A Proposed Media Delivery Index (MDI)
draft-welch-mdi-03

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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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                         A Proposed Media Delivery Index     August 2005 
 
 
    
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 
    
10. 
   Copyright Notice 
    
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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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