ENVS 1000 Lecture Notes - Lecture 23: Internet Protocol Suite
10 May 2018
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ENVS 1000 Lecture 23 Notes – Independent Layers
Introduction
• The independence of each layer means that an individual layer needs to be concerned
only with the interfaces of the layers immediately above it and below it.
• Ideally, the operation of a particular layer is transparent to other layers and could be
modified or replaced without affecting other layers, provided that the layer continues to
provide its required services to the communication process and that there is agreement
between the equivalent, or peer, layers at the sender and receiver end nodes.
• Not surprisingly, the message to be sent through the communication channel gets larger
and larger as it passes down the chain, since each layer in the sender must add its own
component to the message from the previous layer.
• There is an obvious advantage in eliminating layers, whenever they are not needed, to
reduce message traffic and overhead.
• If a message is being sent point-to-point directly between a sender and the ultimate
user of a message (i.e., the message is not being forwarded).
• For example, there is no reason to specify an address for the receiver.
• Therefore, the five layers specified in the protocol suite actually represent a maximum.
• In an earlier, 2001, edition of his book, Fitzgerald [FITZ07] likened the layered model to a
pair of office buildings, where the people on each floor are responsible for a specific set
of business tasks.
• The TCP/IP buildings are each five stories tall.
• The people on the fifth floor of one building put a message for the other building into an
envelope, seal the envelope, and send it down to the fourth floor.
• Each floor adds its own message and puts the previous envelope plus the new message
into another, somewhat larger, envelope.
• When the package reaches the first floor, a messenger person (this is the physical layer)
carries the package across the street to the other building, where the people on each
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