Defining Network Capacity
RFC 5136

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Network Working Group                                        P. Chimento
Request for Comments: 5136                       JHU Applied Physics Lab
Category: Informational                                         J. Ishac
                                              NASA Glenn Research Center
                                                           February 2008

                       Defining Network Capacity

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Abstract

   Measuring capacity is a task that sounds simple, but in reality can
   be quite complex.  In addition, the lack of a unified nomenclature on
   this subject makes it increasingly difficult to properly build, test,
   and use techniques and tools built around these constructs.  This
   document provides definitions for the terms 'Capacity' and 'Available
   Capacity' related to IP traffic traveling between a source and
   destination in an IP network.  By doing so, we hope to provide a
   common framework for the discussion and analysis of a diverse set of
   current and future estimation techniques.

Chimento & Ishac             Informational                      [Page 1]
RFC 5136                    Network Capacity               February 2008

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Links and Paths  . . . . . . . . . . . . . . . . . . . . .  4
     2.2.  Definition: Nominal Physical Link Capacity . . . . . . . .  4
     2.3.  Capacity at the IP Layer . . . . . . . . . . . . . . . . .  5
       2.3.1.  Definition: IP-layer Bits  . . . . . . . . . . . . . .  5
         2.3.1.1.  Standard or Correctly Formed Packets . . . . . . .  5
         2.3.1.2.  Type P Packets . . . . . . . . . . . . . . . . . .  6
       2.3.2.  Definition: IP-type-P Link Capacity  . . . . . . . . .  7
       2.3.3.  Definition: IP-type-P Path Capacity  . . . . . . . . .  7
       2.3.4.  Definition: IP-type-P Link Usage . . . . . . . . . . .  7
       2.3.5.  Definition: IP-type-P Link Utilization . . . . . . . .  8
       2.3.6.  Definition: IP-type-P Available Link Capacity  . . . .  8
       2.3.7.  Definition: IP-type-P Available Path Capacity  . . . .  8
   3.  Discussion . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     3.1.  Time and Sampling  . . . . . . . . . . . . . . . . . . . .  9
     3.2.  Hardware Duplicates  . . . . . . . . . . . . . . . . . . .  9
     3.3.  Other Potential Factors  . . . . . . . . . . . . . . . . .  9
     3.4.  Common Terminology in Literature . . . . . . . . . . . . . 10
     3.5.  Comparison to Bulk Transfer Capacity (BTC) . . . . . . . . 10
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   5.  Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 11
   6.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 11
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 12
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 12

Chimento & Ishac             Informational                      [Page 2]
RFC 5136                    Network Capacity               February 2008

1.  Introduction

   Measuring the capacity of a link or network path is a task that
   sounds simple, but in reality can be quite complex.  Any physical
   medium requires that information be encoded and, depending on the
   medium, there are various schemes to convert information into a
   sequence of signals that are transmitted physically from one location
   to another.

   While on some media, the maximum frequency of these signals can be
   thought of as "capacity", on other media, the signal transmission
   frequency and the information capacity of the medium (channel) may be
   quite different.  For example, a satellite channel may have a carrier
   frequency of a few gigahertz, but an information-carrying capacity of
   only a few hundred kilobits per second.  Often similar or identical
   terms are used to refer to these different applications of capacity,
   adding to the ambiguity and confusion, and the lack of a unified
   nomenclature makes it difficult to properly build, test, and use
   various techniques and tools.

   We are interested in information-carrying capacity, but even this is
   not straightforward.  Each of the layers, depending on the medium,
   adds overhead to the task of carrying information.  The wired
   Ethernet uses Manchester coding or 4/5 coding, which cuts down
   considerably on the "theoretical" capacity.  Similarly, RF (radio
   frequency) communications will often add redundancy to the coding
   scheme to implement forward error correction because the physical