| Rfc | 6792 |
|---|
| Title | Guidelines for Use of the RTP Monitoring Framework |
|---|
| Author | Q. Wu, Ed., G.
Hunt, P. Arden |
|---|
| Date | November 2012 |
|---|
| Format: | TXT, HTML |
|---|
| Status: | INFORMATIONAL |
|---|
|
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.
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
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.
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
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
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
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.
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.
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
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
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
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
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
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.
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.
[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.
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