00      |
    +---------------+---------------+---------------+---------------+
  9 |       00      |      00       |         [EtherType]           |
    +---------------+---------------+---------------+---------------+
 10 |Message octet 0|Message octet 1|Message octet 2| . . .         |
    +---------------+---------------+---------------+---            |
    |                            .  .  .
                                                                    |
    |        -------+---------------+---------------+---------------+
    |         . . . |  octet (n-2)  |  octet (n-1)  |     FILL      |
    +---------------+---------------+---------------+---------------+
 N-1|      FILL     |     FILL      |     FILL      |     FILL      |
    +---------------+---------------+---------------+---------------+
                         HIPPI Packet Format
    Words 0-1:  HIPPI-FP Header
    Words 2-7:  D1 Area (HIPPI-LE Header)
    Words 8-9:  D2 Area (IEEE 802.2 LLC/SNAP)
    Words 10-(N-1):  D2 Area (IP or ARP message)
    (n) is the number of octets in the IP or ARP message.
    +====+ denotes the boundary between D1 and D2 areas.
    [LA] fields are zero unless used otherwise locally.
    Abbreviations:  "W"      = Double_Wide field;
                    "M_Type" = Message_Type field;
                    "D_A_T"  = Destination_Address_Type;
                    "S_A_T"  = Source_Address_Type;
    [FILL] octets complete the HIPPI packet to an even
    number of 32 bit words.  The number of fill octets
    is not counted in the data length.

Renwick & Nicholson [Page 9] RFC 1374 IP and ARP on HIPPI October 1992

 IEEE 802.2 Data
    The IEEE 802.2 Data shall follow the EtherType field immediately.
    Fill octets shall be used following the Data as necessary to make
    the number of octets in the packet a multiple of 8.  In accordance
    with HIPPI-FP, the amount of this fill is not included in the
    D2_Size value in the HIPPI-FP Header.
    The order of the octets in the data stream is from higher numbered
    to lower numbered data signal (left to right) within the HIPPI
    word, as specified in HIPPI-FP Clause 7, "Word and byte formats."
    With the 1600 megabit/second data rate option (64 bit) bits 32
    through 63 are on Cable B, so that the four octets on Cable B come
    logically before those on Cable A.  Within each octet, the most
    significant bit is the highest numbered signal.

48 bit Universal LAN MAC Addresses

 IEEE Standard 802.1A specifies the Universal LAN MAC Address.  The
 globally unique part of the 48 bit space is administered by the IEEE.
 Each node on a HIPPI-SC LAN should be assigned a ULA.  Multiple ULAs
 may be used if a node contains more than one IEEE 802.2 LLC protocol
 entity.
 The format of the address within its 48 bit HIPPI-LE fields follows
 IEEE 802.1A canonical bit order and HIPPI-FP bit and byte order:
 31              23              15               7              0
 +-------------------------------+---------------+---------------+
 |      (not used for ULA)       |ULA octet 0|L|G|  ULA octet 1  |
 +---------------+---------------+---------------+---------------+
 |  ULA octet 2  |  ULA octet 3  |  ULA octet 4  |  ULA octet 5  |
 +---------------+---------------+---------------+---------------+
                  Universal LAN MAC Address Format
    L (U/L bit) = 1 for Locally administered addresses, 0 for
    Universal.
    G (I/G bit) = 1 for Group addresses, 0 for Individual.
 The use of ULAs is optional, but encouraged.  Although ULAs are not
 used by HIPPI-SC switches, they are helpful for HIPPI Switch Address
 resolution, and for distinguishing between multiple logical entities
 that may exist within one node.  They may also be used by gateway
 devices that replace HIPPI hardware headers with the MAC headers of
 other LANs.  Carrying the ULAs in the HIPPI header may simplify these
 devices, and it may also help if HIPPI is used as an interface to
 some future HIPPI based LAN that uses ULAs for addressing.

Renwick & Nicholson [Page 10] RFC 1374 IP and ARP on HIPPI October 1992

Recommended HIPPI-FP Options

 HIPPI-FP allows some flexibility in the construction of a HIPPI
 packet, including placement of short bursts, optional fill and offset
 octets between the D1 and D2 areas and fill following the D2 data.
 For efficiency, Sources should limit the use of these options:
 1.  Send the short burst as the last burst of the packet rather than
     the first.
 2.  Do not place fill octets between the HIPPI-LE header and the
     start of the D2_Area.
 3.  Use no more than seven octets after the D2 Data, as needed to
     make the total packet length a multiple of 8 octets.

