(Item 8296) Clarify DLUR EN tail vector registration processing
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Questions from DLUR implementers concerning DLUR EN tail vector
registration procedures have pointed out a need to specify in the
DLUR architecture document the algorithm for DLUR to use to decide
which session (CP-CP or CP-SVR) session to use to send TDUs.  The
algorithm needs to take into account:
 
        * session outages
        * preferred sessions activating after not-so-preferred sessions
        * initial and subsequent TGV registration
        * adjacent and non-adjacent DLUR/DLUS pairs
 
It turns out that VTAM handles the TDUs the same whether they are
sent over the CP-CP session or the CP-SVR pipe.  Therefore, this item
will also optionally allow the adjacent DLUR to not use the CP-CP
session and only use the CP-SVR pipe for tail vector registration (an
option that many implementers have requested).
 
This clarification includes updating the "DLUR EN Tail Vector
Registration" section of the DLUR Architecture Reference to include
the following algorithm for DLUR to follow to determine which session
(CP-CP or CP-SVR) to use to send TDUs:
 
    1) at initial TGV registration,
 
         a) if the DLUR and the DLUS are adjacent, then the DLUR may
            optionally send the TDUs over the CP-CP session between the
            two nodes
 
         b) otherwise the TDUs will be sent over the CP-SVR pipe
            to the DLUS node
 
    2) if the CP-SVR pipe is selected for initial registration:
 
         a) continue to use it for subsequent TGV registration even if
            a CP-CP session between adjacent DLUR and DLUS is activated
            later
 
         b) if the CP-SVR pipe fails, once the pipe is reactivated (to
            either the same or a different DLUS), return to step 1) and
            send all tail vectors
 
    3) if the CP-CP session is selected for initial registration:
 
         a) continue to use it for subsequent TGV registration
 
         b) if the CP-CP session fails, return to step 1) and send all
            tail vectors
 
         c) if the CP-SVR pipe fails, once the pipe is reactivated (to
            either the same or a different DLUS), return to step 1) and
            send all tail vectors