Network Working Group                                           J. Welch
Request for Comments: 4445                        IneoQuest Technologies
Category: Informational                                         J. Clark
                                                           Cisco Systems
                                                              April 2006
Network Working Group                                           J. Welch
Request for Comments: 4445                        IneoQuest Technologies
Category: Informational                                         J. Clark
                                                           Cisco Systems
                                                              April 2006

A Proposed Media Delivery Index (MDI)


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 (2006).




This RFC is not a candidate for any level of Internet Standard. There are IETF standards which are highly applicable to the space defined by this document as its applicability, in particular, RFCs 3393 and 3611, and there is ongoing IETF work in these areas as well. The IETF also notes that the decision to publish this RFC is not based on IETF review for such things as security, congestion control, MIB fitness, or inappropriate interaction with deployed protocols. The RFC Editor has chosen to publish this document at its discretion. Readers of this document should exercise caution in evaluating its value for implementation and deployment. See RFC 3932 for more information.

本RFC不适用于任何级别的互联网标准。IETF标准高度适用于本文件定义的适用空间,尤其是RFCs 3393和3611,这些领域也正在进行IETF工作。IETF还指出,发布此RFC的决定并非基于IETF对安全性、拥塞控制、MIB适配性或与已部署协议的不当交互等方面的审查。RFC编辑已自行决定发布本文件。本文档的读者在评估其实施和部署价值时应谨慎。有关更多信息,请参阅RFC 3932。



This memo defines a Media Delivery Index (MDI) measurement that 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, Voice over IP, or other information sensitive to arrival time and packet loss. 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 MDI measurement defined in this memo is intended for Information only.


1. Introduction
1. 介绍

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-sensitive 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 display the video properly 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 diagnose issues easily and to 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 composed of 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) that indicates the instantaneous and longer-term behavior of networks carrying streaming media such as MPEG video.


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 document 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.


Network packet delivery time variation and various statistics to characterize the same are described in a generic approach in [i10]. The approach is capable of being parameterized for various purposes with the intent of defining a flexible, customizable definition that can be applied to a wide range of applications and further


experimentation. Other approaches to characterizing jitter behavior are also captured such as in [i12]. A wide-ranging report format [i11] has been described to convey information including jitter for use with the RTP Control Protocol (RTCP) [i12]. The MDI is instead intended to specifically address the need for a scalable, economical-to-compute metric that characterizes network impairments that may be imposed on streaming media, independent of control plane or measurement transport protocol or stream encapsulation protocol. It is a targeted metric for use in production networks carrying large numbers of streams for the purpose of monitoring the network quality of the flows or for other applications intended to analyze large numbers of streams susceptible to IP network device impairments. An example application is the burgeoning deployments of Internet Protocol Television (IPTV) by cable and telecommunication service providers. As described below, MDI provides for a readily scalable per-stream measure focused on loss and the cumulative effects of jitter.


2. Media Delivery Index Overview
2. 媒体交付索引概述

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 that 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 that 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
3. 媒体交付索引组件

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


3.1. Delay Factor
3.1. 延迟因子

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. This assumes the drain rate is the nominal constant traffic rate for constant bit rate streams or the piece-wise computed 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.

延迟因子是在每个媒体流分组结束时观察到的媒体数据到达和媒体数据排出之间的最大差值。这假设漏失率是恒定比特率流的标称恒定业务率或可变速率媒体流分组数据的分段计算业务率。这里的“漏失率”是指有效负载介质速率;e、 例如,对于典型的3.75 Mb/s MPEG视频传输流(TS),漏失率为3.75 Mb/s——解码节点消耗(显示)有效负载的速率。如果在采样时,接收的字节数等于传输的字节数,则瞬时流量平衡为零;然而,最小DF将是线路数据包的媒体数据值,因为这是必须缓冲的最小数据量。

The DF is the maximum observed value of the flow rate imbalance over a calculation interval. 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 is recommended to provide adequate indication of stream variations for monitoring and diagnostic applications for typical stream rates from 1 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 1 Mb/s 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. This time may also be seen as a measure of the network latency that must be induced from buffering, which is required to accommodate stream jitter and prevent loss. The DF`s max and min over the measurement period (multiple intervals) 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.

