Byte and Packet Congestion Notification
RFC 7141

Internet Engineering Task Force (IETF)                        B. Briscoe
Request for Comments: 7141                                            BT
BCP: 41                                                        J. Manner
Updates: 2309, 2914                                     Aalto University
Category: Best Current Practice                            February 2014
ISSN: 2070-1721

                Byte and Packet Congestion Notification

Abstract

   This document provides recommendations of best current practice for
   dropping or marking packets using any active queue management (AQM)
   algorithm, including Random Early Detection (RED), BLUE, Pre-
   Congestion Notification (PCN), and newer schemes such as CoDel
   (Controlled Delay) and PIE (Proportional Integral controller
   Enhanced).  We give three strong recommendations: (1) packet size
   should be taken into account when transports detect and respond to
   congestion indications, (2) packet size should not be taken into
   account when network equipment creates congestion signals (marking,
   dropping), and therefore (3) in the specific case of RED, the byte-
   mode packet drop variant that drops fewer small packets should not be
   used.  This memo updates RFC 2309 to deprecate deliberate
   preferential treatment of small packets in AQM algorithms.

Status of This Memo

   This memo documents an Internet Best Current Practice.

   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).  Further information on
   BCPs is available in 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/rfc7141.

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RFC 7141         Byte and Packet Congestion Notification   February 2014

Copyright Notice

   Copyright (c) 2014 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.

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RFC 7141         Byte and Packet Congestion Notification   February 2014

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Terminology and Scoping . . . . . . . . . . . . . . . . .   6
     1.2.  Example Comparing Packet-Mode Drop and Byte-Mode Drop . .   7
   2.  Recommendations . . . . . . . . . . . . . . . . . . . . . . .   9
     2.1.  Recommendation on Queue Measurement . . . . . . . . . . .   9
     2.2.  Recommendation on Encoding Congestion Notification  . . .  10
     2.3.  Recommendation on Responding to Congestion  . . . . . . .  11
     2.4.  Recommendation on Handling Congestion Indications When
           Splitting or Merging Packets  . . . . . . . . . . . . . .  12
   3.  Motivating Arguments  . . . . . . . . . . . . . . . . . . . .  13
     3.1.  Avoiding Perverse Incentives to (Ab)use Smaller Packets .  13
     3.2.  Small != Control  . . . . . . . . . . . . . . . . . . . .  14
     3.3.  Transport-Independent Network . . . . . . . . . . . . . .  14
     3.4.  Partial Deployment of AQM . . . . . . . . . . . . . . . .  16
     3.5.  Implementation Efficiency . . . . . . . . . . . . . . . .  17
   4.  A Survey and Critique of Past Advice  . . . . . . . . . . . .  17
     4.1.  Congestion Measurement Advice . . . . . . . . . . . . . .  18
       4.1.1.  Fixed-Size Packet Buffers . . . . . . . . . . . . . .  18
       4.1.2.  Congestion Measurement without a Queue  . . . . . . .  19
     4.2.  Congestion Notification Advice  . . . . . . . . . . . . .  20
       4.2.1.  Network Bias When Encoding  . . . . . . . . . . . . .  20
       4.2.2.  Transport Bias When Decoding  . . . . . . . . . . . .  22
       4.2.3.  Making Transports Robust against Control Packet
               Losses  . . . . . . . . . . . . . . . . . . . . . . .  23
       4.2.4.  Congestion Notification: Summary of Conflicting
               Advice  . . . . . . . . . . . . . . . . . . . . . . .  24