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

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

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

I-Field format

 The I-field bits, as defined in HIPPI-SC, shall be set as follows:
    Locally Administered (bit 31) shall be zero.
    Reserved (bits 30, 29) should be zero.  Destinations shall accept
    any value for these bits.

Renwick & Nicholson [Page 11] RFC 1374 IP and ARP on HIPPI October 1992

    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 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.
    Note:  the outcome of Switch Address Resolution (see "Address
    Resolution" below) determines whether to use logical address mode
    or source route mode.  If source route mode is used with multiple
    interconnected switches, different sources may use different
    addresses to reach the same destination, and multicast-based
    address resolution may not be possible because a target node may
    not know the route to itself from a given remote source.
    Regardless of this difficulty, it may be possible to use source
    route mode if the network consists of a single switch, or if
    address resolution is supported by an ARP agent that is able to
    deliver correct routes to each node.  The nodes themselves need
    not be concerned with these problems if they use the addressing

Renwick & Nicholson [Page 12] RFC 1374 IP and ARP on HIPPI October 1992

    mode suggested by the value of the Source_Address_Type field in a
    HIPPI-LE Address Resolution Response packet.

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
 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.

Renwick & Nicholson [Page 13] RFC 1374 IP and ARP on HIPPI October 1992

 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.
    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.

Renwick & Nicholson [Page 14] RFC 1374 IP and ARP on HIPPI October 1992

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

Renwick & Nicholson [Page 15] RFC 1374 IP and ARP on HIPPI October 1992

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

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.

Address Resolution

 The Internet Address Resolution Protocol (ARP) is defined in RFC 826
 [9].  Ethernet, FDDI and 802 networks use ARP to discover another
 host's ULA knowing the Internet address.  Reverse ARP [10] is used to
 discover the Internet address, knowing the ULA.  ARP can be used in
 the conventional way on HIPPI-SC LANs equipped with a multicast
 capability or third party ARP agent.
 HIPPI-LE defines similar lower-level address resolution between ULAs
 and switches.  HIPPI-LE adds a self-address resolution mechanism not
 defined for Internet ARP, which allows a node to discover its own
 switch address dynamically.

Renwick & Nicholson [Page 16] RFC 1374 IP and ARP on HIPPI October 1992

 ARP for the purpose of discovering ULAs is not necessary for the
 operation of a HIPPI-SC LAN, but it serves as the vehicle for
 discovery of HIPPI-SC Switch Addresses, without which the HIPPI-SC
 LAN cannot function.  In other words, at the same time a node is
 using ARP to map another node's IP address to its ULA, it is also
 mapping the ULA to the 12 bit HIPPI Switch Address, from which it
 will construct the I-field value for sending messages to that node.
 This additional level of hardware addressing uses the address fields
 in the HIPPI-LE header.
 In the following discussion, the terms "requester" and "target" are
 used to identify the node requesting address resolution and the node
 whose address it wishes to discover, respectively.  In third party
 ARP (see "ARP Implementation Methods," below) the source of a reply
 is an ARP agent node, not the target node.
 ARP and RARP Message Format
    The HIPPI ARP/RARP protocol uses the same packet format as ARP for
    Ethernet.  ARP packets shall be transmitted with a hardware type
    code of 1 (as for Ethernet).  Furthermore, ARP packets shall be
    accepted if received with hardware type codes of either 1 or 6
    (IEEE 802 networks).
    ar$hrd (16 bits) shall contain 1.
    ar$pro (16 bits) shall contain the IP protocol code 2048
    (decimal).
    ar$hln (8 bits) shall contain 6.
    ar$pln (8 bits) shall contain 4.
    ar$op  (16 bits) shall contain 1 for requests, 2 for responses.
    ar$sha (48 bits) in requests shall contain the requester's ULA.
    In replies it shall contain the target node's ULA.
    ar$spa (32 bits) in requests shall contain the requester's IP
    address if known, otherwise zero.  In replies it shall contain the
    target node's IP address.
    ar$tha (48 bits) in requests shall contain the target's ULA if
    known, otherwise zero.  In replies it shall contain the
    requester's ULA.
    ar$tpa (32 bits) in requests shall contain the target's IP address
    if known, otherwise zero.  In replies it shall contain the

