Networking and Internetworking - Guide to Cloud Computing for Business and Technology Managers - page 42

computers. For example, a transport protocol transmits messages of any
length from a sending process to a receiving process. A process wishing to
transmit a message to another process, issues a call to a transport protocol
module, passing it a message in the specified format. The transport software
then concerns itself with the transmission of the message to its destination,
subdividing it into packets of some specified size and format that can be
transmitted to the destination via the network Protocol—another, lower-
level protocol. The corresponding transport protocol module in the receiv-
ing computer receives the packet via the network-level protocol module and
performs inverse transformations to regenerate the message before passing
it to a receiving process.
2.2.1.2 Protocol Layers
Network software is arranged in a hierarchy of layers. Each layer presents
an interface to the layers above it that extends the properties of the under-
lying communication system. A layer is represented by a module in every
computer connected to the network. Each module appears to communicate
directly with a module at the same level in another computer in the network,
but in reality, data are not transmitted directly between the protocol mod-
ules at each level. Instead, each layer of network software communicates by
local procedure calls with the layers above and below it. On the sending side,
each layer (except the topmost, or application layer) accepts items of data
in a specified format from the layer above it and applies transformations
to encapsulate the data in the format specified for that layer before pass-
ing it to the layer below for further processing. On the receiving side, the
converse transformations are applied to data items received from the layer
below before they are passed to the layer above. The protocol type of the
layer above is included in the header of each layer, to enable the protocol
stack at the receiver to select the correct software components to unpack the
packets. The data items are received and passed upward through the hierar-
chy of software modules, being transformed at each stage until they are in a
form that can be passed to the intended recipient process.
2.2.1.3 Protocol Suite
A complete set of protocol layers is referred to as a protocol suite or a
protocol stack, reflecting the layered structure. Figure 2.1 shows a pro-
tocol stack that conforms to the seven-layer Reference Model for Open
Systems Interconnection (OSI) adopted by the International Organization
for Standardization (ISO). The OSI Reference Model was adopted in order
to encourage the development of protocol standards that would meet the
requirements of open systems. The purpose of each level in the OSI Reference
Model is summarized in Table 2.1. As its name implies, it is a framework for
the definition of protocols and not a definition for a specific suite of protocols.