RFC 3246 (rfc3246) - Page 2 of 16

An Expedited Forwarding PHB (Per-Hop Behavior)

RFC 3246              An Expedited Forwarding PHB             March 2002


Table of Contents

   1      Introduction  ...........................................   2
   1.1    Relationship to RFC 2598  ...............................   3
   2      Definition of EF PHB  ...................................   3
   2.1    Intuitive Description of EF  ............................   3
   2.2    Formal Definition of the EF PHB  ........................   5
   2.3    Figures of merit  .......................................   8
   2.4    Delay and jitter  .......................................   8
   2.5    Loss  ...................................................   9
   2.6    Microflow misordering  ..................................   9
   2.7    Recommended codepoint for this PHB  .....................   9
   2.8    Mutability  .............................................  10
   2.9    Tunneling  ..............................................  10
   2.10   Interaction with other PHBs  ............................  10
   3      Security Considerations  ................................  10
   4      IANA Considerations  ....................................  11
   5      Acknowledgments  ........................................  11
   6      References  .............................................  11
   Appendix: Implementation Examples ..............................  12
   Authors' Addresses .............................................  14
   Full Copyright Statement .......................................  16

1. Introduction

   Network nodes that implement the differentiated services enhancements
   to IP use a codepoint in the IP header to select a per-hop behavior
   (PHB) as the specific forwarding treatment for that packet [3, 4].
   This memo describes a particular PHB called expedited forwarding
   (EF).

   The intent of the EF PHB is to provide a building block for low loss,
   low delay, and low jitter services.  The details of exactly how to
   build such services are outside the scope of this specification.

   The dominant causes of delay in packet networks are fixed propagation
   delays (e.g. those arising from speed-of-light delays) on wide area
   links and queuing delays in switches and routers.  Since propagation
   delays are a fixed property of the topology, delay and jitter are
   minimized when queuing delays are minimized.  In this context, jitter
   is defined as the variation between maximum and minimum delay.  The
   intent of the EF PHB is to provide a PHB in which suitably marked
   packets usually encounter short or empty queues.  Furthermore, if
   queues remain short relative to the buffer space available, packet
   loss is also kept to a minimum.






Davie, et. al.              Standards Track