Network Working Group
Request for Comments: 1044
K. Hardwick
J. Lekashman
February 1988

Internet Protocol on Network Systems HYPERchannel

Protocol Specification


The intent of this document is to provide a complete discussion of the protocols and techniques used to embed DoD standard Internet Protocol datagrams (and its associated higher level protocols) on Network Systems Corporation's HYPERchannel [1] equipment. Distribution of this memo is unlimited.

This document is intended for network planners and implementors who are already familiar with the TCP/IP protocol suite and the techniques used to carry TCP/IP traffic on common networks such as the DDN or Ethernet. No great familiarity with NSC products is assumed; an appendix is devoted to a review of NSC technologies and protocols.

At the time of this first RFC edition, the contents of this document has already been reviewed by about a dozen vendors and users active in the use of TCP/IP on HYPERchannel media. Comments and suggestions are still welcome (and implementable,) however.

Any comments or questions on this specification may be directed to:

Ken Hardwick
Director, Software Technology
Network Systems Corporation MS029
7600 Boone Avenue North
Brooklyn Park, MN 55428


             (612) 424-1607
      John Lekashman
      Nasa Ames Research Center. NAS/GE
      MS 258-6
      Moffett Field, CA, 94035


             (415) 694-4359


    Status of this memo  . . . . . . . . . .  . . . . . . . . . . . .  1
    Goals of this document   . . . . . . . .  . . . . . . . . . . . .  3
    Basic HYPERchannel network messages  . .  . . . . . . . . . . . .  4
      Basic (16-bit address) Message Proper header  . . . . . . . . .  5
      TO addresses and open driver architecture   . . . . . . . . . .  7
      Extended (32-bit address) Message Proper header . . . . . . . .  8
      Address Recognition and message forwarding .  . . . . . . . . . 10
      32-bit message fields   . . . . . . . . . . . . . . . . . . . . 12
    Broadcasting   . . . . . . . . . . . . . . . . . .  . . . . . . . 14
    PROTOCOL SPECIFICATION .  .  .  . . . . . . . . . . . . . . . . . 17
      Basic (16-bit) Message Encapsulation    . . . . . . . . . . . . 18
      Compatibility with existing implementations . . . . . . . . . . 21
      Extended (32-bit) Message Encapsulation   . . . . . . . . . . . 24
      Address Resolution Protocol   . . . . . . . . . . . . . . . . . 27
      Maximum Transmission Unit . . . . . . . . . . . . . . . . . . . 31
    ADDRESS RESOLUTION    . . . . . . . . . . . . . . . . . . . . . . 32
      Local Address Resolution  . . . . . . . . . . . . . . . . . . . 33
      Configuration file format   . . . . . . . . . . . . . . . . . . 34
      ARP servers   . . . . . . . . . . . . . . . . . . . . . . . . . 35
      Broadcast ARP   . . . . . . . . . . . . . . . . . . . . . . . . 36
    Appendix A.
    NSC Product Architecture and Addressing   . . . . . . . . . . . . 38
    Appendix B.
    Network Systems HYPERchannel protocols    . . . . . . . . . . . . 42


In this document, there are four major technical objectives:

   1.  To bless a "de facto" standard for IP on HYPERchannel that  has
       been implemented by Tektronix, Cray, NASA Ames, and others.
       We are attempting to resolve some interoperability problems with
       this standard so as to minimize the changes to existing IP on
       HYPERchannel software.  If any ambiguities remain in the de facto
       standard, we wish to assist in their resolution.
  1. To address larger networks, NSC's newer network products are moving to a 32-bit address from the current 16-bit TO address. This document would introduce the addressing extension to the user community and specify how IP datagrams would work in the new addressing mode.
  1. To define an Address Resolution Protocol for HYPERchannel and other NSC products. It is probably well known that current NSC products do not support the broadcast modes that make ARP particularly useful. However, many have expressed interest in "ARP servers" at a known network address. These servers could fade away as NSC products with broadcast capability come into existence. Host drivers that can generate and recognize this ARP protocol would be prepared to take advantage of it as the pieces fall into place.
  1. Part of this effort is to standardize the unofficial "message type" field that reserves byte 8 of the HYPERchannel network message. To permit better interoperability, NSC will initiate a "network protocol registry" where any interested party may obtain a unique value in byte 8 (or bytes 8 and 9) for their own public, private, commercial or proprietary protocol. Lists of assigned protocol type numbers and their "owners" will be periodically published by NSC and would be available to interested parties.


Unlike most datagram delivery systems, the HYPERchannel network message consists of two parts:

             Message Proper
            |                    |
            |                    |
            |     10-64 bytes    |
            |                    |
            |                    |
             Associated Data
            |                                                    |
            |                                                    |
            |                                                    |
            |                                                    |
            |           Unlimited length                         |
            |                                                    |
            |                                                    |
            |                                                    |
            |                                                    |

The first part is a message header that can be up to 64 bytes in length. The first 10 bytes contain information required for the delivery of the entire message, and the remainder can be used by higher level protocols. The second part of the message, the "Associated Data," can be optionally included with the message proper. In most cases (transmission over HYPERchannel A trunks), the length of the associated data is literally unlimited. Others (such as HYPERchannel B or transmission within a local HYPERchannel A A400 adapter) limit the size of the Associated Data to 4K bytes. If the information sent can be contained within the Message Proper, then the Associated Data need not be sent.

HYPERchannel lower link protocols treat messages with and without Associated Data quite differently; "Message only" transmissions are sent using abbreviated protocols and can be queued in the receiving network adapter, thus minimizing the elapsed time needed to send and receive the messages. When associated data is provided, the HYPERchannel A adapters free their logical resources towards driving the host interface and coaxial trunks.


The first 10 bytes of the network Message Proper are examined by the network adapters to control delivery of the network message. Its format is as follows:

    byte   Message Proper
      0  |      Trunks to Try           |        Message Flags        |
         |   TO trunks  |  FROM trunks  |                 |EXC|BST|A/D|
      2  |                        Access code                         |
         |                                                            |
      4  |       Physical addr of       |                   | TO Port |
         |     destination adapter (TO) |                   | number  |
      6  |  Physical addr of source     |                   |FROM port|
         |        adapter (FROM)        |                   |  number |
      8  |                        Message type                        |
         |                                                            |
     10  |                                                            |
         |            Available for higher level protocols            |
         |                                                            |
         |                                                            |


Consists of two four bit masks indicating which of four possible HYPERchannel A coaxial data trunks are to be used to transmit the message and to return it. If a bit in the mask is ON, then the adapter firmware will logically AND it with the mask of installed trunk interfaces and use the result as a candidate list of interfaces. Whenever one of the internal "frames" are sent to communicate with the destination adapter, the transmission hardware electronically selects the first non-busy trunk out of the list of candidates. Thus, selection of a data trunk is best performed by the adapter itself rather than by the host. "Dedicating" trunks to specific applications only makes sense in very critical real time applications such as streaming data directly from high speed overrunnable peripherals.

A second Trunk mask is provided for the receiving adapter when it sends frames back to the transmitter, as it is possible to build "asymmetric" configurations of data trunks where trunk 1 on one box is connected to the trunk 3 interface of a second. Such configurations are strongly discouraged, but the addressing structure supports it if needed.

The "trunks to try" field is only used by HYPERchannel A. To assure maximum interoperability, a value of 0xFF should be placed in this field to assure delivery over any technology. Other values should only be used if the particular site hardware is so configured to not be physically connected via those trunks.


Contains options in message delivery. In the basic type of message, three bits are used:

ASSOCIATED DATA PRESENT (A/D) is ON if an Associated Data block follows the Message Proper. 0 if only a message proper is present in the network message. The value of this bit is enforced by the network adapter firmware.

