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OSI Layer 3: Routed and Routing Protocols

model is most notably known for the functions it provides by using connec-
tion-oriented or connectionless protocols. Some of the Transport layer's
responsibilities are session establishment, flow and error control, and session
teardown. The following sections begin discussions of protocols that are spe-
cific to the Network and Data-Link layers, respectively.

OSI Layer 3: Routed and Routing Protocols

he Network layer is used by the Transport layer to provide the best

end-to-end services and path for PDU delivery. This means that the Network
layer also uses protocols to accomplish this task. This section discusses seven
different protocols that are used within Layer 3 of the OSI model. Some of
these protocols use protocols within them for finer granularity of certain
functions.

There are also some other differences between these seven protocols.

Some are routing protocols and others are protocols that are routed by the
routing protocols. Routing protocols are used to exchange route informa-
tion and to create a network topology, thus enabling routing decisions to be
made. The routed protocols are protocols that contain information regard-
ing the end systems, how communication is established, and other informa-
tion relevant to the transfer of data.

Internet Protocol (IP)

It is important to distinguish between the Internet Protocol suite and the
actual Internet Protocol that is used in the Network layer of the OSI model.
The IP suite consists of several different protocols that are implemented at
different levels of the OSI model.

The Internet Protocol (IP)

is a Network-layer protocol of the Internet

Protocol suite. It is used to allow routing among internetworks and hetero-
geneous systems. IP is a connectionless protocol, even though it can provide
error reporting, and performs the segmentation and reassembly of PDUs.

IP Packet Structure

Now that you know what IP is, look at the actual packet structure in more
detail. Following is an IP packet that was broken down by EtherPeek. The

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