A Proposed Media Delivery Index     August 2005





   Network Working Group                                        J. Welch
   Internet Draft                                 IneoQuest Technologies
   Intended Category:  Informational                            J. Clark
                                                           Cisco Systems
                                                            August, 2005


                      A Proposed Media Delivery Index
                          draft-welch-mdi-03.txt


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Abstract

   This memo defines a Media Delivery Index (MDI) measurement which can
   be used as a diagnostic tool or a quality indicator for monitoring a
   network intended to deliver applications such as streaming media MPEG
   video and Voice over IP or other arrival time and packet loss
   sensitive information.  It provides an indication of traffic jitter,
   a measure of deviation from nominal flow rates, and a data loss at-a-
   glance measure for a particular flow.  For instance, the MDI may be
   used as a reference in characterizing and comparing networks carrying
   UDP streaming media.

   The Media Delivery Index measurement defined in this memo is intended
   for Information only.



1.
  Introduction

   There has been considerable progress over the last several years in
   the development of methods to provide for Quality of Service (QoS)
   over packet switched networks to improve the delivery of streaming
   media and other time and packet loss sensitive applications such as
   [i1], [i5], [i6], [i7].  QoS is required for many practical networks
   involving applications such as video transport to assure the
   availability of network bandwidth by providing upper limits on the
   number of flows admitted to a network as well as to bound the packet
   jitter introduced by the network.  These bounds are required to
   dimension a receiver`s buffer to properly display the video in real
   time without buffer overflow or underflow.

   Now that large scale implementations of such networks based on RSVP
   and Diffserv are undergoing trials [i3] and being specified by major
   service providers for the transport of streaming media such as MPEG
   video [i4], there is a need to easily diagnose issues and monitor the
   real time effectiveness of networks employing these QoS methods or to
   assess whether they are required. Furthermore, due to the significant
   installed base of legacy networks without QoS methods, a delivery
   system`s transitional solution may be comprised of both networks with
   and without these methods thus increasing the difficulty in
   characterizing the dynamic behavior of these networks.

   The purpose of this memo is to describe a set of measurements that
   can be used to derive a Media Delivery Index (MDI) which indicates
   the instantaneous and longer term behavior of networks carrying
   streaming media such as MPEG video.




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   While this memo addresses monitoring MPEG Transport Stream (TS)
   packets [i8] over UDP, the general approach is expected to be
   applicable to other streaming media and protocols. The approach is
   applicable to both constant and variable bit rate streams though the
   variable bit rate case may be somewhat more difficult to calculate.
   This draft focuses on the constant bit rate case as the example to
   describe the measurement but as long as the dynamic bit rate of the
   encoded stream can be determined (the "drain rate" as described
   below in Section 3), then the MDI provides the measurement of
   network induced cumulative jitter.  Suggestions and direction for
   calculation of MDI for a variable bit rate encoded stream may be the
   subject of a future document.

2.
  Media Delivery Index Overview

   The MDI provides a relative indicator of needed buffer depths at the
   consumer node due to packet jitter as well as an indication of lost
   packets.  By probing a streaming media service network at various
   nodes and under varying load conditions, it is possible to quickly
   identify devices or locales which introduce significant jitter or
   packet loss to the packet stream. By monitoring a network
   continuously, deviations from nominal jitter or loss behavior can be
   used to indicate an impending or ongoing fault condition such as
   excessive load.  It is believed that the MDI provides the necessary
   information to detect all network induced impairments for streaming
   video or voice over IP applications.  Other parameters may be
   required to troubleshoot and correct the impairments.

   The MDI is updated at the termination of selected time intervals
   spanning multiple packets which contain the streaming media (such as
   transport stream packets in the MPEG-2 case.)  The Maximums and
   Minimums of the MDI component values are captured over a measurement
   time.  The measurement time may range from just long enough to
   capture an anticipated network anomaly during a troubleshooting
   exercise to indefinitely long for a long term monitoring or
   logging application.  The Maximums and Minimums may be obtained by
   sampling the measurement with adequate frequency.

3.
  Media Delivery Index Components

   The MDI consists of two components:  the Delay Factor (DF) and the
   Media Loss Rate (MLR).

3.1
   Delay Factor

   The Delay Factor is the maximum difference, observed at the end of
   each media stream packet, between the arrival of media data and the
   drain of media data, assuming the drain rate is the nominal constant
   traffic rate for constant bit rate streams or the piece-wise computed


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   traffic rate of variable rate media stream packet data.  The "drain
   rate" here refers to the payload media rate; e.g., for a typical 3.75
   Mb/s MPEG video Transport Stream (TS), the drain rate is 3.75 Mb/s --
   the rate at which the payload is consumed (displayed) at a decoding
   node.  If, at the sample time, the number of bytes received equals
   the number transmitted, the instantaneous flow rate balance will be
   zero, however the minimum DF will be a line packet's worth of media
   data as that is the minimum amount of data that must be buffered.

