2. 计算机
  3. Which one of the following protocols uses both UDP and TCP ports for the transport layer operation?

Which one of the following protocols uses both UDP and TCP ports for the transport layer operation?

admin2009-05-19  101
问题 Which one of the following protocols uses both UDP and TCP ports for the transport layer operation?
选项 A、FTP
B、TFTP
C、SMTP
D、Telnet
E、DNS
答案E
解析 Explanation:
TCP and UDP must use port numbers to communicate with the upper layers. Port numbers keep track of different conversations crossing the network simultaneously. Originating-source port numbers are dynamically assigned by the source host, which will be some number starting at 1024. 1023 and below are defined in RFC 1700, which discusses what is called well-known port numbers.

Virtual circuits that do not use an application with a well-known port number are assigned port numbers randomly chosen from within a specific range instead. These port numbers identify the source and destination host in the TCP segment. Only DNS uses both UDP and TCP port 53.
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