Audio/Video Transport Working Group Q. Wu, Ed.
Internet-Draft Huawei
Intended status: Informational G. Hunt
Expires: March 3, 2012 Unaffiliated
P. Arden
BT
August 31, 2011
Monitoring Architectures for RTP
draft-ietf-avtcore-monarch-04.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
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 3, 2012.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements notation . . . . . . . . . . . . . . . . . . . . 4
3. RTP monitoring architecture . . . . . . . . . . . . . . . . . 5
3.1. RTCP Metric Block Report and associated parameters . . . . 7
4. Issues with reporting metric block using RTCP XR extension . . 9
5. Guideline for reporting block format using RTCP XR . . . . . . 11
5.1. Using small blocks . . . . . . . . . . . . . . . . . . . . 11
5.2. Correlating identity information with the non-RTP data . . 11
6. An example of a metric block . . . . . . . . . . . . . . . . . 13
7. Application to RFC 5117 topologies . . . . . . . . . . . . . . 14
7.1. Applicability to MCU . . . . . . . . . . . . . . . . . . . 14
7.2. Applicability to Translators . . . . . . . . . . . . . . . 14
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
9. Security Considerations . . . . . . . . . . . . . . . . . . . 17
10. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 18
11. Informative References . . . . . . . . . . . . . . . . . . . . 19
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 21
A.1. draft-ietf-avtcore-monarch-00 . . . . . . . . . . . . . . 21
A.2. draft-ietf-avtcore-monarch-01 . . . . . . . . . . . . . . 21
A.3. draft-ietf-avtcore-monarch-02 . . . . . . . . . . . . . . 21
A.4. draft-ietf-avtcore-monarch-03 . . . . . . . . . . . . . . 22
A.5. draft-ietf-avtcore-monarch-04 . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23
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1. Introduction
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.
In addition, the following terms are defined:
Transport level metrics
A set of metrics which characterise the three transport
impairments of packet loss, packet delay, and packet delay
variation. These metrics should be usable by any application
which uses RTP transport.
Application level metrics
Metrics relating to QoE related parameters. These metrics are
measured at the application level and focus on quality of content
rather than network parameters. One example of such metrics is
the Multimedia Quality Metric specified in [MQ].
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.
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3. RTP monitoring architecture
The RTP monitoring architecture comprises the following two key
functional components shown below:
o Monitor
o Metric Block Structure
Monitor is a functional component defined in RFC3550 that acts as a
source of information gathered for monitoring purposes. It may also
collect statistics from multiple source, stores such information
reported by RTCP XR or other RTCP extension appropriately as base
metric or calculates composite metric. According to the definition
of monitor in RFC3550, the end system that source RTP streams, an
intermediate-system that forwards RTP packets to End-devices or a
third party that does not participate RTP session (i.e., the third
party monitor depicted in figure 1) can be envisioned to act as the
Monitor within the RTP monitoring architecture.
The Metric Block exposes real time Application Quality information in
the appropriate report block format to the Monitor within the RTP
monitoring architecture. Both the RTCP or RTCP XR can be extended to
convey such information. The details on transport protocol for
metric block is described in Section 3.1.
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|---------------+
| Management |
+-------------------+ | System |
| RTP Sender | | +----------+ |
| +-----------+ | | | | |
---------------->| Monitor |---------5------->| Monitor | |
| | | | | | | | |
| | +-----------+ | | +----\-----+ |
| |+-----------------+| | | |
| ||Application || --------|-------+
| ||-Streaming video || |
| |---------|-VOIP || 5
| | ||-Video conference|| |
| | ||-Telepresence || +---------------+
| | ||-Ad insertion || | Third Party |
5 | |+-----------------+| | Monitor |
| | +-------------------+ +---------------+
| 1
| | +Intermediate------------+ |-------------- ---- ----+
| | | RTP System Report Block | RTP Receiver >--4-| |
| | | +---------- transported over| +-----------+ | |
| | | | RTCP extension | | Monitor |<-- |
|------------- Monitor |<--------5------|----| |<------|
| | | | Report Block +----/------+ ||
| | +----------+ transported over | ||
| | RTCP XR | |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
1. RTP communication between real time applications.
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2. Application level metrics collection.
