Thede facto standard Application Program Interface (API) for TCP/IP applicationsis the “sockets” interface. Although this API was developed for （1） in the early 1960s it has also been implemented on a widevariety of no-Unix systems. TCP/IP （2） Writtenusing the sockets API have in the past enjoyed a high degree of portability andwe would like the same （3） with IPv6applications. But changes are required to the sockets API to support IPv6 andthis memo describes these changes. These include a new socket address structureto carry IPv6 （4） ,new address conversion functions,and some new socket options. These extensions are designed to provide access tothe basic IPv6 features requited by TCP and UDP applications, includingmulticasting, while introducing a minimums of change into the system and providingcomplete （5） for existing IPv4 applications.
（1）A．Windows
B．Linux
C．Unix
D．DOS
（2）A．a(chǎn)pplications
B．networks
C．protocols
D．systems
（3）A．portability
B．a(chǎn)vailability
C．calability
D．reliability
（4）A．connections
B．protocols
C．networks
D．a(chǎn)ddress
（5）A．a(chǎn)vailability
B．compatibility
C．capability
D．reliability