Audio/Video Transport Working Group Q. Wu, Ed.
Internet-Draft Huawei
Intended status: Informational April 23, 2011
Expires: October 25, 2011
Monitoring Architectures for RTP
draft-ietf-avtcore-monarch-00.txt
Abstract
This memo proposes an architecture for extending RTCP with a new RTCP
XR (RFC3611) block type to report new metrics regarding media
transmission or reception quality, as proposed in RFC5968. This memo
suggests that a new 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 which 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 which covers their set of concerns. Where
possible, a specific block should be designed to be re-usable across
more than one application, for example, for all of voice, streaming
audio and video.
Status of this Memo
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This Internet-Draft will expire on October 25, 2011.
Copyright Notice
Copyright (c) 2011 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements notation . . . . . . . . . . . . . . . . . . . . 4
3. RTP monitoring architecture . . . . . . . . . . . . . . . . . 5
4. RTCP Metric Block Report and associated parameters . . . . . . 8
4.1. Classification of RTCP Metric Block parameters . . . . . . 9
4.1.1. Application level parameters . . . . . . . . . . . . . 9
4.1.2. Transport level parameters . . . . . . . . . . . . . . 9
4.1.3. End system parameters . . . . . . . . . . . . . . . . 10
5. Monitoring Methodology . . . . . . . . . . . . . . . . . . . . 11
5.1. Option 1 - Monitoring every packet . . . . . . . . . . . . 11
5.2. Option 2 - Real-time histogram methods . . . . . . . . . . 11
5.3. Option 3 - Monitoring by exception . . . . . . . . . . . . 11
5.4. Option 4 - Application-specific monitoring . . . . . . . . 12
6. Issues with RTCP XR extension . . . . . . . . . . . . . . . . 13
7. Guideline for reporting block format using RTCP XR . . . . . . 14
7.1. Using small blocks . . . . . . . . . . . . . . . . . . . . 14
7.2. Sharing the identity block . . . . . . . . . . . . . . . . 14
7.3. Expanding the RTCP XR block namespace . . . . . . . . . . 18
8. An example of a metric block . . . . . . . . . . . . . . . . . 19
9. Application to RFC 5117 topologies . . . . . . . . . . . . . . 20
9.1. Applicability to MCU . . . . . . . . . . . . . . . . . . . 20
9.2. Application to translators . . . . . . . . . . . . . . . . 20
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
11. Security Considerations . . . . . . . . . . . . . . . . . . . 23
12. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 24
13. Informative References . . . . . . . . . . . . . . . . . . . . 25
Appendix A. Appendix A. Change Log . . . . . . . . . . . . . . . 26
A.1. draft-ietf-avtcore-monarch-00 . . . . . . . . . . . . . . 26
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 27
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1. Introduction
Service providers and network providers today suffer from lack of
good service that can monitor the performance at the user's home,
handset or remote office. Without service performance metrics, it is
difficult for network operators to properly locate the problem and
solve service issues before problems impact subscriber/end user. The
resolution generally involves deploying costly field network
technician to conduct on-site troubleshooting and diagnostics. By
reducing the expensive deployments with more automated remote
monitoring capabilities, network operators can save significant
costs, reduce mean time to repair and provider a better service
offering.
As more users and subscribers rely on real time application services,
uncertainties in the performance and availability of these services
are driving the need to support new standard methods for gathering
performance metrics from RTP applications. These rapidly emerging
standards, such as RTCP XR [RFC3611]and other RTCP extension to
Sender Reports(SR), Receiver Reports (RR) [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 QoE.
However the proliferation of RTP/RTCP specific metrics for transport
and application quality monitoring has been identified as a potential
problem for RTP/RTCP interoperability, which attempt to provide full
coverage of all those parameters of concern to a specific
application. Since different applications layered on RTP may have
some monitoring requirements in common, therefore these metrics
should be satisfied by a common design.
The objective of this document is to define an extensible RTP
monitoring framework to provide a small number of re-usable QoS/QoE
metrics which facilitate reduced implementation costs and help
maximize inter-operability. [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
any requirement for a new metric to be transported in RTCP will use a
new RTCP XR block.
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2. Requirements notation
This memo is informative and as such contains no normative
requirements.
