TCB是“计算机系统内保护装置的总体，包括硬件、软件、固件和负责执行安全策略的组合体。它建立了一个基本的保护环境，并提供一个可信计算机信息系统所要求的附加用户服务”。简单地说，TCB描述的是(18)。

(85)

(83)

(84)

(82)

(80)

Rapid advances in computer and communication technologies have resulted in the(71) merger of these two fields. The lines have bluned among computing, switching, and digital transmission equipment, and the same digital techniques are being used for data, voice, and image transmission. Merging and evolving technologies, coupled with increasing demands for efficient and timely collection, processing, and(72) of information, are leading to the development of(73) that transmit and process all types of data. The ultimate goal of this evolution is something its proponents-some of the most powerful forces in the computing and telecommunications industries-call the (74) .The (74) will be a worldwide public telecommunications network that will deliver a wide variety of services. The (74) will be defined by the standardization of user interfaces, and will be implemented as a set of digital switches and paths supporting a broad range of traffic types and providing (75) processing services. Inpractice, there will be multiple networks, implemented within national boundaries, but from the user's point of view there will be a single, uniformly accessible worldwide network.

(78)

(79)

Even though current technologies make the ultimate segmented network impractical, new technologies and products hold the promise of making the concept a reality. Most important of all these new technologies are (66) . The ultimate switched LAN, which is technically possible, now, exhibits the following characteristics:

  1. High-speed yet cost-effective switching devices can be implemented on a wide scale, providing dedicated connections for individual users and supporting the aggregate (67) of all networked users.

  2. The ability to deploy switching capabilities incrementally into the (68) base. Switching implementations support switching of individual users or groups so that existing users of shared media modules can continue to use the existing equipment until they can justify moving to dedicate switched LAN connections.

  3. Support and enhancement of existing routed environments. Switching (69) routint, but rather can be introduced into the network without making any changes to the existing routing equipment or configuration.(70) play a critical role in the integration of switching into existing networks.

(77)