在自治系统内部的各个路由器之间，运行的是内部网关协议IGP。早期的IGP叫做(51)，它执行(52)。当网络规模扩大时，该算法使得传送的路由信息太多，增加了网络负载，后来又出现了执行最短路径优先算法的IGP。按照这种协议，每个路由器向网络中的其他路由器发布(53)，当路由信息改变后，路由器按照(54)算法更新路由表。在不同自治系统的路由器之间，运行外部网关协议EGP，典型的EGP是(55)。

(78)

(79)

(76)

(77)

(71) is a one -way function that takes an arbitrarily long piece of plaintext and from it computes a fixed-length bit string.

  (72) is a protocol that most USENET machines now use it to allow news articles to propagate from one machine to another over a reliable connection.

  (73) is an approach preferred by cable TV providers. In this solution the fiber terminates at an optoelectrical converter and the final segment is a shared coaxial cable.

  (74) is a device used to connect two separate network that use different communication protocol.

  (75) is a digital-to-digital polar encoding method in which the signal level is always either positive or negative.

(73)

(74)

(71)

(72)

In the following essay, each blank has four choices. Choose the most suitable one from the four choices and write down in the answer sheet.

 Open Shortest Path First(OSPF) is a(66)routing algorithm that(67)work done on the OSI IS-IS intradomain routing protocol. This routing, as compared to distance-vector routing, requires(68)processing power. The Dijkstra algorithm is used to calculate(69)OSPF routing table updates only take place when necessary,(70)at regular intervals.