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TOPIC: NACK

NACK 11 years 6 months ago #8523

Im taking my INTRO exam tommorow afternoon and have been reviewing my copy of Exam Cram for the test. I came across a question that I thought was wrong and wanted to know if indeed, it is wrong. The question is:

If a destination did not recieve a segment, it will send back a notification with a ________ that notifies the sender that it needs to resend that segment.

The answer was nack(negative acknowledgement), but I have read extensively about TCP and the flags issued in the communication proccess and no where in my reading have a I come across a NACK flag. I was under the impression that if a destination has a window of 3 and the sender sends all 3 segments and the host did not recieve segment three it would send an ack back of 3, telling the sender to resend the segment. If the the ack was + 1(4), then this would tell the sender that all segments were recieved and to send 4-7 now. If anyone could explain as to if I'm right or wrong and point me to an indepth technical explanation about the NACK flag, I would greatly appreciate it.

Farewell,
Dave
"He who breaks something to find out what it is, has left the path of wisdom."

Gandalf the Grey
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Re: NACK 11 years 4 months ago #9469

  • Ballisaini
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Hello Cyber,
TCP use positive ACK as well Negative ACK;it works on windowing technique and im sure u must be knowing about windowing as tomm is your exam(lol).After recieving each windown reciever sends Positive acknowledgement to sender however if any packet is lost then it send negative acknowledgement as a request to resend that packet.Hope i have cleared your doubt;Best of Luck for your exam.

Your Buddy;
Balli
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Re: NACK 11 years 4 months ago #9475

Thanks for the response, although I aced the exam in June as well as the ICND not to much later. I was just confused because I could not find a specification for it in the rfc for tcp, that is, no packet structure related to a NACK. I knew the entire process of TCP, I just could not find an actual definition or specification anywhere(formally, like an RFC) that used the terminology of "NACK".
"He who breaks something to find out what it is, has left the path of wisdom."

Gandalf the Grey
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Re: NACK 11 years 4 months ago #9490

  • ReX
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I also could not find anything about TCP having a NACK notification, packet or response. What I did find was that TCP uses acknowledgments assuming that if no ACK is receive that the message was not received and should be resent. The only reference to NACK I found was a DHCPDECLINE response to a client requesting an IP that was already in use.

Hope that helps. Congrats on your cert!
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