





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.

























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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























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

   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 High-Level Data Link Control
   (HDLC), PPP, or ATM.

   Aside from questions of coding efficiency, there are issues of how
   access to the channel is controlled, which also may affect the
   capacity.  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 may also vary in capacity, depending on range,
   environmental conditions, mobility, shadowing, etc.






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

2.  Definitions

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

2.1.  Links and Paths

   To define capacity, we need to broaden the notions of link and path
   found in the IP Performance Metrics (IPPM) framework document
   [RFC2330] to include network devices that can impact IP capacity
   without being IP aware.  For example, consider an Ethernet switch
   that can operate ports at different speeds.

   We define nodes as hosts, routers, Ethernet switches, or any other
   device where the input and output links can have different
   characteristics.  A link is a connection between two of these network
   devices or nodes.  We then 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 length is one.

2.2.  Definition: Nominal Physical Link Capacity

   Nominal Physical Link Capacity, NomCap(L), is the theoretical maximum
   amount of data that the link L can support.  For example, an OC-3
   link would be capable of 155.520 Mbit/s.  We stress that this is a
   measurement at the physical layer and not the network IP layer, which
   we will define separately.  While NomCap(L) is typically constant
   over time, there are links whose characteristics may allow otherwise,
   such as the dynamic activation of additional transponders for a
   satellite link.

   The nominal physical link capacity is provided as a means to help
   distinguish between the commonly used link-layer capacities and the
   remaining definitions for IP-layer capacity.  As a result, the value
   of NomCap(L) does not influence the other definitions presented in
   this document.  Instead, it provides an upper bound on those values.



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2.3.  Capacity at the IP Layer

   There are many factors that can reduce the IP information carrying
   capacity of the link, some of which have already been discussed in
   the introduction.  However, the goal of this document is not to
   become an exhaustive list 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 measure capacity accurately.

   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.

2.3.1.  Definition: IP-layer Bits

   IP-layer bits are defined as 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.

   IP-layer bits are recorded at the destination D beginning at time T
   and ending at a time T+I.  Since the definitions are based on
   averages, the two time parameters, T and I, must accompany any report
   or estimate of the following values in order for them to remain
   meaningful.  It is not required that the interval boundary points
   fall between packet arrivals at D.  However, boundaries that fall
   within a packet will invalidate the packets on which they fall.
   Specifically, the data from the partial packet that is contained
   within the interval will not be counted.  This may artificially bias
   some of the values, depending on the length of the interval and the
   amount of data received during that interval.  We elaborate on what
   constitutes correctly received data in the next section.

2.3.1.1.  Standard or Correctly Formed Packets

   The definitions in this document specify that IP packets must be
   received correctly.  The IPPM framework recommends a set of criteria
   for such standard-formed packets in Section 15 of [RFC2330].
   However, it is inadequate for use with this document.  Thus, we
   outline our own criteria below while pointing out any variations or
   similarities to [RFC2330].

   First, data that is in error at layers below IP and cannot be
   properly passed to the IP layer must not be counted.  For example,
   wireless media often have a considerably larger error rate than wired
   media, resulting in a reduction in IP link capacity.  In accordance
   with the IPPM framework, packets that fail validation of the IP




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   header must be discarded.  Specifically, the requirements in
   [RFC1812], Section 5.2.2, on IP header validation must be checked,
   which includes a valid length, checksum, and version field.

   The IPPM framework specifies further restrictions, requiring that any
   transport header be checked for correctness and that any packets with
   IP options be ignored.  However, the definitions in this document are
   concerned with the traversal of IP-layer bits.  As a result, data
   from the higher layers is not required to be valid or understood as
   that data is simply regarded as part of the IP packet.  The same
   holds true for IP options.  Valid IP fragments must also be counted
   as they expend the resources of a link even though assembly of the
   full packet may not be possible.  The IPPM framework differs in this
   area, discarding IP fragments.

   For a discussion of duplicates, please see Section 3.2.

   In summary, any IP packet that can be properly processed must be
   included in these calculations.

2.3.1.2.  Type P Packets

   The definitions in this document refer to "Type P" packets to
   designate a particular type of flow or sets of flows.  As defined in
   RFC 2330, Section 13, "Type P" is a placeholder for what may be an
   explicit specification of the packet flows referenced by the metric,
   or it may be a very loose specification encompassing aggregates.  We
   use the "Type P" designation in these definitions in order to
   emphasize two things: First, that the value of the capacity
   measurement depends on the types of flows referenced in the
   definition.  This is because networks may treat packets differently
   (in terms of queuing and scheduling) based on their markings and
   classification.  Networks may also arbitrarily decide to flow-balance
   based on the packet type or flow type and thereby affect capacity
   measurements.  Second, the measurement of capacity depends not only
   on the type of the reference packets, but also on the types of the
   packets in the "population" with which the flows of interest share
   the links in the path.