BURST MODE (BST) Enables a special mode for time critical transfers where a single HYPERchannel A coaxial trunk is dedicated during transmission of the network message. Not recommended for anything that won't cause peripheral device overruns if data isn't delivered once message transmission starts.

EXCEPTION (EXC) Indicates to some channel programmed host interfaces that the message is "out of band" in some way and requires special processing.


A feature to permit adapters to share use of a cable yet still permit an "access matrix" of which adapter boxes and physically talk to which others. Not currently in use by anyone, support is being discontinued.


Consists of three parts. The high order 8-bits contains the physical address of the network adapter box which is to receive the message. The low order 8-bits are interpreted in different ways depending on the nature of the receiving network adapter. If the receiving adapter has different host "ports," then the low order bits of the TO field are used to designate which interface is to receive the message. On IBM data channels, the entire "logical" TO field is interpreted as the subchannel on which the incoming data is to be presented. Parts of the logical TO field that are not interpreted by the network adapter are passed to the host for further interpretation.


The FROM address is not physically used during the process of transmitting a network message, but is passed through to the receiving host so that a response can be returned to the point of origin. In general, reversing the TO and FROM 16-bit address fields and the TO and FROM trunk masks can reliably return a message to its destination.


The following two bytes are reserved for NSC. Users have been encouraged to put a zero in byte 8 and anything at all in byte 9 so as to not conflict with internal processing of messages by NSC firmware. In the past, this field has been loosely defined as carrying information of interest to NSC equipment carrying the message and not as a formal protocol type field. For example, 0xFF00 in bytes 8 and 9 of the message will cause the receiving adapter to "loop back" the message without delivering it to the attached host.

Concurrent with this document, it is NSC's intent to use both bytes 8 and 9 as a formal "protocol type" designator. Major protocols will be assigned a unique value in byte 8 that will (among good citizens) not duplicate a value generated by a different protocol. Minor protocols will have 16-bit values assigned to them so that we won't run out when 256 protocols turn up. Any interested party could obtain a protocol number or numbers by application to NSC. In this document, protocol types specific to IP protocols are assigned.


Since not all 16-bits of the TO address are used for the physical delivery of the network message, the remainder are considered "logical" in that their meaning is physically determined by host computer software or (in cases such as the FIPS data channel) by hardware in the host interface.

Since HYPERchannel is and will be used to support a large variety of general and special purpose protocols, it is desirable that several independent protocol servers be able to independently share the HYPERchannel network interface. The implementation of many of NSC's device drivers as well as those of other parties (such as Cray Research) support this service. Each protocol server that wishes to send or receive HYPERchannel network messages logically "connects" to a HYPERchannel device driver by specifying the complete 16-bit TO address it will "own" in the sense that any network message with that TO address will be delivered to that protocol server.

The logical TO field serves a function similar to the TYPE byte in the Ethernet 802.2 message header, but differs from it in that the width of the logical TO field varies from host to host, and that no values of the logical TO address are reserved for particular protocols. On the other hand, it is possible to have several "identical" protocols (such as two independent copies of IP with different HYPERchannel addresses) sharing the same physical HYPERchannel interface. This makes NSC's addressing approach identical to the OSI concept that the protocol server to reach is embedded within the address, rather than the IP notion of addressing a "host" and identifying a server through a message type.

Since the HYPERchannel header also has a "message type" field, there is some ambiguity concerning the respective roles of the message type and logical TO fields:

    o   The logical TO field is always used to identify the protocol
        server which will receive the message.  Once a server has
        specified the complete TO address for the messages it wishes to
        receive, the message will not be delivered to a different
        protocol server regardless of the contents of the message type
    o   Although the "type" field cannot change the protocol server at
        the final destination of the message, the type field can be used
        by intermediate processes on the network to process the message
        before it reaches the server destination.  An obvious example is
        the 0xFF00 message loopback type function, where network
        processing to loop back the message results in nondelivery to
        the TO address.  In the future, intermediate nodes may process
        "in transit" messages based on the message type only for
        purposes such as security validation, aging of certain
        datagrams, and network management.


In the original days of HYPERchannel, the limitation of 256 adapter "boxes" that could be addressed in a network message was deemed sufficient as 40 or so adapters was considered a "large" network. As with the Ethernet, more recent networks have resulted in a need to address larger networks. Although a few ad hoc modes have existed to address larger HYPERchannel networks for some years, newer technologies of HYPERchannel equipment have logically extended the network message to support 32-bits of addressing, with 24 of those bits to designate a physical network adapter.

This 32-bit header has been designed so that existing network adapters are capable of sending and receiving these messages. Only the network bridges need the intelligence to select messages designated for them.

     0  |      Trunks to Try           |        Message Flags        |
        |   TO trunks  |  FROM trunks  |GNA|CRC|     |SRC|EXC|BST|A/D|
     2  |         TO Domain #          |         TO Network #        |
        |                              |                             |
     4  |O|    Physical addr of        |                   | TO Port |
        |N|  destination adapter (TO)  |                   | number  |
     6  |O| Physical addr of source    |                   |FROM port|
        |N|     adapter (FROM)         |                   |  number |
     8  |                         Message type                       |
        |                                                            |
     10 |          FROM Domain #       |       FROM Network #        |
        |                              |                             |
     12 |          - reserved -        |         age count           |
        |                              |                             |
     14 |      Next Header Offset      |      Header End Offset      |
        |        (normally 16)         |        (normally 16)        |
     16 |                  Start of user protocol                    |
        |              bytes 16 - 64 of message proper               |
        |                                                            |
          Associated Data
   |                                                                 |
   |     As with basic format network messages                       |
   |                                                                 |


With the 32-bit form of addressing, NSC is keeping with the premise that the native HYPERchannel address bears a direct relation to the position of the equipment in an extended HYPERchannel network.

Each collection of "locally" attached NSC network adapters that are connected by coax or fiber optic cable (with the possible addition of nonselective repeaters such as the ATRn series) is considered a "network". Each network can have up to 256 directly addressable adapters attached to it which can be reached by the basic format network message.

Existing bridges or "link adapters" can be programmed to become "selective repeaters" in that they can receive network messages containing a subset of network addresses send them over the bridge medium (if present) and reintroduce them on the other network. Such interconnected local area networks are considered a single network from an addressing point of view.

A large NSC network can have up to 64K networks which can be complexly interconnected by network bridges and/or "backbone" networks which distribute data between other networks. To simplify the mechanics of message forwarding, the 16-bit network field is divided into two eight quantities, a "network number" identifying which network is to receive the message and a "domain number" which specifies which network of networks is the recipient.

The bridge technology adapters which move messages between networks have address recognition hardware which examines all the 24-bits in bytes 2-5 of the network message header to determine if the bridge should accept the message for forwarding. At any given instant of time in the network, each bridge will have a list of networks and domains that it should accept for forwarding to a network at the other end of the bridge. Each Adapter (Including Newer Technology host adapters) contains in address recognition hardware:

    o   domainmask -- a 256-bit mask of domain numbers that should  be
        accepted for forwarding (not local processing) by this adapter.
    o   MyDomain  --  the  value  of the domain on which this host
        adapter or bridge end is installed.
    o   NetworkMask -- a 256-bit mask of network numbers that should be
        accepted for forwarding by this adapter.
    o   MyNetwork  -  the  value of the network on which this host
        adapter or bridge end is installed.
    o   AddressMask -- A 256-bit mask of the local network addresses
        that should be accepted by the adapter.
    o   MyAddress -- the "base address" of the box, which must be
        supplied in any message that is directed to control processes
        within the adapter, such as a loopback message.

