





Network Working Group                                          K. Dubray
Request for Comments: 2432                           IronBridge Networks
Category: Informational                                     October 1998


               Terminology for IP Multicast Benchmarking

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.

Copyright Notice

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

Abstract

   The purpose of this document is to define terminology specific to the
   benchmarking of multicast IP forwarding devices. It builds upon the
   tenets set forth in RFC 1242, RFC 2285, and other IETF Benchmarking
   Methodology Working Group (BMWG) efforts.  This document seeks to
   extend these efforts to the multicast paradigm.

   The BMWG produces two major classes of documents: Benchmarking
   Terminology documents and Benchmarking Methodology documents. The
   Terminology documents present the benchmarks and other related terms.
   The Methodology documents define the procedures required to collect
   the benchmarks cited in the corresponding Terminology documents.

1.  Introduction

   Network forwarding devices are being required to take a single frame
   and support delivery to a number of destinations having membership to
   a particular group. As such, multicast support may place a different
   burden on the resources of these network forwarding devices than with
   unicast or broadcast traffic types.

   Such burdens may not be readily apparent at first glance - the IP
   multicast packet's Class D address may be the only noticeable
   difference from an IP unicast packet.  However, there are many
   factors that may impact the treatment of IP multicast packets.

   Consider how a device's architecture may impact the handling of a
   multicast frame.  For example, is the multicast packet subject to the
   same processing as its unicast analog?  Or is the multicast packet
   treated as an exeception and processed on a different data path?



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   Consider, too, how a shared memory architecture may demonstrate a
   different performance profile than an architecture which explicitly
   passes each individual packet between the processing entities.

   In addition to forwarding device architecture, there are other
   factors that may impact a device's or system's multicast related
   performance.  Protocol requirements may demand that routers and
   switches consider destination and source addressing in its multicast
   forwarding decisions.  Capturing multicast source/destination
   addressing information may impact forwarding table size and lengthen
   lookups.  Topological factors such as the degree of packet
   replication, the number of multicast groups being supported by the
   system, or the placement of multicast packets in unicast wrappers to
   span non-multicast network paths may all potentially affect a
   system's multicast related performance. For an overall understanding
   of IP multicasting, the reader is directed to [Se98], [Hu95], and
   [Mt98].

   By clearly identifying IP multicast benchmarks and related
   terminology in this document, it is hoped that detailed methodologies
   can be generated in subsequent documents.  Taken in tandem, these two
   efforts endeavor to assist the clinical, empirical, and consistent
   characterization of certain aspects of multicast technologies and
   their individual implementations.  Understanding the operational
   profile of multicast forwarding devices may assist the network
   designer to better deploy multicast in his or her networking
   environment.

   Moreover, this document focuses on one source to many destinations
   profiling.  Elements of this document may require extension when
   considering multiple source to multiple destination IP multicast
   communication.

2.  Definition Format

   This section cites the template suggested by RFC 1242 in the
   specification of a term to be defined.

   Term to be defined.

   Definition:
      The specific definition for the term.

   Discussion:
      A brief discussion of the term, its application, or other
      information that would build understanding.





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   Measurement units:
      Units used to record measurements of this term, if applicable.

   [Issues:]
      List of issues or conditions that affect this term. This field can
      present items the may impact the term's related methodology or
      otherwise restrict its measurement procedures.  This field is
      optional in this document.

   [See Also:]
      List of other terms that are relevant to the discussion of this
      term. This field is optional in this document.

2.1 Existing Terminology

   This document draws on existing terminology defined in other BMWG
   work.  Examples include, but are not limited to:

   Throughput                [RFC 1242, section 3.17]
   Latency                   [RFC 1242, section 3.8]
   Constant Load             [RFC 1242, section 3.4]
   Frame Loss Rate           [RFC 1242, section 3.6]
   Overhead behavior         [RFC 1242, section 3.11]
   Forwarding Rates          [RFC 2285, section 3.6]
   Loads                     [RFC 2285, section 3.5]
   Device Under Test (DUT)   [RFC 2285, section 3.1.1]
   System Under Test (SUT)   [RFC 2285, section 3.1.2]

   Note: "DUT/SUT" refers to a metric that may be applicable to a DUT or
   SUT.

