IP Performance Metrics Working                               P. Chimento
Group                                            JHU Applied Physics Lab
Internet-Draft                                                  J. Ishac
Expires: December 23, 2005                    NASA Glenn Research Center
                                                           June 21, 2005

                       Defining Network Capacity

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   Copyright (C) The Internet Society (2005).


   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 build a common

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   language that can be used when discussing and analyzing a diverse set
   of current and future estimation techniques.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3

   2.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  5

   3.  Discussion . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     3.1   Common Literature Terminology  . . . . . . . . . . . . . .  7
     3.2   Type P Packets . . . . . . . . . . . . . . . . . . . . . .  8

   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9

   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10

   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11

   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     7.1   Normative References . . . . . . . . . . . . . . . . . . . 12
     7.2   Informative References . . . . . . . . . . . . . . . . . . 12

       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 12

       Intellectual Property and Copyright Statements . . . . . . . . 14

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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 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

   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
   medium (air) is lossy.  This can further decrease the information

   In addition to coding schemes, usually the physical layer and the
   link layer add framing bits for multiplexing and control purposes.
   For example, on SONET there is physical layer framing and typically
   also some layer 2 framing such as HDLC, PPP or even ATM.

   Aside from questions of coding efficiency, there are issues of how
   access to the channel is controlled which may affect the capacity
   also.  For example, a multiple-access medium with collision
   detection, avoidance and recovery mechanisms has a varying capacity
   from the point of view of the users.  This varying capacity depends
   upon the total number of users contending for the medium, how busy
   the users are, and bounds resulting from the mechanisms themselves.
   RF channels are also varying capacity, depending on range,
   environmental conditions, mobility and shadowing, etc.

   The important points to derive from this discussion are these: First,
   capacity is only meaningful when defined relative to a given protocol
   layer in the network.  It is meaningless to speak of "link" capacity

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   without qualifying exactly what is meant.  Second, capacity is not
   necessarily fixed, and consequently, a single measure of capacity at
   whatever layer may in fact provide a skewed picture (either
   optimistic or pessimistic) of what is actually available.

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2.  Definitions

   In this section, we specify definitions for capacity.  We begin by
   first defining a baseline capacity that is simply tied to the
   physical properties of the link.

   Nominal Physical Link Capacity:
      Or NomCap(L) is the theoretical maximum amount of data that the
      link can support.  For example, an OC-3 link would be capable of
      roughly 155 Mbps and does not vary with time.  We stress that this
      is a measurement at the physical layer and not the network IP
      layer, which we will define separately.

   There are many factors that can reduce the information carrying
   capacity of the link, some of which have already been discussed
   earlier in the introduction.  However, the goal of this document is
   not to become an exhaustive list of of such factors.  Rather, we
   outline some of the major examples in the following section, thus
   providing food for thought to those implementing the algorithms or
   tools that attempt to accurately quantify these values.

   The remaining definitions are all given in terms of "IP layer bits"
   in order to distinguish these definitions from the nominal physical
   capacity of the link.  Also, given a time T and a time interval I, we
   define a path P of length n as a series of links (L1, L2, ..., Ln)
   connecting a sequence of nodes (N1, N2, ..., Nn+1).  A source S and
   destination D reside at N1 and Nn+1 respectively.  Furthermore, we
   define a link L as a special case where the path size is one.

   IP layer bits:
      Eight (8) times the number of octets in all IP packets received,
      from the first octet of the IP header to the last octet of the IP
      packet payload, inclusive.

   The definitions are also defined as an average, not as an
   instantaneous value.  Thus, the time parameters, T and I, must
   accompany any report or estimate of the following values in order for
   them to remain meaningful.

   IP Layer Link Capacity:
      We define the IP Layer link capacity, C(L,T,I), to be the maximum
      number of IP layer bits that can be transmitted from S and
      correctly received by D over the link L during the interval T to
      T+I, divided by I.

   Using this, we can then extend this notion to an entire path, such
   that the IP layer path capacity simply becomes that of the link with
   the smallest capacity along that path.

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   IP Layer Path Capacity:
      C(P,T,I) = min {1..n} {C(Ln,T,I)}

   The previous definitions specify a link's capacity, namely the IP
   information bits that can be transmitted across a link or path should
   the resource be free of any contention.  Determining how much
   capacity is available for use on a congested link is potentially much
   more useful.  However, in order to define the available capacity we
   must first specify how much is being used.

   IP Layer Link Usage:
      The average usage of a link L, Used(L,T,I), is the actual number
      of IP layer bits correctly transmitted from any source over link L
      from time T to time T+I, divided by I.

   An important distinction between usage and capacity is that
   Used(L,T,I) is not the maximum amount, but rather, the actual amount
   of IP bits that are sent.  The information transmitted across the
   link can be generated by any source, including those who may not be
   directly attached to either side of the link.  In addition, each
   information flow from these sources may share any number (from one to
   n) of links in the overall path between S and D. Next, we express
   usage as a fraction of the overall IP layer link capacity.

   Average IP Layer Link Utilization:
      Util(L,T,I) = ( Used(L,T,I) / C(L,T,I) )

   Thus, the utilization now represents the fraction of the capacity
   that is being used and is a value between zero, meaning nothing is
   used, and one, meaning the link is fully saturated.  Multiplying the
   utilization by 100 yields the percent utilization of the link.  By
   using the above, we can now define the capacity available over the
   link as well as the path between S and D. Note that this is
   essentially the definition in [PDM].

