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What is TCP seq no

Posted on 2013-06-29
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What is TCP seq no,if possible than plz explain in context of two hosts

Host A - 192.168.44.140
Host B - 192.168.44.146

Host A sends syn packet,Host B replies with syn ack,Host A sends ack to Host B.three way handshake completed. keeping this scenario in mind plz explain Tcp seq no.


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Question by:pawanopensource
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by:aadih
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by:Akinsd
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In simple terms, view sequence number as consignment number. Sequence number works in conjunction with throttling.

After the handshake, the device  sending packets (package) the following are literal conversation and transaction.

Sender: I am computer1, here is my IP address 192.168.1.1  and here is my mac address aaa.bbb.ccc
Receiver: thanks for contacting me, I am computer2, my IP is 192.168.1.2, my MAC address is bbb.ccc.ddd
Computer1. Nice to meet you I have this order for you. The order numberi is ABC123.
Computer2. Very well, send me the package.
Computer1. It seems the package won't fit in one container, I will send them to you in consignments. Ther are 1000 consignments altogether. Here is consignment 1 to  2.
Computer2. Thank you, the last sequence number (consignment) I received was 2, send me 3. By the way it's going to take forever to finish this transaction. Can we send more at once.
Computer1. Not a problem, looks like we have more than enough bandwidth. I'm going to increase the throttle and send 10 at a time. Here are consignments 3 to 12.
Conmputer2. I receive. Sequence number 12. Send me 13, maybe 100 at a time this time.
Computer1. No problem, here is sequence number 13 to 112.
Computer2. I received sequence number 69, send me 70
Computer1. Looks like we lost some consignments. No problem, I will just send 50 at a time , here are seq numbers 70 to 119.
Computer2. I acknowledge receipt of  seq number 119, send me seq n 120
Computer1. That went well, let me throttle to 80 at a time. Here are seq numbers 121 to 200.  
Etc till all 1000 sequence numbers are delivered. At which point computer 2 will send ack message for seq 1000 at the same time send a close request

I hope this analogy helps.
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by:pawanopensource
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Thx Akinsd,

can u please explain in this scenario what will happen to a tcp seq no

if pc1 is watching any movie or downloading any  data from pc2, than what will happen here.suppose data will take 2 hrs to get downloaded at pc1.will  tcp seq no will remain same until data is downloaded ?


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Sequence numbers are usually applied to tcp (reliable) packets.

Videos and voice by default run on udp (unreliable)

Reliability does not mean they are useless but means there is no means to confirm if packets are dropping or not.

The number of time it took to download packets does not affect the sequence number.

On a rainy day, for instance, it may take a truck 4 hours to run 4 trips when on a good day may just take him 15 minutes running the same trip.

.....And yes, the sequence number will not change from the total number already sent in the initial tcp header

consignments. There are 1000 consignments altogether. Here is consignment 1 to  2.

In this case download will not complete until the 1000th seq number is received.

On a passing note. Have you ever wondered when downloading a file, you may initially see 4hours or some value, all of a sudden, it changes to 20 minutes, then to 40 minutes remaining etc. That's the result of throttling. It's the same when you are traveling to a place. Your initial TOA (time of arrival) may be 20 minutes, then you hit traffic and so on.

In summary, how long it takes is a reflection of how fast the transmission is. How often the time remaining changes also reflects how often the speed changes.
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