





Internet Engineering Task Force (IETF)                        Q. Wu, Ed.
Request for Comments: 6792                                        Huawei
Category: Informational                                          G. Hunt
ISSN: 2070-1721                                             Unaffiliated
                                                                P. Arden
                                                                      BT
                                                           November 2012


           Guidelines for Use of the RTP Monitoring Framework

Abstract

   This memo proposes an extensible Real-time Transport Protocol (RTP)
   monitoring framework for extending the RTP Control Protocol (RTCP)
   with a new RTCP Extended Reports (XR) block type to report new
   metrics regarding media transmission or reception quality.  In this
   framework, a new XR block should contain a single metric or a small
   number of metrics relevant to a single parameter of interest or
   concern, rather than containing a number of metrics that attempt to
   provide full coverage of all those parameters of concern to a
   specific application.  Applications may then "mix and match" to
   create a set of blocks that cover their set of concerns.  Where
   possible, a specific block should be designed to be reusable across
   more than one application, for example, for all of voice, streaming
   audio, and video.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6792.









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Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................3
   2. Terminology .....................................................3
   3. RTP Monitoring Framework ........................................5
      3.1. Overview of the RTP Monitoring Framework ...................5
      3.2. Location of Monitors .......................................7
   4. Issues with Reporting Metrics Blocks Using RTCP XR Extensions ...8
      4.1. Using a Compound Metrics Block .............................8
      4.2. Correlating RTCP XR with Non-RTP Data ......................8
      4.3. Measurement Information Duplication ........................9
      4.4. Consumption of XR Block Code Points ........................9
   5. Guidelines for Reporting Metrics Blocks Using RTCP XR ...........9
      5.1. Use a Single Metric in the Metrics Block ...................9
      5.2. Include the Payload Type in the Metrics Block .............10
      5.3. Use RTCP SDES to Correlate XRs with Non-RTP Data ..........10
      5.4. Reduce Measurement Information Repetition across
           Metrics Blocks ............................................11
   6. An Example of a Metrics Block ..................................11
   7. Application to RFC 5117 Topologies .............................12
      7.1. Applicability to Translators ..............................13
      7.2. Applicability to MCUs .....................................13
   8. Security Considerations ........................................14
   9. Acknowledgements ...............................................14
   10. Informative References ........................................15











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1.  Introduction

   Multimedia services using the Real-time Transport Protocol (RTP) are
   seeing increased use.  Standard methods for gathering RTP performance
   metrics from these applications are needed to manage uncertainties in
   the behavior and availability of their services.  Standards such as
   "RTP Control Protocol Extended Reports (RTCP XR)" [RFC3611] as well
   as other RTCP extensions to sender reports (SRs) and receiver reports
   (RRs) [RFC3550] are being developed for the purpose of collecting and
   reporting performance metrics from endpoint devices that can be used
   to correlate the metrics, provide end-to-end service visibility, and
   measure and monitor Quality of Experience (QoE) [RFC6390].

   However, the proliferation of RTP-/RTCP-specific metrics for
   transport and application quality monitoring has been identified as a
   potential problem for interoperability when using RTP/RTCP to
   communicate all the parameters of concern to a specific application.
   Given that different applications layered on RTP may have some
   monitoring requirements in common, these metrics should be satisfied
   by a common design.

   The objective of this document is to describe an extensible RTP
   monitoring framework to provide a small number of reusable Quality of
   Service (QoS) / QoE metrics that facilitate reduced implementation
   costs and help maximize interoperability.  "Guidelines for Extending
   the RTP Control Protocol (RTCP)" [RFC5968] has stated that where RTCP
   is to be extended with a new metric, the preferred mechanism is by
   the addition of a new RTCP XR [RFC3611] block.  This memo assumes
   that all the guidelines from RFC 5968 must apply on top of the
   guidelines in this document.  Guidelines for developing new
   performance metrics are specified in [RFC6390].  New RTCP XR report
   block definitions should not define new performance metrics but
   should rather refer to metrics defined elsewhere.

2.  Terminology

   This memo is informative and as such contains no normative
   requirements.

