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    A Programmer's Guide to the EUCSD Ethernet

    Introduction

    CLIENT, STATION, PACKET, PORT

       Ethernet "Stations" are devices connected to  "Clients"  (computers).
    It  is  the  job  of  stations  to  forward messages across the ethernet
    between and on behalf of clients or processes within clients.

       Physically, messages are transmitted in "packets" which consist of  a
    15-byte  header,  the data part containing the actual message (up to 532
    bytes) and a 4-byte trailer.   Client software has very  little  control
    over  the  actual  scheduling  of transmissions, or of what exactly goes
    into the header and trailer.   These details are left up  to  "firmware"
    running within the station.

       The  interface presented to the client is of a number (32) of "ports"
    which are endpoints  of  transmissions.   These  ports  are  addressable
    objects, each having a six-byte address.  Within the EUCSD ethernet, the
    first  address  byte (usually) uniquely identifies a station, the second
    byte (usually) identifies a port within a  station,  and  the  remaining
    four  bytes must all be zero.   Client software treats ports number 1 to
    31 of its station as independent general-purpose bi-directional  streams
    through  which messages of between 0 and 532 bytes in length may be sent
    or received.   Port number 0  is  rather  more  special-purpose  and  is
    discussed later.

       The   services  offered  by  the  station  fall  into  one  of  three
    categories:

    CONNECTION, DATAGRAM, BROADCAST

       (1) The virtual connection provides a  reliable  bi-directional  link
    between  a  port  in  one  station  and a port in another station.   The
    connection is reliable in the sense that  the  station  deals  with  any
    requirement for re-transmitting packets which have collided or have been
    lost  for  some  other  reason  (usually  congestion  at the destination
    station).   Packets are guaranteed to arrive at their destination in the
    same  order  in  which  they  were  transmitted  and  receiving stations
    guarantee  to  suppress  the  forwarding  of  duplicated  packets.   The
    transmitting  client  is  notified  when  a  packet  just  sent has been
    accepted  by  the  receiving  station  (not   the   receiving   client).
    Alternatively,  when  the  transmitting  station  reckons  that  further
    retransmissions due to absence of acknowledgements would be futile,  the
    transmitting   client   is   notified   of   the  transmission  failure.
    Transmissions of datagrams and broadcasts are not reliable in the  sense
    that  connections are; packets merely arrive at their destination with a
    high probability and are duplicated with low probability.

       In a  virtual  connection  there  is  always  a  unique  "other  end"
    associated  with  each  end-point (port).   So there is the concept of a
    port being "closed", i.e. having no other end  associated  with  it,  or
    "open to station S port P", i.e. having a particular endpoint associated
    with  it.   A port which is closed will not accept any packets addressed
    to it, and a port which is open will only accept packets coming from the
    particular other end in which it is interested.







       Once various bits of  software  running  in  different  clients  have
    established  that  they  wish to communicate they would normally seek to
    establish a reliable connection by agreeing which  of  their  respective
    ports  to  open.   Such  agreement  can be established by the programmer
    beforehand, if  he  knows  which  physical  machines  are  going  to  be
    involved.

       For example, this approach, rather inelegant but dictated at the time
    for  various  reasons, is taken in the way the Fred-machines talk to the
    1976 Departmental filestore.   The machines know that the filestore  has
    address 16_70, the filestore knows that the Fred-machines have addresses
    in  the  range 16_11 to 16_2F, so the filestore starts up by opening its
    port I to machine 16_10+I, port F, for I between 1 and 16_1F,  and  each
    Fred-machine  (with  address  J) opens its port F to machine 16_70, port
    J-16_10.

       (2) The datagram is a message sent through port  0  of  any  machine.
    Port  0  is  in a sense deemed permanently open to any other end, and so
    always accepts  any  packet  addressed  to  it.   Because  there  is  no
    permanent remote address information associated with port 0, any message
    sent  through  port  0  must  be prefixed by six address bytes, i.e. the
    first six bytes of a message (which must be between 6 and 538  bytes  in
    length) are interpreted by the station as a destination address, and any
    message  received  through  port 0 will have the six-byte source address
    prefixed to it  by  the  station.   This  allows  receiving  clients  to
    determine  where  the  datagram  came  from  and transmitting clients to
    instruct the station where a message is to go to.

