Low Extra Delay Background Transport (LEDBAT)
RFC 6817

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Internet Engineering Task Force (IETF)                       S. Shalunov
Request for Comments: 6817                                      G. Hazel
Category: Experimental                                  BitTorrent, Inc.
ISSN: 2070-1721                                               J. Iyengar
                                           Franklin and Marshall College
                                                           M. Kuehlewind
                                                 University of Stuttgart
                                                           December 2012

             Low Extra Delay Background Transport (LEDBAT)

Abstract

   Low Extra Delay Background Transport (LEDBAT) is an experimental
   delay-based congestion control algorithm that seeks to utilize the
   available bandwidth on an end-to-end path while limiting the
   consequent increase in queueing delay on that path.  LEDBAT uses
   changes in one-way delay measurements to limit congestion that the
   flow itself induces in the network.  LEDBAT is designed for use by
   background bulk-transfer applications to be no more aggressive than
   standard TCP congestion control (as specified in RFC 5681) and to
   yield in the presence of competing flows, thus limiting interference
   with the network performance of competing flows.

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/rfc6817.

Shalunov, et al.              Experimental                      [Page 1]
RFC 6817                         LEDBAT                    December 2012

Copyright Notice

   Copyright (c) 2012 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.

Shalunov, et al.              Experimental                      [Page 2]
RFC 6817                         LEDBAT                    December 2012

Table of Contents

   1. Introduction ....................................................4
      1.1. Requirements Notation ......................................4
      1.2. Design Goals ...............................................4
      1.3. Applicability ..............................................5
   2. LEDBAT Congestion Control .......................................6
      2.1. Overview ...................................................6
      2.2. Preliminaries ..............................................6
      2.3. Receiver-Side Operation ....................................7
      2.4. Sender-Side Operation ......................................7
           2.4.1. An Overview .........................................7
           2.4.2. The Complete Sender Algorithm .......................8
      2.5. Parameter Values ..........................................11
   3. Understanding LEDBAT Mechanisms ................................13
      3.1. Delay Estimation ..........................................13
           3.1.1. Estimating Base Delay ..............................13
           3.1.2. Estimating Queueing Delay ..........................13
      3.2. Managing the Congestion Window ............................14
           3.2.1. Window Increase: Probing for More Bandwidth ........14
           3.2.2. Window Decrease: Responding to Congestion ..........14
      3.3. Choosing the Queuing Delay Target .........................15
   4. Discussion .....................................................15
      4.1. Framing and ACK Frequency Considerations ..................15
      4.2. Competing with TCP Flows ..................................15
      4.3. Competing with Non-TCP Flows ..............................16
      4.4. Fairness among LEDBAT Flows ...............................16