3. Table of Defined Terms

   3.1 General Nomenclature

     3.1.1 Traffic Class. (TC)
     3.1.2 Group Class. (GC)
     3.1.3 Service Class. (SC)

   3.2 Forwarding and Throughput
     3.2.1 Mixed Class Throughput (MCT).
     3.2.2 Scaled Group Forwarding Matrix (SGFM).
     3.2.3 Aggregated Multicast Throughput (AMT)
     3.2.4 Encapsulation Throughput (ET)
     3.2.5 Decapsulation Throughput (DT)
     3.2.6 Re-encapsulation Throughput (RET)





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   3.3 Forwarding Latency
     3.3.1 Multicast Latency (ML)
     3.3.2 Min/Max Multicast Latency (Min/Max ML)

   3.4 Overhead
     3.4.1 Group Join Delay. (GJD)
     3.4.2 Group Leave Delay. (GLD)

   3.5 Capacity
     3.5.1 Multicast Group Capacity. (MGC)

   3.6 Interaction
     3.6.1 Burdened Response
     3.6.2 Forwarding Burdened Multicast Latency (FBML)
     3.6.3 Forwarding Burdened Join Delay (FBJD)

3.1 General Nomenclature

   This section will present general terminology to be used in this and
   other documents.

3.1.1 Traffic Class. (TC)

   Definition:
      An equivalence class of packets comprising one or more data
      streams.

   Discussion:
      In the scope of this document, Traffic Class will be considered a
      logical identifier used to discriminate between a set or sets of
      packets offered the DUT.

      For example, one Traffic Class may identify a set of unicast
      packets offered to the DUT.  Another Traffic Class may
      differentiate the multicast packets destined to multicast group X.
      Yet another Class may distinguish the set of multicast packets
      destined to multicast group Y.

      Unless otherwise qualified, the usage of the word "Class" in this
      document will refer simply to a Traffic Class.

   Measurement units:
      Not applicable.








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3.1.2 Group Class. (GC)

   Definition:
      A specific type of Traffic Class where the packets comprising the
      Class are destined to a particular multicast group.

   Discussion:

   Measurement units:
      Not applicable.

3.1.3 Service Class. (SC)

   Definition:
      A specific type of Traffic Class where the packets comprising the
      Class require particular treatment or treatments by the network
      forwarding devices along the path to the packets' destination(s).

   Discussion:

   Measurement units:
      Not applicable.

3.2 Forwarding and Throughput.

   This section presents terminology related to the characterization of
   the packet forwarding ability of a DUT/SUT in a multicast
   environment.  Some metrics extend the concept of throughput presented
   in RFC 1242.  The notion of Forwarding Rate is cited in RFC 2285.

3.2.1 Mixed Class Throughput (MCT).

   Definition:
      The maximum rate at which none of the offered frames, comprised
      from a unicast Class and a multicast Class, to be forwarded are
      dropped by the device across a fixed number of ports.

   Discussion:
      Often times, throughput is collected on a homogenous traffic class
      - the offered load to the DUT is either singularly unicast or
      singularly multicast.  In most networking environments, the
      traffic mix is seldom so uniformly distributed.

      Based on the RFC 1242 definition for throughput, the Mixed Class
      Throughput benchmark attempts to characterize the DUT's ability to
      process both unicast and multicast frames in the same aggregated
      traffic stream.




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   Measurement units:
      Frames per second

   Issues:
      Related methodology may have to address the ratio of unicast
      packets to multicast packets.

      Since frame size can sometimes be a factor in frame forwarding
      benchmarks, the corresponding methodology for this metric will
      need to consider frame size distribution(s).

3.2.2 Scaled Group Forwarding Matrix (SGFM).

   Definition:
      A table that demonstrates Forwarding Rate as a function of tested
      multicast groups for a fixed number of tested DUT/SUT ports.

   Discussion:
      A desirable attribute of many Internet mechanisms is the ability
      to "scale." This benchmark seeks to demonstrate the ability of a
      SUT to forward as the number of multicast groups is scaled
      upwards.

   Measurement units:
      Packets per second, with corresponding tested multicast group and
      port configurations.

   Issues:
      The corresponding methodology may have to reflect the impact that
      the pairing (source, group) has on many multicast routing
      protocols.

      Since frame size can sometimes be a factor in frame forwarding
      benchmarks, the corresponding methodology for this metric will
      need to consider frame size distribution(s).

