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What is a TCP Duplicate ACK?

Posted on 2008-09-29
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Hi,
  I'm writing a web service client and my wireshark output shows MANY TCP DUP ACKs.  What are they and does this indicate a problem somewhere?

Thanks!
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Question by:pbenito
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rweil earned 2000 total points
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Apparently there is either transmisison problems (lost packets), orpakets being sent out of order.

from http://www.isi.edu/nsnam/DIRECTED_RESEARCH/DR_WANIDA/DR/JavisInActionFastRetransmitFrame.html

The idea of fast retransmit is straightforward. It only adds a tiny thing to the normal operation of TCP. Every time a packet with sequence number x arrives correctly at the receiver; the receiver acknowledges the packet #x by sending an ACK packet (containing the sequence number of another packet which it is waiting for - this number may or may not be "x+1") back to the sender. Therefore, when a packet arrives out of order (for example, 1 2 3 ... 5 : "4" is missing), TCP at the receiving side resends the last ACK packet to portray the expected packet again. This causes a duplicate ACK at the sending side. ("duplicate ACK" means the second, third, fourth, ... transmission of the same acknowledgement.) Fast retransmit plays an important role here. After receiving some numbers of duplicate ACKs, TCP at the sending side retransmits the missing packet without waiting for the timer to be expired. Moreover, receiving some numbers of duplicate ACKs means that the network congestion has been occured. Thus, TCP at the sending side resets cwnd to 1 and sets ssthresh to (old cwnd / 2) due to the congestion control algorithm; then starts slow-start again. In the practical TCP, the third duplicate ACKs triggers fast retransmit.

 

Note ! The reason that the sending side has to wait until the third duplicate ACK is described in RFC2001 as follows:

    " Since TCP does not know whether a duplicate ACK is caused by a lost segment or just a reordering of segments, it waits for a small number of duplicate ACKs to be received. It is assumed that if there is just a reordering of the segments, there will be only one or two duplicate ACKs before the reordered segment is processed, which will then generate a new ACK. If three or more duplicate ACKs are received in a row, it is a strong indication that a segment has been lost. "

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