【71-75】the TCP protocolis a （71） layer protocol. each connection connects two TCPs thatmaybejustone physical network apart or located on opposite sides ofthe globe. in other words， eachconnectioncreates a （72） witha length that may be totally different from another path created byanotherconnection. this means that TCP cannot use the same retransmission time for all connections.selectingafixed retransnussion time for all connections can result in serious consequences. iftheretransmission time does not allow enough time for a （73） to reach the destination and an acknowledgmentto 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 indelayforthe application programs.even for one single connection， the retransmission time should notbefixed.aconnection may be able to send segments and receive （74）faster during nontraffic periodthanduringcongested periods. TCP uses the dynamic retransmission time，a transmission time is differentfor each connection and which may be changed during the same connection. retransmission timecanbemade（75） by basing it on the round-trip time （rtt）. several formulas are used for thispurpose.