Renwick & Nicholson [Page 17] RFC 1374 IP and ARP on HIPPI October 1992

    requester's IP address.
    The format of the six octets of the ULA shall be the same as
    required in the HIPPI-LE header (see "48 bit Universal LAN MAC
    Addresses" above), except for the alignment of the Source ULA with
    respect to the 32 bit HIPPI word, which is different between ARP
    and HIPPI-LE.  No bit reversal is necessary as is required with
    FDDI [11].
    31    28        23  21          15        10     7         2   0
    +-----+---------+-+-+-----------+---------+-----+---------+-----+
  0 |      04       |1|0|         000         |      03       |  0  |
    +---------------+-+-+---------------------+---------------+-----+
  1 |                              36                               |
    +-----+-+-------+-----------------------+-----------------------+
  2 |[LA] |W|   1   |          000          |   Target Switch Addr  |
    +-----+-+-------+-----------------------+-----------------------+
  3 |   2   |   2   |          000          |Requester's Switch Addr|
    +---------------+---------------+-------+-----------------------+
  4 |             00 00             |                               |
    +-------------------------------+                               |
  5 |                           Target ULA                          |
    +-------------------------------+-------------------------------+
  6 |             [LA]              |                               |
    +-------------------------------+                               |
  7 |                        Requester's ULA                        |
    +===============+===============+===============+===============+
  8 |       AA      |      AA       |       03      |       00      |
    +---------------+---------------+---------------+---------------+
  9 |       00      |      00       |        EtherType (2054)       |
    +---------------+---------------+-------------------------------+
 10 |            hrd (1)            |           pro (2048)          |
    +---------------+---------------+-------------------------------+
 11 |    hln (6)    |    pln (4)    |            op (1)             |
    +---------------+---------------+-------------------------------+
 12 |                 Requester's ULA octets 0 - 3                  |
    +-------------------------------+-------------------------------+
 13 | Requester's ULA octets 4 - 5  | Requester's IP Address upper  |
    +-------------------------------+-------------------------------+
 14 | Requester's IP Address lower  |    Target ULA octets 0 - 1    |
    +-------------------------------+-------------------------------+
 15 |                   Target ULA octets 2 - 5                     |
    +---------------------------------------------------------------+
 16 |                       Target IP Address                       |
    +---------------------------------------------------------------+
              HIPPI ARP/RARP Request (logical address mode)

Renwick & Nicholson [Page 18] RFC 1374 IP and ARP on HIPPI October 1992

 All ARP requests shall be sent with the I-field bit 28 set to zero,
 i.e. requesting a 32 bit connection.
 Unless another convention is locally defined for ARP requests, the
 I-field Path Selection bits may be set to binary 01 or 11 (logical
 address mode), and Destination Address field set to the HIPPI-SC
 address reserved for traffic conventionally directed to the IEEE
 802.1[12] broadcast address (which HIPPI-SC defines as FE0, hex).
 Reply packets shall be sent with I-field Path Selection and Routing
 Control fields set according to the Source_Address_Type and
 Source_Switch_Address fields in the request.
 In the HIPPI-LE header of ARP/RARP requests and replies the following
 fields shall be set:
 Double-Wide should be 1 if the HIPPI Destination at the sending node
 can accept 64 bit HIPPI connections.
 Message_Type shall contain an address resolution type code as defined
 in HIPPI-LE.  It shall be set appropriately to the value of the ARP
 operation code (ar$op) in piggybacked ARP messages:
       +-----------------------+-----------------------+
       |       ARP ar$op       | HIPPI-LE Message_Type |
       +=======================+=======================+
       |ARP Request (1)        |AR_Request (1)         |
       |ARP Reply (2)          |AR_Response (2)        |
       +-----------------------+-----------------------+
       |Reverse ARP Request (3)|AR_Request (1)         |
       |Reverse ARP Reply (4)  |AR_Response (2)        |
       +-----------------------+-----------------------+
 There is no ARP message corresponding to HIPPI-LE self address
 discovery; these packets are sent without ULP data.
 Destination_Switch_Address in requests shall be the Switch Address of
 the target node if known, otherwise zero.  In replies it shall be the
 requesting node's Switch Address
 Destination_Address_Type shall be 1 if the Destination_Switch_Address
 is a source route, 2 if it is a 12 bit address.
 Source_Address_Type shall be 1 if the Source_Switch_Address is a
 source route, 2 if it is a 12 bit address.
 Source_Switch_Address in requests shall be the Switch Address of the
 requesting node if known, otherwise zero.  In replies it shall be the