   In addition, the following terms are defined:

   Transport-level metrics

      A set of metrics that characterize the three transport impairments
      of packet loss, packet delay, and jitter (also known as delay
      variation).  These metrics should be usable by any application
      that uses RTP transport.




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   Application-level metrics

      Metrics relating to application-specific parameters or QoE-related
      parameters.  Application-specific parameters are measured at the
      application level and focus on quality of content rather than
      network performance.  QoE-related parameters reflect the end-to-
      end performance at the services level and are usually measured at
      the user endpoint.  One example of such metrics is the QoE metric
      as specified in the QoE Metrics Report Block; see [QOE_BLOCK].

   End-system metrics

      Metrics relating to the way a terminal deals with transport
      impairments affecting the incident RTP stream.  These may include
      de-jitter buffering, packet loss concealment, and the use of
      redundant streams (if any) for correction of error or loss.

   Direct metrics

      Metrics that can be directly measured or calculated and are not
      dependent on other metrics.

   Interval metrics

      Metrics measured over the course of a single reporting interval
      between two successive report blocks.  This may be the most recent
      RTCP reporting interval ([RFC3550], Section 6.2) or some other
      interval signaled using an RTCP Measurement Information XR Block
      [RFC6776].  An example interval metric is the count of the number
      of RTP packets lost over the course of the last RTCP reporting
      interval.

   Cumulative metrics

      Metrics measured over several reporting intervals for accumulating
      statistics.  The time period over which measurements are
      accumulated can be the complete RTP session, or some other
      interval signaled using an RTCP Measurement Information XR Block
      [RFC6776].  An example cumulative metric is the total number of
      RTP packets lost since the start of the RTP session.

   Sampled metrics

      Metrics measured at a particular time instant and sampled from the
      values of a continuously measured or calculated metric within a
      reporting interval (generally, the value of some measurement as
      taken at the end of the reporting interval).  An example is the
      inter-arrival jitter reported in RTCP SR and RR packets, which is



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      continually updated as each RTP data packet arrives but is only
      reported based on a snapshot of the value that is sampled at the
      instant the reporting interval ends.

3.  RTP Monitoring Framework

   There are many ways in which the performance of an RTP session can be
   monitored.  These include RTP-based mechanisms such as the RTP MIB
   module [RFC2959]; or the Session Initiation Protocol (SIP) event
   package for RTCP summary reports [RFC6035]; or non-RTP mechanisms
   such as generic MIBs, NetFlow [RFC3954], IP Flow Information Export
   (IPFIX) [RFC5101] [RFC5102], and so on.  Together, these provide
   useful mechanisms for exporting data on the performance of an RTP
   session to non-RTP network management systems.  It is desirable to
   also perform in-session monitoring of RTP performance.  RTCP provides
   the means to do this.  In the following, we review the RTP Monitoring
   Framework, and give guidance for using and extending RTCP for
   monitoring RTP sessions.  One major benefit of such a framework is
   ease of integration with other RTP/RTCP mechanisms.

3.1.  Overview of the RTP Monitoring Framework

   The RTP monitoring Framework comprises the following two key
   functional components described below:

   o  Monitor

   o  RTP Metrics Block

   "Monitor" is the functional component defined in the RTP
   specification [RFC3550].  It acts as a repository of information
   gathered for monitoring purposes.

   According to the definition of "monitor" in [RFC3550], the end system
   that runs an application program that sends or receives RTP data
   packets, an intermediate system that forwards RTP packets to end
   devices, or a third party that observes the RTP and RTCP traffic but
   does not make itself visible to the RTP Session participants can play
   the role of the monitor within the RTP monitoring framework.  As
   shown in Figure 1, the third-party monitor can be a passive monitor
   that sees the RTP/RTCP stream pass it, or a system that gets sent
   RTCP reports but not RTP and uses that to collect information.  The
   third-party monitor should be placed on the RTP/RTCP path between the
   sender, the intermediate system, and the receiver.