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3. RTP monitoring architecture
The RTP monitoring architecture comprises the following two
functional components shown below:
o Monitor
o Metric Block Structure
Monitor is a functional component that acts as a source of
information gathered for monitoring purposes. It may also collects
statistics from multiple source, stores such information reported by
RTCP XR or other RTCP extension appropriately as base metric or
calculates composite metric. The end system that source RTP streams,
or an intermediate-system that forwards RTP packets to End-devices
that send or receive metric block can be envisioned to act as Monitor
within the RTP monitoring architecture.
The Metric Block exposes real time Application Quality information in
the report block format to monitor within the Network Management
System. The RTCP or RTCP XR can be extended to convey such
information.
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+-------------------+
| RTP Sender |
| +-----------+ |
| | Monitor | --- --------|
| | | | |
| +-----------+ | |
|+-----------------+| | +--------------+
||Application || | |Management |
||-streaming video || | | System |
|---|-VOIP || | | |
| ||-video conference|| 5 | | +----------+ |
| ||-telepresence || --------->| | |
| ||-ad insertion || | | Monitor | |
| |+-----------------+| |--------->| | |
| +-------------------+ | | +-------\--+ |
| Report Block | +---------|----+
| transported over | Report Block |
| RTCP extension | transported over|5
| 1 | RTCP XR |
| +------ -----------------+ | +--------------|---- ----+
| | RTP System | | | RTP Receiver >--4-|--- |
| | +----------+ | 5 | | +-----------+ | |
| | | |------------| | | Monitor |<-- |
| | | Monitor | | | | |<------|
| | | | | | +----/------+ ||
| | +----------+ | | | ||
| | | | |2 ||
| | +-----------------+ | | +-------/---------+ ||
| | |Application | | | |Application | ||
| | |-streaming video | | | |-streaming video | ||
| | |-VOIP | | 1 | |-VOIP | 3|
---->-Video conference|--------------->|-Video conference ||
| |-Telepresence | | | |-Telepresence | ||
| |-Ad insertion | | | |-Ad insertion | ||
| +-----------------+ | | +-----------------+ ||
| +-----------------+ | | +-----------------+ ||
| |Transport | | | |Transport | ||
| |-IP/UDP/RTP | | | |-IP/UDP/RTP >---||
| |-IP/TCP/RTP | | | | -IP/TCP/RTP | |
| |-IP/TCP/RTSP/RTP | | | |-IP/TCP/RTSP/RTP | |
| +-----------------+ | | +-----------------+ |
+------------------------+ +------------------------+
Figure 1: RTP Monitoring Architecture
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1. RTP communication between real time applications
2. Application layer metrics
3. Transport layer metrics
4. End System metrics
5. Reporting Session- metrics transmitted over specified interfaces
6. Management application- monitor interaction using northbound
interface. - Monitor outputs reports to the management
application. The management application collects raw data from
monitor, organizes database, conducts data analysis and creates
alerts to the users. Note that Management application
interaction with Monitor is out of scope of this document.
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4. RTCP Metric Block Report and associated parameters
The basic RTCP Reception Report (RR) conveys reception statistics in
metric block report format for multiple RTP media streams including
o transport level statistics
o the fraction of packet lost since the last report
o the cumulative number of packets lost
o the highest sequence number received
o an estimate of the inter-arrival jitter
o and information to allow senders to calculate the network round
trip time.
The RTCP XRs [RFC3611] supplement the existing RTCP packets and
provide more detailed feedback on reception quality in several
categories:
o Loss and duplicate RLE reports
o Packet-receipt times reports
o Round-trip time reports
o Statistics Summary Reports
There are also various other scenarios in which it is desirable to
send RTCP Metric reports more frequently. The Audio/Video Profile
with Feedback [RFC4585]extends the standard A/V Profile[RFC3551] to
allow RTCP reports to be sent early provided RTCP bandwidth
allocation is respected. There are four use cases but are not
limited to:
o RTCP NACK is used to provide feedback on the RTP sequence number
of the lost packets.
o RTCP XR is extended to provide feedback on multicast acquisition
statistics information and parameters.
o RTCP is extended to convey requests for full intra-coded frames or
select the reference picture, and signalchanges in the desired
temporal/spatial trade-off and maximum media bit rate.
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o RTCP or RTCP XR is extended to provide feedback on ECN statistics
information.
4.1. Classification of RTCP Metric Block parameters
4.1.1. Application level parameters
Measured data at the application level, i.e., QoE related parameters
which focus on quality of content rather than network parameters.