Renwick & Nicholson [Page 19] RFC 1374 IP and ARP on HIPPI October 1992

 target node's Switch Address.
 Destination_IEEE_Address shall be the same as the ar$tha field in the
 ARP message.
 Source_IEEE_Address shall be the same as the ar$sha field in the ARP
 message.
    31    28        23  21          15        10     7         2   0
    +-----+---------+-+-+-----------+---------+-----+---------+-----+
  0 |      04       |1|0|         000         |      03       |  0  |
    +---------------+-+-+---------------------+---------------+-----+
  1 |                              36                               |
    +-----+-+-------+-----------------------+-----------------------+
  2 |[LA] |W|   2   |          000          |Requester's Switch Addr|
    +-----+-+-------+-----------------------+-----------------------+
  3 |   2   |   2   |          000          | Target Switch Address |
    +---------------+---------------+-------+-----------------------+
  4 |             00 00             |                               |
    +-------------------------------+                               |
  5 |                        Requester's ULA                        |
    +-------------------------------+-------------------------------+
  6 |             [LA]              |                               |
    +-------------------------------+                               |
  7 |                           Target ULA                          |
    +===============+===============+===============+===============+
  8 |       AA      |      AA       |       03      |       00      |
    +---------------+---------------+---------------+---------------+
  9 |       00      |      00       |        EtherType (2054)       |
    +---------------+---------------+-------------------------------+
 10 |            hrd (1)            |           pro (2048)          |
    +---------------+---------------+-------------------------------+
 11 |    hln (6)    |    pln (4)    |            op (2)             |
    +---------------+---------------+-------------------------------+
 12 |                    Target ULA octets 0 - 3                    |
    +-------------------------------+-------------------------------+
 13 |    Target ULA octets 4 - 5    |    Target IP Address upper    |
    +-------------------------------+-------------------------------+
 14 |    Target IP Address lower    | Requester's ULA octets 0 - 1  |
    +-------------------------------+-------------------------------+
 15 |                 Requester's ULA octets 2 - 5                  |
    +---------------------------------------------------------------+
 16 |                    Requester's IP Address                     |
    +---------------------------------------------------------------+
                   HIPPI ARP/RARP Reply (logical address mode)

Renwick & Nicholson [Page 20] RFC 1374 IP and ARP on HIPPI October 1992

ARP procedure

 The combined HIPPI-LE/ARP packet contains six addresses, three each
 for the requester and the target:
    Requester's IP Address          (ARP)
    Requester's ULA                 (ARP and HIPPI-LE)
    Requester's Switch Address      (HIPPI-LE)
    Target's IP Address             (ARP)
    Target's ULA                    (ARP and HIPPI-LE)
    Target's Switch Address         (HIPPI-LE)
 Internet ARP concerns the IP Address and ULA; HIPPI-LE address
 resolution concerns the ULA and Switch Address.  Thus the ULA appears
 in both parts of the packet.
 Successful ARP results in tables in each node that map remote nodes'
 IP addresses to ULAs and ULAs to Switch Addresses, so that when an
 application requests a connection to a remote node by its IP address,
 both the remote ULA and Switch Address can be determined, a correct
 HIPPI-LE header can be built, and a connection to the node can be
 established using the correct Switch Address in the I-field.  Any
 recipient of an ARP request or reply may use information in the
 packet to augment its tables, even if it is neither the target node
 nor the requester.
 Note that the use of ULAs with HIPPI is not required.  In both the
 HIPPI-LE header and the Internet ARP message, the fields that contain
 ULAs should be set to zero when the ULA is not known.  Address
 resolution consists of two separate protocols, HIPPI-LE address
 resolution and Internet ARP, neither of which can function
 independently without ULAs.  However HIPPI Switch Address resolution
 can work without ULAs if the two protocols are piggybacked and
 treated as one operation in which Internet addresses are mapped
 directly to switch addresses.  With the exception of the optional
 self-address resolution request, which has no analogous Internet
 protocol, HIPPI-LE address resolution and Internet ARP messages
 should be sent together as a single HIPPI packet.
 If ULAs are used, the HIPPI-LE address resolution request can be sent
 without a piggybacked 802.2 LLC PDU, so it is possible to map ULAs to
 HIPPI Switch Addresses without using ARP.  Nodes shall accept both
 piggybacked and non-piggybacked forms of HIPPI-LE address resolution
 messages.
 The recipient of an address resolution request, having first updated
 its address mapping tables with any new information it can find in