   The DF is the maximum observed value of the flow rate imbalance.
   This buffered media data in bytes is expressed in terms of how long,
   in milliseconds, it would take to drain (or fill) this data at the
   nominal traffic rate to obtain the DF.  Display of DF with a
   resolution of tenths of milliseconds provides adequate indication of
   stream variations for monitoring and diagnostic applications for
   typical stream rates of up to 40 Mb/s.  The DF value must be updated
   and displayed at the end of a selected time interval.  The selected
   time interval is chosen to be long enough to sample a number of TS
   packets and will, therefore, vary based on the nominal traffic rate.
   For typical stream rates of 64 Kbps and up, an interval of 1 second
   provides a long enough sample time and should be included for all
   implementations.  The Delay Factor indicates how long a data stream
   must be buffered (i.e. delayed) at its nominal bit rate to prevent
   packet loss.  Another perspective of this time is as a measure of the
   network latency that must be induced from buffering that is required
   to accommodate stream jitter and prevent loss.  The DF`s max and min
   over the measurement period may also be displayed to show the worst
   case arrival time deviation, or jitter, relative to the nominal
   traffic rate in a measurement period.  It provides a dynamic flow
   rate balance indication with its max and min showing the worst
   excursions from balance.  To arrive at a bounded DF, the long term
   flow rate deviation (LFRD) must be 0, where LFRD is a running
   deviation of flow rate from expected nominal traffic rate over a
   measurement period.  A large positive or negative LFRD usually
   indicates a source flow failure or misconfiguration and would cause
   the DF value to steadily increase from interval to interval.

   The Delay Factor gives a hint of the minimum size of the buffer
   required at the next downstream node.  As a stream progresses, the
   variation of the Delay Factor indicates packet bunching (jitter).
   Greater DF values also indicate more network latency necessary to
   deliver a stream due to the need to prefill a receive buffer before
   beginning the drain to guarantee no underflow.   The DF comprises a
   fixed part based on packet size and a variable part based on the
   various network component switch elements` buffer utilization that
   comprise the switched network infrastructure [i2].

3.2
   Media Loss Rate



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   The Media Loss Rate is the count of lost or out of order flow packets
   over a selected time interval, where the flow packets are packets
   carrying streaming application information.  There may be zero or
   more streaming packets in a single IP packet.  For example, it is
   common to carry seven 188 Byte MPEG Transport Stream packets in an IP
   packet.  In such a case, a single IP packet loss would result in 7
   lost packets counted for the case where the 7 lost packets did not
   include null packets.  Including out of order packets is important as
   many stream consumer type devices do not attempt to reorder packets
   that are received out of order.

3.3
   Media Delivery Index

   Combining the Delay Factor and Media Loss Rate quantities for
   presentation results in the MDI:

                                  DF:MLR

   Where:

                          DF is the Delay Factor
                        MLR is the Media Loss Rate

   At a receiving node, knowing its nominal drain bit rate, the DF`s max
   indicates the size of required buffer to accommodate packet jitter.
   Or, in terms of Leaky Bucket [i9] parameters, DF indicates bucket
   size b expressed in time to transmit bucket traffic b, at the given
   nominal traffic rate, r.

3.4
   MDI Application Examples

   In the case where a known, well characterized receive node is
   separated from the data source by unknown or less well characterized
   nodes such as intermediate switch nodes, the MDI measured at
   intermediate data links provides a relative indication of the
   behavior of upstream traffic flows.  DF difference indications
   between one node and another in a data stream for a given constant
   interval of calculation can indicate local areas of traffic
   congestion or possibly misconfigured QoS flow specification(s)
   leading to greater filling of measurement point local device buffers,
   resultant flow rate deviations, and possible data loss.

   For a given MDI, if DF is high and/or the DF Max-Min captured over a
   significant measurement period is high, jitter has been detected but
   the longer term, average flow rate may be nominal.  This could be the
   result of a transient flow upset due to a coincident traffic stream
   unrelated to the flow of interest causing packet bunching.  A high DF
   may cause downstream buffer overflow or underflow or unacceptable
   latency even in the absence of lost data.


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   Due to transient network failures or DF excursions, packets may be
   lost within the network.  The MLR component of the MDI shows this
   condition.