   IP Layer Available Link Capacity
      AvailCap(L,T,I) = C(L,T,I) * ( 1 - Util(L,T,I) )

   IP Layer Available Path Capacity
      AvailCap(P,T,I) = min {1..n} {AvailCap(Ln,T,I)}

   Since measurements of available capacity are more volatile that that
   of capacity, it is important that both the time and interval be
   specified as their values have a great deal of influence on the
   results.  In addition, a range of measurements may be beneficial in
   offsetting the volatility when attempting to characterize available

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3.  Discussion

   The base definitions specify that the IP layer packets must be
   received correctly.  This is deliberate, because of the possible
   presence of lossy multiple-access media.  On optical media, at a
   nominal bit error rate of 10**-15, a stream transmitted at 10 Gbps
   will experience an error about every 1 2/3 weeks on the average.  On
   wireless media, the error rate is considerably larger than that,
   reducing the effective capacity accordingly.  Note that the reception
   of a partial packet cannot be checked for correctness, and
   consequently the definitions count as capacity only bits received as
   part of whole IP packets.

   The base definitions also make no mention of hardware duplication of
   packets.  While hardware duplication has no impact on the nominal
   capacity, it can impact the IP link layer capacity.  For example,
   consider a link which can normally carry a capacity of 2X on average.
   However, the link has developed a syndrome where it duplicates every
   incoming packet.  The link would still technically carry a capacity
   of 2X, however the link has a effective capacity of X or lower,
   depending on framing overhead to send the duplicates, etc.  Thus, a
   value for C(L,T,I) and AvailCap(L,T,I) will reflect the duplication
   with the lower value.

   IP encapsulation does not impact the definitions as all IP header and
   payload bits should be counted regardless of content.  However,
   different sized IP packets can lead to a variation in the amount of
   overhead needed at the lower layers to transmit the data, thus
   altering the overall IP link layer capacity.

   Should the link happen to employ a compression scheme such as ROHC
   [RFC3095] or V.44 [V44], some of the original bits are not
   transmitted across the link.  However, the inflated (not compressed)
   number of IP-layer bits should be counted.

3.1  Common Literature Terminology

   Certain terms are often used to characterize specific aspects of the
   presented definitions.  The link with the smallest capacity is
   commonly referred to as the "narrow link" of a path.  Also, the value
   of n that satisfies AvailCap(P,T,I), is often referred to as the
   "tight link" within a path.  So, while Ln may have a very large
   capacity, the overall congestion level on the link makes it the
   likely bottleneck of a connection.  Conversely, a link that has the
   smallest capacity may not be a bottleneck should it be lightly
   congested in relation to the rest of the path.

   Also, common literature often overloads the term "bandwidth" to refer

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   to what we have described as capacity in this document.  For example,
   when inquiring about the bandwidth of a 802.11b link, a network
   engineer will likely answer with 11 Mbps.  However, an electrical
   engineer may answer with 25 MHz, and an end user may tell you that
   his observed bandwidth is 8 Mbps.  In contrast, the term capacity is
   not quite as overloaded and is an appropriate term that better
   reflects what is actually being measured.

3.2  Type P Packets

   Note that these definitions do not make mention of "Type P" packets,
   while other IPPM definitions do.  We could add the packet type as an
   extra parameter.  This would have the effect of defining a large
   number of quantities, relative to the QoS policies that a given
   network or concatenation of networks may have in effect in the path.
   It would produce metrics such as "estimated EF IP Link/Path

   Such metrics may indeed be useful.  For example, this would yield
   something like the sum of the capacities of all the QoS classes
   defined along the path as the link or path capacity.  The breakdown
   then gives the user an analysis of how the link or path capacity (or
   at least the "tight link" capacity) is allocated among classes.

   These QoS-based capacities become difficult to measure on a path if
   there are different capacities defined per QoS class on different
   links in the path.  Possibly the best way to approach this would be
   to measure each link in a path individually, and then combine the
   information from individual links.  However, such an approach is
   extremely difficult in practice.

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4.  IANA Considerations

   This document makes no request of IANA.

   Note to RFC Editor: this section may be removed on publication as an

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5.  Security Considerations

   This document specifies definitions regarding IP traffic traveling
   between a source and destination in an IP network.  These definitions
   do not raise any security issues and do not have a direct impact on
   the networking protocol suite.

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6.  Acknowledgements

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7.  References

7.1  Normative References

7.2  Informative References

   [PDM]      Dovrolis, C., Ramanathan, P., and D. Moore, "Packet
              Dispersion Techniques and a Capacity Estimation
              Methodology", IEEE/ACM Transactions on Networking 12(6):
              963-977, December 2004.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3095]  Bormann, C., Burmeister, C., Degermark, M., Fukushima, H.,
              Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T., Le,
              K., Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K.,
              Wiebke, T., Yoshimura, T., and H. Zheng, "RObust Header
              Compression (ROHC): Framework and four profiles: RTP, UDP,
              ESP, and uncompressed", RFC 3095, July 2001.

   [V44]      ITU Telecommunication Standardization Sector (ITU-T)
              Recommendation V.44, "Data Compression Procedures",
              November 2000.

Authors' Addresses

   Phil Chimento
   JHU Applied Physics Lab
   11100 Johns Hopkins Road
   Laurel, Maryland  20723-6099

   Phone: +1-240-228-1743
   Fax:   +1-240-228-0789
   Email: Philip.Chimento@jhuapl.edu

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   Joseph Ishac
   NASA Glenn Research Center
   21000 Brookpark Road
   Cleveland, Ohio  44135

   Phone: +1-216-433-6587
   Fax:   +1-216-433-8705
   Email: jishac@grc.nasa.gov

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