Simple Lost Retransmission Detection with SACK TCP
draft-scheffenegger-tcpm-lrd-00

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Author Richard Scheffenegger 
Last updated 2021-03-12
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TCPM                                                    R. Scheffenegger
Internet-Draft                                                    NetApp
Intended status: Experimental                             March 12, 2021
Expires: September 13, 2021

           Simple Lost Retransmission Detection with SACK TCP
                    draft-scheffenegger-tcpm-lrd-00

Abstract

   Lost Retransmissions are a major source of latency for TCP transfers.
   This note specifies how selective acknowledgment (SACK) information
   can be used to timely recover from lost retransmissions.  In
   addition, it codifies the congestion control reaction on lost
   retransmissions.

Note to Readers

   Discussion of this draft takes place on the TCPM working group
   mailing list [1], which is archived at
   <https://mailarchive.ietf.org/arch/browse/tcpm/>.

   Working Group information can be found at
   <https://datatracker.ietf.org/wg/tcpm/>;

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on September 13, 2021.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Definitions . . . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Design Considerations . . . . . . . . . . . . . . . . . . . .   4
     5.1.  Recovery Initiation . . . . . . . . . . . . . . . . . . .   4
     5.2.  Detection of lost retransmissions . . . . . . . . . . . .   4
     5.3.  Reordering  . . . . . . . . . . . . . . . . . . . . . . .   5
     5.4.  Ordering of retransmitted segments  . . . . . . . . . . .   6
   6.  Algorithm . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     6.1.  Lost Retransmission Detection . . . . . . . . . . . . . .   7
     6.2.  LRD Algorithm Detail  . . . . . . . . . . . . . . . . . .   8
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     10.2.  Informative References . . . . . . . . . . . . . . . . .  10
     10.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  11
   Appendix A.  Lost Retransmission Detection Example  . . . . . . .  12
     A.1.  Lost Retransmission, Mid-Stream . . . . . . . . . . . . .  12
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   Selective Acknowledgement (SACK) is widely used to identify exactly
   which TCP segment was lost and only send these missing segments
   during a recovery episode.  This helps improve the effectiveness of
   loss recovery and aligns with the principle of packet conservation.
   In addition, SACK information can also be used to infer about lost
   retransmissions.  When this information is not used, TCP senders
   revert to the retransmission timeout (RTO) scheme to recover from
   lost retransmissions.

   Current SACK implementations, with one widely deployed exception, do
   not perform lost retransmission detection.  Lost retransmission

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   detection (LRD) in the one implementation that performs it was
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