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HP X.25/9000 Programmer's Guide

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Chapter 6 95
Extended Features
Using Fast Select
Using Fast Select
The use of fast select in X.25 is described in X.25: The PSN Connection.
This section describes how to access this facility through X.25.
The fast select facility lets processes specify up to 128 bytes of call user
data in the CALL REQUEST, CALL ACCEPTED, and CLEAR
REQUEST packets. (Usually, only 16 bytes are permitted in the CALL
REQUEST packet, and no user data is permitted in the CALL
ACCEPTED and CLEAR REQUEST packets).
This facility is called “fast” because it lets developers specify user data in
the same packet that is requesting or acknowledging a connection
request. Usually, the CALL REQUEST packet contains all of the
information the server side needs to service the request, and it
terminates the connection and places all the information that the client
needs regarding the results of its request in the CLEAR REQUEST
packet. The circuit is never fully set up and is then quickly torn down,
hence the term “fast select.”
The ioctl() calls used for fast select are listed below:
• X25_WR_USER_DATA (for writing extended call user data).
• X25_RD_USER_DATA (for reading extended call user data).
• X25_WR_FACILITIES (for writing facilities).
• X25_RD_FACILITIES (for reading facilities).
The next two subsections provide a summary of the steps needed to
enable fast select on the calling and receiving side of a connection.
Fast select on the calling side
To implement fast select on the calling side, issue the
ioctl(X25_WR_FACILITIES) call with the fast select facilities code
defined in CCITTX.25 recommendations. Then, specify the call user data
for the packet with the ioctl(X25_WR_USER_DATA). These calls must be
executed before issuing the connect() call.
If the calling side is using protocol IDs for address matching, the protocol
ID must be specified in the call user data field before any other call user
data is added with the ioctl(X25_WR_USER_DATA) call. The data may be
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