These include but are not limited to:
o Sound/Noise Level
o Echo return lost
o Statistics Summary Info, e.g.,key frame lost key frame lost rate/
discard rate, key frame burst severity
o Codec Control
o Estimated Mean Opinion Score (MOS)
4.1.2. Transport level parameters
Measured data at the transport level. These include but are not
limited to:
o Lost packets
o Round trip delay
o Jitter
o Congestion info
o FEC
o Codec Control
o Media Synchronization info
o Retransmission Info
o RAMS info
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4.1.3. End system parameters
Measured data from application residing in that device. These
include but are not limited to:
o Error Concealment
o FEC
o Media Synchronization info
o Jitter Buffer Lost
o Jitter Buffer Delay
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5. Monitoring Methodology
5.1. Option 1 - Monitoring every packet
The aim of "monitoring every packet" is to ensure that the
information reported is not dependent on the application. In this
scheme, RTP systems will report arrival data for each individual RTP
packet. RTP (or other) systems receiving this "raw" data may use it
to calculate any preferred heuristic metrics, but such calculations
and the reporting of the results (e.g. to a session control layer or
a management layer) are outside the scope of RTP and RTCP.
5.2. Option 2 - Real-time histogram methods
There are several potentially useful metrics which rely on the
accumulation of a histogram in real time, so that a packet arrival
results in a counter being incremented rather than in the creation of
a new data item. These metrics may be gathered with a low and
predictable storage requirement. Each counter corresponds to a
single class interval or "bin" of the histogram. Examples of metrics
which may be accumulated in this way include the observed
distribution of packet delay variation, and the number of packets
lost per unit time interval.
Different networks may have very different expected and achieved
levels of performance, but it may be useful to fix the number of
class intervals in the reported histogram to give a predictable
volume of data. This can be achieved by starting with small class
intervals ("bin widths") and automatically increasing the width (e.g.
by factors of two) if outliers are seen beyond the current upper
limit of the histogram. Data already accumulated may be assigned
unambiguously to the new set of bins, given some simple conditions on
the relationship between the old and new origins and bin widths.
A significant disadvantage of the histogram method is the loss of any
information about time-domain correlations between the samples which
build the histogram. For example, a histogram of packet delay
variation provides no indication of whether successive samples of
packet delay variation were uncorrelated, or alternatively that the
packet delay variation showed a highly-correlated low-frequency
wander.
5.3. Option 3 - Monitoring by exception
An entity which both monitors the packet stream, and has sufficient
knowledge of the application to know when transport impairments may
have degraded the application's performance, may choose to send
exception reports containing details of the transport impairments to
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a receiving system. The crossing of a transport impairment
threshold, or some application-layer event, would trigger such
reports. RTP end systems and mixers are likely to contain
application implementations which may, in principle, identify this
type of exception.
It is likely that RTP translators will not contain suitable
implementations which could identify such exceptions.
On-path devices such as routers and switches are not likely to be
aware of RTP at all. Even if they are aware of RTP, they are
unlikely to be aware of the RTP-level performance required by
specific applications, and hence they are unlikely to be able to
identify the level of impairment at which exceptional transport
conditions may start to affect application performance.
This type of monitoring typically requires the storage of recent data
in a FIFO (e.g. a circular buffer) so that data relevant to the
period just before and just after the exception may be reported. It
is not usually helpful to report transport data only from the period
following an exception event detected by an application. This
imposes some storage requirement (though less than needed for Option
1). It also implies the existence of additional cross-layer
primitives or APIs to trigger the transport layer to generate and
send its exception report. Such a capability might be considered
architecturally undesirable, in that it complicates one or more
interfaces above the RTP layer.
5.4. Option 4 - Application-specific monitoring
This is a business-as-usual option which suggests that the current
approach should not be changed, based on the idea that previous
application-specific approaches such as that of [RFC3611] were valid.
If a large category of RTP applications (such as VoIP) has a
requirement for a unique set of transport metrics, arising from its
different requirements of the transport, then it seems reasonable for
each application category to define its preferred set of metrics to
describe transport impairments. We expect that there will be few
such categories, probably less than 10.
It may be easier to achieve interworking for a well-defined set of
application-specific metrics than it would be in the case that
applications select a profile from a palette of many independent re-
usable metrics.
