Editing Transmission Control Protocol (section)

=====Duplicate-ACK-based retransmission=====
If a single segment (say segment number 100) in a stream is lost, then the receiver cannot acknowledge packets above that segment number (100) because it uses cumulative ACKs. Hence the receiver acknowledges packet 99 again on the receipt of another data packet. This duplicate acknowledgement is used as a signal for packet loss. That is, if the sender receives three duplicate acknowledgments, it retransmits the last unacknowledged packet. A threshold of three is used because the network may reorder segments causing duplicate acknowledgements. This threshold has been demonstrated to avoid spurious retransmissions due to reordering.<ref>{{cite journal|last1=Mathis|last2=Mathew|last3=Semke|last4=Mahdavi|last5=Ott|title=The macroscopic behavior of the TCP congestion avoidance algorithm|journal=ACM SIGCOMM Computer Communication Review|volume=27|issue=3|pages=67–82|year=1997|doi=10.1145/263932.264023|citeseerx=10.1.1.40.7002|s2cid=1894993}}</ref> Some TCP implementations use [[selective acknowledgement]]s (SACKs) to provide explicit feedback about the segments that have been received. This greatly improves TCP's ability to retransmit the right segments.

Retransmission ambiguity can cause spurious fast retransmissions and congestion avoidance if there is reordering beyond the duplicate acknowledgment threshold.{{sfn|RFC 3522|p=4}} In the last two decades more packet reordering has been observed over the Internet<ref>{{cite journal |last1=Leung |first1=Ka-cheong |last2=Li |first2=Victor O.k. |last3=Yang |first3=Daiqin |date=2007 |title=An Overview of Packet Reordering in Transmission Control Protocol (TCP): Problems, Solutions, and Challenges |url=https://ieeexplore.ieee.org/document/4118693 |journal=IEEE Transactions on Parallel and Distributed Systems |volume=18 |issue=4 |pages=522–535 |doi=10.1109/TPDS.2007.1011}}</ref> which led TCP implementations, such as the one in the Linux Kernel to adopt heuristic methods to scale the duplicate acknowledgment threshold.<ref>{{cite thesis |last=Johannessen |first=Mads |date=2015 |title=Investigate reordering in Linux TCP |publisher=University of Oslo |url=http://urn.nb.no/URN:NBN:no-51662 |degree=MSc}}</ref> Recently, there have been efforts to completely phase out duplicate-ACK-based fast-retransmissions and replace them with timer based ones.<ref>{{cite conference |url=https://www.ietf.org/proceedings/94/slides/slides-94-tcpm-6.pdf |title=RACK: a time-based fast loss detection for TCP draft-cheng-tcpm-rack-00 |last1=Cheng |first1=Yuchung |date=2015 |publisher=IETF |location=Yokohama |conference=IETF94}}</ref> (Not to be confused with the classic RTO discussed below). The time based loss detection algorithm called Recent Acknowledgment (RACK){{Sfn|RFC 8985}} has been adopted as the default algorithm in Linux and Windows.<ref>{{cite conference |url=https://datatracker.ietf.org/meeting/100/materials/slides-100-tcpm-draft-ietf-tcpm-rack-01.pdf |title=RACK: a time-based fast loss recovery draft-ietf-tcpm-rack-02 |last1=Cheng |first1=Yuchung |last2=Cardwell |first2=Neal |last3=Dukkipati |first3=Nandita |last4=Jha |first4=Priyaranjan |date=2017 |publisher=IETF |location=Yokohama |conference=IETF100}}</ref>