HP-UX SNAplus2 CPI-C Programmer's Guide (June 2003)

Concepts
Basic Conversations
Chapter 150
Basic Conversations
Basic conversations are normally used by service programs. These are
programs that provide services to other local programs. They are more
complex than mapped conversations but provide an experienced LU 6.2
programmer with a greater degree of control over the transmission and
handling of data. This section summarizes the characteristics of basic
conversations.
Logical Records
In a basic conversation, data is sent in the form of logical records. A
logical record is a record that has the general data stream (GDS) syntax
described in this section. For more information about GDS syntax, refer
to IBM Systems Network Architecture: Formats.
The sending TP must format the data into multiple logical records, and
the receiving TP must decode the logical records into usable data.
If a logical record is a single record, it consists of the following fields:
• A 2-byte record-length (LL) field
• A 2-byte GDS identifier (ID) field (for example, 0x12FF identifies the
data as application data)
• A data field that can range in length from 0 to 32,763 bytes
The first four bytes are called the LLID.
If a logical record has multiple parts, the first part has the same format
as a single record, and all subsequent parts consist of the following fields:
• A 2-byte record-length (LL) field
• A data field that can range in length from 0 to 32,765 bytes
The length recorded in the LL field includes the two bytes of the LL field
(and the two bytes of the ID field, if it is present). For example, a single
part GDS with no data has a value of 0x0004 for its LL field. The LL field
must be in high-low format, rather than byte-swapped format. For
example, a length of 230 bytes is represented as 0x00E6, rather than
0xE600.