Internet Architecture- TCP/IP

Comparison between OSI and TCP/IP Protocol Suite
When we compare the two models, we find that two layers, session and presentation, are
missing from the TCP/IP protocol suite. These two layers were not added to the TCP/IP protocol
suite after the publication of the OSI model. The application layer in the suite is usually considered
to be the combination of three layers in the OSI model,
Two reasons were mentioned for this decision. First, TCP/IP has more than one transportlayer
protocol. Some of the functionalities of the session layer are available in some of the
transport layer protocols. Second, the application layer is not only one piece of software. Many
applications can be developed at this layer. If some of the functionalities mentioned in the session
and presentation are needed for a particular application, it can be included in the development of
that piece of software.
TCP/IP is a hierarchical protocol made up of interactive modules, the term hierarchical
means that each upper level protocol is supported by one or more lower level protocols.
Layers in the TCP/IP Protocol Suite
When we study the purpose of each layer, it is easier to think of a private internet, instead
of the global Internet. We assume that we want to use the TCP/IP suite in a small, private internet.
Such an internet is made up of several small networks, which we call links.
Physical Layer
TCP/IP does not define any specific protocol for the physical layer. It supports all of the
standard and proprietary protocols. At this level, the communication is between two hops or nodes,
either a computer or router. The unit of communication is a single bit. When the connection is
established between the two nodes, a stream of bits is flowing between them.
Note that if a node is connected to n links, it needs n physical-layer protocols, one for each
link. The reason is that different links may use different physical-layer protocols.