Address recognition takes place using the algorithm:

IF Domain IN DomainMask OR

IF (Domain = MyDomain AND Network IN NetworkMask) OR

IF (Domain = MyDomain AND Network = MyNetwork AND

                    Address IN AddressMask) THEN accept-message
                                            ELSE ignore-message.

This algorithm means that an adapter's hardware address recognition logic will accept any messages to the box itself, any secondary or aliased local addresses owned by the adapter, and any message directed to a remote network or domain that that particular adapter is prepared to forward.



Is as in the basic network message. Messages that are to be delivered outside the immediate network should have 0xFF in this byte so that all possible trunks in intermediate networks should be tried. Locally delivered 32-bit messages may still contain specially tailored trunk masks to satisfy local delivery needs.


The currently defined bits remain as before. Three new bits have been defined since that time.

CRC (END-END MESSAGE INTEGRITY). Newer technology host adapters are capable of generating a 32-bit CRC for the entire network message as soon as it is received over the channel or bus interface from the host. This 32-bit CRC is appended to the end of the associated data block and is preserved through the entire delivery process until it is checked by the host adapter that is the ultimate recipient of the message, which removes it. This end to end integrity checking is designed to provide a high degree of assurance that data has been correctly moved through all intermediate LAN's, geographic links, and internal adapter hardware and processes.

SRC (SOURCE FROM ADDRESS CORRECT). This bit is provided to take advantage of the physical nature of the network address to optionally verify that the 32-bit FROM address provided in the network message is in fact the location that the message originated. If the bit is not set by the transmitting host, no particular processing occurs on the message. If the bit is set, then all intermediate adapters involved in the delivery of the message have the privilege of turning the bit off if the received message FROM address is not a TO address that would be delivered to the originator if the message were going the opposite direction.

If the message is received by a host computer with this bit still set, then the FROM address is guaranteed correct in the sense that returning a message with TO and FROM information reversed will result in delivery of the message to the process that actually originated it. By careful attention to the physical security of adapters and intermediate links between networks, a high degree of security can be built into systems that simply examine the FROM address of a message to determine the legitimacy of its associated request.

GNA (GLOBAL NETWORK ADDRESSING). This bit ON indicates that 32-bit addressing is present in the message. When this bit is on, bytes 2-3 (Domain and Network numbers) should also be nonzero.


Four bytes contain the TO address, which is used to deliver the network message as described in "Address Recognition and Message Forwarding" on page 8. The "logical" part of the TO address is used to designate a protocol server exactly as in the basic format network message header.

The existing "address" field has its high order bit reserved as an outnet bit for compatibility with existing A-series network adapter equipment. Were it not for this bit, the A-series adapters would attempt to accept messages that were "passing through" the local network on their way elsewhere simply because the address field matched while the the Domain and Network numbers (ignored by the A- series adapters) were quite different.

This "outnet" bit is used in the following way:

    o   All network adapters (of  any type) in an extended set of
        networks containing A-Series adapters that will ever use 32-bit
        addressing must have their addresses in the range 00-7F (hex.)
    o   If a message is to be sent to a destination on a nonlocal
        network and domain on such an extended network, then the
        high order bit of the address field is turned on.
    o   When the last bridge in the chain realizes that it is about to
        forward the message to its final destination (the Domain and
        Network numbers are local), then it turns the Outnet bit off.
        This will result in local delivery to the destination adapter.


The FROM address follows the same logic as the TO address in that any message can be returned to its source by reversing the FROM and TO fields of the message. Since so many protocols examine byte 8 of the message to determine its type, the FROM field has been split so that the Domain and Network numbers extend into bytes 10-11.


This field (informally defined in the past) has been extended to 16- bits so that a unique value can be assigned to any present or future protocol which is layer on HYPERchannel messages for either private or public use.


This field serves the same purpose as the IP "time to live" in that it prevents datagrams from endlessly circulating about in an improperly configured network. Each time a 32-bit message passes through a bridge, the Age Count is decremented by one. When the result is zero, the message is discarded by the bridge.


These are used as fields to optionally provide "loose source routing", where a list of 32-bit TO addresses can be provided by the transmitter to explicitly determine the path of a message through the network. If this feature is not used, both these fields would contain the value 16 (decimal) to both indicate extra TO addresses are absent and that the beginning of protocol data following the HYPERchannel header is in byte 16.

Although it is conceivable that a HYPERchannel IP process could use this source routing capability to direct messages to hosts or gateways, this capability is not felt to be of sufficient value to IP to build it into a HYPERchannel IP protocol.

In the future, all higher level protocols should be able to examine Header End Offset to determine the start of the higher level protocol information.


NSC message forwarding protocols use low level link protocols to negotiate transmission of a message to its next destination on the network. Furthermore, NSC network boxes often "fan out" so that several hosts share the same network transmission equipment as in the A400 adapter. Both these characteristics mean that providing a genuine broadcast capability is not a trivial task, and in fact no current implementations of NSC technology support a broadcast capability.

The last several years have seen broadcast applications mature to the point where they have virtually unquestioned utility on a local and sometimes campuswide basis. Accordingly, new NSC technologies will support a broadcast capability. Information on the use of this capability is included here as it is essential to the discussion of the Address Resolution Protocol later in this document.

Broadcast capability will be supported only with the extended (32-bit address) message format. A broadcast message will have the following general appearance:

    byte   Message Proper
      0  |      Trunks to Try           |        Message Flags        |
         |   TO trunks  |  FROM trunks  |GNA|CRC|     |SRC|EXC|BST|A/D|
      2  |       TO Domain Number       |      TO Network Number      |
         |          or 0xFF             |          or 0xFF            |
      4  |           0xFF               |   Broadcast channel number  |
         |                              |                             |
      6  |O| Physical addr of source    |                   |FROM port|
         |N|     adapter (FROM)         |                   |  number |
      8  |                         Message type                       |
         |                                                            |
      10 |     FROM Domain Number       |    FROM Network Number      |
         |                              |                             |
      12 |          - reserved -        |         age count           |
         |                              |                             |
      14 |      Next Header Offset      |      Header End Offset      |
         |        (normally 16)         |        (normally 16)        |
      16 |                  Start of user protocol                    |
         |              bytes 16 - 64 of message proper               |
         |                                                            |
          Associated Data
    |                                                                 |
    |     As with basic format network messages                       |
    |     Maximum associated data size 1K bytes.                      |
    |                                                                 |


These fields are defined just as with a normal 32-bit message. All bits in the Message Flags field are valid with broadcast modes.


For Domain, Network and Adapter Address fields, the value 0xFF is reserved for use by the broadcast mechanism. A value of 0xFF in the adapter address field indicates to the local network hardware that this message is to be sent to all connected network equipment on the individual network.

A value of 0xFF in the network or domain fields, respectively indicates a request that the scope of the broadcast exceed the local network. The bridging link adapters will receive the broadcast message along with everyone else and will examine the "Broadcast Channel" field and their internal switches to determine if the message should be forwarded to other remote networks.

If the Network and Domain fields contain the local network and domain, then the broadcast message will only be broadcast within the local network. If a remote Network and Domain is specified, then the message will be delivered as a single message to the remote network and broadcast there.


Since individual hosts and protocol servers generally are not interested in all broadcast messages that float about the network, a filtering mechanism is provided in the header and network adapter equipment so that only proper classes of broadcast messages are delivered to the end point.