   Through automated or manual flow detection and identification and
   subsequent MDI calculations for real time statistics on a flow, the
   DF can indicate the dynamic deterioration or increasing burstiness of
   a flow which can be used to anticipate a developing network operation
   problem such as transient oversubscription.  Such statistics can be
   obtained for flows within network switches using available switch cpu
   resources due to the minimal computational requirements needed for
   small numbers of flows.  Statistics for all flows present on, say, a
   gigabit Ethernet network, will likely require dedicated hardware
   facilities though these can be modest as buffer requirements and the
   required calculations per flow are minimal.  By equipping network
   switches with MDI measurements, flow impairment issues can quickly be
   identified, localized, and corrected.  Until switches are so equipped
   with appropriate hardware resources, dedicated hardware tools can
   provide supplemental switch statistics by gaining access to switch
   flows via mirror ports, link taps, or the like as a transition
   strategy.

   The MDI figure can also be used to characterize a flow decoder's
   acceptable performance.  For example, an MPEG decoder could be
   characterized as tolerating a flow with a given maximum DF and MLR
   for acceptable display performance (acceptable on-screen artifacts).
   Network conditions such as Interior Gateway Protocol (IGP)
   reconvergence might also be included in the flow tolerance resulting
   in a higher quality user experience.

4.
  Summary

   The MDI combines the Delay Factor which indicates potential for
   impending data loss and Media Loss Rate as the indicator of lost
   data.  By monitoring the DF and MLR and their min and max excursions
   over a measurement period and at multiple strategic locations in a
   network, traffic congestion or device impairments may be detected and
   isolated for a network carrying streaming media content.

5.
  Security Considerations


   The measurements identified in this document do not directly affect
   the security of a network or user.  Actions taken in response to
   these measurements which may affect the available bandwidth of the
   network or availability of a service is out of scope for this
   document.



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   Performing the measurements described in this document only requires
   examination of payload header information such as MPEG transport
   stream headers or RTP headers to determine nominal stream bit rate
   and sequence number information.  Content may be encrypted without
   affecting these measurements.  Therefore, content privacy is not
   expected to be a concern.

6.
  Normative References


7.
  Informative References

   i1. R. Braden et al., `Resource Reservation Protocol ` Version 1
      Functional Specification`, RFC 2205, 1997.
   i2. C. Partridge, `A Proposed Flow Specification`, RFC 1363, 1992.
   i3. R. Fellman, `Hurdles to Overcome for Broadcast Quality Video
      Delivery over IP` VidTranS 2002.
   i4. CableLabs `PacketCable Dynamic Quality-of-Service Specification`,
      PKT-SP-DQOS-I06-030415, 2003.
   i5. S. Shenker, C. Partridge, R. Guerin, `Specification of Guaranteed
      Quality of Service`, RFC 2212, 1997.
   i6. J. Wroclawski, `Specification of the Controlled-Load Network
      Element Service`, RFC 2211, 1997.
   i7. R. Braden, D. Clark, S. Shenker, `Integrated Services in the
      Internet Architecture: an Overview` RFC 1633, 1994.
   i8. ISO/IEC 13818-1 (MPEG-2 Systems)
   i9. V. Raisanen, `Implementing Service Quality in IP Networks`, John
      Wiley & Sons Ltd., 2003.

8.
  Acknowledgments

   The authors gratefully acknowledge the contributions of Marc Todd and
   Jesse Beeson of IneoQuest Technologies, Inc., Bill Trubey and John
   Carlucci of Time Warner Cable, Nishith Sinha of Cox Communications,
   Ken Chiquoine of SeaChange International, Phil Proulx of Bell Canada,
   Dr Paul Stallard of TANDBERG Television, Gary Hughes of Broadbus
   Technologies, Brad Medford of SBC Laboratories, John Roy of Adelphia
   Communications, Cliff Mercer, PhD of Kasenna, Mathew Ho of Rogers
   Cable, and Irl Duling of Optinel Systems for reviewing and evaluating
   early drafts of this document and implementations for MDI.

9.
  Authors' Address

   James Welch
   IneoQuest Technologies, Inc
   170 Forbes Blvd
   Mansfield, Massachusetts 02048
   508 618 0312
   Jim.Welch@ineoquest.com


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   James Clark
   Cisco Systems, Inc
   500 Northridge Road
   Suite 800
   Atlanta, Georgia 30350
   678 352 2726
   jiclark@cisco.com

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TO BE DELETED BY THE RFC EDITOR UPON PUBLICATION:
     Changes from draft-welch-mdi-02.txt:

     *removed representative MIB that could be used for export since
   focus of document is the MDI measurement and suggested MIB did not
   comply with MIB review guidelines.
     *clarified recommended common measurement period and quantization
   to promote common implementations










































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