       For  example,  the  1982  mini-filestore  and  the  1983  replacement
    filestore  use  datagrams  from  a  prospective  customer machine to the
    filestore (the station address of which  is  known  to  the  prospective
    customer) to convey a request to establish a connection.   The filestore
    responds with a datagram saying which port it has allocated, and opened,
    to the prospective customer.

       (3) The broadcast is a message sent to (port 0 of)  all  machines  on
    the ethernet.  Address filtering hardware in the ether receiver of every
    station makes the station deaf to all packets other than those which are
    either  addressed  specifically  to  that station (the first byte of the
    header is in fact the destination station address), or else addressed to
    pseudo-station number 0 (deemed the broadcast address).   Since  packets
    are  always  addressed  to  a  remote  object  (port)  with  a  six-byte
    (effectively two-byte) address, and the first byte being zero designates
    a broadcast, and since broadcasts are always accepted by port zero,  the
    meaning  of  the  second address byte of a broadcast message cannot be a
    port number.   Instead it is taken to be a notional broadcast  "channel"
    number.   Receiving stations may be configured to tune in to all, no, or
    an arbitrary selection of these broadcast channels, currently numbered 0
    to 63.

       Broadcasts are sent in the same way as datagrams, i.e.  if  they  are
    sent  through  port 0 they are prefixed by the six-byte address in which
    the pseudo station number is zero and the  pseudo  port  number  is  the
    channel  number, if they are sent through a non-zero port then that port
    must have been opened to  remote  "station"  zero  and  "port"  &lt;channel
    number&gt;.   Reception of broadcasts, however, is different from datagrams
    in that, though they both come through port 0, there must be some way of
    deciding whether an incoming packet  was  a  datagram  or  a  broadcast.
    Since of the six-byte prefix the first is the source station address and
    must  hence be non-zero, a broadcast is further prefixed by two bytes of






    which the first is zero and the second is the channel number.   In other
    words, a datagram is prefixed by the six bytes &lt;source station&gt;, &lt;source
    port&gt;,  &lt;four  zeroes&gt;,  and  a broadcast is prefixed by the eight bytes
    &lt;zero&gt;,  &lt;channel  number&gt;,  &lt;source  station&gt;,  &lt;source  port&gt;,   &lt;four
    zeroes&gt;.

       For example, where a particular service (such as the CS4 X25 exercise
    "other  end" machine) is provided by software running in some machine on
    the ethernet, but where it is not known in which particular machine that
    software happens to be running, it would be appropriate to  allocate  (a
    priori)  a  broadcast  channel  number  to  that  service, and to expect
    customer  machines  to  broadcast   on   that   channel   to   establish
    communication  with the server.   Once the server responds, the customer
    will know  which  machine  the  server  is  running  in  and  subsequent
    communication may be by datagrams or virtual connections.

    High-Level Programming

       Under  the  current  (December  1983)  software  environment  on  the
    Fred-machines the following procedures are available  for  communicating
    across the ether:
    %routine etheropen(%integer localport,remoteport)
      LOCALPORT should be in the range 1 to 31,  REMOTEPORT is the sum of a
      number in the range 0 to 31 (the remote port number) and another number
      (the remote station address) multiplied by 256 (shifted left 8).
      Calling ETHEROPEN has the obvious effect of opening port LOCALPORT in
      this client's station to port REMOTEPORT&amp;31 in remote station numbmer
      REMOTEPORT&gt;&gt;8.

    %routine etherwrite(%integer port, %bytename buffer, %integer size)
      PORT must be zero or the number of a local port which has been opened.
      BUFFER points to the first byte of the message to be transmitted, but
      if PORT=0 it points to the first byte of the six-byte address field,
      to be immediately followed by the actual message.
      SIZE conveys the size of the message (inclusive of address field if
      PORT=0). It must be in the range 0 to 532 if PORT#0 or 6 to 538 if
      PORT=0.
      The effect is to wait until transmission of any packet previously sent
      on the same port has been acknowleged, then to transmit this packet.

