The TCP/IP Reference Model

* Describes how the internet works.

* There are 4 layers: the host to network layer, the internet layer, the transport layer, the application layer.

Host to Network Layer

* The TCP/IP model says very little about the host to network layer.

* It says that the host has to connect to the network using some protocol so it can send IP packets over it.

* The protocol is not defined and varies from host to host, and network to network.

The Internet Layer

* The job of the internet layer is to allow a host to inject packets into any network and have them travel independently to the destination.

* Packet routing is a major issue.

* Also defines an official packet format and protocol called IP (Internet Protocol).

The Transport Layer

* Designed to allow peer entities on the source and destination hosts to carry on a conversation (end-to-end).

* Transmission Control Protocol (TCP) is a reliable connection-oriented protocol that allows a byte stream originating on one machine to be delivered without error to any other machine on the internet.

* The byte stream is fragmented into discrete messages which are passed onto the internet layer.

* At the receiving end, the messages are reassembled into the original byte stream.

The Application Layer

* No need for session or presentation layers was perceived.

* Contains all the higher level protocols including FTP, SMTP (email), NNTP (news), HTTP (web).