在OSI参考模型当中，物理层通过物理连接在(29)之间提供比特流的传输；数据链路层通过数据链路在(30)之间提供数据链路服务单元透明传输；网络层通过网络连接在(31)之间交换网络服务数据单元；传输实体向(32)提供面向连接的传输服务。在应用层提供的公共应用服务元素CASE、特定应用服务元素SASE和用户元素UE中，作为系统与用户进程交互手段的是(33)。

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VRML is a(71)for 3D multimedia and shared virtual worlds on the WWW. In comparison to HTML, VRML adds the next level of interaction, structured graphics, and extra(72)(z and time)to the presentation of documents. The applications of VRML are(73), ranging from simple business graphics to entertaining WWW page graphics, manufacturing, scientific, entertainment, and educational applications, and 3D shared virtual worlds and communities.

 X3D is the name under which the development of VRML is continued. X3D is based on XML and is backwards(74)with VRML. Furthermore, it is componentized, profiled, and extensible, which makes it possible to use X3D in very different(75), from high-end visualizations to lightweight applications.

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Most IP layer-based proxy mechanisms, such as network address translation(NAT), only support uni-directional proxy, from the internal network to external network(the Internet). The proxy establishes the communication channel by(66)IP addresses. The IP addresses must be(67). These proxy mechanisms prevent applications that must establish return data channels from establishing their data channels, such as, multimedia applications. IP layer-based proxy mechanisms need additional software(68)for each application that uses(69)channels. SOCKS identifies communication targets through domain names, overcoming the restriction of using private IP addresses. SOCKS can use domain names to establish communication between separate LANs, with(70)IP addresses.