Introduction to Message Sets
Chapter 7
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In the communication protocols for the message sets we describe in the
following chapters, there are differences in the layers above the HDLC
data-link layer. However, there are functions of these layers (network
layer through application layer) that apply to all message-set protocols.
The network layer protocol defines a network packet format for
interaction between application programs. The HDLC data-link layer
protocol merely serves to carry these packets as data fields in frames
between two nodes. The application programs may be located at opposite
ends of the point-to-point full-duplex link. However, the network
protocol can even handle the transfer of messages between application
programs in the same device.
The network layer ignores the internal functioning of data-link protocols.
It requires that the data-link driver accepts a message for delivery, tries to
send it, and indicates whether it was delivered.
Network Model
To implement your network layer software, use a routing subroutine and a
queue. Messages created by the application software are sent to the router
for transmission over the network. Messages that are delivered by the
network are placed on an incoming message queue that is unique for each
application. Figure 7.1 illustrates this model.
Figure 7.1
Network Model
Command
Initiator
R
Commands
Replies
Network
Q
Command
Executor
R
Replies
Commands
Q
R
= Routing Subroutine Entry
Q
= Incoming Message Queue
Network through Application
Layer