Modelled after the postal service

Service user specifies full destination address for each message

Service user establishes, uses, and releases a connection

Service provider makes a "best effort" attempt to communicate

Connection-oriented

Connectionless

Reliable

Unreliable

Application layer

Transport layer

Session layer

Physical layer

Application layer

Transport layer

Network layer

Link layer

TCP

IP

UDP

HTTP

To encode the message

To add extra information such as sender and receiver details

To increase the size of the message

To remove unnecessary data from the message