   The RTP Metrics Block (MB) conveys real-time application QoS/QoE
   metric information and is used by the monitor to exchange information
   with other monitors in the appropriate report block format.  The



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   information contained in the RTP MBs is collected by monitors and can
   be formulated as various types of metrics, e.g., direct metrics/
   composed performance metrics [RFC6390] or interval metrics/cumulative
   metrics/sampled metrics, etc.  Both the RTCP and RTCP XR can be
   extended to transport these metrics, e.g., the basic RTCP reception
   report [RFC3550] that conveys reception statistics (i.e., transport-
   level statistics) for multiple RTP media streams, the RTCP XRs
   [RFC3611] that supplement the existing RTCP packets and provide more
   detailed feedback on reception quality, and an RTCP NACK [RFC4585]
   that provides feedback on the RTP sequence numbers for a subset of
   the lost packets or all the currently lost packets.  Ultimately, the
   metric information collected by monitors within the RTP monitoring
   framework may go to the network management tools beyond the RTP
   monitoring framework; e.g., as shown in Figure 1, the monitors may
   export the metric information derived from the RTP monitoring
   framework to the management system using non-RTP means.

                  +-----------+                  +----------+
                  |Third-Party|                  |Management|
                  |  Monitor  |          >>>>>>>>|  System  |<<<<<
                  +-----------+          ^       +----------+    ^
                      :   ^              ^                       ^
                      :   |              ^                       ^
   +---------------+  :   |       +-------------+        +-------------+
   | +-----------+ |  :   |       |+-----------+|        |+-----------+|
   | |  Monitor  | |..:...|.......||  Monitor  ||........||  Monitor  ||
   | +-----------+ |      |       |+-----------+|        |+-----------+|
   |               |------+------>|             |------->|             |
   | RTP Sender    |              |RTP Mixer or |        |RTP Receiver |
   |               |              |Translator   |        |             |
   +---------------+              +-------------+        +-------------+

   ----> RTP media traffic
   ..... RTCP control channel
   >>>>> Non-RTP/RTCP management flows

                 Figure 1: Example Showing the Components
                      of the RTP Monitoring Framework

   RTP may be used with multicast groups: both Any-Source Multicast
   (ASM) and Source-Specific Multicast (SSM).  These groups can be
   monitored using RTCP.  In the ASM case, the monitor is a member of
   the multicast group and listens to RTCP reports from all members of
   the ASM group.  In the SSM case, there is a unicast feedback target
   that receives RTCP feedback from receivers and distributes it to
   other members of the SSM group (see Figure 1 of [RFC5760]).  The
   monitor will need to be co-located with the feedback target to




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   receive all feedback from the receivers (this may also be an
   intermediate system).  In both ASM and SSM scenarios, receivers can
   send RTCP reports to enhance reception-quality reporting.

3.2.  Location of Monitors

   As shown in Figure 1, there are several possible locations from which
   RTP sessions can be monitored.  These include end systems that
   terminate RTP sessions, intermediate systems that are an active part
   of an RTP session, and third-party devices that passively monitor an
   RTP session.  Not every RTP session will include monitoring, and
   those sessions that are monitored will not all include each type of
   monitor.  The performance metrics collected by monitors can be
   divided into end-system metrics, application-level metrics, and
   transport-level metrics.  Some of these metrics may be specific to
   the measurement point of the monitor or may depend on where the
   monitors are located in the network, while others are more general
   and can be collected in any monitoring location.

   End-system monitoring is monitoring that is deployed on devices that
   terminate RTP flows.  Flows can be terminated in user equipment, such
   as phones, videoconferencing systems, or IPTV set-top boxes.
   Alternatively, they can be terminated in devices that gateway between
   RTP and other transport protocols.  Transport-level metrics, end-
   system metrics, and application-level metrics that don't reflect the
   end-to-end user experience may be collected at all types of end
   systems, but some application-level metrics (i.e., quality of
   experience (QoE) metrics) may only be applicable for user-facing end
   systems.

   RTP sessions can include intermediate systems that are an active part
   of the system.  These intermediate systems include RTP mixers and
   translators, Multipoint Control Units (MCUs), retransmission servers,
   etc.  If the intermediate system establishes separate RTP sessions to
   the other participants, then it must act as an end system in each of
   those separate RTP sessions for the purposes of monitoring.  If a
   single RTP session traverses the intermediate system, then the
   intermediate system can be assigned a synchronization source (SSRC)
   in that session, which it can use for its reports.  Transport-level
   metrics may be collected at such an intermediate system.

