HP HP-UX SNAplus2, Administration Manual

Page: 41 / 398

Chapter 1 41
SNA Terms and Concepts
Basic SNA Concepts
A logical record consists of a two- or four-byte header starting with a
two-byte length field, often represented as “LL,” followed by up to
32,765 bytes of data. Logical records can be grouped together and
sent as a block, transmitting more than one logical record with a
single call to the
SEND
function.
• In a mapped conversation, information is passed to the
SEND
function
as a pointer to a single, unformatted block of data; the length of the
block is passed as another parameter. The block cannot be received as
one or more logical records; the receiving TP must do whatever
record-level formatting is required.
NOTE
Only LU type 6.2 supports mapped conversations.
Modes
Each LU-LU session has an associated mode that defines a set of session
characteristics. These session characteristics include throughput
parameters, session limits (such as the maximum number of sessions
between two LUs), message sizes, and routing parameters.
Each mode is identified by a unique mode name. The mode name must be
the same on all SNA nodes that use that mode.
Route Selection
To establish an LU-LU session, a route must be calculated between the
nodes where the two LUs reside. A route is an ordered sequence of links
and nodes that represents a path between the two nodes.
SNA networks support the following methods of route selection:
• For subarea networks, you must predefine all routes between subarea
nodes.
• For peer networks that do not support APPN, type 2.1 nodes can
support sessions only with adjacent nodes; their sessions cannot be
routed through intermediate nodes.
• For APPN networks, SNA can compute routes dynamically at the
time of session initiation, using a class of service specified for the
mode used by the session (see “Class of Service”).