Broadcast channel numbers in the range 00-0xFF will be assigned by NSC much like the "message type" field. Host protocol servers specify a specific TO address containing a channel number (such as 0xFF04) when they bind themselves to the HYPERchannel device driver. The driver and the underlying equipment will deliver only broadcast messages with the correct channel number to the protocol server. If a protocol server wishes to receive several different broadcast messages, it must bind itself to the driver several times with the desired addresses.

Link adapters that are prepared to handle multinetwork broadcast messages may be equipped with switches to determine which broadcast channels will be propagated into the next network. Since multinetwork broadcast is an arrangement that must be configured with care, these switches are off by default.


The FROM address is constructed just as with a normal 32-bit network message. The Source Address Correct bit is processed just as with a normal message.


Message type is defined as with normal messages. Presumably broadcast applications will have unique message types that are not generally found in normal messages.


Age count is vitally important in a multinetwork broadcast as "loops" in the network can cause a great deal of activity until all the progeny of the original broadcast message die out.


This section contains information on the technique used to encapsulate IP datagrams on the HYPERchannel network message. It contains three sections to describe three protocol packagings:

    o   The technique used to encapsulate IP datagrams on the basic
        16-bit network message.  This is a de facto standard that has
        been in use for several years and is documented here to make it
    o   The encapsulation technique for IP datagrams on 32 bit network
    o   The definition of an Address Resolution Protocol on


           Message Proper
      0  |      Trunks to Try           |        Message Flags        |
         |   TO trunks  |  FROM trunks  |GNA|CRC|     |SRC|EXC|BST|A/D|
      2  |                      Access code 0000                      |
         |                   (no longer supported)                    |
      4  |       Physical addr of       |  Protocol server  |Dest Port|
         |     destination adapter      |  logical address  | number  |
      6  |       Physical addr of       |    Originating    | Src Port|
         |       source  adapter        |  server address   |  number |
      8  |    IP on HYPERchannel        |   Offset to start of IP     |
         |    type code  0x05           |  header from message start  |
     10  |      IP type designator      |   Offset to start of IP     |
         |           0x34               |    header from byte 12      |
     12  |          Padding (variable length incl. zero bytes)        |
         |                                                            |
     Off |          First (64-Offset) bytes of IP datagram            |
         |                                                            |
         |                                                            |
         |                                                            |
           Associated Data
         |                                                            |
         |                Remainder of IP datagram                    |
         |                                                            |
         |            No associated data is present if IP             |
         |            datagram fits in the Message Proper             |
         |                                                            |


From the vantage of an IP driver, any trunk mask is valid so long as it results in successful delivery of the HYPERchannel network message to its destination. There is no reason to check this field for validity on reception of the message. Specification of the Trunk Mask on output is a local affair that could be specified by the transmitting driver's address resolution tables.


No use is made of the Flags field (byte 1) other than to appropriately set the Associated Data bit. Burst Mode and the Exception bit should not be used with IP.


Although some current implementations of IP on HYPERchannel support the access code, no one appears to be using it at the current time. Since this field is currently reserved for the use of 32-bit addresses, no value other than 0000 should be placed in this field.


The TO field is generally obtained by a local IP driver through a table lookup algorithm where a 16-bit TO address is found that corresponds to the IP address of a local host or gateway. The high order bits of the TO address of course refer to the adapter number the adapter attached to the destination host.

The logical TO field should contain the protocol server address of the HYPERchannel IP driver for that host as determined by the host's system administrator. Many HYPERchannel TCP/IP drivers in the field today are not "open" in that any network message delivered to that host will be presumed to be an IP datagram regardless of the logical TO field; however any transmitting IP process should be capable of generating the entire 16-bit TO field in order to generate a message capable of reaching a destination IP process.

The process of determining which HYPERchannel address will receive an IP datagram based on its IP address is a major topic that is covered in "Address Resolution".


The FROM address is filled in with the address that the local driver expects to receive from the network, but no particular use is make of the FROM address.


Network Systems requests that a value of 5 (decimal) be placed in this byte to uniquely indicate that the network message is being used to carry IP traffic. No other well-behaved protocol using HYPERchannel should duplicate this value of 5.

Many current implementations of IP on HYPERchannel place a zero or other values in this field simply because no value was reserved for IP usage. Transmitting versions of IP should always place a 5 in this field; receiving IP's should presume a delivered message to be an IP datagram until proven otherwise regardless of the contents of the Message Type field.

   Developers should note that it is often convenient to permit
   reception of the value 0xFF00 in bytes 8 and 9 of the IP datagram.
   Transmitting a message with this value will cause it to be looped
   back at the destination adapter and returned to the protocol server
   designate in the FROM address.  This permits the developer have host
   applications talk to others on the same host for purposes of network
   interface or other protocol debugging.

Byte 9 contains the offset to the start of the IP header within the message proper, such that the Message Proper address plus the IP header offset generates the address of the first byte of the IP header (at least on byte addressable machines.)

This field is redundant with the offset field in byte 11, and is present for cosmetic compatibility with 32-bit implementations. On reception, the value in byte 11 should take precedence.

As part of the migration to larger HYPERchannel headers, this field will become significant with the 32-bit addressing format, as the length of the header is no longer 10 bytes and byte 11 is used for other purposes.


Early implementations of IP drivers on HYPERchannel wanted to leave bytes 8 and 9 alone for NSC use and place a "message type" field in later in the message. A value of 0x34 had been selected by earlier developers for reasons that are now of only historical interest. Once again, implementations should generate this value on transmission, but not check it on input, assuming that an IP datagram is present in the message.


This value is used by a number of commercial implementations of IP on HYPERchannel to align the start of the IP header within the network message. This offset is relative to byte 12 of the network message so that a value of zero indicates that the IP header begins in byte 12. This value should be both correctly generated on transmission, and always respected on input processing.

The maximum permissible offset in this field is 52 indicating that the IP header begins at the start of the associated data block.


Beginning at the offset designated in byte 11, the IP datagram is treated as a contiguous block of data that flows from byte 63 of the message proper into the first byte of associated data, so that the entire message plus data is treated as a single contiguous block.

If the IP header is small enough to fit within the entire network message, then only the message proper is transmitted. The length of the message proper sent should always be 64 bytes, even if the IP datagram and HYPERchannel header do not occupy all 64 bytes of the message proper.

If the datagram flows over into the associated data, then both message and data are sent. Since a number of machines cannot send a length of data to the HYPERchannel that is an exact number of bytes (due to 16-64 bits on the channel bus,) the length of the associated data received should not be used as a guide to the length of the IP datagram -- this should be extracted from the IP header. A driver should verify, of course, that the associated data received is at least as long as is needed to hold the entire IP datagram.


The basic format described here is clearly a compromise between several implementations of IP on HYPERchannel. Not all existing implementations are interoperable with the standard described above. Currently there are two known "families" of IP HYPERchannel drivers in existence:


This protocol is in the widest production use and has the largest number of supported drivers in existence. It is interoperable and identical with the standard described above with the sole exception that bytes 8 and 9 are set to zero by these drivers. As these bytes are ignored by most implementations of this driver, they have been assigned values to formalize the use of the message type field and to make it consistent with the 32-bit protocol.


This protocol was historically the first IP on HYPERchannel implementation developed (at Tektronix) and subsequently made its way to Berkeley and BSD UNIX. This protocol is not interoperable with the standard described above due to several distinct differences.

First, bytes 8 through 11 are always zero. The IP header always starts on byte 12. Comments in some of these drivers designate byte 11 as an "IP header offset" field, but apparently this value is never processed.