Renwick & Nicholson [Page 21] RFC 1374 IP and ARP on HIPPI October 1992

 the request, checks to see if it is the target node.  If it is, it
 generates a reply by filling in the unknown target address fields
 according to the HIPPI-LE message type and the ARP operation code,
 and swapping the four pairs of source/target address fields.  Then it
 connects to the requesting node with the Source Switch Address from
 the request, and sends the reply packet.
 A node is the target of an address resolution request if the request
 contains one of the following:
 1.  the node's ULA in the Destination_IEEE_Address field of a HIPPI-
     LE AR_Request message
 2.  the node's IP address in the target protocol address field
     (ar$tpa) of a piggybacked Internet ARP message
 If two target fields are known but are not mapped together in the
 recipient's address mapping tables, it may do one of three things:
 1.  treat the request as having two targets, and send correct replies
     for both to the requester.
 2.  assume its own tables are invalid and ignore the request.
 3.  assume one of the "known" target fields is correct and respond as
     if the other had been unknown.
 The best choice depends on which fields conflict and the nature of
 the implementation.  Choice 3 is probably best for ordinary nodes,
 but third party ARP agents may have reason to use one of the other
 two.  Future experience may shed light on this.

ARP Implementation Methods

 The requirements for nodes to handle address resolution messages
 depend on the means by which address resolution is implemented on the
 LAN.
 In conventional networks ARP is a distributed function. ARP requests
 are broadcast; each host may update its address mappings with any ARP
 request or reply it sees, and responds to ARP requests that contain
 its own address in the target protocol address field.  HIPPI-SC
 switches are not required to provide multicast service, although some
 do.  Even if the switches do not multicast, one or more nodes can act
 as multicast servers, receiving packets sent to the HIPPI-SC
 broadcast address and repeating them to each other node in turn.
 Either way, if multicast exists on a HIPPI-SC LAN, ARP can be a
 distributed function.  In this situation each node is required to

Renwick & Nicholson [Page 22] RFC 1374 IP and ARP on HIPPI October 1992

 respond correctly to address resolution requests for which it is the
 target.
 Third party ARP is a second method that does not depend on multicast.
 The switches can map the HIPPI ARP multicast address to a node that
 acts as an ARP agent, replying to ARP requests on behalf of the real
 target nodes.  Ordinary nodes never receive ARP requests or generate
 replies and never have the opportunity to update mapping tables based
 on ARP requests from other nodes, as usually happens on conventional
 networks.  Each node must request any address information it needs,
 but never has to process ARP information it doesn't need.  Under
 third party ARP a node should not receive address resolution
 requests, and each node that is not an ARP agent should ignore those
 that it does receive.
 As a third possibility, one can omit the implementation of ARP
 entirely, choosing instead to build address mapping tables in each
 node from information available to a network administrator.  Such a
 technique is implementation dependent and outside the scope of this
 memo.  It may be helpful in prototype networks without multicast
 where no ARP agent is yet implemented.  In this case, nodes are not
 required to respond to address resolution requests, but must accept
 any they may receive.

ARP Example

 Assume a HIPPI-SC switch is installed with two nodes, X and Y,
 connected.  Each node has a unique Switch Address.  Both nodes have
 access to the host data base (e.g. /etc/hosts) in which the network
 administrator has configured the network and given the two nodes IP
 addresses.  There is an ARP agent connected to a switch port that is
 mapped to the address FE0 (hex).  The ARP agent contains no mappings
 of any IP, IEEE or Switch addresses.  Both nodes know their own ULAs
 and Switch Addresses.  They want to talk to each other; each knows
 the other's IP address (from the host data base) but neither knows
 the other's ULA or Switch Address.  Node X starts:
 1.  Node X connects to FE0 and sends a piggyback ARP Request
     requesting addresses for Y:
         HIPPI-LE Message_Type is 1, AR_Request
         HIPPI-LE Destination_Switch_Address = 0
         HIPPI-LE Source_Switch_Address = X's Switch Address
         HIPPI-LE Destination_IEEE_Address = 0
         HIPPI-LE Source_IEEE_Address = X's ULA
         ARP ar$op = 1 (request)
         ARP ar$sha = X's ULA
         ARP ar$spa = X's IP Address

