The corporate head office of TestKing has a teleconferencing system that uses VOIP (voice over IP) technology. This system uses UDP as the transport for the data transmissions. If these UDP datagrams arrive at their destination out of sequence, what will happen?

A. UDP will send an ICMP Information Request to the source host.
B. UDP will pass the information in the datagrams up to the next OSI layer in the order
that they arrive.
C. UDP will drop the datagrams.
D. UDP will use the sequence numbers in the datagram headers to reassemble the data in
the correct order.
E. UDP will not acknowledge the datagrams and wait for a retransmission of the
datagrams.

Answer: B

Explanation:

VOIP systems utilize UDP because it is faster and uses less overhead. In addition, the
reliable transport mechanism used in TCP is useless to VOIP because if a packet gets
dropped and needs to be resent, it will be already too late.

UDP provides a service for applications to exchange messages. Unlike TCP, UDP is
connectionless and provides no reliability, no windowing, and no reordering of the
received data. However, UDP provides some functions of TCP , such as data transfer,
segmentation, and multiplexing using port numbers, and it does so with fewer bytes of
overhead and with less processing required. UDP data transfer differs from TCP data
transfer in that no reordering or recovery is accomplished. Applications that use UDP are
tolerant of lost data, or they have some application mechanism to recover data loss.

Reference: CCNA Self-Study CCNA INTRO exam certification Guide (Cisco Press,
ISBN 1-58720-094-5) Page 161.