A TCP connection is in the ESTABLISHED state The following events o

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A TCP connection is in the ESTABLISHED state. The

following events occur one after another: First, the

application sends a “close” message. Second, an ACK

segment is received. a. What is the state of the connection

after the first event? b. What is the action after the first

event? c. What is the state of the connection after the

second event? d. What is the action after the second

event?

Solution

A) The following shows the case. It happens when an ACK

is delayed and the time-out occurs. The sender resends

the packet that is already acknowledged by the receiver.

The receiver discards the duplicate packet, but resends

the previous ACK to inform the sender that there is a delay

in the network

Receiving a duplicate ACK may alert the sender to

increase the time-out to prevent resending the packets

prematurely. Note that if the receiver ignores the duplicate

packet and does not send the second ACK, the sender

believes that the first packet is lost and the first ACK is

actually acknowledging the packet that is resent. Duplicate

ACKs give the clue about what is happening.

B) If the receiver expects a packet with sequence number

64 and packets with sequence numbers 62 to 65 are

already sent but not acknowledged, it means that two

packets with sequenc numbers 64 and 65 are in transit

from the sender to the receiver. If the sender expects the

acknowledgment for packet 62, but the value of Rn = 64, it

means that the ACK packets with acknowledgment

numbers 62 and 63 are in transit from the receiver to the

sender.

If the sender expects the acknowledgment for packet 62,

but the value of Rn = 64, it means that the ACK packets

with acknowledgment numbers 62 and 63 are in transit

from the receiver to the sender.

c) When a duplicate segment arrives, the receiver TCP

drops the packet and sends an ACK that defines the

segment expected. This is also a duplicate ACK that gives

a clue to the sender that its timer may have timed out

prematurely.

D)One might ask how the receiver could know whether the

duplicate ACK is for an out-of-order segment or a duplicate

segment. To trigger a fast retransmission, the sender

needs to receive three duplicate ACKs (four ACKs with the

same sequence number); a duplicate ACK per se does not

trigger a fast retransmission

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A TCP connection is in the ESTABLISHED state. The following events occur one after another: First, the application sends a “close” message. Second, an ACK segment is received. a. What is the state of the connection after the first event? b. What is the action after the first event? c. What is the state of the connection after the second event? d. What is the action after the second event? Solution A) The following shows the case. It happens when an ACK is delayed and the time-out occurs. The sender resends the packet that is already acknowledged by the receiver. The receiver discards the duplicate packet, but resends the previous ACK to inform the sender that there is a delay in the network Receiving a duplicate ACK may alert the sender to increase the time-out to prevent resending the packets prematurely. Note that if the receiver ignores the duplicate packet and does not send the ...
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