The Open Systems Interconnection model is comprised of seven layers
that computer systems use to communicate over a network. It divides these
communication functions into seven separate layers. TCP/IP has its own stack
that correlate to these layers (Weaver et al., 2014). I'll briefly discuss the
third layer, the network layer.
The network layer defines how internetworks function. It is the lowest
layer in the OSI model that is concerned with getting data from one
computer to another. In contrast the data link layer (layer 2) deals with
devices that are local to each other. The network layer is important in acting
as a fence between the layers below and above it. It is at this layer that the
more abstract functions of the higher layers transition into the specific tasks
required to get data to its destination. The transport layer, which is similar to
the network layer, continues this transition as you go up the OSI protocol
stack.
Some specific jobs performed at this layer include logical addressing,
routing, datagram encapsulation, fragmentation and reassembly, and error
handling and diagnostics. Logical addressing on the internet follows the
Internet Protocol (IP) and every machine has an IP address. Logical addresses
are independent of hardware and must be unique across an entire
internetwork. Routing involves the moving of data across a series of
interconnected networks and is probably the defining function of the network
layer. More specifically routing is the job of devices and software routines to
handle incoming packets from various sources and determine their final
destination. Datagram encapsulation deals with encapsulating messages
received from higher layers and placing them into datagrams (or packets)
with a network layer header. Fragmentation and reassembly happen when
the network layer sends messages down to the data link layer for
transmission. Since some data link layer technologies have limits on the
length of message that can be sent, the network layer must split any
message too large, send each piece and then have them reassembled once
they arrive at the destination machine. Lastly, error handling and diagnostics
is observed when special protocols are used to allow devices that are
logically connected to exchange information about the status of hosts on the
network or devices (Kozierok, 2005).
References
Kozierok, C. M. (2005, October 20). Network Layer (Layer 3). The TCP/IP Guide - Network
Layer (layer 3). Retrieved October 27, 2021, from
http://tcpipguide.com/free/t_NetworkLayerLayer3.htm.
Weaver, R., Weaver, D., & Farwood, D. (2014). Ch. 2: TCP/IP. In Guide to network defense and
countermeasures (pp. 35–77). essay, Course Technology, Cengage Learning.
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