Renwick & Nicholson [Page 23] RFC 1374 IP and ARP on HIPPI October 1992

         ARP ar$tha = 0
         ARP ar$tpa = Y's IP Address
 2.  The ARP agent receives the ARP request and adds an entry for X to
     its address mapping table.  It does not know about Y, so it does
     not generate a reply.
 3.  Node X waits for a reply.  It may set a timer to retransmit the
     request periodically, but its requests will be ignored until node
     Y sends an ARP request.
 4.  Node Y connects to FE0 and sends an ARP request requesting
     addresses for X:
         HIPPI-LE Message_Type is 1, AR_Request
         HIPPI-LE Destination_Switch_Address = 0
         HIPPI-LE Source_Switch_Address = Y's Switch Address
         HIPPI-LE Destination_IEEE_Address = 0
         HIPPI-LE Source_IEEE_Address = Y's ULA
         ARP ar$op = 1 (request)
         ARP ar$sha = Y's ULA
         ARP ar$spa = Y's IP Address
         ARP ar$tha = 0
         ARP ar$tpa = X's IP Address
 5.  The ARP agent receives Y's request and adds an entry for Y to its
     address mapping table.  It knows about the target node, X, so it
     connects to Y (using the Source_Switch_Address given in the
     request) and sends an ARP Reply:
         HIPPI-LE Message_Type is 2, AR_Reply
         HIPPI-LE Destination_Switch_Address = Y's Switch Address
         HIPPI-LE Source_Switch_Address = X's Switch Address
         HIPPI-LE Destination_IEEE_Address = Y's ULA
         HIPPI-LE Source_IEEE_Address = X's ULA
         ARP ar$op = 2 (reply)
         ARP ar$sha = X's ULA
         ARP ar$spa = X's IP Address
         ARP ar$tha = Y's ULA
         ARP ar$tpa = Y's IP Address
 6.  Node Y receives the ARP reply and builds its address mapping
     table entry for Node X.
 7.  Node Y connects to node X and transmits an IP packet with the
     following information in the HIPPI-LE header:
         HIPPI-LE Message_Type is 0, AR_Data

Renwick & Nicholson [Page 24] RFC 1374 IP and ARP on HIPPI October 1992

         HIPPI-LE Destination_Switch_Address = X's Switch Address
         HIPPI-LE Source_Switch_Address = Y's Switch Address
         HIPPI-LE Destination_IEEE_Address = X's ULA
         HIPPI-LE Source_IEEE_Address = Y's ULA
 8.  Node X receives the IP packet.  Since the ARP agent now knows
     about node Y, node X can retransmit its ARP request (repeating
     step 1) and receive an ARP reply:
         HIPPI-LE Message_Type is 2, AR_Reply
         HIPPI-LE Destination_Switch_Address = X's Switch Address
         HIPPI-LE Source_Switch_Address = Y's Switch Address
         HIPPI-LE Destination_IEEE_Address = X's ULA
         HIPPI-LE Source_IEEE_Address = Y's ULA
         ARP ar$op = 2 (reply)
         ARP ar$sha = Y's ULA
         ARP ar$spa = Y's IP Address
         ARP ar$tha = X's ULA
         ARP ar$tpa = X's IP Address
 Address resolution is now complete for both nodes.
 If there had been a multicast facility instead of the ARP agent in
 the configuration, the target nodes themselves would have received
 the requests and responded to them in the same way the ARP agent did.

Discovery of One's Own Switch Address

 The ARP example above assumed that each node had prior knowledge of
 its own switch address.  This may be manually configured, by means
 that are outside the scope of this memo, when each node is connected
 to the switch.  If a multicast capability exists, the node may
 discover its own address automatically when it starts up, using a
 protocol defined in HIPPI-LE.
 In the self-address discovery protocol, a node connects to a
 multicast address and sends a HIPPI-LE message containing its own
 ULA.  It receives a multicast copy of its own message, and learns its
 own switch address from the destination address field of the received
 I-field.
 HIPPI-LE self address resolution uses the same HIPPI-LE message
 format described in "ARP and RARP Message Format," above, with the
 AR_S_Request and AR_S_Response message type codes and no piggybacked
 ULP data.  The HIPPI-LE header contents for the request are:
     Message_Type is 3, AR_S_Request
     Destination_Address_Type = 0 (undefined)

