The TCP protocolis a （1）layer protocol. Each connection connects two TCPs that may be just one physical network apart or located on opposite sides of the globe. In other words, each connection creates a （2） 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 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.
（1）是（）
A.physical
B.network
C.transport
D.application