   Third-party monitors may be deployed that passively monitor RTP
   sessions for network management purposes.  Third-party monitors often
   do not send reports into the RTP session being monitored but instead
   collect transport-level metrics, end-system metrics, and application-
   level metrics.  In some cases, however, third-party monitors can send
   reports to some or all participants in the session being monitored.




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   For example, in a media streaming scenario, third-party monitors may
   be deployed that passively monitor the session and send reception-
   quality reports to the media source but not to the receivers.

4.  Issues with Reporting Metrics Blocks Using RTCP XR Extensions

   The following sections discuss four issues that have come up in the
   past with reporting metrics blocks using RTCP XR extensions.

4.1.  Using a Compound Metrics Block

   A compound metrics block is designed to contain a large number of
   parameters from different classes for a specific application in a
   single block.  For example, "RTP Control Protocol Extended Reports
   (RTCP XR)" [RFC3611] defines seven report block formats for network
   management and quality monitoring.  Some of these block types defined
   in the RTCP XRs [RFC3611] are only specifically designed for
   conveying multicast inference of network characteristics (MINC) or
   voice over IP (VoIP) monitoring.  However, different applications
   layered on RTP may have different monitoring requirements.  Designing
   a compound metrics block only for specific applications may increase
   implementation costs and minimize interoperability.

4.2.  Correlating RTCP XR with Non-RTP Data

   The Canonical End-Point Identifier SDES Item (CNAME), as defined in
   RTP [RFC3550], is an example of an existing tool that allows binding
   an SSRC that may change to a name that is fixed within one RTP
   session.  The CNAME may also be fixed across multiple RTP sessions
   from the same source.  However, there may be situations where RTCP
   reports are sent to other participating endpoints using a non-RTP
   protocol in a session.  For example, as described in [RFC6035] in
   relation to summary reports, the data contained in RTCP XR VoIP
   metrics reports [RFC3611] is forwarded to a central collection server
   system using SIP.  In such a case, there is a large portfolio of
   quality parameters that can be associated with real-time
   applications, e.g., VOIP applications, but only a minimal number of
   parameters are included in the RTCP XRs.  With this minimal number of
   RTCP statistical parameters mapped to non-RTCP measurements, it is
   hard to provide accurate measurements of real-time application
   quality, conduct detailed data analysis, and create timely alerts for
   users.  Therefore, a correlation between RTCP XRs and non-RTP data
   should be provided.








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4.3.  Measurement Information Duplication

   We may set a measurement interval for the session and monitor RTP
   packets within one or several consecutive report intervals.  In such
   a case, extra measurement information (e.g., extended sequence number
   of the first packet, measurement period) may be expected.  However,
   if we put such extra measurement information into each metrics block,
   there may be situations where an RTCP XR packet that contains
   multiple metrics blocks will report on the same streams from the same
   source.  In other words, duplicated data for the measurement is
   provided multiple times, once in every metrics block.  Though this
   design ensures immunity to packet loss, it may result in more
   packetization complexity, and this processing overhead is not
   completely trivial in some cases.  Therefore, a compromise between
   processing overhead and reliability should be taken into account.

4.4.  Consumption of XR Block Code Points

   The RTCP XR block namespace is limited by the 8-bit block type field
   in the RTCP XR header.  Space exhaustion may be a concern in the
   future.  In anticipation of the potential need to extend the block
   type space, it is noted that Block Type 255 is reserved for future
   extensions in [RFC3611].

