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Chapter 2

Protocol Attributes

Session

This layer is responsible for coordinating communication

between applications, which it does through dialog-control methods.

Transport

This layer takes care of end-to-end communications. It is

responsible for connection to the destination system, as well as packet seg-
mentation and assembly. The transport layer includes both connection-
oriented and connectioness protocols (for example, TCP and UDP).

Network

This layer defines the topology of the network through the use

of logical addressing. Routing protocols use this information to route
packets.

Data-Link

This layer takes all of the data that is accumulated as packets

are handed from one layer to the next, and then packages it into frames.
This layer equates the Network layer address (IP address) to a data-link
address, or MAC address, of the next hop. Once the physical address
is known, the frame is sent to that address. The receiving interface uses
the Data-Link layer to extract the packet from the frame, discards the
frame, and then sends the packet up to the Network layer.

Physical

This layer sends and receives bits with values of ones or zeroes.

The Physical layer is in charge of determining how it sends these values.
If the physical connection between two machines is fiber optic, then the
Physical layer has to use light to transmit the ones and zeroes. If the
connection is electrical, then electrical signals are sent to represent the
ones and zeros.

You saw how the logical data flow of the OSI model works, but look at

Figure 2.2, in which you can see the actual data flow. The figure depicts data
that is handed from the Application layer all the way down to the Physical
layer. At that point, the data is transmitted across any variety of physical
media to the next hop, or destination system. Once the ones and zeroes
arrive at the Physical layer of the destination system, the information is sent
to Layer 2. This layer discards the frame, and then the extracted packet is
handed up to the Network layer. The network packet header is stripped off,
and the resulting packet is handed up to the Transport layer. This process is
repeated for each layer until it arrives at the Application layer.

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