The major difference (and the incompatibility) concerns the packaging of the IP datagram into the network message. Due to historical difficulties in the early 80's with the sending and receiving of very small blocks of associated data on VAXes, this protocol the takes a curious approach to the placement of the IP header and the headers of higher level protocols (such as TCP or UDP.)

    o   If the entire length of the IP datagram is 54 bytes or less,
        it is possible to fit the entire datagram and the HYPERchannel
        header in the 64 byte message proper.  In this case, no
        associated data is sent; only a message proper is used to carry
        the data.  The length of the message proper transmitted is the
        exact length needed to enclose the IP datagram; no padding bytes
        are sent at the end of the message.
    o   If the length of the IP header is greater than 54 bytes, then:
        -   All higher level protocol information (TCP/UDP header and
            their associated  data fields) are placed in the associated
            data block, with the TCP/UDP header beginning at the start
            of the associated data block.
        -   On transmission, the length of the message proper
            transmitted is set to the length of the HYPERchannel header
            plus the IP header --  it is not padded out to 64 bytes.
            The length of the associated data sent should be sufficient
            to accommodate the TCP/UDP header and its data fields.


In choosing which to follow, the "Cray-Ames" approach was taken for several reasons:

  1. Cray Research has performed exemplary work in dealing with other vendors to provide IP on HYPERchannel from the Cray computers to other hosts. As a result, there are 4 or 5 vendor supported implementations of IP on HYPERchannel that use this approach.
  1. The two part structure of the message proper has its uses when a machine wishes to make protocol decisions before staging the transfer of an immense block of associated data into memory. Many network coprocessors and intelligent I/O subsystems find it simpler to read in the entire network message before deciding what to do with it. Arbitrarily catenating the two components does this best and permits streaming of messages from future technology network adapters.
    3.  Some TCP users (mostly  secure  DoD  sites) intend to load up IP
        datagrams with optional fields in the future.  The
        Tektronix-Berkeley implementation has problems if the IP header
        length exceeds 54 bytes.


           Message Proper
      0  |      Trunks to Try           |1|       Message Flags       |
         |   TO trunks  |  FROM trunks  |GNA|CRC|     |SRC|EXC|BST|A/D|
      2  |    Destination  Domain       |    Destination  Network     |
         |         Number               |           Number            |
      4  |O|     Physical addr of       |  Protocol server  |Dest Port|
         |N|  destination adapter       |  logical address  | number  |
      6  |O|     Physical addr of       |    Originating    | Src Port|
         |N|     source  adapter        |  server address   |  number |
      8  |    IP on HYPERchannel        |   Offset to start of IP     |
         |    type code  0x06           |      datagram header        |
      10 |    Source Domain Number      |   Source Network Number     |
         |                              |                             |
      12 |          - reserved -        |         Age Count           |
      14 |      Next Header Offset      |      Header End Offset      |
         |                              |       (usually 16)          |
      16 |         Padding to IP header start (usually 0 bytes)       |
         |                                                            |
      Off|     Entire IP datagram if datagram length <= (64-Offset)   |
         |                                                            |
         |        else first (64-Offset) bytes of IP datagram         |
           Associated Data
         |                                                            |
         |                   Remainder of IP datagram                 |
         |                                                            |
         |            No associated data is present if IP             |
         |            datagram fits in the Message Proper             |
         |                                                            |


From the vantage of an IP driver, any trunk mask is valid so long as it results in successful delivery of the HYPERchannel network message to its destination. There is no reason to check this field for validity on reception of the message. Specification of the Trunk Mask on output is a local affair that can be specified by the transmitting driver's address resolution tables.

The use of 0xFF in this value is strongly encouraged for any message other than those using exotic trunk configurations on a single local network.


Several new bits have been defined here.

EXTENDED ADDRESSING. This bit should be set ON whenever a 32-bit address (Network and/or Domain numbers nonzero) is present in the message. It should always be OFF with the 16-bit message header. If this bit is improperly set, delivery of the message to the (apparent) destination is unlikely.

END-TO-END CRC. Some newer technology adapters are equipped to place a 32-bit CRC of the associated data at the end of the associated data block when this bit is on. Similarly equipped adapters will examine the trailing 32-bits of associated data (when the bit is on) to determine if the message contents have been corrupted at any stage of the transmission.

Transmitting device drivers should include the ability to set this bit on transmission as a configuration option similar to the specific HYPERchannel device interface used. The bit should be generated to be turned ON if the HYPERchannel IP driver is attached to an adapter equipped to generated CRC information -- it should be left OFF in all other circumstances.

If a message arrives at the host with the CRC bit still on, this indicates that the CRC information was placed at the end of associated data by the transmitting adapter and not removed by the receiving adapter; thus the associated data will be four bytes longer than otherwise expected. Since the IP datagram length is self contained in the network message, this should not impact IP drivers.

It is possible for host computers to both generate and check this CRC information to match the hardware assisted generation and checking logic in newer network adapters. Contact NSC if there are particular applications requiring exceptional data integrity that could benefit from host generation and checking.

FROM ADDRESS CORRECT. This bit should be set by all transmitting IP drivers who have endeavored to provide a completely correct FROM address that properly reflects the adapter interface used. No action should be taken on this bit by the receiving IP driver at this time. Additional work needs to be done to determine the action an IP driver should take if it detects a real or imagined "security violation" should a message arrive with this bit absent.


The TO address logically constitutes bytes 2-5 of the network message.

NETWORK AND DOMAIN NUMBERS. The Network and Domain numbers should both be nonzero when 32-bit addressing is used. If the message is local in nature, then the local Network and Domain numbers should be placed in this field.

ADAPTER ADDRESS. Contains the adapter address as in the basic message. The high order bit of this eight bit field (the "outnet" bit) should be set to zero if the destination network and domain are the same as the transmitting host's. The high order bit should be set to one if the destination host is not in the local network or domain.

LOGICAL TO AND SUBADDRESS. The logical TO field should contain the protocol server address of the HYPERchannel IP driver for that host as determined by the host's system administrator.


The FROM address is filled in with the address that the local driver expects to receive from the network, but no particular use is made of the FROM address.


The value 6 must be placed in this byte to uniquely indicate that the network message is being used to carry IP traffic. No other well- behaved protocol using HYPERchannel should duplicate this value of 6.

Note that all IP drivers should be prepared to send and receive the basic format network messages using the 16-bit HYPERchannel addresses. The driver can distinguish an incoming network message by the value of byte 8 -- 32-bit messages will always have a 6 in byte 8, while 16-bit messages should have a 5 here. For interoperability with older drivers, a value of 0 here should be treated as 16 address bit messages.


Byte 9 contains the offset to the start of the IP header within the message proper, such that the Message Proper address plus the IP header offset generates the address of the first byte of the IP header (at least on byte addressable machines.)

Unlike the 16-bit header, receiving IP drivers should assume that this field contains a correct offset to the IP header and examine the information at that offset for conformance to an IP datagram header.

Valid offsets are in the range of 16 through 44 bytes, inclusive. The limitation of 44 bytes is imposed so that routing decisions on the vast majority of IP datagrams can be made by examining only the message proper, as the basic IP datagram will fit into the message proper if it begins at an offset of 44.


The message and data are treated as logically contiguous entities where the first byte of associated data immediately follows the 64th byte of the message proper.

If the entire IP datagram is less than or equal to (64-offset) bytes in length it will fit into the Message Proper. If so, only a message proper containing the HYPERchannel header and IP datagram is sent on the network.

If the IP datagram is greater than this length, the IP datagram spills over into the associated data. On transmission, a 64 byte message proper is sent followed by as many bytes of associated data as are needed to send the entire datagram.

On reception, the message proper can be read into the start of an IP input buffer and the associated data read into memory 64 bytes from the start of the message. If the received message is in fact a 32- bit address message, no "shuffling" of the message will be required to build a contiguous IP datagram -- it's right there at buffer+16.