5.  Guidelines for Reporting Metrics Blocks Using RTCP XR

5.1.  Use a Single Metric in the Metrics Block

   Different applications using RTP for media transport certainly have
   differing requirements for metrics transported in RTCP to support
   their operation.  For many applications, the basic metrics for
   transport impairments provided in RTCP SR and RR packets [RFC3550]
   (together with source identification provided in RTCP Source
   Description (SDES) packets) are sufficient.  For other applications,
   additional metrics may be required or at least may be sufficiently
   useful to justify the overhead, in terms of both processing in
   endpoints and of increased session bandwidth.  For example, an IPTV
   application using Forward Error Correction (FEC) might use either a
   metric of post-repair loss or a metric giving detailed information
   about pre-repair loss bursts to optimize payload bandwidth and the
   strength of FEC required for changing network conditions.  However,
   there are many metrics available.  It is likely that different
   applications or classes of applications will wish to use different
   metrics.  Any one application is likely to require metrics for more
   than one parameter, but if this is the case, different applications
   will almost certainly require different combinations of metrics.  If





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   larger blocks are defined containing multiple metrics to address the
   needs of each application, it becomes likely that many such different
   larger blocks are defined, which poses a danger to interoperability.

   To avoid this pitfall, this memo recommends the definition of metrics
   blocks containing a very small number of individual metrics
   characterizing only one parameter of interest to an application
   running over RTP.  For example, at the RTP transport layer, the
   parameter of interest might be packet delay variation, and
   specifically the metric "IP Packet Delay Variation (IPDV)" defined by
   [Y1540].  See Section 6 for architectural considerations for a
   metrics block, using as an example a metrics block to report packet
   delay variation.  Further, it is appropriate to not only define
   report blocks separately but also to do so in separate documents
   where possible.  This makes it easier to evolve the reports (i.e., to
   update each type of report block separately) and also makes it easier
   to require compliance with a particular report block.

5.2.  Include the Payload Type in the Metrics Block

   There are some classes of metrics that can only be interpreted with
   knowledge of the media codec that is being used (audio mean opinion
   scores (MOSs) were the triggering example, but there may be others).
   In such cases, the correlation of an RTCP XR with RTP data is needed.
   Report blocks that require such correlation need to include the
   payload type of the reported media.  In addition, it is necessary to
   signal the details and parameters of the payload format to which that
   payload type is bound using some out-of-band means (e.g., as part of
   a Session Description Protocol (SDP) offer/answer exchange).

5.3.  Use RTCP SDES to Correlate XRs with Non-RTP Data

   There may be situations where more than one media transport protocol
   is used by one application to interconnect to the same session in the
   gateway.  For example, one RTCP XR packet is sent to the
   participating endpoints using non-RTP-based media transport (e.g.,
   using SIP) in a VoIP session.  One crucial factor lies in how to
   handle the different identities that correspond to these different
   media transport protocols.

   This memo recommends an approach to facilitate the correlation of the
   RTCP session with other session-related non-RTP data.  That is to
   say, if there is a need to correlate RTP sessions with non-RTP
   sessions, then the correlation information needed should be conveyed
   in a new RTCP SDES item, since such correlation information describes
   the source rather than providing a quality report.  An example use
   case is where a participant endpoint may convey a call identifier or
   a global call identifier associated with the SSRC of a measured RTP



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   stream.  In such a case, the participant endpoint uses the SSRC to
   bind the call identifier using the SDES item in the SDES RTCP packet
   and sends this correlation to the network management system.  A flow
   measurement tool that is configured with the 5-tuple and is not call-
   aware then forwards the RTCP XRs along with the SSRC of the measured
   RTP stream, which is included in the XR Block header and 5-tuple to
   the network management system.  The network management system can
   then correlate this report using SSRC with other diagnostic
   information, such as call detail records.