Renwick & Nicholson [Page 25] RFC 1374 IP and ARP on HIPPI October 1992

     Destination_Switch_Address = 0 (unknown)
     Source_Address_Type = 0 (undefined)
     Source_Switch_Address = 0 (unknown)
     Destination_IEEE_Address = my ULA
     Source_IEEE_Address = my ULA
 There is no D2 data; the packet contains only the HIPPI-FP header and
 D1_Area containing the HIPPI-LE header.
 The node that wants to discover its address connects to the multicast
 address for this purpose (hex FE0 in HIPPI-SC) and transmits the
 request packet.  What happens next depends on the particular network:
 With multicast:
    The node receives its own request and can learn its own switch
    address from the I-field it receives.  This is the only time a
    node should use an address from a received I-field.
 With an ARP agent:
    The node may receive an AR_S_Response message with its own ULA in
    the Destination_IEEE_Address field and its own switch address in
    the Destination_Switch_Address field.  This address may be
    different from the address contained in the I-field, and should be
    used instead.
 The ARP agent response alternative requires that the agent have prior
 knowledge of the node's location and ULA through some process not
 specified by this memo.  The node may receive both its request and
 the agent's response if both an ARP agent and multicast are active.
 In this case the address it learns from the I-field is later replaced
 by the address given by the ARP agent in the response.  Agents may
 assign new addresses to nodes and inform them by sending unsolicited
 AR_S_Response messages.  Any node whose switch address is updated in
 this way should invalidate the switch addresses it has saved for
 other nodes, and use ARP to rediscover them.
 If the node reacts correctly to either the multicast request or
 agent-generated response, it can discover its address without having
 to know whether or not an ARP agent is active.  The full procedure
 is:
 1.  Transmit the AR_S_Request

Renwick & Nicholson [Page 26] RFC 1374 IP and ARP on HIPPI October 1992

 2.  When a connection arrives, accept it and save the I-field for
     later analysis.
 3.  Receive the message and look at the HIPPI-LE header.  If the
     message is Message Type AR_S_Request, analyze the I-field to
     discover the node's own switch address.  HIPPI-SC I-field formats
     suggest the following:
        if bit 25 == 1
                Address type is HIPPI-SC logical address.
                if bit 27 == 1
                        take address from bits 23-12
                else
                        take address from bits 11-00
                endif
        else
                Address is unusable (source route)
        endif
     This is a one-time operation.  Once the node knows an address for
     itself, it should not take any new address from a received I-
     field.
 4.  If a message of type AR_S_Response arrives and the
     Destination_IEEE_Address field contains the node's own ULA, take
     the new switch address from the Destination_Switch_Address field
     and its type from the Destination_Address_Type field.
 5.  The node should invalidate its ARP tables when an AR_S_Response
     changes its own switch address, to force retransmission of ARP
     requests containing its new address to all the remote nodes with
     which it communicates.

Path MTU Discovery

 RFC 1191 [13] 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.

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

Renwick & Nicholson [Page 27] RFC 1374 IP and ARP on HIPPI October 1992

 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     |
         +----+-----+-------------------+-------------------+
                    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

Renwick & Nicholson [Page 28] RFC 1374 IP and ARP on HIPPI October 1992

 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.

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.
 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

Renwick & Nicholson [Page 29] RFC 1374 IP and ARP on HIPPI October 1992

 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:
                   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

Renwick & Nicholson [Page 30] RFC 1374 IP and ARP on HIPPI October 1992

 which a Source or Destination can process small packets; such limits
 may further reduce the available throughput if small packets are
 used.

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.

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:

Renwick & Nicholson [Page 31] RFC 1374 IP and ARP on HIPPI October 1992

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

Renwick & Nicholson [Page 32] RFC 1374 IP and ARP on HIPPI October 1992

       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.

Renwick & Nicholson [Page 33] RFC 1374 IP and ARP on HIPPI October 1992

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

Renwick & Nicholson [Page 34] RFC 1374 IP and ARP on HIPPI October 1992

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

Renwick & Nicholson [Page 35] RFC 1374 IP and ARP on HIPPI October 1992

    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.
    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.

Renwick & Nicholson [Page 36] RFC 1374 IP and ARP on HIPPI October 1992

    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.

Appendix B – How to Build a Practical HIPPI LAN

 "IP and ARP 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.
 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.
 6.  A server system is attached to the hub.  It provides multicast
     and ARP services.
 7.  All options of the Internet Draft are supported.  Hosts may use
     any form of address resolution: manual configuration, ARP with
     multicast, or ARP with a server.
 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.

Renwick & Nicholson [Page 37] RFC 1374 IP and ARP on HIPPI October 1992

    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
    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.