Address Resolution Protocol has achieved a great deal of success on the Ethernet as it permits a local IP network to configure itself simply by having each node know its own IP address. Those unfamiliar with the intent, protocol, and logic of the Address Resolution Protocol should refer to RFC-826, "An Ethernet Address Resolution Protocol" [2].

A later section of this document describes four techniques where a target HYPERchannel address is to obtained given the destination's IP address. The protocol is defined in this section for completeness.

           Message Proper
      0  |      Trunks to Try           |1|       Message Flags       |
         |   TO trunks  |  FROM trunks  |GNA|CRC|     |SRC|EXC|BST|A/D|
      2  |      Server Domain or        |      Server Network or      |
         |          0xFF                |           0xFF              |
      4  |   Server Adapter Address or  | Server logical addr/port or |
         |           0xFF               |             07              |
      6  |O|     Physical addr of       |    Originating    | Src Port|
         |N|     source  adapter        |  server address   |  number |
      8  |                      NSC ARP type code                     |
         |             07               |             00              |
      10 |         Source Domain        |       Source Network        |
      12 |          - reserved -        |         Age Count           |
      14 |      Next Header Offset      |      Header End Offset      |
         |        (usually 16)          |       (usually 16)          |
      16 |        Padding to start of IP info (usually 0 bytes)       |
     Off |          ARP hardware address type for HYPERchannel        |
         |                              8                             |
      +2 |                 HYPERchannel protocol type                 |
         |             06                           00                |
      +4 | HYPERchannel address length  |     IP address length       |
         |             6                |           4                 |
      +6 |               ARP opcode (request or reply)                |
      +8 |          Domain              |           Network           |
         +-           Sender's 32-bit HYPERchannel address           -+
     +10 |       Adapter address        |     Logical addr/port       |
     +12 |                      Source's MTU size                     |
     +14 |                              |                             |
         +-                Sender's 32-bit IP address                -+
     +16 |                                                            |
     +18 |          Domain              |           Network           |
         +-        Destination's 32-bit HYPERchannel address         -+
     +20 |                (to be determined on request)               |
         |       Adapter address        |     Logical addr/port       |
     +22 |                  Destination's MTU size                    |
         |               (to be determined on request)                |
     +24 |                              |                             |
         +-             Destination's 32-bit IP address              -+
     +26 |                                                            |

Layout of the fields of this ARP message is a fairly straightforward exercise given the standards of ARP and the 32-bit message header. A few fields are worth remarking upon:


The TO address of an ARP message will be one of two classes of address. A "normal" address indicates that the message is an ARP response, or that it is an ARP request directed at an ARP server at a well known address on the local network. For those HYPERchannel networks which are equipped to broadcast, a value of 0xFFFFFF07 in the TO address will (by convention) be picked up only by those protocol servers prepared to interpret and respond to ARP messages.

The issue of which address to use in an ARP request is discussed in the Address Resolution section.


Must be the correct FROM address of the user protocol server issuing an ARP request. The Source Correct bit in the Message Flags byte should be set by this requesting server, as some ARP servers may someday choose to issue ARP information on an "need to know" basis in secure environments. With an ARP response, the FROM address will contain the "normal" HYPERchannel address of the protocol server responding to the ARP address, even if that server was reached via broadcast mechanisms.

ARP responses are returned to the party specified in the FROM address specified in the message header, rather than the address in the "Source HYPERchannel Address" field within the body of the ARP message.


The 16-bit value 0x0700 is reserved for the exclusive use of ARP. Unlike IP messages, no provision is made for the ARP message to begin at an arbitrary offset within the message proper, so the value in byte 9 is an extension of the message type.


ARP uses the 32-bit addressing convention that byte 15 contains the offset to the start of user protocol (and hence the end of user protocol information). Note that this is not a substitute for the IP offset fields, as this field also serves as the end of HYPERchannel header information -- future NSC message processing code may well take exception to "garbage" between the actual header end and the start of user data.


This 16-bit number is assigned a formal ARP hardware type of 8.


On the Ethernet, this field is used to distinguish IP from all other protocols that may require address resolution. To be logically consistent, this field is identical to bytes 8 and 9 0x0600 in a 32- bit address HYPERchannel message carrying an IP datagram.


This contains the value 6, a sufficient number of bytes to accommodate the four byte HYPERchannel address and 2 bytes to indicate the largest IP datagram size that source and destination can handle.


This field contains the Domain, Network, and Adapter/port address of source and destination, respectively. A value of 0000 in the Domain and Network fields has special significance as this is interpreted as a request to send and receive 16-bit HYPERchannel headers rather than 32-bit headers. If 32-bit headers are to be used within a single HYPERchannel network, then the local domain and network numbers may be specified.


HYPERchannel LAN technology is such that messages of unlimited length may be sent between hosts. Since host throughput on a network is generally limited by the rate the network equipment can be functioned, larger transmission sizes result in higher bulk transfer performance. Since not every host will be able to handle the maximum size IP datagram, a more flexible means of MTU (maximum transmission unit) size negotiation than simply wiring the same value into every network host is needed. With this field, each host declares the maximum IP datagram size (not the associated data block size) it is prepared to receive. Transmitting IP drivers should be prepared to send the minimum of the source and destination IP sizes negotiated at ARP time.

The MTU size sent refers to the maximum size of IP header + data. It does not include the length of the HYPERchannel Hardware header or any offset between the header and the start of the IP datagram. Since it is the option of the transmitting hosts to use an offset of up to 44 bytes a receiving host must in any event be prepared to receive a 64 byte Message Proper and an Associated Data block of MTU-20 (that is 64 - 44, or the length of the basic IP header).

An example of a typical 16-bit packet is:

            12 bytes hardware header.
            12 bytes offset.
            40 bytes IP/TCP header.
          4096 bytes of data.
       This gives an MTU of 4136.

An example of a typical 32-bit packet is:

            16 bytes hardware header.
             8 bytes offset.
            40 bytes  IP/TCP header.
          4096 bytes of associated data,
       This also gives an MTU of 4136.

The offset values are chosen so that the typical packet causes user data to be page aligned at the start of the associated data area. This is an implementation decision, which can certainly be modified as required.

The maximum maximum transmission unit is 65536, the current largest size IP datagram. In order to allow this value to fit into a 16-bit field, the offset length is not included in the MTU. This MTU size is not a requirement that a local host be equipped to send or receive datagrams of that size; it simply indicates the maximum capacity of the receiving host.

A note on trunk masks:

There is no field for specifying trunk masks. This is intentional, as new NSC hardware will contain trunk reachability information, eliminating the need for the host to maintain hardware configuration layouts. All HYPERchannel messages generated as a result of an ARP response should use 0xFF in the trunk mask.


This section describes techniques used by an IP driver to determine the HYPERchannel address and header that a message should contain given an IP datagram containing an IP address. It describes techniques that are local to specific hosts (and hence can be modified without regard to the activities or techniques of other hosts) as well as techniques to use the Address Resolution Protocol on existing HYPERchannel equipment to better manage IP addresses.

It also discusses the migration of name resolution on one of four steps.

  1. Truncation of the IP address to form a HYPERchannel address.
  1. Local resolution of HYPERchannel addresses through configuration files.
  1. Centralized resolution of HYPERchannel addresses through an "ARP server" driven by a configuration file.
  1. Distributed resolution of HYPERchannel addresses using a "real" address Resolution Protocol on future HYPERchannel media supporting a broadcast mode.


