单选题

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. 空白处（25）应选择（）

A sequence

B acknowledgment

C connection

D message

解析： 分析当传输实体发送失败，系统随后重新启动时，所有活动连接的状态信息都会丢失。受影响的连接则变成半开放的，没有失败的一端并不知道另一端发生的问题。半开放连接中活动的一端可以使用“放弃”计时器来关闭连接。当传输机器把一个包重新发送最大次数后，将持续等待传输包的回应，这时计时器就计算时间。当时间终了时，传输实体就假设另一个端的传输实体或中间的网络失败。因而，计时器关闭连接，且给TS用户发送一个异常关闭的信号。在一个传输实体失败并立即重新开始的事件中，可以使用RST包更快地结束半开放的连接。失败的一端返回一个RST（i）给它收到的每一个包i。当这个RST到达另一端时，必须进行基于序号i的有效性检查，就像这个RST响应原来的包一样。如果重置无效，则传输实体异常终止。