Renwick & Nicholson [Page 38] RFC 1374 IP and ARP on HIPPI October 1992

    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
    Internet Draft, 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.
    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 & Nicholson [Page 39] RFC 1374 IP and ARP on HIPPI October 1992

 Server Algorithm
    Different host implementations may take any of three approaches to
    address resolution:
    1.  Manual configuration, no ARP
    2.  Send ARP requests but expect a server to reply on its behalf
    3.  Send ARP requests and expect to receive them via multicast.
    If the network includes a server that is capable of both multicast
    and ARP service, and that knows the services expected by each
    host, all can coexist on the same net.
    The HIPPI-SC compliant switches are programmed to route the
    HIPPI-SC "broadcast" address FE0 (hex) to the server's port.  It
    is initially given the following information by a human network
    administrator:
    1.  The list of all addresses eligible to be used by network hosts
    2.  The list of addresses that should not receive multicast
        messages (a subset of list 1).  This is also the list of all
        hosts that either do manual configuration or expect a server
        to answer ARP requests.
    3.  The list of addresses of hosts that do manual configuration
        and do not send ARP requests (a subset of list 2) with the IP
        address corresponding to each one.
    The server maintains an address resolution cache that it
    initializes from list 3 (the manually configured hosts).  It will
    add to its cache as other hosts send ARP requests.
    When the server receives a message sent to the broadcast address
    FE0, it
    1.  Repeats the message to all addresses in list 1 but not in list
        2
    2.  If the message is a HIPPI-LE AR_Request with a piggybacked ARP
        Request, update the cache with information about the sender.
    3.  If the message is a HIPPI-LE AR_Request with a piggybacked ARP
        Request, the target system has an entry in the cache and the
        target is in list 2, respond to the ARP request.

Renwick & Nicholson [Page 40] RFC 1374 IP and ARP on HIPPI October 1992

    Server Optimizations
       1.  The server could be given a topological map of the hub and
           satellites from which it could construct list 1.
       2.  If all the hosts in list 2 ignore ARP messages as required
           in the Internet Draft, list 2 may be eliminated and the
           server can respond to all ARP requests (redundant replies
           may be sent).
 Sharing Switch Hardware With Other Devices
    Some host channels and peripheral devices that are connected to
    the switches may use protocols other than IP, and not participate
    in the LAN.  Since connections in a switch are independent, these
    applications can share switch hardware with no effect on LAN
    operation.  To ensure success:
       The server's lists of addresses should not include addresses
       for ports that are not used by LAN links or hosts.
       If non-LAN applications use paths between switches, separate
       links should be installed for them so that they do not use the
       same inter-switch links the LAN does.

References

[1] ANSI X3.183-1991, High-Performance Parallel Interface - Mechanical,

   Electrical and Signalling Protocol Specification (HIPPI-PH).

[2] ANSI X3.210-199X, High-Performance Parallel Interface - Framing

   Protocol (HIPPI-FP).

[3] ANSI X3.218-199X, 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 X3.222-199X, High-Performance Parallel Interface - Physical

   Switch Control (HIPPI-SC).

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

   Institute, September 1981.

Renwick & Nicholson [Page 41] RFC 1374 IP and ARP on HIPPI October 1992

[6] Postel, J., and Reynolds, J., "A Standard for the Transmission of

   IP Datagrams over IEEE 802 Networks", RFC 1042, USC/Information
   Sciences Institute, February 1988.

[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", RFC 1340,

   USC/Information Sciences Institute, July 1992.

[9] Plummer, D., "An Ethernet Address Resolution Protocol - or -

   Converting Network Protocol Addresses to 48.bit Ethernet Address
   for Transmission on Ethernet Hardware", RFC 826, MIT, November
   1982.

[10] Finlayson, R., Mann, T., Mogul, J., and Theimer, M., "A Reverse

   Address Resolution Protocol", RFC 903, Stanford University, June
   1984.

[11] Katz, D., "A Proposed Standard for the Transmission of IP Datagrams

   over FDDI Networks", RFC 1188, Merit/NSFNET, October, 1990.

[12] IEEE, "Draft Standard P802.1A–Overview and Architecture", 1989.

[13] Mogul, J.C., and Deering, S.E., "Path MTU discovery", RFC 1191,

   Stanford University, November, 1990.

Security Considerations

 Security issues are not discussed in this memo.

Authors' Addresses

 John K. Renwick
 Cray Research, Inc.
 655F Lone Oak Drive
 Eagan, MN 55121
 Phone: (612) 683-5573
 Mailing List: (none)
 EMail: jkr@CRAY.COM

Renwick & Nicholson [Page 42] RFC 1374 IP and ARP on HIPPI October 1992

 Andy Nicholson
 Cray Research, Inc.
 655F Lone Oak Drive
 Eagan, MN 55121
 Phone: (612) 683-5473
 Mailing List: (none)
 EMail: droid@CRAY.COM

Renwick & Nicholson [Page 43]

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