Unreliable Transport Service

Unreliable transport service involves two aspects: user multiplexing and unreliable data transfer between users. A transport protocol can achieve this simply by adding user multiplexing to the message transfer service provided by the network layer. When a user generates a data segment destined to a remote user, the transport protocol gives the network layer a packet containing the user's local port number, the user's remote port number, the user's remote host IP address, the user's transport protocol number, and the data segment.

When the network entity at a host receives a packet, it first looks for a local user with local port number equal to the packet's destination port number, remote port number equal to the packet's sender port number, remote host address equal to the packet's sender IP address and transport protocol number equal to the packet's transport protocol number. If it finds such a user, it passes the packet's data segment to the user. Otherwise, it looks for a local user (presumably a server) with local port number equal to the packet's destination port number, remote port number and host address equal to nil, and transport protocol number equal to the packet's transport protocol number. It it finds such a user, it passes the packet's data segment to the user. Otherwise, it discards the packet.