2. 计算机
  3. If two communicating entities are in different hosts connected by a network，there is a risk that PDUs will not arrive in the ord

If two communicating entities are in different hosts connected by a network，there is a risk that PDUs will not arrive in the ord

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问题   If two communicating entities are in different hosts connected by a network，there is a risk that PDUs will not arrive in the order in which they were sent，because thev may traverse (71)____ paths through the network．If each PDUs is given a unique number．and numbers are assigned (72)____，then it is a logically simple task for the receiving entity to reorder (73)____ PDUs on the basis of sequence number．A problem with this scheme is that,with a(74)____．sequence number field，sequence number will repeat．Evidently，the maximum sequence number must be (75)____ than the maximum number of PDUs that could be outstanding at any time．
(71)
选项 A、same
B、different
C、single
D、unique
答案B
解析  如果两个通信实体处于通过网络连接的不同主机上，PDU可能不会按照发送时的顺序有序地到达目的端点，因为它们在网络中可能会经过不同的路径。如果给每个PDU赋予一个唯一的序号，而且序号是连续的，那么接收实体就可以很方便地根据序号来重新调整PDU的顺序。但是这种机制存在一个问题，即如果序号的数目有限，就会发生序号重复的情况。因此在任何情况下，最大序号数都必须大于PDU的最大数量。
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