Because the UDP response is faster than setting up a TCP session, UDP makes sense in these situations. The DNS server would then respond by telling the originating system the IP address of the Web server. The query is sent over a single UDP packet. When you attempt to use your Web browser to access it must first resolve the name to an IP address. An example of a use of the UDP protocol is DNS queries. UDP is a very simple protocol and is used when speed is an issue. Data simply begins to flow between the two systems. When a UDP connection occurs, there is no beginning, middle, or end to the conversation. The User Datagram Protocol is a connectionless protocol that is designed to stream data. Ralph Bonnell, in Configuring Juniper Networks NetScreen & SSG Firewalls, 2007 UDP Communications Delivery of UDP datagrams is not guaranteed and UDP datagrams may arrive out of order and in any number of copies due to the nature of IP. The checksum does not make UDP reliable however, since UDP datagrams with a failing checksum are dropped without notifying the application process. The checksum covers the UDP header and data as well as a pseudo-header consisting of certain fields of the IP header, including the IP source and destination addresses. UDP also calculates a checksum over the datagram. Each UDP datagram is addressed to a specific port at the end host and incoming UDP datagrams are demultiplexed between the recipients. The ports are 16-bit values used to distinguish between different senders and receivers at each end point. Where IP provides addressing of a specific host in an Internet, UDP provides per-process addressing by the use of ports. UDP provides an extra layer of multiplexing on top of IP. There are also no guarantees that the datagrams are delivered in the same order as they were sent. This mechanism is best-effort because the underlying IP network does its best to deliver the datagram, but does not guarantee that the datagrams are delivered at the destination. UDP provides a best-effort datagram delivery service. Jean-Philippe Vasseur, Adam Dunkels, in Interconnecting Smart Objects with IP, 2010 6.1.1 Best-effort Datagram Delivery
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