Because the transport layer operates over a network

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Because the transport layer operates over a network or internet, the amount of the transmission delay may be highly variable. This makes it difficult to effectively use a timeout mechanism for retransmission of lost data. There are two reasons why one transport entity would want to restrain the rate of segment transmission over a connection from another transport entity: The user of the receiving transport entity cannot keep up with the flow of data. The receiving transport entity itself cannot keep up with the flow of segments. A transport entity has a certain amount of buffer space. Incoming segments are added to the buffer. Each buffer segment is processed and data sent to TS user. Thus, the transport entity needs to take steps to stop or slow the flow of segments to prevent buffer overflow. Four ways of coping with the flow control requirement. The receiving transport entity can a. Do nothing. b. Refuse to accept further segments from the network service. c. Use a fixed sliding-window protocol. d. Use a credit scheme. The sender has increased its output to include new segments plus retransmitted old segments. The second alternative is a backpressure mechanism that relies on the network service to do the work. When a buffer of a transport entity is full, it refuses additional data from the network service and refuses additional segments from its transport entity. Fig. 17.3 Sending and Receiving Flow Control Perspectives 8. Explain briefly Crash Recovery. ( 20 marks) Solution Crash Recovery
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When the system upon which a transport entity is running fails and subsequently restarts, the state information of all active connections is lost. The affected connections become half open because the side that did not fail does not yet realize the problem. The still active side of a half-open connection can close the connection using a persistence timer. When the timer expires, the transport entity assumes that the other transport entity or the interviewing network has failed, closes the connection, and signals an abnormal close to TS user. If a transport entity fails and quickly restarts, half-open connections can be terminated more quickly by the use of RST segment. The failed side returns an RST i to every segment i that it receives. When RST i reaches the other side, it must be checked for validity based on the sequence number i, because RST could be in response to an old segment. If the reset is valid, the transport entity performs an abnormal termination. These measures clean up the situation at the transport level.The problem is one of synchronization.At the time of failure, there may have been one or more outstanding segments in either direction.TS user on the side that did not fail knows how much data it has received, but the other user may not, if state information were lost. There is the danger that some user data will be lost or duplicated.
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