单选题 选出应填入下面一段英语中______内的正确答案。
   When the system upon which a transport entity is running fails and subsequently restarts, the (1) 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 (2) timer. This timer measures the time transport machine will continue to await an (3) of a transmitted segment after the segment has been retransmitted the maximum number of times. When the timer (4) 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 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 (5) 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.