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