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6. Issues with RTCP XR extension
Issues that have come up in the past with extensions to RTCP or RTCP
XR include (but are probably not limited to) the following:
o Repeating this information in a number of metrics blocks within
the same RTCP packet may bring inefficiency when multiple small
blocks in the same RTCP XR packet contain measurement data for the
same stream and period.
o RFC 3611 [RFC3611] defines seven report block formats for network
management and quality monitoring. However some of these block
types defined in [RFC3611]are only specifically designed for
conveying multicast inference of network characteristics(MINC) or
voice over IP (VoIP) monitoring.
o Designing a single report block or metric containing a large
number of parameters in different classes for a specific
application may increase implementation cost and minimize
interoperability.
o The RTCP XR block namespace is limited by the 8-bit block type
field in the RTCP XR header Under current allocation pressure, we
expect that the RTCP XR Block Type space will be exhausted soon.
We therefore need a way to extend the block type space, so that
new specifications may continue to be developed.
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7. Guideline for reporting block format using RTCP XR
7.1. Using small blocks
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 SDES packets)
are sufficient. For other applications additional metrics may be
required or at least sufficiently useful to justify the overheads,
both of 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 optimise 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 different such larger blocks are defined, which
becomes a danger to interoperability.
To avoid this pitfall, this memo proposes the use of small RTCP XR
metrics blocks each containing a very small number of individual
metrics characterising 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 "IPDV" defined by [Y1540]. See Section 8 for
architectural considerations for a metrics block, using as an example
a metrics block to report packet delay variation.
7.2. Sharing the identity block
Any measurement must be identified. However if metrics are delivered
in small blocks there is a danger of inefficiency arising from
repeating this information in a number of metrics blocks within the
same RTCP packet, in cases where the same identification information
applies to multiple metrics blocks.
An instance of a metric must be identified using information which is
likely to include most of the following:
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o the node at which it was measured,
o the source of the measured stream (for example, its CNAME),
o the SSRC of the measured stream,
o the sequence number of the first packet of the RTP session,
o the extended sequence numbers of the first packet of the current
measurement interval, and the last packet included in the
measurement,
o the duration of the most recent measurement interval and
o the duration of the interval applicable to cumulative measurements
(which may be the duration of the RTP session to date).
Note that this set of information may overlap with, but is more
extensive than, that in the union of similar information in RTCP RR
packets. However we can not assume that RR information is always
present when XR is sent, since they may have different measurement
intervals. Also the reason for the additional information carried in
the XR is the perceived difficulty of "locating" the *start* of the
RTP session (sequence number of 1st packet, duration of interval
applicable to cumulative measurements) using only RR. However when
an RTCP XR packet containing more than two metrics blocks, reporting
on the same streams from the same source, each metric block should
have the same measurement identify, if each metric block carry the
duplicated data for the measurement identity ,it leads to redundant
information in this design since equivalent information is provided
multiple times, once in *every* identification packet. Though this
ensures immunity to packet loss, the design bring more complexity and
the overhead is not completely trivial.
This section proposes an approach to minimise the inefficiency of
providing this identification information, assuming that an
architecture based on small blocks means that a typical RTCP packet
will contain more than one metrics block needing the same
identification. The choice of identification information to be
provided is discussed in [IDENTITY] (work in progress).
The approach is to define a stand-alone block containing only
identification information, and to tag this identification block with
a number which is unique within the scope of the containing RTCP XR
packet. The "containing RTCP XR packet" is defined here as the RTCP
XR header with PT=XR=207 defined in Section 2 of [RFC3611] and the
associated payload defined by the length field of this RTCP XR
header. The RTCP XR header itself includes the SSRC of the node at
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which all of the contained metrics were measured, hence this SSRC
need not be repeated in the stand-alone identification block. A
single containing RTCP XR packet may contain multiple identification
blocks limited by the range of the tag field. Typically there will
be one identification block per monitored source SSRC, but the use of
more than one identification block for a single monitored source SSRC
within a single containing RTCP XR packet is not ruled out.
There will be zero or more metrics blocks dependent on each
identification block. The dependence of an instance of a metrics
block on an identification block is established by the metrics
block's having the same numeric value of the tag field as its
identification block (in the same containing RTCP XR packet).
Figure 2 below illustrates this principle using as an example an RTCP
XR packet containing four metrics blocks, reporting on streams from
two sources. The measurement identity information is provided in two
blocks with Block Type NMI, and tag values 0 and 1 respectively.