   All of this indicates two different approaches to measuring: One is
   to measure capacity using a broad spectrum of packet types,
   suggesting that "Type P" should be set as generic as possible.  The
   second is to focus narrowly on the types of flows of particular
   interest, which suggests that "Type P" should be very specific and
   narrowly defined.  The first approach is likely to be of interest to
   providers, the second to application users.





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   As a practical matter, it should be noted that some providers may
   treat packets with certain characteristics differently than other
   packets.  For example, access control lists, routing policies, and
   other mechanisms may be used to filter ICMP packets or forward
   packets with certain IP options through different routes.  If a
   capacity-measurement tool uses these special packets and they are
   included in the "Type P" designation, the tool may not be measuring
   the path that it was intended to measure.  Tool authors, as well as
   users, may wish to check this point with their service providers.

2.3.2.  Definition: IP-type-P 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 the source S and
   correctly received by the destination D over the link L during the
   interval [T, T+I], divided by I.

   As mentioned earlier, this definition is affected by many factors
   that may change over time.  For example, a device's ability to
   process and forward IP packets for a particular link may have varying
   effect on capacity, depending on the amount or type of traffic being
   processed.

2.3.3.  Definition: IP-type-P Path Capacity

   Using our definition for IP-layer link capacity, 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.

   C(P,T,I) = min {1..n} {C(Ln,T,I)}

   The previous definitions specify the number of IP-layer bits that can
   be transmitted across a link or path should the resource be free of
   any congestion.  It represents the full capacity available for
   traffic between the source and destination.  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.

2.3.4.  Definition: IP-type-P Link Usage

   The average usage of a link L, Used(L,T,I), is the actual number of
   IP-layer bits from any source, correctly received over link L during
   the interval [T, T+I], divided by I.






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   An important distinction between usage and capacity is that
   Used(L,T,I) is not the maximum number, but rather, the actual number
   of IP bits sent that are correctly received.  The information
   transmitted across the link can be generated by any source, including
   those sources that 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.

2.3.5.  Definition: IP-type-P Link Utilization

   We express usage as a fraction of the overall IP-layer link capacity.

   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].

2.3.6.  Definition: IP-type-P Available Link Capacity

   We can now determine the amount of available capacity on a congested
   link by multiplying the IP-layer link capacity with the complement of
   the IP-layer link utilization.  Thus, the IP-layer available link
   capacity becomes:

   AvailCap(L,T,I) = C(L,T,I) * ( 1 - Util(L,T,I) )

2.3.7.  Definition: IP-type-P Available Path Capacity

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

   AvailCap(P,T,I) = min {1..n} {AvailCap(Ln,T,I)}

   Since measurements of available capacity are more volatile than that
   of link capacity, we stress the importance that both the time and
   interval be specified as their values have a great deal of influence
   on the results.  In addition, a sequence of measurements may be
   beneficial in offsetting the volatility when attempting to
   characterize available capacity.




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3.  Discussion

3.1.  Time and Sampling

   We must emphasize the importance of time in the basic definitions of
   these quantities.  We know that traffic on the Internet is highly
   variable across all time scales.  This argues that the time and
   length of measurements are critical variables in reporting available
   capacity measurements and must be reported when using these
   definitions.

   The closer to "instantaneous" a metric is, the more important it is
   to have a plan for sampling the metric over a time period that is
   sufficiently large.  By doing so, we allow valid statistical
   inferences to be made from the measurements.  An obvious pitfall here
   is sampling in a way that causes bias.  For example, a situation
   where the sampling frequency is a multiple of the frequency of an
   underlying condition.

3.2.  Hardware Duplicates

   We briefly consider the effects of paths where hardware duplication
   of packets may occur.  In such an environment, a node in the network
   path may duplicate packets, and the destination may receive multiple,
   identical copies of these packets.  Both the original packet and the
   duplicates can be properly received and appear to be originating from
   the sender.  Thus, in the most generic form, duplicate IP packets are
   counted in these definitions.  However, hardware duplication can
   affect these definitions depending on the use of "Type P" to add
   additional restrictions on packet reception.  For instance, a
   restriction only to count uniquely-sent packets may be more useful to
   users concerned with capacity for meaningful data.  In contrast, the
   more general, unrestricted metric may be suitable for a user who is
   concerned with raw capacity.  Thus, it is up to the user to properly
   scope and interpret results in situations where hardware duplicates
   may be prevalent.

