Experimental Physics and Industrial Control System

  
  Marty, Jeff, Bob, & John:
      Here's my notes (finally) from our discussion at LANL (blurred somewhat
  by time) on the subject of extending CA beyond a single subnet:
  
      1. Extension should support the possibility of read/write or read-only
         to remote PV's without administrative overhead (that is, it should
         be possible without having to edit a gateway PV list).
  
      2. Read-only, read/write, and no-access mix in both directions.
  
      3. (Optional -- this idea was considered not important during
         discussions) Possibility of imposing access control by a simple
         list.
  
      4. No remote subnet message for local PV's (that is, the first broadcast
         must be constrained to the local subnet if desired).
  
      5. CA able to handle one or more local networks on the first broadcast
         (needed for OPI's with multiple network interfaces)
  
      6. User configurable list of subnets or addresses, with control over
         which addresses are used in the first broadcast.
  
      7. Must be ready by June 1, preferably earlier for beta test.  (This
         constraint was requested by CEBAF to support end station to accelerator
         data exchanges during commissioning).
  
  It was agreed that 7 precludes developing a full blown name server.  Jeff's
  idea (developed by the group) was for CA to support a list of addresses
  to contact for PV's.  These addresses could be either subnet broadcast
  addresses, or IP addresses of specific IOC's.  The list would be user
  specifiable at run time (not compiled in), with a delimiter to indicate
  which addresses are used on the first try.  E.g. a list might be of the
  form:
  
  	ip1, ip2; ip3; ip4, ip5
  
  In this case, the initial broadcast is sent to ip1 and ip2.  After a timeout,
  it is sent to ip1, ip2 and ip3.  On the next timeout it would be sent to all
  five.  Other discussion also suggested that the time interval between
  retries also be run-time configurable.
  
  Just think:  you could make an OPI display which showed the up status of
  all EPICS collaboration IOC's !!!
  
  Chip

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