3. Transport level metrics collection.
4. End System metrics collection.
5. Reporting Session- metrics transmitted over specified interfaces.
3.1. 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. [RFC4585]
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o RTCP XR is extended to provide feedback on multicast acquisition
statistics information and parameters.[RFC6332]
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. [RFC5104]
o RTCP or RTCP XR is extended to provide feedback on ECN statistics
information. [ECN]
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4. Issues with reporting metric block using RTCP XR extension
Issues that have come up in the past with reporting metric block
using RTCP XR extensions include (but are probably not limited to)
the following:
o Using large block. A single report block or metric is designed to
contain a large number of parameters in different classes for a
specific application. For example, 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. However different applications layered on RTP may
have some monitoring requirements in common, design large block
only for specific applications may increase implementation cost
and minimize interoperability.
o Correlating RTCP XR with the non-RTP data. CNAME [RFC3550] is an
example of existing tool that allows to bind an SSRC that may
change to a fixed source name in one RTP session. It is also
fixed across multiple RTP sessions from the same source. However
there may be situations where RTCP reports are sent to other
participating endpoints using non-RTP protocol in a session. For
example, as described in [RFC6035], the data contained in RTCP XR
VoIP metrics reports [RFC3611] are forwarded to a central
collection server systems using SIP. In such case, there is a
large portfolio of quality parameters that can be associated with
real time application,e.g., VOIP application, but only a minimal
number of parameters are included on the RTCP-XR reports.
Therefore correlation between RTCP XR and non-RTP data should be
concerned if administration or management systems need to rely on
the mapping RTCP statistics to non-RTCP measurements to conducts
data analysis and creates alerts to the users. Without such
correlation, it is hardly to provide accurate measures of real
time application quality with a minimal number of parameters
included on the RTCP-XR reports in such case.
o Identity Information duplication. Identity information is used to
identify an instance of a metric block. The SSRC of the measured
stream as part of the metric block is one example of Identity
information. However in some cases, Identity information may be
not part of metric and include information more than the SSRC in
the metric block, e.g., when we set a metric interval for the
session and monitor RTP packets within one or several consecutive
metric interval, extra identity information (e.g., sequence number
of 1st packet) is expected, if we put such extra identity
information into each metric block, there may be situations where
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an RTCP XR packet containing more than two metric blocks including
the duplicated extra identity information, reports on the same
streams from the same source. each block have the same extra
identity information for measurement, if each metric block carry
such duplicated data for the measurement, it leads to redundant
information in this design since equivalent information is
provided multiple times, once in *every* metric block. Though
this ensures immunity to packet loss, the design may bring more
complexity and the overhead is not completely trivial in some
cases.
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5. Guideline for reporting block format using RTCP XR
5.1. 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 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 "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.
5.2. Correlating identity information with the non-RTP data
When more than one media transport protocols are used by one
application to interconnected to the same session (in gateway),e.g.,
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 their different identities that
are corresponding to different media transport.
This memo proposes an approach to facilitate the correlation of the
RTCP Session with other session-related non-RTP data, i.e., if there
is a need to correlate RTP sessions with non-RTP sessions, then the
correlation information needed should be conveyed in RTCP SDES
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packets since such correlation information describes the source,
rather than providing a quality report. An example use case is for a
participant endpoint may convey a call identifier or a global call
identifier associated with the SSRC of measured RTP stream . In such
case, the participant endpoint uses SSRC of source to bind the call
identifier in each chunk of the SDES RTCP packet and send such
correlation using the chunk containing SDES item to the network
management system. A flow measurement tool that is not call-aware
then forward the RTCP XR reports along with SSRC of the measured RTP
stream which is included in the XR Block header to the network
management system. Network management system can then correlate this
report using SSRC with other diagnostic information such as call
detail records.
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6. 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 5.1.
