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7 layer OSI Model

The Transport Layer

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The addressing part in transport layer is handled by port numbers, the addressing part in the network layer is handled by IP address in most cases and the addressing part at the link layer is handled by the link address (ex mac address, dlci, vpi/vci). The transport layer functionality is three fold

Addressing (fulfilled by the concept of ports)

Transport (duh obviously, handled by protocols like TCP, UDP, RTP et al)

Error correction

I have already explained the part of addressing; let me get on to the concept of transport. In general there are two kinds of transport possible reliable and unreliable. Depending on the application the transport required may vary. For example SMNP which is a network management application uses unreliable transport whereas HTTP uses reliable transport. TCP (Transmission control protocol) defines the protocol for reliable transport and UDP (User datagram protocol) defines the protocol for unreliable transport. SNMP uses UDP at the transport layer whereas HTTP uses TCP at the transport layer. There is also a concept of real time transport and RTP (Real-time transport protocol) defines the protocol for real-time transport.

Lets us take it one at a time starting with TCP.

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