A number of IP on HYPERchannel implementations support modes where the HYPERchannel address is generated by placing the low order 16- bits of the IP address in the TO address of the message proper. This more or less treats a set of HYPERchannel boxes addressable through 16-bit HYPERchannel addresses as a Class B IP network.

   This approach certainly offers simplicity:  IP addresses are simply
   chosen to match HYPERchannel addresses and no IP address
   "configuration files" need be kept.  Although this approach works in
   an environment where the HYPERchannel completely constitutes a Class
   B network, or where connection to a larger IP network is not a
   concern, its long term use is discouraged for several reasons:
    o   It simply will not work with any Class C address (the physical
        TO address is not controllable) or a Class A address (where host
        addresses are generally carefully administered.)  In addition,
        it will not support subnetworks.  It is quite incompatible with
        32-bit HYPERchannel addresses.
    o   By decoupling the IP and HYPERchannel addresses through more
        complex address resolution, the characters of the two addresses
        allow greater site flexibility:  the IP address becomes
        "logical" in character so that an address can move about a site
        with the user or host; the HYPERchannel address maintains its
        physical character so that a HYPERchannel address carefully
        identifies the physical location of the source and destination
        within the extended HYPERchannel network.


   The current state of address resolution art with IP on HYPERchannel
   works as follows:  given an arbitrary IP address, the IP HYPERchannel
   driver looks up an entry with that address in a (generally hashed)
   table.  If found, the table entry contains the first 6 bytes of the
   HYPERchannel header that is used to send the IP datagram to the next
   IP node on the internet.  Since implementations such as the 4.3BSD
   UNIX IP are clever enough to provide its lower level drivers with the
   IP address of the next gateway as well as the destination address on
   the internet (assuming the message is not delivered locally on the
   HYPERchannel,) the number of entries in this table is more or less
   manageable, as it must only contain the IP hosts and gateway
   addresses that are directly accessible on the HYPERchannel.


So long as this technique of address resolution is used, the techniques used are exclusively local to the host in the sense that the techniques used to generate and store the information in the table are irrelevant to other hosts.

Shown here is a typical file format. This file should probably be program generated from a database, as asymmetric trunk configurations and multiply homed hosts can cause differences in physical routing and trunk usage. This format is documented here to illustrate what sort of information must be kept at the link layer.

The file consists of source lines each with the form:

<type> <hostname> <trunks/flags> <domain/net> <addr> <MTU>

an example:

           <type>  <hostname>             <t/f> <dom/net> <addr>  <MTU>
           # Random front end
           host          FF88    0103    3702    4148
           # because we want to show the 4 byte format
           host            FF00    0000    2203    1024
           # Small packets, interactive traffic.
           host     FF88    0103    4401    4148
           # The other interface, for big packets.
           ahost     FF88    0103    4501    32768
           # A loopback interface, (What else)
           loop         FF00    0000    3700    4148
           # And of course an example of arp service.
           arpserver       FF88    0103    7F07
    Comments may begin with  either # or ;.
    Case is not significant in any field.

<type> indicates the type of entity to be defined.

Currently defined types are "host," "ahost", "loop," "address," and "arpserver".

      host    This token indicates an IP  host.  The following field  is
              expected to be a name that can be resolved to an IP
      ahost   This field indicates an additional network interface to
              the same host.  This may be used for performance
      loop    Sets a flag in the entry for that host so that  0xFF00 is
              placed in bytes 8 and 9 of the message.  This will cause
              the IP datagram  to be directed towards the specified host
              (which must still be a valid host name) and looped back
              within the remote adapter.  This facility serves both as a
              debugging aid and as a crude probe of the availability of
              the remote network adapter.

arpserver This indicates an address to use for directing ARP

              requests to the network.  If several arpserver addresses
              are specified, they will be tried in turn until a response
              is received (or we run out of servers.)  An arpserver with
              the  appropriate  broadcast address of FFFF FF07 would
              cause an ARP broadcast to take place when broadcasting
              becomes available.  Broadcast and specific addresses may
              be used in combination.

<hostname> This field is the logical name of the destination. For a host it is the logical name to be given to the local naming service to determine the associated IP address. This field may contain four decimal numbers separated by dots, in which case it is assumed to be the explicit IP address.

<trunks/flags> This field is the value to be placed in bytes 0 and 1 of the message header when sending to this host. The associated data bit need not be supplied as the driver must control it. All other bits are sent as provided. This field is a hexidecimal number.

<domain/net> This field is the value to be placed in the Domain and Network number field of the message. A value of 0000 in this field indicates that the destination should be reached by constructing a 16-bit HYPERchannel header, rather than a 32-bit header.

<address> This field is the value to be placed in the 16-bit TO field to reach <hostname>. This field is a hexidecimal number.

<MTU> This field contains the largest size IP datagram that the destination host is prepared to receive. This field is a decimal number. This field is optional. If not present, a value of 4148 is assumed. See the earlier discussion on Maximum Transmission Unit for more detail.


The primary problem with local host address resolution is that changes or additions to hosts on the local net must be replicated to every HYPERchannel host in that network. While this is manageable for up to half a dozen hosts, it becomes quite unmanageable for larger networks. An approach that can be implemented using existing HYPERchannel technology is to have a server on the HYPERchannel network provide the HYPERchannel destination address that is associated with an IP address.

Although this is strictly a point-to-point request/response dialogue between two network nodes, the Address Resolution Protocol which was originally designed for Ethernet (but thoughtfully constructed to work with any pair of link and network addresses) performs an excellent job.

ARP servers can be reached simply by placing the address of the server in the 32-bit TO address of the network message. ARP servers only "listen" to messages that arrive on their well known normal address; they do not respond to ARP broadcast messages. Properly equipped IP drivers should respond to the broadcast messages when they appear.

If an ARP server receives a message containing an IP address it does not know how to resolve, it ignores the message so that another ARP server might be addressed at the source's next attempt.

If the address is resolvable, it places the known HYPERchannel address and MTU size in the response and returns it to the location in the HYPERchannel header FROM address.

Unlike a broadcast ARP, the ARP server will be required to service two requests when two hosts that are initially unknown to one another attempt to get in touch. Since the destination did not receive the ARP request, it must contact the ARP server when its higher level protocols first generate a datagram to respond to the the source's first IP datagram to go through to the destination.

The source configuration file described in the previous section was explicitly designed so that it could be sufficient as a data base for an ARP server as well as an individual host.


When a local HYPERchannel network contains a broadcast capability, any IP driver wishing to perform HYPERchannel address resolution may be configured to emit the ARP message on a broadcast instead of a well known address. IP drivers on other hosts are presumed to know if their local HYPERchannel interface can send broadcast messages; if so, they arrange to "listen" on the FF07 broadcast TO address for ARP.

Processing of a received ARP broadcast message is otherwise identical to RFC-826:

    o   Messages are responded to if and only if the destination IP
        driver is authoritative for the designated IP address.
    o   Whenever an ARP message is processed, the IP driver takes the
        source HYPERchannel address and MTU size and adds it to its
        address resolution tables.  Thus the driver is equipped to
        turn around the IP datagram that arrives from the destination
        host when contact is made.

Each IP driver may have address resolutions that are set through a static routing table (the configuration file specified above). If ARP information arrives that contradicts a static entry (as opposed to previously set dynamic ARP information) then the ARP information should be ignored. This decision is made on the premise that the only useful purpose of static routing in a broadcast ARP environment is to add authentication, as it's easy to lie with ARP.


This section is intended to be a concise review of the state of the art in NSC networks and the techniques they provide for the delivery of messages. Those who are thoroughly familiar with HYPERchannel may wish to only skim this section; however, there is material on new technologies and addressing formats that are not yet generally known to most of NSC's customers.