5.4.  Reduce Measurement Information Repetition across Metrics Blocks

   When multiple metrics blocks are carried in one RTCP XR packet,
   reporting on the same stream from the same source for the same time
   period, RTCP should use the SSRC to identify and correlate the
   multiple metrics blocks placed between Measurement Information
   Blocks; see "Measurement Identity and Information Reporting Using a
   Source Description (SDES) Item and an RTCP Extended Report (XR)
   Block" [RFC6776].  [RFC6776] enables an RTCP sender to convey the
   common time period and the number of packets sent during this period.
   If the measurement interval for a metric is different from the RTCP
   reporting interval, then this measurement duration in the Measurement
   Information Block should be used to specify the interval.  When there
   may be multiple Measurement Information Blocks with the same SSRC in
   one RTCP XR compound packet, the Measurement Information Block should
   be put in order and followed by all the metrics blocks associated
   with this Measurement Information Block.  New RTCP XR metrics blocks
   that rely on the Measurement Information Block must specify the
   response in case the new RTCP XR metrics block is received without an
   associated Measurement Information Block.  In most cases, it is
   expected that the correct response is to discard the received metric.
   In order to reduce measurement information repetition in one RTCP XR
   compound packet containing multiple metrics blocks, the measurement
   information shall be sent before the related metrics blocks that are
   from the same reporting interval.  Note that for packet loss
   robustness, if the report blocks for the same interval span more than
   one RTCP packet, then each block must have the measurement identity
   information sent together with itself in the same RTCP compound
   packet, even though the information will be the same.

6.  An Example of a Metrics Block

   This section uses the example of an existing proposed metrics block
   to illustrate the application of the principles set out in Section 5.

   The example [RFC6798] is a block to convey information about packet
   delay variation (PDV) only, consistent with the principle that a
   metrics block should address only one parameter of interest.  One



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   simple metric of PDV is available in the RTCP RR packet as the
   "inter-arrival jitter" field.  There are other PDV metrics with a
   certain similarity in metric structure that may be more useful to
   certain applications.  Two such metrics are the IPDV metric ([Y1540]
   [RFC3393]) and the mean absolute packet delay variation 2 (MAPDV2)
   metric [G1020].  The use of these metrics is consistent with the
   principle in Section 5 of the RTCP guidelines document [RFC5968] that
   metrics should usually be defined elsewhere, so that RTCP standards
   define only the transport of the metric rather than its nature.  The
   purpose of this section is to illustrate the architectural
   considerations, using the example of [RFC6798], rather than to
   document the design of the PDV metrics block or to provide a tutorial
   on PDV in general.

   Given the availability of at least three metrics for PDV, there are
   design options for the allocation of metrics to RTCP XR blocks:

   o  Provide an RTCP XR block per metric.

   o  Provide a single RTCP XR block that contains all three metrics.

   o  Provide a single RTCP block to convey any one of the three
      metrics, together with an identifier to inform the receiving RTP
      system of the specific metric being conveyed.

   In choosing between these options, extensibility is important,
   because additional metrics of PDV may well be standardized and
   require inclusion in this framework.  The first option is extensible
   but only by the use of additional RTCP XR blocks, which may consume
   the limited namespace for RTCP XR blocks at an unacceptable rate.
   The second option is not extensible and so could be rejected on that
   basis, but in any case a single application is quite unlikely to
   require the transport of more than one metric for PDV.  Hence, the
   third option was chosen.  This implies the creation of a subsidiary
   namespace to enumerate the PDV metrics that may be transported by
   this block, as discussed further in [RFC6798].

7.  Application to RFC 5117 Topologies

   The topologies specified in [RFC5117] fall into two categories.  The
   first category relates to the RTP system model utilizing multicast
   and/or unicast.  The topologies in this category are specifically
   Topo-Point-to-Point, Topo-Multicast, Topo-Translator (both variants
   Topo-Trn-Translator and Topo-Media-Translator as well as combinations
   of the two), and Topo-Mixer.  These topologies use RTP end systems,
   RTP mixers, and RTP translators as defined in [RFC3550].  For the
   purposes of reporting connection quality to other RTP systems, RTP
   mixers and RTP end systems are very similar.  Mixers resynchronize



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   packets and do not relay RTCP reports received from one cloud towards
   other cloud(s).  Translators do not resynchronize packets and should
   forward certain RTCP reports between clouds.  In this category, the
   RTP system (end system, mixer, or translator) that originates,
   terminates, or forwards RTCP XR blocks is expected to handle RTCP,
   including RTCP XR, according to RTP [RFC3550].  Provided this
   expectation is met, an RTP system using RTCP XR is architecturally no
   different from an RTP system of the same class (end system, mixer, or
   translator) that does not use RTCP XR.  The second category relates
   to deployed system models used in many H.323 [H323] videoconferences.
   The topologies in this category are Topo-Video-switch-MCU and
   Topo-RTCP-terminating-MCU.  Such topologies based on systems (e.g.,
   MCUs) do not behave according to RTP [RFC3550].

