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  3. The corporate head office of Ezonexam has a teleconferencing system that uses VOIP (voice over IP) technology. This system uses

The corporate head office of Ezonexam has a teleconferencing system that uses VOIP (voice over IP) technology. This system uses

admin2009-05-19  32
问题 The corporate head office of Ezonexam 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?
选项 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.
答案B
解析 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.
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