3.2.3 Aggregated Multicast Throughput (AMT)

   Definition:
      The maximum rate at which none of the offered frames to be
      forwarded through N destination interfaces of the same multicast
      group are dropped.

   Discussion:
      Another "scaling" type of exercise, designed to identify the
      DUT/SUT's ability to handle traffic as a function of the multicast
      destination ports it is required to support.




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   Measurement units:
      The ordered pair (N,t) where,

         N = the number of destination ports of the multicast group.
         t = the throughput, in frames per second, relative to the
         source stream.

   Issues:
      Since frame size can sometimes be a factor in frame forwarding
      benchmarks, the corresponding methodology for this metric will
      need to consider frame size distribution(s).

3.2.4 Encapsulation Throughput (ET)

   Definition:
      The maximum rate at which frames offered a DUT are encapsulated
      and correctly forwarded by the DUT without loss.

   Discussion:
      A popular technique in presenting a frame to a device that may not
      support a protocol feature is to encapsulate, or tunnel, the
      packet containing the unsupported feature in a format that is
      supported by that device.

      More specifically, encapsulation refers to the act of taking a
      frame or part of a frame and embedding it as a payload of another
      frame. This benchmark attempts to characterize the overhead
      behavior associated with that translational process.

   Measurement units:
      Frames per second.

   Issues:
      Consideration may need to be given with respect to the impact of
      different frame formats on usable bandwidth.

      Since frame size can sometimes be a factor in frame forwarding
      benchmarks, the corresponding methodology for this metric will
      need to consider frame size distribution(s).

3.2.5 Decapsulation Throughput (DT)

   Definition:
      The maximum rate at which frames offered a DUT are decapsulated
      and correctly forwarded by the DUT without loss.






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   Discussion:
      A popular technique in presenting a frame to a device that may not
      support a protocol feature is to encapsulate, or tunnel, the
      packet containing the unsupported feature in a format that is
      supported by that device. At some point, the frame may be required
      to be returned its orginal format from its encapsulation wrapper
      for use by the frame's next destination.

      More specifically, decapsulation refers to the act of taking a
      frame or part of a frame embedded as a payload of another frame
      and returning it to the payload's appropriate format. This
      benchmark attempts to characterize the overhead behavior
      associated with that translational process.

   Measurement units:
      Frames per second.

   Issues:
      Consideration may need to be given with respect to the impact of
      different frame formats on usable bandwidth.

      Since frame size can sometimes be a factor in frame forwarding
      benchmarks, the corresponding methodology for this metric will
      need to consider frame size distribution(s).

3.2.6 Re-encapsulation Throughput (RET)

   Definition:
      The maximum rate at which frames of one encapsulated format
      offered a DUT are converted to another encapsulated format and
      correctly forwarded by the DUT without loss.

   Discussion:
      A popular technique in presenting a frame to a device that may not
      support a protocol feature is to encapsulate, or tunnel, the
      packet containing the unsupported feature in a format that is
      supported by that device. At some point, the frame may be required
      to be converted from one encapsulation format to another
      encapsulation format.

      More specifically, re-encapsulation refers to the act of taking an
      encapsulated payload of one format and replacing it with another
      encapsulated format - all the while preserving the original
      payload's contents.  This benchmark attempts to characterize the
      overhead behavior associated with that translational process.

   Measurement units:
      Frames per second.



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   Issues:
      Consideration may need to be given with respect to the impact of
      different frame formats on usable bandwidth.

      Since frame size can sometimes be a factor in frame forwarding
      benchmarks, the corresponding methodology for this metric will
      need to consider frame size distribution(s).

3.3 Forwarding Latency.

   This section presents terminology relating to the characterization of
   the forwarding latency of a DUT/SUT in a multicast environment.  It
   extends the concept of latency presented in RFC 1242.

3.3.1 Multicast Latency. (ML)

   Definition:
      The set of individual latencies from a single input port on the
      DUT or SUT to all tested ports belonging to the destination
      multicast group.

   Discussion:
      This benchmark is based on the RFC 1242 definition of latency.
      While it is useful to collect latency between a pair of source and
      destination multicast ports, it may be insightful to collect the
      same type of measurements across a range of ports supporting that
      Group Class.

