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User Datagram Protocol

UDP provides a service for applications to exchange messages. Unlike TCP, UDP is connectionless
and provides no reliability, no windowing, no reordering of the received data, and
no segmentation of large chunks of data into the right size for transmission. However, UDP
provides some functions of TCP, such as data transfer and multiplexing using port numbers,
and it does so with fewer bytes of overhead and less processing required than TCP.

UDP data transfer differs from TCP data transfer in that no reordering or recovery is
accomplished. Applications that use UDP are tolerant of the lost data, or they have some
application mechanism to recover lost data. For example, VoIP uses UDP because if a voice
packet is lost, by the time the loss could be noticed and the packet retransmitted, too much
delay would have occurred, and the voice would be unintelligible. Also, DNS requests use
UDP because the user will retry an operation if the DNS resolution fails. As another example,
the Network File System (NFS), a remote file system application, performs recovery
with application layer code, so UDP features are acceptable to NFS.

Figure 5-10 shows the UDP header format. Most importantly, note that the header includes
source and destination port fields, for the same purpose as TCP. However, the UDP has
only 8 bytes, in comparison to the 20-byte TCP header shown in Figure 5-1. UDP needs a
shorter header than TCP simply because UDP has less work to do.
Free CISCO CCNA Routing and Switching ICND1 Study Guide
Figure 5-10 UDP Header

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