3.3.  Other Potential Factors

   IP encapsulation does not affect the definitions as all IP header and
   payload bits must be counted regardless of content.  However, IP
   packets of different sizes 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.







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   Should the link happen to employ a compression scheme such as RObust
   Header Compression (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.

3.4.  Common Terminology in Literature

   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 link
   with the smallest available capacity is often referred to as the
   "tight link" within a path.  So, while a given link 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 the bottleneck should it be lightly
   loaded in relation to the rest of the path.

   Also, literature often overloads the term "bandwidth" to refer 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 Mbit/s.  However, an electrical
   engineer may answer with 25 MHz, and an end user may tell you that
   his observed bandwidth is 8 Mbit/s.  In contrast, the term "capacity"
   is not quite as overloaded and is an appropriate term that better
   reflects what is actually being measured.

3.5.  Comparison to Bulk Transfer Capacity (BTC)

   Bulk Transfer Capacity (BTC) [RFC3148] provides a distinct
   perspective on path capacity that differs from the definitions in
   this document in several fundamental ways.  First, BTC operates at
   the transport layer, gauging the amount of capacity available to an
   application that wishes to send data.  Only unique data is measured,
   meaning header and retransmitted data are not included in the
   calculation.  In contrast, IP-layer link capacity includes the IP
   header and is indifferent to the uniqueness of the data contained
   within the packet payload.  (Hardware duplication of packets is an
   anomaly addressed in a previous section.)  Second, BTC utilizes a
   single congestion-aware transport connection, such as TCP, to obtain
   measurements.  As a result, BTC implementations react strongly to
   different path characteristics, topologies, and distances.  Since
   these differences can affect the control loop (propagation delays,
   segment reordering, etc.), the reaction is further dependent on the
   algorithms being employed for the measurements.  For example,
   consider a single event where a link suffers a large duration of bit
   errors.  The event could cause IP-layer packets to be discarded, and
   the lost packets would reduce the IP-layer link capacity.  However,
   the same event and subsequent losses would trigger loss recovery for



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   a BTC measurement resulting in the retransmission of data and a
   potentially reduced sending rate.  Thus, a measurement of BTC does
   not correspond to any of the definitions in this document.  Both
   techniques are useful in exploring the characteristics of a network
   path, but from different perspectives.

4.  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.

   Tools that attempt to implement these definitions may introduce
   security issues specific to each implementation.  Both active and
   passive measurement techniques can be abused, impacting the security,
   privacy, and performance of the network.  Any measurement techniques
   based upon these definitions must include a discussion of the
   techniques needed to protect the network on which the measurements
   are being performed.

5.  Conclusion

   In this document, we have defined a set of quantities related to the
   capacity of links and paths in an IP network.  In these definitions,
   we have tried to be as clear as possible and take into account
   various characteristics that links and paths can have.  The goal of
   these definitions is to enable researchers who propose capacity
   metrics to relate those metrics to these definitions and to evaluate
   those metrics with respect to how well they approximate these
   quantities.

   In addition, we have pointed out some key auxiliary parameters and
   opened a discussion of issues related to valid inferences from
   available capacity metrics.

6.  Acknowledgments

   The authors would like to acknowledge Mark Allman, Patrik Arlos, Matt
   Mathis, Al Morton, Stanislav Shalunov, and Matt Zekauskas for their
   suggestions, comments, and reviews.  We also thank members of the
   IETF IPPM Mailing List for their discussions and feedback on this
   document.








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7.  References

7.1.  Normative References

   [RFC1812]  Baker, F., "Requirements for IP Version 4 Routers",
              RFC 1812, June 1995.

   [RFC2330]  Paxson, V., Almes, G., Mahdavi, J., and M. Mathis,
              "Framework for IP Performance Metrics", RFC 2330,
              May 1998.

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.

   [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.

   [RFC3148]  Mathis, M. and M. Allman, "A Framework for Defining
              Empirical Bulk Transfer Capacity Metrics", RFC 3148,
              July 2001.

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



















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Authors' Addresses

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

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


   Joseph Ishac
   NASA Glenn Research Center
   21000 Brookpark Road, MS 54-5
   Cleveland, Ohio  44135
   USA

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





























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