Improving the Robustness of TCP to Non-Congestion Events
RFC 4653

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Network Working Group                                      S. Bhandarkar
Request for Comments: 4653                                A. L. N. Reddy
Category: Experimental                              Texas A&M University
                                                               M. Allman
                                                               ICIR/ICSI
                                                              E. Blanton
                                                       Purdue University
                                                             August 2006

        Improving the Robustness of TCP to Non-Congestion Events

Status of This Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document specifies Non-Congestion Robustness (NCR) for TCP.  In
   the absence of explicit congestion notification from the network, TCP
   uses loss as an indication of congestion.  One of the ways TCP
   detects loss is using the arrival of three duplicate acknowledgments.
   However, this heuristic is not always correct, notably in the case
   when network paths reorder segments (for whatever reason), resulting
   in degraded performance.  TCP-NCR is designed to mitigate this
   degraded performance by increasing the number of duplicate
   acknowledgments required to trigger loss recovery, based on the
   current state of the connection, in an effort to better disambiguate
   true segment loss from segment reordering.  This document specifies
   the changes to TCP, as well as the costs and benefits of these
   modifications.

Bhandarkar, et al.            Experimental                      [Page 1]
RFC 4653            Improving the Robustness of TCP          August 2006

Table of Contents

   1. Introduction ....................................................2
      1.1. Terminology ................................................4
   2. NCR Description .................................................5
   3. Algorithm .......................................................6
      3.1. Initialization .............................................8
      3.2. Terminating Extended Limited Transmit and
           Preventing Bursts ..........................................9
      3.3. Extended Limited Transmit .................................10
      3.4. Entering Loss Recovery ....................................11
   4. Advantages .....................................................12
   5. Disadvantages ..................................................12
   6. Related Work ...................................................13
   7. Security Considerations ........................................14
   8. Acknowledgments ................................................14
   9. IANA Considerations ............................................14
   10. References ....................................................14
      10.1. Normative References .....................................14
      10.2. Informative References ...................................15

1.  Introduction

   One strength of TCP [RFC793] lies in its ability to adjust its
   sending rate according to the perceived congestion in the network
   [Jac88, RFC2581].  In the absence of explicit notification of
   congestion from the network, TCP uses segment loss as an indication
   of congestion (i.e., assuming queue overflow).  TCP receivers send
   cumulative acknowledgments (ACKs) indicating the next sequence number
   expected from the sender for arriving segments [RFC793].  When
   segments arrive out of order, duplicate ACKs are generated.  As
   specified in [RFC2581], a TCP sender uses the arrival of three
   duplicate ACKs as an indication of segment loss.  The TCP sender
   retransmits the lost segment and reduces the load imposed on the
   network, assuming the segment loss was caused by resource contention
   within the network path.  The TCP sender does not assume loss on the
   first or second duplicate ACK, but waits for three duplicate ACKs to
   account for minor packet reordering.  However, the use of this
   constant threshold of duplicate ACKs has several problems that can be
   mitigated with a dynamic threshold.

   The following is an example of TCP's behavior:

     + TCP A is the data sender, and TCP B is the data receiver.

     + TCP A sends 10 segments, each consisting of a single data byte
       (i.e., transmits bytes 1-10 in segments 1-10).

Bhandarkar, et al.            Experimental                      [Page 2]
RFC 4653            Improving the Robustness of TCP          August 2006

     + Assume segment 3 is dropped in the network.

     + TCP B cumulatively acknowledges segments 1 and 2, making the
       cumulative ACK transmitted to the sender 3 (the next expected