Increasing TCP's Initial Window
RFC 6928

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Internet Engineering Task Force (IETF)                            J. Chu
Request for Comments: 6928                                  N. Dukkipati
Category: Experimental                                          Y. Cheng
ISSN: 2070-1721                                                M. Mathis
                                                            Google, Inc.
                                                              April 2013

                    Increasing TCP's Initial Window

Abstract

   This document proposes an experiment to increase the permitted TCP
   initial window (IW) from between 2 and 4 segments, as specified in
   RFC 3390, to 10 segments with a fallback to the existing
   recommendation when performance issues are detected.  It discusses
   the motivation behind the increase, the advantages and disadvantages
   of the higher initial window, and presents results from several
   large-scale experiments showing that the higher initial window
   improves the overall performance of many web services without
   resulting in a congestion collapse.  The document closes with a
   discussion of usage and deployment for further experimental purposes
   recommended by the IETF TCP Maintenance and Minor Extensions (TCPM)
   working group.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Engineering
   Task Force (IETF).  It represents the consensus of the IETF
   community.  It has received public review and has been approved for
   publication by the Internet Engineering Steering Group (IESG).  Not
   all documents approved by the IESG are a candidate for any level of
   Internet Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6928.

Chu, et al.                   Experimental                      [Page 1]
RFC 6928             Increasing TCP's Initial Window          April 2013

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................3
      1.1. Terminology ................................................4
   2. TCP Modification ................................................4
   3. Implementation Issues ...........................................5
   4. Background ......................................................6
   5. Advantages of Larger Initial Windows ............................7
      5.1. Reducing Latency ...........................................7
      5.2. Keeping Up with the Growth of Web Object Size ..............8
      5.3. Recovering Faster from Loss on Under-Utilized or
           Wireless Links .............................................8
   6. Disadvantages of Larger Initial Windows for the Individual ......9
   7. Disadvantages of Larger Initial Windows for the Network ........10
   8. Mitigation of Negative Impact ..................................11
   9. Interactions with the Retransmission Timer .....................11
   10. Experimental Results From Large-Scale Cluster Tests ...........11
      10.1. The Benefits .............................................11
      10.2. The Cost .................................................12
   11. Other Studies .................................................13
   12. Usage and Deployment Recommendations ..........................14
   13. Related Proposals .............................................15
   14. Security Considerations .......................................16
   15. Conclusion ....................................................16
   16. Acknowledgments ...............................................16
   17. References ....................................................16
      17.1. Normative References .....................................16
      17.2. Informative References ...................................17
   Appendix A. List of Concerns and Corresponding Test Results .......21

Chu, et al.                   Experimental                      [Page 2]
RFC 6928             Increasing TCP's Initial Window          April 2013