Citrix System Inc Interview Question for Software Engineer / Developers






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in TCP the method of ensuring reliability is through acknowledgements and time-out mechanisms. For every segment sent, the sender waits for an ACK for that segment. It resends the packet if it does not receive an ACK before the time-out occurs.
In fast retransmit, if the sender gets 3 NACKS ( negative acknowledgement) then it immediately retransmits the packet without waiting for the time-out to occur. This concept is called Fast retransmit in TCP.

- vp January 09, 2011 | Flag Reply
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Does tcp has negative acknowledgements?

- Anonymous February 06, 2011 | Flag Reply
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No tcp does not have negative acknowledgements. By NACKS i mean 3 duplicate ACKS.

- vp February 07, 2011 | Flag
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This is correct
for example

packet1
packet2
packet3
packet4
packet5

if you get ACK = 2 back when you just send out packet5, which means the third times that you get ACK = 2, in other words, this means packet3 is lost (time out), and then you retransmit packet3 and keep moving

- Teng March 14, 2011 | Flag
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When the sender receives 3 duplicate ACKs then it does not wait for the timer to be finished, instead it just sends the whole window starting from that segment.
This is fast retransmit.
And then it goes into the slow start mode after that.

- alexander June 24, 2012 | Flag Reply


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