2. 计算机
  3. The TCP protocolis a (1) layer protocol．Each connection connects two TCPs that may bejust one physical network apart or locate

The TCP protocolis a (1) layer protocol．Each connection connects two TCPs that may bejust one physical network apart or locate

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问题 The TCP protocolis a  (1)  layer protocol．Each connection connects two TCPs that may bejust one physical network apart or located on opposite sides of the globe．In other words，each connection creates a  (2)  with a 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 a fixed retransnussion time for all connections can result in serious consequences．If the retransmission time does not allow enough time for a  (3)  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 retransmission 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  (4)  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  (5)  by basing it on the round-trip time(RTT)．Several formulas are used for this purpose．
(5)
选项 A、long
B、short
C、fixed
D、dynamic
答案D
解析 TCP是一种传输层协议，每个TCP连接都连接着两个TCP，这两个TCP可能是在一个物理网络里，但是是分开的或者位于全局网络的对立面。换句话说，每个连接创建一个路径的长度可能完全不同于南另外一个连接创建的路径。这意味着TCP不能为所有的连接使用相同的转发时间。为所有的连接选择一个固定的转发时间可能会导致严重的后果。如果这个转发时间并不为一个段提供足够的时间去到达目的地确认获取来源，将导致段转发的部分仍在路上。相反，如果转发时间超过了必要的短路径，它可能会导致延迟的应用程序，甚至一个单独的连接，转发时间也应该是不同定的，相比于拥挤的时期，一个连接在不拥挤的时期可能能够更快地发送部分和接收确认。TCP使用动态转发时间，每个连接的传输时间是不同的，而且可能在相同的连接中变化。转发时间可能是动态的基于往返时间。有几个准则用于这一目的。
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