Network Systems manufactures several different network technologies that use very different media and link controls, but still provide a common host interface in both the protocol and hardware sense of the term. These four technologies are:

    o   HYPERchannel A -- A 50-megabit, baseband, CSMA with collision
        avoidance  network using a coaxial cable bus.  Individual
        HYPERchannel "network adapters" can control up to 4 of these
        coaxial cable "trunks,"  providing up to 200 megabits of
        capacity on a fully interconnected network.  HYPERchannel A
        is NSC's earliest product and has been in production since
        1977.  It is principally used to interconnect larger
        mainframe computers and high speed mainframe peripherals such
        as tape drives and laser printers.
    o   HYPERchannel  B -- a 10-megabit, baseband, CSMA with collision
        avoidance network using a single coaxial cable bus.  This
        technology is used for direct host to host communications under
        the name HYPERchannel B, and for terminal connections under the
        name HYPERbus.  It is currently used for three major
        applications -- local networks of ASCII terminals, networks
        of IBM 3270 terminals, and host to host communications of
        smaller computers.
    o   DATAPIPE[3]  --  a 275-megabit fiber optic "backbone" network
        that interconnects lower speed local area networks within a 20
        mile range, and to provide an ultra-high-performance network for
        the next generation of supercomputers and optical storage
        systems.  A prototype version of DATApipe is currently under
        development at a customer site.
    o   Bridges and Network Distance Extensions -- NSC quickly
        discovered that its customers wanted very high speeds over
        geographic areas, not just within the range of several miles
        that is conceivable with a coaxial cable network.  Starting
        in 1978, NSC began to build a series of "link adapters" that
        are integral bridges between local area networks.  These link

adapters support common high speed communications media such

as Telco T1 circuits, private microwave, high speed

satellite links, and fiber optic point to point connections.


Network Systems' high speed interfaces use the attachment techniques of the manufacturer's highest speed peripheral controllers in order to achieve burst transfer rates of tens of megabits per second. These attachment techniques fall into three categories:


      +-----------+-------+                   +------------+  | | | |
      |           |       |                   |HYPERchannel+--+ | | |
      |           |       +-------------------+  Network   +--|-+ | |
      | Host      |  I/O  +-------------------+  Adapter   +--|-|-+ |
      |           |       |   Standard host   |            +--|-|-|-+
      | Computer  |Control|    data channel   +------------+  | | | |
      |           |       |
      |           |       |
      |           |       |
      |           |       |

The network adapter contains interface boards and firmware to be cabled to the manufacturer's data channel, such as would be done with a disk or tape controller. Mainframe network adapters do not emulate an existing manufacturer's device (such as a tape drive) but are supported by software which functions the channel and adapter to send and receive network messages.

Models of HYPERchannel adapters are available for essentially all large scale computers worldwide.


Since the network adapter contains lots of expensive, high speed logic, a different technique is used to provide attachment to minicomputers and workstations.

      +-------------+        +---------------+       +--------------+
      |             |        |               |       |              |
      | Minicomputer|        |  Supermini    |       | Workstation  |
      |             |        |               |       |              |
      +-----+-------+        +-------+-------+       +-------+------+
      |     |  DMA  |        |       |  DMA  |       |  DMA  |      |
      |     |control|        |       |control|       |control|      |
      +-----+---++--+        +-------+--++---+       +--++---+------+
                ||                      ||              ||
                ||                      ||              ||
                |+----------+           ||    +---------+|
                +----------+|           ||    |+---------+
                           ||           ||    ||
                         |     |     |     |     |
                         |         x400          |
                         |    Network Adapter    |
                         |                       |
                                 | | | |
   In this case, NSC provides a DMA controller designed for direct
   connection to that minicomputer's backplane bus.  These DMA
   controllers accept functions and burst blocks of data from host
   memory to a channel cable that is connected to one of four ports on a
   "general purpose computer adapter."  This adapter multiplexes
   transmissions to and from the HYPERchannel trunks from up to four
   attached processors.


For about 10 different bus systems, Network systems provides a "smart" DMA controller containing onboard memory and a Motorola 68010 protocol processor.

       |            |     |   Coprocessor |       |        +--------+
       |            |Host |    MC 68010   |Adapter+--------+  x400  |
       |    HOST    |DMA  |   256K memory |  DMA  +--------+ Adapter|
       |            |     |               |       |        +--------+
       |    Memory  +-----+---------------+-------+
       |            |

This class of interface works through the network coprocessor's direct access to host memory. Network transmit and receive request packets are placed in a common "mailbox" area and extracted by the coprocessor. The coprocessor reads and writes system memory as required to service network requests in the proper order. The coprocessors currently provide a service to read or write network messages (called Driver service as it is more or less identical to HYPERchannel dumb DMA drivers) and a service for NETEX, which is NSC's OSI-like communications protocol.


The protocols implemented by NSC within its own boxes are designed for the needs of the different technologies. A compact summation of these protocols is:

      HYPERchannel B         HYPERchannel A            DATApipe
     10 Mbits/second       50-200 Mbits/second     275 Mbits/second
 |                                                                   |
 |                  HYPERchannel network message                     |
 |                 connectionless datagram protocol                  |
 |                                                                   |
 |    "HYPERchannel     |                      |                     |
 |  compatibility mode" |    HYPERchannel A    |     DATApipe        |
 |   Virtual circuit    |   reservation and    |   acknowledgment    |
 |   estab. & control   |    flow control      |    & flow control   |
 +----------------------+      protocol        |      protocol       |
 |                      |                      |                     |
 |  Virtual Circuits    |                      |                     |
 |    Flow Control      |                      |                     |
 |    CSMA / VT         |      CSMA / CA       |                     |
 |  frame (datagram)    |  frame (datagram)    | TDMA packet delivery|
 |    delivery and      |   delivery and       |                     |
 |   acknowledgment     |  acknowledgment      |                     |
 |                      |                      |    Fiber optics     |
 |     75 ohm coax      |  1-4 75 ohm coax     | (various cable sizes|
 |       cable          |      cables          |  and xmission modes)|

Without getting into great detail on these internal protocols, a few points are particularly interesting to system designers:

    o   All three technologies supply the same interface to the host
        computer or network coprocessor, a service to send and receive
        network messages that are datagrams from the host's vantage in
        that each contains sufficient information to deliver the message
        in and of itself.  Since this datagram and its header fields are
        of paramount interest to the host implementor, it is discussed
        in detail below.
    o   All technologies use acknowledgments at a very low level to
        determine if packets  have been successfully delivered.  In
        addition to permitting  a highly tuned contention mechanism for
        the coax medium, it also permits HYPERchannel A to balance the

load over several coax cables -- a feat that has proven very difficult on, for example, Ethernet.

    o   All boxes go to some lengths to assure that resources exist
        in the receiving box before actual transmission takes place.
        HYPERchannel B uses a virtual circuit that endures for several
        seconds of inactivity after one host first attempts to send a
        message to the other.  Traffic over this "working virtual
        circuit" is flow controlled from source to destination and
        buffer resources are reserved for the path.

HYPERchannel A exchanges frames at very high rates to determine that the receiver is ready to receive data and to control its flow as data moves through the network.

DATApipe propagation time is relatively long compared to the time needed to send an internal packet of 2K-4K bytes. As a result, DATApipe controllers use a streamlined TP4-like transport protocol to assure delivery of frames between DATApipe boxes.


      [1]   HYPERchannel is a trademark of Network Systems Corporation.
      [2]   Plummer, D., "An Ethernet Address Resolution Protocol",
            RFC-826, Symbolics, September 1982.
      [3]   DATApipe is a registered trademark of Network Systems