HP X.25/9000 Programmer's Guide

54 Chapter4

Sending and Receiving Data

Data Transmission Requirements

automatically scattered across several DATA packets as part of the X.25

protocol. Each packet will have its M bit (“More” bit) set, except for the

last packet, to indicate that there is at least one more packet to follow.

Once the last packet (the one without an M bit) has been received, the

message is assembled again.

If X.25 receives a series of packets with the M bit set, the packets must

be read with a single system call, or as a series of message fragments. If

the application is reading message fragments the

ioctl(X25_SET_FRAGMENT_SIZE) must be set.

The MDTF Bit

Although the M bit can not be set by the application, the MDTF (More

Data to Follow) bit—an extension of the X.25 protocol speciﬁc to this X.25

implementation—can be used by an application to force the M bit to be

set. This allows an application to use several send() calls to send a

message, even if the message is small enough to be sent as a single

packet. The packets will be assembled and interpreted as a single

message at the other end.

To send a message with several send() calls, use the

ioctl(X25_SEND_TYPE) call with the

X25_MDTF_BIT

set to 1. All

subsequent send() or write() calls will be treated as fragments of a

large message. That is, all of the packets used to send the data have their

M bits set.

To clear the M bit on a message use ioctl(X25_SEND_TYPE) with the

X25_MDTF_BIT

call cleared (set to 0). This must be done before the last

send() or write() call in the message series so that the ﬁnal packet

does not have the M bit set. Refer to “Controlling the MDTF, D, and Q

bits” below for the syntax for ioctl(X25_SEND_TYPE).

Unless the ioctl(X25_SET_FRAGMENT_SIZE) system call is used with

the MDTF bit, messages must be received with a single recv() or

read() system call. If the ioctl(X25_SET_FRAGMENT_SIZE) system call

is used, however, the message is read as a series of message fragments,

allowing very long messages to be read.

Using the D (Delivery Conﬁrmation) Bit

When the D bit (Delivery Conﬁrmation bit) is set in a DATA packet, it

requires that conﬁrmation be sent upon its arrival at the remote DTE. To

use the D bit in data transmission, the D bit must be set at connection

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