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off-the-shelf purchased application, such as an X.400 mail application or an
X.500 directory server.
•
Application Support has two divisions, called the Application Programming
Interface (abbreviated API-A for a particular application 'A'), and Application
Support (abbreviated SUPP-A). The API and the application support are
closely tied together, and are chosen by the application programmer based
upon the requirements of the application.
Examples of application programming interfaces include CPI-C (Common
Programming Interface for Communications), RPC (Remote Procedure Call),
and MQI (Message Queue Interface). Depending upon the API selected, the
application services may be quite different. For instance, CPI-C utilizes
Advanced Program-to-Program Communication (APPC) and SNA logical unit
6.2 (LU 6.2) services, which includes the protocol flows between two
applications for establishing a conversation, exchanging data, ensuring
commitment of resources, and terminating a conversation. RPC does
networking through program stubs that are customized for each application
program and then attached (linked). RPC usually operates over TCP/IP
protocols. MQI provides queue-to-queue communication, in lieu of direct
program-to-program communication over a dedicated logical connection; it is
a form of networking middleware with resource commitment and assured
message delivery. MQI operates over LU 6.2, TCP/IP, and other networking
protocols.
•
Transport Network, which corresponds to the critical Transport and Network
OSI layers, is abbreviated TPORT-A for a particular application 'A.' These
two layers work closely together to ensure that user data is transmitted with
a predictable level of service from the source node to an end node, perhaps
through a set of intermediate nodes.
Depending upon the specific protocol chosen, these layers provide such
functions as optimal route determination, error detection and recovery, and
congestion control. Examples of transport protocols include TCP/IP and SNA
Advanced Peer-to-Peer Networking (APPN*). Each of these protocols utilizes
unique addressing structures, protocol flows between peer transport layers
in end nodes, and routing protocols between intermediate nodes. Please
note that throughout this book the term “transport protocol” will refer to the
combination of these two OSI layers (unless explicitly identified as OSI layer
4) to match nomenclature commonly used in the industry.
Also note that, historically, the Application Support and Transport Network
have been very closely tied together, and the selection of a particular API
forced the selection of a particular network protocol, or, conversely, a
programmer was forced to select an API based on the currently supported
transport protocol in the network. For instance, Remote Procedure Call
(RPC) and the TCP sockets interface are closely associated with the TCP/IP
transport protocol, and would be the application programming interface of
choice for a TCP/IP-based transport network; however, if a CPI-C-based
application might solve a particular business requirement, then SNA
transport would have to be added to this TCP/IP network to support the
CPI-C-based application, which might involve substantial effort.
•
Subnetworking, abbreviated SNETG, corresponds to the OSI Data Link
Control and Physical layers. These layers are concerned with getting data
on the physical media of the network, and then getting it reliably and
efficiently from one physical node to the next physical node in the network.
Chapter 1. Fundamental Concepts of Technologies
7