The de facto standard application program interface (API) for TCP/IP applications is the "sockets" interface. Although this API was developed for ___71___ in the early 1980s. it has also been implemented on a wide variety of non-Unix systems. TCP/IP ___72___ written using the sockets API have in the past enjoyed a high degree of portability and we would like the same ___73___ with IPv6 applications. But changes are required to the sockets API to support IPv6 and this memo describes thesechanges. These include a new socket address structure to carry IPv6 ___74___. new address conversion functions, and some new socket options. These extensions are designed to provide Access tothe basic IPv6 features required by TCP and UDP applications, including multicasting, while introducing a minimum of change into the system and providing complete ___75___ for existing IPv4 applications.

75（）

A．availability

B．compatibility

C．capability

D．reliability