To identify different connections to and from the same IP address, we use port numbers. UDP is used here mainly, because it has low overhead: sending 1 small query in 1 datagram and receiving an answer takes less time and needs to transmit less data than making a TCP connection (exchanging 3 segment between hosts): sending a query from a client, sending a confirmation from the server, sending an answer from the server, and then sending a confirmation from a client and disconnecting the connection (4 segments). In this case a protocol of the higher layer has to resend a query after not receiving an answer in the given amount of time. ![]() UDP is used sometimes by other protocols in the application layer - for instance, in the case of DNS. Because of that, UDP is mainly used to transmit real-time multimedia data - VoIP talks, videoconferences, audio and video. If packets are not received in the correct order, UDP will, nonetheless give them to an application in the order that they were received. It doesn't provide retransmissioning for lost datagrams. If any segment will be lost in the network, it will be sent again if it does not receive the confirmation in time. TCP always provides data in the correct order. TCP protocol is connection-oriented, which means that before sending data, it is required to create a connection between the remote machines. Those protocols are used to transport data through the Internet - they are working in transport layer. Next, messages of the given protocol has to be packed into a TCP segment or UDP datagram (in transport layer).
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