      A variety of statistical exercises can be applied to the set of
      latencies measurements.

   Measurement units:
      Time units with enough precision to reflect a latency measurement.

3.3.2 Min/Max Multicast Latency. (Min/Max ML)

   Definition:
      The difference between the maximum latency measurement and the
      minimum latency measurement from the set of latencies produced by
      the Multicast Latency benchmark.

   Discussion:
      This statistic may yield some insight into how a particular
      implementation handles its multicast traffic.  This may be useful
      to users of multicast synchronization types of applications.

   Measurement units:
      Time units with enough precision to reflect latency measurement.



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3.4  Overhead

   This section presents terminology relating to the characterization of
   the overhead delays associated with explicit operations found in
   multicast environments.

3.4.1 Group Join Delay. (GJD)

   Definition:
      The time duration it takes a DUT to start forwarding multicast
      packets from the time a successful IGMP group membership report
      has been issued to the DUT.

   Discussion:
      Many factors can contribute to different results, such as the
      number or type of multicast-related protocols configured on the
      device under test. Other factors are physical topology and "tree"
      configuration.

      Because of the number of variables that could impact this metric,
      the metric may be a better characterization tool for a device
      rather than a basis for comparisons with other devices.

   Issues:
      A consideration for the related methodology:  possible need to
      differentiate a specifically-forwarded multicast frame from those
      sprayed by protocols implementing a flooding tactic to solicit
      prune feedback.

      While this metric attempts to identify a simple delay, the
      underlying and contributing delay components (e.g., propagation
      delay, frame processing delay, etc.) make this a less than simple
      measurement.  The corresponding methodology will need to consider
      this and similar factors to ensure a consistent and precise metric
      result.

   Measurement units:
      Microseconds.

3.4.2 Group Leave Delay. (GLD)

   Definition:
      The time duration it takes a DUT to cease forwarding multicast
      packets after a corresponding IGMP "Leave Group" message has been
      successfully offered to the DUT.






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   Discussion:
      While it is important to understand how quickly a device can
      process multicast frames; it may be beneficial to understand how
      quickly that same device can stop the process as well.

      Because of the number of variables that could impact this metric,
      the metric may be a better characterization tool for a device
      rather than a basis for comparisons with other devices.

   Measurement units:
      Microseconds.

   Issues:
      The Methodology may need to consider protocol-specific timeout
      values.

      While this metric attempts to identify a simple delay, the
      underlying and contributing delay components (e.g., propagation
      delay, frame processing delay, etc.) make this a less than simple
      measurement.  Moreover, the cessation of traffic is a rather
      unobservable event (i.e., at what point is the multicast forwarded
      considered stopped on the DUT interface processing the Leave?).
      The corresponding methodology will need to consider this and
      similar factors to ensure a consistent and precise metric result.

3.5 Capacity

   This section offers terms relating to the identification of multicast
   group limits of a DUT/SUT.

3.5.1 Multicast Group Capacity. (MGC)

   Definition:
      The maximum number of multicast groups a SUT/DUT can support while
      maintaining the ability to forward multicast frames to all
      multicast groups registered to that SUT/DUT.

   Discussion:

   Measurement units:
      Multicast groups.

   Issues:
      The related methodology may have to consider the impact of
      multicast sources per group on the ability of a SUT/DUT to "scale
      up" the number of supportable multicast groups.





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3.6 Interaction

   Network forwarding devices are generally required to provide more
   functionality than than the forwarding of traffic.  Moreover, network
   forwarding devices may be asked to provide those functions in a
   variety of environments.  This section offers terms to assist in the
   charaterization of DUT/SUT behavior in consideration of potentially
   interacting factors.

3.6.1 Burdened Response.

   Definition:
      A measured response collected from a DUT/SUT in light of
      interacting, or potentially interacting, distinct stimulii.

   Discussion:
      Many metrics provide a one dimensional view into an operating
      characteristic of a tested system.  For example, the forwarding
      rate metric may yield information about the packet processing
      ability of a device.  Collecting that same metric in view of
      another control variable can oftentimes be very insightful. Taking
      that same forwarding rate measurement, for instance, while the
      device's address table is injected with an additional 50,000
      entries may yield a different perspective.

   Measurement units:
      A burdened response is a type of metric.  Metrics of this this
      type must follow guidelines when reporting results.

