The main objective of Quality of Service is to transfer certain data packets either particularly safe or as immediately as possible:
- It may happen during a data transfer that data packets are not delivered to the addressee. But for some applications it is very important that all sent packets really do arrive. An e-mail, for example, divided into several small data packets, can only be assembled together again, when all parts have arrived completely. Whether one or an other packet arrives with little time delay does not make any difference. These applications often count on the connection-orientated Transmission Control Protocol (TCP). This protocol ensures that data will be transferred correctly and chronologically via the net. It automatically adjusts the sending rate downwards if the confirmation of sent data packets is outstanding for longer times, and also takes care of repeated transmission in case of packet losses.
- In other applications, e.g. telephony via the Internet (Voice-over-IP, VoIP), it is - differently to the case above - very important that the data packets arrive at the addressee with only little time delay. But it really doesn’t matter if once a data packet gets lost in this case. The participant at the other end of the connection will understand the caller, even if small parts of the speech got lost. This application aims at the fastest sending of data packets as possible. The connectionless User Datagram Protocol (UDP) is often used for this kind of application. Also this protocol has very little administrative overhead. But chronological delivery of packets is not guaranteed, data packets are simply sent out. Because no confirmation receipt exists, lost packets never get delivered again.