HP X.25/9000 Programmer's Guide

56 Chapter4

Sending and Receiving Data

Data Transmission Requirements

with a second ioctl(X25_SEND_TYPE) call. Refer to “Controlling the

MDTF, D, and Q bits” below for the syntax for ioctl(X25_SEND_TYPE)

Detecting D Bit Arrival and Conﬁrmation

If you have not set the D bit during connection establishment, you will

not be able to determine if the D bit is set on an incoming message with

ioctl(X25_NEXT_MSG_STAT) (described later in this chapter). When a

data packet arrives with the D bit set, X.25 level 3 acknowledges the

arrival of the packet automatically. The X.25/300 subsystem

acknowledges the D bit immediately upon receipt of the packet. The

X.25/800 subsystem acknowledges the D bit when the packet is received

(without

MSG_PEEK

) by the application.

If the socket is in blocking mode, the process sending data blocks until D

bit conﬁrmation arrives. If the socket is in nonblocking mode, the

X.25/800 subsystem sends an out-of-band event (

OOB_VC_DBIT_CONF

) to

the process. The arrival of a D bit conﬁrmation can also be detected with

ioctl(X25_NEXT_MSG_STAT).

If the peer process is not using the X.25/800 subsystem, the D bit does

not imply that the process has read the data. It implies only that the

data has been received by the remote interface. If an end-to-end or

disk-to-disk data conﬁrmation protocol is required, it must be developed

between the application processes. Refer to “Controlling the MDTF, D,

and Q bits” below for the syntax for ioctl(X25_SEND_TYPE)

Using the Q (Qualiﬁer) Bit

The Q bit (Qualiﬁer bit) is used to indicate a PAD CONTROL packet and

is used when the remote DTE is a Packet Assembler/Disassembler

(PAD). PAD CONTROL packets can control the operation of the PAD.

PAD CONTROL packets received indicate the success or failure of the

control operations sent. For a complete description of how to control a

PAD and the meaning of the response packets, consult the CCITT X.29

and X.3 Recommendations.

The use of this bit is undeﬁned if the remote DTE is not a PAD or

emulating a PAD. Therefore, the application may use it for its own

purposes when PAD CONTROL packets are not used on the VC.

Controlling the MDTF, D, and Q Bits

The MDTF, D, and Q bits are used to indicate special usage of DATA

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