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
architecture 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 standardized 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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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, and combinations of
the two), and Topo-Mixer. These topologies use RTP end systems, RTP
mixers and RTP translators defined in [RFC3550]. For purposes of
reporting connection quality to other RTP systems, RTP mixers and RTP
end systems are very similar. Mixers resynchronize audio 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) which originates,
terminates or forwards RTCP XR blocks is expected to handle RTCP,
including RTCP XR, according to [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) which does not use RTCP XR. The second category relates
to deployed system models used in many H.323 [H323] video
conferences. The topologies in this category are Topo-Video-Switch-
MCU and Topo-RTCP-terminating-MCU. Such topologies based on systems
do not behave according to [RFC3550].
Considering the translator and MCU are two typical topologies in the
two categories mentioned above, this document will take them as two
typical examples to explain how RTCP XR report works in different
RFC5117 topologies.
7.1. Applicability to MCU
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.
7.2. Applicability to Translators
Section 7.2 of [RFC3550] describes processing of RTCP by translators.
RTCP XR is within the scope of the recommendations of [RFC3550].
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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
collocated with itself so that the monitor may populate the SSRC in
the RTCP XR packet header as packet sender SSRC and send it
out(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
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 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 also a possibility of a receive-only end system
being permanently mis-categorised as a translator sending XR report,
i.e.,the monitor sending XR report within the translator. Hence it
is desirable for a translator that sends XR report to have a way to
declare itself explicitly.
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8. IANA Considerations
None.
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9. 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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10. Acknowledgement
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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11. Informative References
[ECN] Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P.,
and K. Carlberg, "Explicit Congestion Notification (ECN)
for RTP over UDP", ID draft-ietf-avtcore-ecn-for-rtp-04,
July 2011.
[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.
[MQ] Wu, Q., Zorn, G., Schott, R., and K. Lee, "RTCP XR Blocks
for multimedia quality metric reporting",
ID draft-wu-xrblock-rtcp-xr-quality-monitoring-02,
May 2011.
[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.
[RFC5104] Wenger, S., Chandra, U., Westerlund, M., and B. Burman,
"Session Initiation Protocol Event Package for Voice
Quality Reporting", RFC 5104, February 2008.
[RFC5117] Westerlund, M., "RTP Topologies", RFC 5117, January 2008.
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[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.
[RFC6332] Begen, A. and E. Friedrich, "Multicast Acquisition Report
Block Type for RTP Control Protocol (RTCP) Extended
Reports (XRs)", RFC 6332, July 2011.
[Y1540] ITU-T, "ITU-T Rec. Y.1540, IP packet transfer and
availability performance parameters", November 2007.
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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 to acknowledgement section.
A.2. draft-ietf-avtcore-monarch-01
The following are the major changes compared to 00:
o Restructure the document by merging section 4 into section 3.
o Remove section 4.1,section 5 that is out of scope of this
document.
o Remove the last bullet in section 6 and section 7.3 based on
conclusion of last meeting.
o Update figure 1 and related text in section 3 according to the
monitor definition in RFC3550.
o Revise section 9 to address monitor declaration issue.
o Merge the first two bullet in section 6.
o Add one new bullet to discuss metric block association in section
6.
A.3. draft-ietf-avtcore-monarch-02
The following are the major changes compared to 01:
o Deleting first paragraph of Section 1.
o Deleting Section 3.1, since the interaction with the management
application is out of scope of this draft.
o Separeate identity information correlation from section 5.2 as new
section 5.3.
o Remove figure 2 and related text from section 5.2.
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o Editorial changes in the section 4 and the first paragraph of
section 7.
A.4. draft-ietf-avtcore-monarch-03
The following are the major changes compared to 02:
o Update bullet 2 in section 4 to explain the ill-effect of Identity
Information duplication.
o Update bullet 3 in section 4 to explain why Correlating RTCP XR
with the non-RTP data is needed.
o Update section 5.2 to focus on how to reduce the identity
information repetition
o Update section 5.3 to explain how to correlate identity
information with the non-RTP data
A.5. draft-ietf-avtcore-monarch-04
The following are the major changes compared to 03:
o Update section 5.2 to clarify using SDES packet to carry
correlation information.
o Remove section 5.3 since additional identity information goes to
SDES packet and using SSRC to identify each block is standard RTP
feature.
o Swap the last two paragraphs in the section 4 since identity
information duplication can not been 100% avoided.
o Other editorial changes.
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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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