SNAX/APN Application Programming Manual

The SNALU Interface
SNAX/APN Application Programming Manual—420111-001
4-20
Brackets
•
If immediate request mode is used, the sender of an RU must specify that it desires a
definite response (ERI=0, DR1I and/or DR2I = 1).
•
If delayed request mode is used, the sender of an RU can request:
•
No response
•
A definite response
•
An exception response
The use of these modes is enforced by the data flow control (DFC) portion of the user’s
application. Request/response modes are discussed in the IBM Systems Network
Architecture: Format and Protocol Reference: Architectural Logic manual.
Brackets
Brackets group a sequence of normal-flow request chains and their responses into a
logical unit of work. The bracket protocols allow the two LUs to contend for activating
a bracket, and provide a means for resolving such contention.
The BBI and EBI bits in the RH (bits 0 and 1 respectively of byte 8 in the SNAX
header) are used to initiate and terminate brackets:
•
When an LU wants to initiate a bracket, it sets the BBI bit to 1 in the SNAX header
of the first RU in the bracket.
•
When an LU wants to terminate a bracket, it sets the EBI bit to 1 in the SNAX
header of the first RU of the last chain in the bracket.
Whether or not brackets are to be used is determined by the particular FM profile
specified in the BIND request at session initiation. Bracket protocols are discussed in
the IBM Systems Network Architecture: Format and Protocol Reference: Architectural
Logic manual.
Send/Receive Modes
The two LUs can communicate with one another in any of the following send/receive
modes:
•
Half-duplex flip-flop (HDX-FF) not using the bracket protocol
•
Half-duplex flip-flop (HDX-FF) using the bracket protocol
•
Half-duplex contention (HDX-CONT)
•
Full duplex (FDX)
The Change Direction Indicator (CDI)—bit 2 of byte 8 in the RH of the SNAX
header—is used in the HDX-FF modes to control the direction of RU flow across the
link. The CDI may also be used in the HDX-CONT mode. When an LU sets the CDI
bit (CDI=1), it effectively passes control to the other LU. An LU can only transmit RUs
when it “owns” the CDI bit.
If chaining is being used, only the final request in a chain may carry the CDI bit. The
CDI bit cannot be conveyed by way of a request that also carries EBI=1 or by way of a
response.