单选题
When the system upon which a transport entity is running
fails and subsequently restarts, the{{U}} {{U}} 16 {{/U}}
{{/U}}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{{U}} {{U}} 17 {{/U}} {{/U}}timer.
This timer measures the time the transport machine will continue to await
an{{U}} {{U}} 18 {{/U}} {{/U}}of a transmitted segment after the
segment has been retransmitted the maximum number of times.0
When the timer{{U}} {{U}} 19 {{/U}} {{/U}}, 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{{U}} {{U}} 20 {{/U}} {{/U}}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.