DF是计算间隔内流量不平衡的最大观测值。以字节为单位的缓冲媒体数据表示为以标称流量消耗(或填充)该数据以获得DF所需的时间(以毫秒为单位)。建议以十分之一毫秒的分辨率显示DF,以便为监测和诊断应用程序提供足够的流变化指示,典型流速率为1到40 Mb/s。DF值必须在选定的时间间隔结束时更新和显示。所选择的时间间隔被选择为足够长以对多个TS分组进行采样,并且因此将基于标称业务速率而变化。对于1 Mb/s及以上的典型流速率,1秒的间隔提供了足够长的采样时间,并且应该包括在所有实现中。延迟因子指示数据流必须以其标称比特率缓冲(即延迟)多长时间以防止数据包丢失。这段时间也可以被视为必须由缓冲引起的网络延迟的度量,缓冲是适应流抖动和防止丢失所必需的。还可以显示测量周期(多个间隔)内的DF最大值和最小值,以显示相对于测量周期内标称流量的最坏情况到达时间偏差或抖动。它提供了动态流量平衡指示,其最大和最小值显示了与平衡的最差偏差。

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 or packet


gaps (jitter). Greater DF values also indicate that more network latency is necessary to deliver a stream due to the need to pre-fill 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 buffer utilization of the various network component switch elements that comprise the switched network infrastructure [i2].


To further detail the calculation of DF, consider a virtual buffer VB used to buffer received packets of a stream. When a packet P(i) arrives during a calculation interval, compute two VB values, VB(i,pre) and VB(i,post), defined as:


   VB(i,pre) = sum (Sj) - MR * Ti; where j=1..i-1
   VB(i,post) = VB(i,pre) + Si
   VB(i,pre) = sum (Sj) - MR * Ti; where j=1..i-1
   VB(i,post) = VB(i,pre) + Si

where Sj is the media payload size of the jth packet, Ti is the relative time at which packet i arrives in the interval, and MR is the nominal media rate.


VB(i,pre) is the Virtual Buffer size just before the arrival of P(i). VB(i,post) is the Virtual Buffer size just after the arrival of P(i).


The initial condition of VB(0) = 0 is used at the beginning of each measurement interval. A measurement interval is defined from just after the time of arrival of the last packet during a nominal period (typically 1 second) as mentioned above to the time just after the arrival of the last packet of the next nominal period.


During a measurement interval, if k packets are received, then there are 2*k+1 VB values used in deriving VB(max) and VB(min). After determining VB(max) and VB(min) from the 2k+1 VB samples, DF for the measurement interval is computed and displayed as:

在测量间隔期间,如果接收到k个数据包,则在推导VB(max)和VB(min)时使用2*k+1 VB值。从2k+1 VB样本中确定VB(最大)和VB(最小)后,计算测量间隔的DF并显示为:

   DF = [VB(max) - VB(min)]/ MR
   DF = [VB(max) - VB(min)]/ MR

As noted above, a measurement interval of 1 second is typically used. If no packets are received during an interval, the last DF calculated during an interval in which packets did arrive is displayed. The time of arrival of the last previous packet is always retained for use in calculating VB when the next packet arrives (even if the time of the last received packet spans measurement intervals). For the first received measurement interval of a measurement period, no DF is calculated; however, packet arrival times are recorded for use in calculating VB during the following interval.


3.2. Media Loss Rate
3.2. 媒体丢失率

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 (if those 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
3.3. 媒体交付指数

Combining the Delay Factor and Media Loss Rate quantities for presentation results in the following 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 buffer required 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.

在接收节点上,知道其标称漏比特率后,DF's max指示容纳数据包抖动所需的缓冲区大小。或者,就漏桶[i9]参数而言,DF表示桶大小b,以给定标称流量率r下传输桶流量b的时间表示。

3.4. MDI Application Examples
3.4. MDI应用程序示例

If 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 of multiple intervals 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.

对于给定的MDI,如果DF较高和/或在多个间隔的重要测量周期内捕获的DF Max Min较高,则已检测到抖动,但长期平均流速可能为标称值。这可能是由于与感兴趣的流无关的重合业务流导致数据包聚束而导致的瞬态流混乱的结果。即使在没有丢失数据的情况下,高DF也可能导致下游缓冲区溢出或下溢或不可接受的延迟。

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 time then might also be included in the flow tolerance DF resulting in a higher-quality user experience.