    %integerfn etherread(%integer port, %bytename buffer, %integer maxsize)
      PORT must be zero or the number of a local port which has been opened.
      BUFFER points to the first byte of a buffer into which a message is to
      be received.
      MAXSIZE indicates how big the buffer is. Incoming messages longer than
      this value will be read but the caller's memory is not overwritten past
      the end of the buffer.
      The %result of the function is the actual size of the message received
      (inclusive of the six-or-eight-byte prefix if PORT=0), and may be
      larger than maxsize.
      The effect is to wait until a packet arrives on this port, then to
      read it.

    %routine enable broadcasts(%integer channel)
      CHANNEL should be in the range 0 to 63.
      The effect is to instruct the station to tune in to the specified
      channel. It does not affect the "tuned-in-ness" to any other channel,
      i.e. to tell the station to tune in to all channels, this procedure
      must be called 64 times with CHANNEL taking every value between 0 and
      63.







    %routine disable broadcasts
      There is no parameter and the effect is to instruct the station to
      disregard broadcasts on all channels.

    %integerfn etherstation
      The %result is the station address of this client's ether station.

    ** NOTE **

       Unless  you know what you're doing, steer clear of attempting to talk
    to stations 16_70, which is the filestore,  and  16_72,  which  is  VAX.
    Also steer clear of port 16_0F, which is used by the Fred-machine system
    to communicate with the filestore.

    ** NOTE **

       The  standard firmware in the EPROM of the ether station is not up to
    date and does not work properly with regard to datagrams and broadcasts.
    To use these services, it is necessary (until such time  as  the  EPROMs
    can  be  changed  on  all machines) to load the up-to-date firmware into
    your local ether station.   This is done by issuing the  system  command
    ETHER:LOAD,  which will respond by saying "Done" followed by the address
    of your station.   This will have to be repeated if for any reason  your
    station has been reset, such as when you re-load the Fred-machine.

    Advanced Facilities

       It  is  not always convenient to call ETHERREAD or ETHERWRITE because
    they will wait until it is possible to proceed with  the  transfer.   To
    allow  user  programs  to  determine  whether such transfers can proceed
    there exist a number of integer variables which are treated  as  boolean
    arrays  (0:31)  which  indicate whether a particular condition holds for
    each of the 32 ports.

       Bit P of variable DTX indicates, when set, that a packet has  arrived
    on  port P, and that it is therefore possible to do an ETHERREAD on that
    port without waiting.   In fact what ETHERREAD does is to wait until bit
    P  of  DTX  becomes  set,  it then clears that bit and reads the packet.
    Setting of the bit is done by the interrupt handler in the system.

       Bit  P  of  variable  ACK  indicates,  when  set,  that   either   an
    acknowledgement for the packet last sent on port P has been received (if
    bit  P  of  variable  NAK  is  clear)  or  that the station has given up
    re-transmitting (if bit P  of  variable  NAK  is  set).   In  fact  what
    ETHERWRITE  does is to wait until bit P of ACK is set, it then clears it
    and sends the packet.   It ignores NAK.   Whenever the station sends  an
    ACK  control  character  to  its  client, the interrupt handler sets the
    appropriate bit in variable ACK; when the station sends a NAK character,
    the interrupt handler sets the appropriate bits in  both  NAK  and  ACK.
    Therefore,  to  determine  whether  your transmission has completed, you
    should wait until the bit in ACK sets, then look at NAK.  If the NAK bit
    is set, the transmission failed (and you should then clear the NAK bit),
    otherwise the packet sent was accepted by the destination (except in the
    case of broadcasts, which are never acknowledged by receiving  stations,
    but  the  transmitting  station nevertheless sends a proforma ACK to its
    client to keep the protocol consistent).

       In the above, "bit P of XXX is set" means that "XXX&amp;(1&lt;&lt;P)#0", "bit P
    of XXX is clear" means "XXX&amp;(1&lt;&lt;P)=0", in other words bit 0 is the least






    significant bit of the word.

       All  the  above  procedures  and  variables  are  specified  in   IMP
    Include-file   "i:fs.inc".    This  file  also  contains  references  to
    variables STX and RDY, these are of relevance  only  to  the  ETHERREAD,
    ETHERWRITE,  and ETHEROPEN routines and should not be used by high-level
    user programs running under the standard Fred-machine system.

    Low-Level Programming

       [This section is of interest to those writing programs which  do  not
    expect to run under the standard system.  It has not been written yet.]

    Rainer Thonnes,  16/12/83.


















































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