Note: in this example, RTCP XR block type values for four proposed
new block types (work in progress) are given as NMI, NPDV, NBGL and
NDEL. These represent numeric block type codepoints to be allocated
by IANA at the conclusion of the work.
Each of these two identity blocks will specify the SSRC of one of the
monitored streams, as well as information about the span of the
measurement. There are two metrics blocks with tag=0 indicating
their association with the measurement identity block which also has
tag=0. These are the two blocks following the identity block with
tag=0, though this positioning is not mandatory. There are also two
metrics blocks with tag=1 indicating their association with the
measurement identity block which also has tag=1, and these are the
two blocks following the identity block with tag=1.
Note that if metrics blocks associated with an identity block must
always follow the identity block, we could save the tag field and
possibly simplify processing. However depending on ordering of
metric block and identity block may bring inefficiency since you do
not know which block is the last metric block associated with
identity block. Hence it is more desirable to to cross-referencing
with a numeric tag,i.e., using tag field to associated metric block
with identity block.
In the example, the block types of the metrics blocks associated with
tag=0 are BT=NPDV (a PDV metrics block) and BT=NBGL (a burst and gap
loss metrics block). The block types of the metrics blocks
associated with tag=1 are BT=NPDV (a second PDV metrics block) and
BT=NDEL (a delay metrics block). This illustrates that:
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o multiple instances of the same metrics block may occur within a
containing RTCP XR packet, associated with different
identification information, and
o differing measurements may be made, and reported, for the
different streams arriving at an RTP system.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|reserved | PT=XR=207 | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of RTCP XR packet sender |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BT=NMI |0|tag=0| resv | block length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of stream source 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. ...measurement identity information, source 1... .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BT=NPDV |I|tag=0|pdvtyp | block length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. ...PDV information for source 1... .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BT=NBGL |I|tag=0| resv | block length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. ...burst-gap-loss information for source 1... .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BT=NMI |0|tag=1| resv | block length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of stream source 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. ...measurement identity information, source 2... .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BT=NPDV |I|tag=1|pdvtyp | block length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. ...PDV information for source 2... .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BT=NDEL |I|tag=1| resv | block length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. ...delay information for source 2... .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: RTCP XR block with identity blocks
This approach of separating the identification information is more
costly than providing identification in each metrics block if only a
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single metrics block is sent in an RTCP packet, but becomes
beneficial as soon as more than one metrics block shares common
identification.
7.3. Expanding the RTCP XR block namespace
The RTCP XR block namespace is limited by the 8-bit block type field
in the RTCP XR header (Section 3 of [RFC3611]). IESG have noted that
this is potentially restrictive. It would be possible to standardize
an expansion mechanism, probably based on use of a new field near the
start of the variable-length "type-specific block contents" field.
Clearly this could apply only to new block types, so might be
standardized to apply to some subrange of the current 8-bit range,
for example the range 128 through 191 might be used. At time of
writing, block types 12 to 254 are unassigned and 255 is reserved for
future expansion. One potential use is through hierarchical control,
where one or a few codepoints at the top level are given to other
SDOs who may then define a number of metrics distinguished by values
in the (so far hypothetical) new field.
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8. An example of a metric block
This section uses the example of an existing proposed metrics block
to illustrate the application of the principles set out in
Section 7.1.
The example [PDV] (work in progress) 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 "jit" field. There are other PDV metrics which may be
more useful to certain applications. Two such metrics are the IPDV
metric ([Y1540], [RFC3393]) and the MAPDV2 metric [G1020]. Use of
these metrics is consistent with the principle in Section 5 of
[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
architecure using the example of [PDV] (work in progress) 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 which contains all three metrics
o provide a single RTCP block to convey any one of the three
metrics, together with a 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 standardised and
require inclusion in this framework. The first option is extensible
but only by 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, so could be rejected on that basis,
but in any case a single application is quite unlikely to require
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 which may be transported by this block, as
discussed further in [PDV] (work in progress).
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9. Application to RFC 5117 topologies
An RTP system (end system, mixer or translator) which originates,
terminates or forwards RTCP XR blocks is expected to handle RTCP,
including RTCP XR, as specified in [RFC3550] for that class of RTP
systems. 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) which does not use RTCP XR.
This statement applies to the topologies investigated in [RFC5117],
where they use RTP end systems, RTP mixers and RTP translators as
these classes are defined in [RFC3550].