   Considering that the translator and MCU are two typical intermediate
   systems in these two categories mentioned above, this document will
   take them as two typical examples to explain how RTCP XR works in
   different [RFC5117] topologies.

7.1.  Applicability to Translators

   Section 7.2 of the RTP specification [RFC3550] describes the
   processing of RTCP by translators.  RTCP XR is within the scope of
   the recommendations of [RFC3550].  Some RTCP XR metrics blocks may
   usefully be measured at, and reported by, translators.  As described
   in [RFC3550], this creates a requirement for the translator to
   allocate an SSRC for the monitor co-located with itself so that the
   monitor may populate the SSRC in the RTCP XR packet header as the
   packet sender SSRC and send it out (although the translator is not a
   synchronization source in the sense of originating RTP media
   packets).  It must also supply this SSRC and the corresponding CNAME
   in RTCP SDES packets.

   In RTP sessions where one or more translators generate any RTCP
   traffic towards their next-neighbor RTP system, other translators in
   the session have a choice as to whether they forward a translator's
   RTCP packets.  Forwarding may provide additional information to other
   RTP systems in the connection but increases RTCP bandwidth and may in
   some cases present a security risk.  RTP translators may have
   forwarding behavior based on local policy, which might differ between
   different interfaces of the same translator.

7.2.  Applicability to MCUs

   Topo-Video-switch-MCU and Topo-RTCP-terminating-MCU suffer from the
   difficulties described in [RFC5117].  These difficulties apply to
   systems sending, and expecting to receive, RTCP XR blocks as much as
   to systems using other RTCP packet types.  For example, a participant



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   RTP end system may send media to a video switch MCU.  If the media
   stream is not selected for forwarding by the switch, neither RTCP RR
   packets nor RTCP XR blocks referring to the end system's generated
   stream will be received at the RTP end system.  Strictly speaking,
   the RTP end system can only conclude that its RTP has been lost in
   the network, though an RTP end system complying with the robustness
   principle of [RFC1122] should survive with essential functions (i.e.,
   media distribution) unimpaired.

8.  Security Considerations

   This document focuses on the RTCP reporting extension using RTCP XR
   and should not give rise to any new security vulnerabilities beyond
   those described in RTCP XRs [RFC3611].  However, it also describes
   the architectural framework to be used for monitoring at the RTP
   layer.  The security issues with monitoring need to be considered.

   In RTP sessions, an RTP system may use its own SSRC to send its
   monitoring reports towards its next-neighbor RTP system.  Other RTP
   systems in the session may have a choice as to whether they forward
   this RTP system's RTCP packets.  This presents a security issue,
   since the information in the report may be exposed by the other RTP
   system to any malicious node.  Therefore, if the information is
   considered sensitive, the monitoring reports should be secured to the
   same extent as the RTP flows that they measure.  If encryption is
   used and the encrypted monitoring report is received by the RTP
   system that deploys the third-party monitor, the RTP system may
   decrypt the monitor report for the third-party monitor based on local
   policy (e.g., third-party monitors are allowed access to the metric)
   and forward it to the third-party monitor; otherwise, the third-party
   monitor should discard the received encrypted monitoring report.

9.  Acknowledgements

   The authors would like to thank Colin Perkins, Charles Eckel, Robert
   Sparks, Salvatore Loreto, Graeme Gibbs, Debbie Greenstreet, Keith
   Drage, Dan Romascanu, Ali C. Begen, Roni Even, Magnus Westerlund,
   Meral Shirazipour, Tina Tsou, Barry Leiba, Benoit Claise, Russ
   Housley, and Stephen Farrell for their valuable comments and
   suggestions on early versions of this document.











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10.  Informative References

   [G1020]      ITU-T, "Performance parameter definitions for quality of
                speech and other voiceband applications utilizing IP
                networks", ITU-T Rec. G.1020, July 2006.

