What is duplicate packets?


Although rare, the IP protocol may deliver a single packet more than one time. This can happen, for example, when a link-layer network protocol performs a retransmission and creates two copies of the same packet. When duplicates are created, TCP can become confused in some of the ways we have seen already. Consider the case shown in following figure in which packet number 3 has been duplicated three times.

As we can see, the effect of packet 3 being duplicated is to produce a series of duplicate ACKs from the receiver. This is enough to trigger a spurious fast retransmit, as the non-SACK sender may mistakenly believe that packets 5 and 6 have arrived earlier. With SACK (and DSACK, in particular) this is more easily diagnosed at the sender. With DSACK, each of the duplicate ACKs for A3 contains DSACK information that segment 3 has already been received. Furthermore, none of them contains an indication of any out-of-order data, meaning the arriving packets (or their ACKs) must have been duplicates. TCP can often suppress spurious retransmissions in such cases.



Packet analysis for the duplicate ACK scenario:

Duplicate ACK issue occurs because has not seen the reply its previous sent ACK frame.



Referenced Links and documents:

  • TCP.IP.Illustrated.Volume.1.2nd.Edition

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