When the system upon which a transport entity is running fails and subsequently reatarts, the ______ 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 promble.
The still active side of a half-open connections using a ______ timer. This timermeasures the time transport machine will continue to await an ______ of a transmitted segment after the segment has been retransmitted the maximum number of times. When the timer ______, 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 restart, half-open connections can be teminated more quickly by the use of the RST segment. The failed side returns an RSTi to every segment i that it receives. When the RSTi reaches the other side, it must be checked for validity dased on the ______ 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.
A. connection B. acknowledgment C. sequenceD. message
参考答案：C
解析： 当传输实体所在的系统失效并且重新启动后，所有活动连接的状态(state)信息都会丢失。受其影响的连接形成了半开放状态，但是没有失效的一方，还没有认识到这个问题。
半连接中仍旧活动的一方可以使用放弃(give-up)定时器来关闭连接。该定时器测量一个段重发最大次数后等待应答(acknowledgment)的时间。当该定时器超时(expires)后，传输实体假定对方传输实体或中向的网络己经失效。结果是，定时器关闭连接，并向传输用户发出非正常关闭的通知。
在一个传输实体失效并且很快重启动的情况下，使用RST段可以更快地终止半连接。失效一方对它接收到的每一个段i，返回一个RSTi。当RSTi到达另一边时，应该验证它的序号(sequence)i，因为RST有可能是对老数据段的响应。如果重启是有效的，传输实体将完成非正常终止过程。