The TCP protocol is a (  ) layer protoco

考试题库2022-08-02  54

问题 The TCP protocol is a (  ) layer protocol. Each connection connects two TCPs that may be just one physical network apart or located on opposite sides ofthe globe. In other words, each connection creates a(  )witha length that may be totally different from another path created by another connection. This means that TCP cannot use the same retransmission time for all connections. Selecting afixed retransnussion time for all connections can result in serious consequences. Ifthe retransmission time does not allow enough time for a(  )to reach the destination and an acknowledgment to reach the source, it can result in retransmission of segments that are still on the way. Conversely, if the retransnussion time is longer than necessary for a short path, it may result in delay for the application programs.Even for one single connection, the retransmission time should not be fixed.A connection may be able to send segments and receive(  )faster during nontraffic period than during congested periods. TCP uses the dynamic retransmission time,a transmission time is different for each connection and which may be changed during the same connection. Retransmission time can be made(  )by basing it on the round-trip time (RTT). Several formulas are used for this purpose。问题1选项A.physicalB.networkC.transportD.application问题2选项A.pathB.windowC.responseD.process问题3选项A.processB.segmentC.programD.user问题4选项A.connectionsB.requestsC.acknowledgmentsD.datagrams问题5选项A.longB.shortC.fixedD.dynamic

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答案 CABCD

解析 TCP是一种传输层协议。每一个连接都连接了两个TCP实体,这两个TCP实体可能存在于同一个物理网络中,也可能是分局于地球的两边。换言之,每一个连接都产生了一条通道,其长度与另外一个连接产生的通路完全不同。这就意味着,TCP不能对所有的连接使用同样的重传时间。对所有的连接选择一个固定的重传时间可能产生严重的后果。如果重传时间不足以使一个段到达目标,或者不足以是一个应答到达源站,这就可能对尚在路途中的段产生重传。反之,如果重传时间比一条短通路所需要的时间长,则可能对应用程序产生延迟。
   即使对单个连接,重传时间也不应该固定。一个连接应该能够在非峰值时段比拥堵时段更快地发送数据和接收应答。TCP使用了动态重传时间,重传时间对每一个连接是不同的,在同一个连接持续期间也是可以改变的。重传时间可以动态地根据环回时间(RTT)而改变,为此建立了几个公式。
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