4. Summary
4. 总结

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
5. 安全考虑

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


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. Informative References
6. 资料性引用

[i1] Braden, R., Zhang, L., Berson, S., Herzog, S., and S. Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1 Functional Specification", RFC 2205, September 1997.

[i1] Braden,R.,Zhang,L.,Berson,S.,Herzog,S.,和S.Jamin,“资源预留协议(RSVP)——版本1功能规范”,RFC 22052997年9月。

[i2] Partridge, C., "A Proposed Flow Specification", RFC 1363, September 1992.

[i2] 帕特里奇,C.,“拟定流量规范”,RFC 1363,1992年9月。

[i3] R. Fellman, `Hurdles to Overcome for Broadcast Quality Video Delivery over IP` VidTranS 2002.

[i3] R.Fellman,`通过IP进行广播质量视频传输需要克服的障碍',VidTranS 2002。

[i4] CableLabs `PacketCable Dynamic Quality-of-Service Specification`, PKT-SP-DQOS-I06-030415, 2003.

[i4] CableLabs“PacketCable动态服务质量规范”,PKT-SP-DQOS-I06-030415,2003年。

[i5] Shenker, S., Partridge, C., and R. Guerin, "Specification of Guaranteed Quality of Service", RFC 2212, September 1997.

[i5] Shenker,S.,Partridge,C.和R.Guerin,“保证服务质量规范”,RFC 2212,1997年9月。

[i6] Wroclawski, J., "Specification of the Controlled-Load Network Element Service", RFC 2211, September 1997.

[i6] Wroclawski,J.,“受控负荷网元服务规范”,RFC2211,1997年9月。

[i7] Braden, R., Clark, D., and S. Shenker, "Integrated Services in the Internet Architecture: an Overview", RFC 1633, June 1994.

[i7] Braden,R.,Clark,D.,和S.Shenker,“互联网体系结构中的综合服务:概述”,RFC16331994年6月。

[i8] ISO/IEC 13818-1 (MPEG-2 Systems)

[i8] ISO/IEC 13818-1(MPEG-2系统)

[i9] V. Raisanen, "Implementing Service Quality in IP Networks", John Wiley & Sons Ltd., 2003.

[i9] V.Raisanen,“在IP网络中实施服务质量”,John Wiley&Sons有限公司,2003年。

[i10] Demichelis, C. and P. Chimento, "IP Packet Delay Variation Metric for IP Performance Metrics (IPPM)", RFC 3393, November 2002.

[i10]Demichelis,C.和P.Chimento,“IP性能度量的IP数据包延迟变化度量(IPPM)”,RFC 3393,2002年11月。

[i11] Friedman, T., Caceres, R., and A. Clark, "RTP Control Protocol Extended Reports (RTCP XR)", RFC 3611, November 2003.

[i11]Friedman,T.,Caceres,R.,和A.Clark,“RTP控制协议扩展报告(RTCP XR)”,RFC 3611,2003年11月。

[i12] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, July 2003.

[i12]Schulzrinne,H.,Casner,S.,Frederick,R.,和V.Jacobson,“RTP:实时应用的传输协议”,STD 64,RFC 35502003年7月。

7. Acknowledgements
7. 致谢

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 versions of this document and implementations for MDI.

作者衷心感谢IneoQuest Technologies,Inc.的Marc Todd和Jesse Beeson、时代华纳有线电视公司的Bill Trubey和John Carlucci、Cox Communications公司的Nishith Sinha、SeaChange International公司的Ken Chiquoine、加拿大贝尔公司的Phil Proulx、坦堡电视公司的Paul Stallard博士以及,Broadbus Technologies的Gary Hughes、SBC实验室的Brad Medford、Adelphia Communications的John Roy、Kasena的Cliff Mercer博士、Rogers Cable的Mathew Ho以及Optinel Systems的Irl Duling,用于审查和评估本文件的早期版本以及MDI的实施。

Authors' Addresses


James Welch IneoQuest Technologies, Inc 170 Forbes Blvd Mansfield, Massachusetts 02048

马萨诸塞州曼斯菲尔德福布斯大道170号詹姆斯·韦尔奇IneoQuest Technologies,Inc.02048

Phone: 508 618 0312 EMail:


James Clark Cisco Systems, Inc 500 Northridge Road Suite 800 Atlanta, Georgia 30350

James Clark Cisco Systems,Inc.乔治亚州亚特兰大市北岭路500号800室30350

Phone: 678 352 2726 EMail:


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