RFC 3708 (rfc3708) - Page 2 of 9

Using TCP Duplicate Selective Acknowledgement (DSACKs) and Stream Control Transmission Protocol (SCTP) Duplicate Transmission Sequence Numbers (TSNs) to Detect Spurious Retransmissions

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RFC 3708           TCP DSACKs and SCTP Duplicate TSNs      February 2004


   This document is intended to outline reasonable and safe algorithms
   for detecting spurious retransmissions and discuss some of the
   considerations involved.  It is not intended to describe the only
   possible method for achieving the goal, although the guidelines in
   this document should be taken into consideration when designing
   alternate algorithms.  Additionally, this document does not outline
   what a TCP or SCTP sender may do after a spurious retransmission is
   detected.  A number of proposals have been developed (e.g.,
   [RFC 3522], [SK03], [BDA03]), but it is not yet clear which of these
   proposals are appropriate.  In addition, they all rely on detecting
   spurious retransmits and so can share the algorithm specified in this
   document.

   Finally, we note that to simplify the text much of the following
   discussion is in terms of TCP DSACKs, while applying to both TCP and
   SCTP.

   Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC 2119].

2.  Counting Duplicate Notifications

   For certain applications a straight count of duplicate notifications
   will suffice.  For instance, if a stack simply wants to know (for
   some reason) the number of spuriously retransmitted segments,
   counting all duplicate notifications for retransmitted segments
   should work well.  Another application of this strategy is to monitor
   and adapt transport algorithms so that the transport is not sending
   large amounts of spurious data into the network.  For instance,
   monitoring duplicate notifications could be used by the Early
   Retransmit [AAAB03] algorithm to determine whether fast
   retransmitting [RFC 2581] segments with a lower than normal duplicate
   ACK threshold is working, or if segment reordering is causing
   spurious retransmits.

   More speculatively, duplicate notification has been proposed as an
   integral part of estimating TCP's total loss rate [AEO03] for the
   purposes of mitigating the impact of corruption-based losses on
   transport protocol performance.  [EOA03] proposes altering the
   transport's congestion response to the fraction of losses that are
   actually due to congestion by requiring the network to provide the
   corruption-based loss rate and making the transport sender estimate
   the total loss rate.  Duplicate notifications are a key part of
   estimating the total loss rate accurately [AEO03].




Blanton & Allman              Experimental
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