Let's assume that A is connected to B, and A has received 100 bytes until now. So, the next packet he will receive should have a SEQ number of 100 or a little more if the packets don't reach A in order.

What happens if he receives a significantly different value, for example, 800?

1 Answer 1


Queue the traffic waiting for the rest of the traffic to arrive. If that traffic doesn't appear within some hold-off period, it will re-ACK the last segment it received. This is all well documented TCP processing. (read: use any search engine, or visit a book store.)

  • nice answer ricky Dec 20, 2013 at 20:08
  • Thank you, but I didn't find the answer despite using a search engine, most results explain how it works, not what happens in special situations like this.
    – Ulysse
    Dec 20, 2013 at 20:10
  • 3
    It's not a special situation. It's an integral part of the protocol -- reliable, error free, in order traffic delivery. 'tho I'll agree it's faster to ask someone who knows rather than research it yourself.
    – Ricky
    Dec 20, 2013 at 20:21
  • 2
    IMHO it's important to add that this only happens when the received SEQ falls within the receive window, otherwise the segment is dropped. Also, if both A and B support SACK (selective ACK) then instead of saying "I'm waiting for 100", A can also tell B "I'm waiting for 100 but I already have 800-2099" allowing B to just retransmit 100-799 instead of retransmitting everything from 100 on.
    – hertitu
    Sep 27, 2016 at 7:01

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