Subject: DLSw Version 0.16 Chapter 4

From: "Alan K. Bartky, Manager of Technology" <[email-masked]>
Date: Thu, 22 Dec 94 10:27:39 PST

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This is overall part 4 of 12

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RFC XXXX      DLSw: Switch-to-Switch Protocol  December 1994

4. Circuit Priority

   At circuit start time, each circuit end point will provide priority
   information to its circuit partner.  The initiator of the circuit will
   choose which circuit priority will be effective for the life of the
   circuit.  If Priority is not implemented by the Data Link Switch, then
   "Unsupported" priority is used.

4.1. Frame format

   Circuit priority will be valid in the CANUREACH_cs, ICANREACH_cs, and
   REACH_ACK frames only. The relevant header field is shown below.  The
   Circuit Priority value is a byte value at offset 22 in an SSP Control
   Message.

   The following describes the format of the Circuit Priority byte.

     7   6   5   4   3   2   1   0
   +-------------------+-----------+
   |   reserved        |    CP     |
   +-------------------+-----------+

   CP: Circuit Priority bits
           000 - Unsupported       (note 1)
           001 - Low Priority
           010 - Medium Priority
           011 - High Priority
           100 - Highest Priority
           101 to 111 are reserved for future use

   Note 1: Unsupported means that the Data Link Switch that originates the
   circuit does not implement priority.  Actions taken on Unsupported
   priority are vendor specific.

4.2. Circuit Startup

   The sender of a CANUREACH_cs is responsible for setting the CP bits to
   reflect the priority it would like to use for the circuit being
   requested.  The mechanism for choosing an appropriate value is
   implementation dependent.  The sender of an ICANREACH_cs frame will set
   the CP bits to reflect the priority it would like to use for the circuit
   being requested, with the mechanism for choosing the appropriate value
   being implementation dependent.  The receiver of the ICANREACH_cs will
   select from the priorities in the CANUREACH_cs and ICANREACH_cs frames,
   and will set the value in the CP field of the REACH_ACK frame that
   follows to the value to be used for this circuit.  This priority will be
   used for the life of the circuit.  A CANUREACH_cs or ICANREACH_cs with
   the circuit priority value set to Unsupported (CP=000) indicates that
   the sender does not support the circuit priority function.

AIW DLSw Related Interest Group           DLSw version 0.16 [Page 21]

RFC XXXX      DLSw: Switch-to-Switch Protocol  December 1994

   Flow:

      DLSw A               DLSw B

   CANUREACH_cs (CP=011) ----->           Circuit initiator requests
                                          high Priority.

        <--------- ICANREACH_cs (CP=010)  Circuit target requests
                                          medium priority.

   REACH_ACK (CP=010) -------->           Circuit initiator sets
                                          the priority for this
                                          circuit to medium. The
                                          circuit initiator could
                                          choose either high or
                                          medium in this example.

AIW DLSw Related Interest Group           DLSw version 0.16 [Page 22]