單選題：When the system upon which a transport entity is running fails and subsequently restarts,the(21) information of all active connections is lost. the affected connections become half-open, as the side that did not fail does not yet realize the problem.the still active side of a half-open connections can close the connection using a(22)timer. this timer measures the time the transport machine will continue to await an (23) of a transmitted segment after the segment has been retransmitted the maximum number of times.when the timer (24),the transport entity assumes that either the other transport entity or the intervening network has failed. as a result, the timer closes the connection, and signals an abnormal close to the ts user. in the event that a transport entity fails and quickly restarts, half-open connections can be terminated more quickly by the use of the rst segment, the failed side returns an rst i to every segment i that it receives. when the rst i reaches the other side, it must be checked for validity based on the (25) number i, as the rst could be in response to an old segment. if the reset is valid, the transport entity performs an abnormal termination
空白處（25）應選擇（）
A、sequence
B、acknowledgment
C、connection
D、message

答案：A

解析：分析當傳輸實體發送失敗,系統隨後重新啟動時,所有活動連接的狀態信息都會丟失.受影響的連接則變成半開放的,沒有失敗的一端並不知道另一端發生的問題.半開放連接中活動的一端可以使用「放棄」計時器來關閉連接.當傳輸機器把一個包重新發送最大次數後,將持續等待傳輸包的回應,這時計時器就計算時間.當時間終了時,傳輸實體就假設另一個端的傳輸實體或中間的網絡失敗.因而,計時器關閉連接,且給ts用戶發送一個異常關閉的信號.在一個傳輸實體失敗並立即重新開始的事件中,可以使用rst包更快地結束半開放的連接.失敗的一端返回一個rst（i）給它收到的每一個包i.當這個rst到達另一端時,必須進行基於序號i的有效性檢查,就像這個rst響應原來的包一樣.如果重置無效,則傳輸實體異常終止