After the protocols of the switching layer have established the basis for communication, another protocol is needed so that the data packets reach the corresponding applications. With the OSI model, this forwarding is carried out on the transport layer (layer 4). Each stack also has its own protocols. For the internet protocol family, these are particularly
- TCP (Transmission Control Protocol)
- and UDP (User Datagram Protocol).
Since the great success of the internet, the first mentioned TCP is equal to IP as a standard for network connections. In most cases it builds directly on IP, which is why TCP/IP networks are often used. As a connection-oriented protocol, TCP requires an existing connection between the communication users for the transport of data packets. It guarantees reliable transport of the data and that all of the packets will arrive complete and in the correct order. To do this, the protocol adds additional information, such as a sequence number and proof sum to the data.
UDP is the TCP counterpart of the internet protocol family for the simple and quick transfer of smaller data packets without a connection. UDP connections don’t offer any security for a packet arriving at the addressee, but thanks to the low administration data (additional information in the header), there isn’t a clear speed advantage for data transfer where smaller transmission errors aren’t a problem. For this reason, the User Datagram Protocol is used for audio and video streaming, DNS queries, and VPN (Virtual Private Network) connections.
Like the internet protocol family, other protocol stacks also have specific transmission protocols based on their network protocols and largely similar to TCP. Novell networks, for example, wait in the transport layer with the protocol SPX (Sequenced Packet Exchange). With the AppleTalk stack, the data packets can be transported using the ATP (AppleTalk Transaction Protocol).