The document discusses the TCP/IP and OSI models. It describes that TCP/IP was developed before OSI and has 5 layers that do not exactly match OSI's layers. While OSI specifies functions for each layer, TCP/IP layers contain independent protocols that can be mixed depending on system needs. Both models are based on independent protocol stacks and have similar functionality below the transport layer, but OSI protocols are better hidden and replaceable while TCP/IP was modeled after existing protocols. OSI supports both connectionless and connection-oriented communication at the network layer but only connection-oriented at transport, while TCP/IP is connectionless at network but supports both modes at transport.

1. Lecture 4
TCP/IP protocol suite
The TCP/IP protocol suite, was developed prior to OSI model. Therefore the
layers in the Transmission Control Protocol/Internet Protocol don’t match with that of
the OSI model. The TCP/IP protocol is made up of 5 layers
1. Physical Layer
2. Data Link Layer
3. Network Layer
4. Transport Layer
5. Application Layer
OSI model specifies which function belongs to each of its layers, the layers of the TCP/IP
protocol suite contain relatively independent protocols that can be mixed and matched
depending on the needs of the system.

2. Difference between TCP/IP and ISO/OSI model
The TCP/IP and OSI models have much in common. Both are based on the
concept of a stack of independent protocols. Also, the functionality of the layers is
roughly similar. For example, in both models the layers below and including transport
layer are there to provide an end-to-and, network-independent transport service. Also in
both models, layers above the transport layer are application-oriented users of transport
service.
The protocols in the OSI model are better hidden than in the TCP/IP model and can
be replaced relatively easily as the technology changes.
The OSI model was devised before the corresponding protocols were invented.
This means that the model was not biased toward one particular set of protocols. With
TCP/IP model the reverse was true: the protocols came first and the model was really just
a description of the existing protocols.
Also, the OSI model supports both connectionless and connection-oriented
communication in the network layer, but only connection-oriented communication in the
transport layer. While, the TCP/IP model has only one mode in the network layer
(connectionless) but supports both modes the transport layer.