   [H323]       ITU-T, "Packet-based multimedia communications systems",
                ITU-T Rec. H.323, December 2009.

   [QOE_BLOCK]  Clark, A., Wu, Q., Schott, R., and G. Zorn, "RTP Control
                Protocol (RTCP) Extended Report (XR) Blocks for QoE
                Metric Reporting", Work in Progress, October 2012.

   [RFC1122]    Braden, R., "Requirements for Internet Hosts -
                Communication Layers", STD 3, RFC 1122, October 1989.

   [RFC2959]    Baugher, M., Strahm, B., and I. Suconick, "Real-Time
                Transport Protocol Management Information Base",
                RFC 2959, October 2000.

   [RFC3393]    Demichelis, C. and P. Chimento, "IP Packet Delay
                Variation Metric for IP Performance Metrics (IPPM)",
                RFC 3393, November 2002.

   [RFC3550]    Schulzrinne, H., Casner, S., Frederick, R., and V.
                Jacobson, "RTP: A Transport Protocol for Real-Time
                Applications", STD 64, RFC 3550, July 2003.

   [RFC3611]    Friedman, T., Caceres, R., and A. Clark, "RTP Control
                Protocol Extended Reports (RTCP XR)", RFC 3611,
                November 2003.

   [RFC3954]    Claise, B., "Cisco Systems NetFlow Services Export
                Version 9", RFC 3954, October 2004.

   [RFC4585]    Ott, J., Wenger, S., Sato, N., Burmeister, C., and J.
                Rey, "Extended RTP Profile for Real-time Transport
                Control Protocol (RTCP)-Based Feedback (RTP/AVPF)",
                RFC 4585, July 2006.

   [RFC5101]    Claise, B., "Specification of the IP Flow Information
                Export (IPFIX) Protocol for the Exchange of IP Traffic
                Flow Information", RFC 5101, January 2008.

   [RFC5102]    Quittek, J., Bryant, S., Claise, B., Aitken, P., and J.
                Meyer, "Information Model for IP Flow Information
                Export", RFC 5102, January 2008.




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   [RFC5117]    Westerlund, M. and S. Wenger, "RTP Topologies",
                RFC 5117, January 2008.

   [RFC5760]    Ott, J., Chesterfield, J., and E. Schooler, "RTP Control
                Protocol (RTCP) Extensions for Single-Source Multicast
                Sessions with Unicast Feedback", RFC 5760,
                February 2010.

   [RFC5968]    Ott, J. and C. Perkins, "Guidelines for Extending the
                RTP Control Protocol (RTCP)", RFC 5968, September 2010.

   [RFC6035]    Pendleton, A., Clark, A., Johnston, A., and H.
                Sinnreich, "Session Initiation Protocol Event Package
                for Voice Quality Reporting", RFC 6035, November 2010.

   [RFC6390]    Clark, A. and B. Claise, "Guidelines for Considering New
                Performance Metric Development", BCP 170, RFC 6390,
                October 2011.

   [RFC6776]    Clark, A. and Q. Wu, "Measurement Identity and
                Information Reporting Using a Source Description (SDES)
                Item and an RTCP Extended Report (XR) Block", RFC 6776,
                October 2012.

   [RFC6798]    Clark, A. and Q. Wu, "RTP Control Protocol (RTCP)
                Extended Report (XR) Block for Packet Delay Variation
                Metric Reporting", RFC 6798, November 2012.

   [Y1540]      ITU-T, "IP packet transfer and availability performance
                parameters", ITU-T Rec. Y.1540, March 2011.





















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Authors' Addresses

   Qin Wu (editor)
   Huawei
   101 Software Avenue, Yuhua District
   Nanjing, Jiangsu  210012
   China

   EMail: sunseawq@huawei.com


   Geoff Hunt
   Unaffiliated

   EMail: r.geoff.hunt@gmail.com


   Philip Arden
   BT
   Orion 3/7 PP4
   Adastral Park
   Martlesham Heath
   Ipswich, Suffolk  IP5 3RE
   United Kingdom

   Phone: +44 1473 644192
   EMail: philip.arden@bt.com
























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