These topologies are specifically Topo-Point-to-Point, Topo-
Multicast, Topo-Translator (both variants, Topo-Trn-Translator and
Topo-Media-Translator, and combinations of the two), and Topo-Mixer.
9.1. Applicability to MCU
The topologies based on systems which do not behave according to
[RFC3550], that is 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 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 unimpaired.
9.2. Application to translators
Section 7.2 of [RFC3550] describes 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 itself so that
it may populate the SSRC in the RTCP XR packet header (although the
translator is not a Synchronisation 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-neighbour 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
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RTP systems in the connection but increases RTCP bandwidth and may in
some cases present a security risk. RTP translators may have
forwarding behaviour based on local policy, which might differ
between different interfaces of the same translator.
For bidirectional unicast, an RTP system may usually detect RTCP XR
from a translator by noting that the sending SSRC is not present in
any RTP media packet. However even for bidirectional unicast there
is a possibility of a source sending RTCP XR before it has sent any
RTP media (leading to transient mis-categorisation of an RTP end
system or RTP mixer as a translator), and for multicast sessions - or
unidirectional/streaming unicast - there is a possibility of a
receive-only end system being permanently mis-categorised as a
translator. Hence it is desirable for a translator that send XR to
have a way to declare itself explicitly.
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10. IANA Considerations
None.
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11. Security Considerations
This document itself contains no normative text and hence should not
give rise to any new security considerations, to be confirmed.
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12. Acknowledgement
Geoff Hunt and Philip Arden wrote the initial draft for this document
and provided useful reviews. Many thanks to them. The authors would
also like to thank Colin Perkins, Graeme Gibbs, Debbie Greenstreet,
Keith Drage,Dan Romascanu, Ali C. Begen, Roni Even for their valuable
comments and suggestions on the early version of this document.
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13. Informative References
[G1020] ITU-T, "ITU-T Rec. G.1020, Performance parameter
definitions for quality of speech and other voiceband
applications utilizing IP networks", July 2006.
[H323] ITU-T, "ITU-T Rec. H.323, Packet-based multimedia
communications systems", June 2006.
[IDENTITY]
Hunt, G., "RTCP XR Report Block for Measurement Identity",
ID draft-ietf-avt-rtcp-xr-meas-identity-02, May 2009.
[PDV] Hunt, G., "RTCP XR Report Block for Packet Delay Variation
Metric Reporting", ID draft-ietf-avt-rtcp-xr-pdv-03,
May 2009.
[RFC1122] Braden, R., "Requirements for Internet Hosts --
Communication Layers", RFC 1122, October 1989.
[RFC3393] Demichelis, C., "IP Packet Delay Variation Metric for IP
Performance Metrics (IPPM)", RFC 3393, November 2002.
[RFC3550] Schulzrinne, H., "RTP: A Transport Protocol for Real-Time
Applications", RFC 3550, July 2003.
[RFC3551] Schulzrinne , H. and S. Casner, "Extended RTP Profile for
Real-time Transport Control Protocol (RTCP)-Based Feedback
(RTP/AVPF)", RFC 3551, July 2003.
[RFC3611] Friedman, T., "RTP Control Protocol Extended Reports (RTCP
XR)", RFC 3611, November 2003.
[RFC4585] Ott, J. and S. Wenger, "Extended RTP Profile for Real-time
Transport Control Protocol (RTCP)-Based Feedback (RTP/
AVPF)", RFC 4585, July 2006.
[RFC5117] Westerlund, M., "RTP Topologies", RFC 5117, January 2008.
[RFC5968] Ott, J. and C. Perkins, "Guidelines for Extending the RTP
Control Protocol (RTCP)", RFC 5968, September 2010.
[Y1540] ITU-T, "ITU-T Rec. Y.1540, IP packet transfer and
availability performance parameters", November 2007.
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Appendix A. Appendix A. Change Log
Note to the RFC-Editor: please remove this section prior to
publication as an RFC.
A.1. draft-ietf-avtcore-monarch-00
The following are the major changes compared to
draft-hunt-avtcore-monarch-02:
o Move Geoff Hunt and Philip Arden from author list to
acknowledgement section.
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Author's Address
Qin Wu (editor)
Huawei
101 Software Avenue, Yuhua District
Nanjing, Jiangsu 210012
China
Email: sunseawq@huawei.com
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