      The metric's principal result MUST be reported in conjunction with
      the contributing factors.

      For example, in reporting a Forwarding Burdened Latency, the
      latency measurement should be reported with respect to
      corresponding Offered Load and Forwarding Rates.

   Issues: A Burdened response may be very illuminating when trying to
      characterize a single device or system.  Extreme care must be
      exercised when attempting to use that characterization as a basis
      of comparison with other devices or systems.  Test agents must
      ensure that the measured response is a function of the controlled
      stimulii, and not secondary factors.  An example of of such an
      interfering factor would be configuration mismatch of a timer
      impacting a response process.







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3.6.2 Forwarding Burdened Multicast Latency. (FBML)

   Definition:
      A multicast latency taken from a DUT/SUT in the presence of a
      traffic forwarding requirement.

   Discussion:
      This burdened response metric builds on the Multicast Latency
      definition offered in section 3.3.1.  It mandates that the DUT be
      subjected to an additional measure of traffic not required by the
      non-burdened metric.

      This metric attempts to provide a means by which to evaluate how
      traffic load may or may not impact a device's or system's packet
      processing delay.

   Measurement units:
      Time units with enough precision to reflect the latencies
      measurements.

      Latency measurements MUST be reported with the corresponding
      sustained Forwarding Rate and associated Offered Load.

3.6.3 Forwarding Burdened Group Join Delay. (FBGJD)

   Definition:
      A multicast Group Join Delay taken from a DUT in the presence of a
      traffic forwarding requirement.

   Discussion:
      This burdened response metric builds on the Group Join Delay
      definition offered in section 3.4.1.  It mandates that the DUT be
      subjected to an additional measure of traffic not required by the
      non-burdened metric.

      Many factors can contribute to different results, such as the
      number or type of multicast-related protocols configured on the
      device under test. Other factors could be physical topology or the
      logical multicast "tree" configuration.

      Because of the number of variables that could impact this metric,
      the metric may be a better characterization tool for a device
      rather than a basis for comparisons with other devices.

   Measurement units:
      Time units with enough precision to reflect the delay
      measurements.




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      Delay measurements MUST be reported with the corresponding
      sustained Forwarding Rate and associated Offered Load.

   Issues:
      While this metric attempts to identify a simple delay, the
      underlying and contributing delay components (e.g., propagation
      delay, frame processing delay, etc.) make this a less than simple
      measurement.  The corresponding methodology will need to consider
      this and similar factors to ensure a consistent and precise metric
      result.

4. Security Considerations

   This document addresses metrics and terminology relating to the
   performance benchmarking of IP Multicast forwarding devices.  The
   information contained in this document does not impact the security
   of the Internet.

   Methodologies regarding the collection of the metrics described
   within this document may need to cite security considerations.  This
   document does not address methodological issues.

5. Acknowledgments

   The IETF BMWG participants have made several comments and suggestions
   regarding this work.  Particular thanks goes to Harald Alvestrand,
   Scott Bradner, Brad Cain, Eric Crawley, Bob Mandeville, David Newman,
   Shuching Sheih, Dave Thaler, Chuck Winter, Zhaohui Zhang, and John
   Galgay for their insightful review and assistance.






















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6. References

   [Br91] Bradner, S., "Benchmarking Terminology for Network
          Interconnection Devices", RFC 1242, July 1991.

   [Br96] Bradner, S., and J. McQuaid, "Benchmarking Methodology for
          Network Interconnect Devices", RFC 1944, May 1996.

   [Hu95] Huitema, C.  "Routing in the Internet."  Prentice-Hall, 1995.

   [Se98] Semeria, C. and Maufer, T.  "Introduction to IP Multicast
          Routing."  http://www.3com.com/nsc/501303.html  3Com Corp.,
          1998.

   [Ma98] Mandeville, R., "Benchmarking Terminology for LAN Switching
          Devices", RFC 2285, February 1998.

   [Mt98] Maufer, T.  "Deploying IP Multicast in the Enterprise."
          Prentice-Hall, 1998.

7. Author's Address

   Kevin Dubray
   IronBridge Networks
   55 Hayden Avenue
   Lexington, MA 02421
   USA

   Phone: 781 372 8118
   EMail: kdubray@ironbridgenetworks.com





















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8.  Full Copyright Statement

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
























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