Audio/Video Transport Working G. Hunt
Group BT
Internet-Draft A. Clark
Intended status: Standards Track Telchemy
Expires: August 29, 2009 February 25, 2009
RTCP XR Report Block for Packet Delay Variation Metric Reporting
draft-ietf-avt-rtcp-xr-pdv-01.txt
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Abstract
This document defines an RTCP XR Report Block that allows the
reporting of Packet Delay Variation metrics for a range of RTP
applications.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Packet Delay Variation Metrics Block . . . . . . . . . . . 3
1.2. RTCP and RTCP XR Reports . . . . . . . . . . . . . . . . . 3
1.3. Performance Metrics Framework . . . . . . . . . . . . . . 3
1.4. Applicability . . . . . . . . . . . . . . . . . . . . . . 3
2. Packet Delay Variation Metrics Block . . . . . . . . . . . . . 4
2.1. Report Block Structure . . . . . . . . . . . . . . . . . . 4
2.2. Definition of Fields in PDV Metrics Block . . . . . . . . 4
2.3. Guidance on use of PDV metrics . . . . . . . . . . . . . . 7
2.4. Examples of use . . . . . . . . . . . . . . . . . . . . . 7
3. SDP Signaling . . . . . . . . . . . . . . . . . . . . . . . . 9
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
4.1. New RTCP XR Block Type value . . . . . . . . . . . . . . . 10
4.2. New RTCP XR SDP Parameter . . . . . . . . . . . . . . . . 10
4.3. Contact information for registrations . . . . . . . . . . 10
4.4. New registry of PDV types . . . . . . . . . . . . . . . . 10
5. Security Considerations . . . . . . . . . . . . . . . . . . . 12
6. Changes from previous version . . . . . . . . . . . . . . . . 13
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
7.1. Normative References . . . . . . . . . . . . . . . . . . . 14
7.2. Informative References . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15
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1. Introduction
1.1. Packet Delay Variation Metrics Block
This draft defines a new block type to augment those defined in
[RFC3611], for use in a range of RTP applications.
The new block type provides information on Packet Delay Variation
using one of several standard metrics.
The metrics belong to the class of transport metrics defined in
[MONARCH] (work in progress).
Instances of this Metrics Block refer by tag to the separate
auxiliary Measurement Identity block [MEASIDENT] which contains
information such as the SSRC of the measured stream, and RTP sequence
numbers and time intervals indicating the span of the report.
1.2. RTCP and RTCP XR Reports
The use of RTCP for reporting is defined in [RFC3550]. [RFC3611]
defined an extensible structure for reporting using an RTCP Extended
Report (XR). This draft defines a new Extended Report block that
MUST be used as defined in [RFC3550] and [RFC3611].
1.3. Performance Metrics Framework
The Performance Metrics Framework [PMOLFRAME] provides guidance on
the definition and specification of performance metrics. Metrics
described in this draft either reference external definitions or
define metrics generally in accordance with the guidelines in
[PMOLFRAME].
1.4. Applicability
These metrics are applicable to a range of RTP applications.
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2. Packet Delay Variation Metrics Block
Metrics in this block report on packet delay variation in the stream
arriving at the RTP system.
2.1. Report Block Structure
PDV metrics block
0 1 2 3
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BT=NPDV |I| tag |pdvtyp | block length=3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pos Threshold/Peak PDV | Pos PDV Percentile |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Neg Threshold/Peak PDV | Neg PDV Percentile |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Mean PDV | unused |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Report Block Structure
2.2. Definition of Fields in PDV Metrics Block
block type (BT): 8 bits
A Packet Delay Variation Metrics Report Block is identified by the
constant NPDV.
[Note to RFC Editor: please replace NPDV with the IANA provided RTCP
XR block type for this block.]
Interval Metric flag (I): 1 bit
This field is used to indicate whether the Packet Delay Variation
metrics block is an Interval or a Cumulative report, that is,
whether the reported values apply to the most recent measurement
interval duration between successive metrics reports (I=1) (the
Interval Duration) or to the accumulation period characteristic of
cumulative measurements (I=0) (the Cumulative Duration).
Numerical values for both these intervals are provided in the
Measurement Identifier block referenced by the tag field below.
Measurement Identifier association (tag): 3 bits
This field is used to identify the Measurement Identifier block
[MEASIDENT] which describes this measurement. The relevant
Measurement Identifier block has the same tag value as the Packet
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Delay Variation Metrics block. Note that there may be more than
one Measurement Identifier block per RTCP packet.
Packet Delay Variation Metric Type (pdvtyp): 4 bits
This field is used to identify the Packet Delay Variation Metric
Type used in this report block, according to the following code:
bits 014-017
0: PPDV according to [RFC3550],
1: MAPDV according to [G.1020],
2: IPDV according to [Y.1540]
Other values as registered by IANA in new registry "RTCP XR PDV
block - PDV type", see Section 4.4.
block length: 16 bits
The length of this report block in 32-bit words, minus one. For
the Packet Delay Variation Metrics block, the block length is
equal to 3.
Positive Threshold/Peak PDV: 16 bit, S11:4 format
The PDV associated with the Positive PDV percentile expressed in
milliseconds. The term Positive is associated with packets
arriving later than the expected time.
If the measured value is more negative than -2047.9375 (the value
which would be coded as 0x8001), the value 0x8000 SHOULD be
reported to indicate an over-range negative measurement. If the
measured value is more positive than +2047.8125 (the value which
would be coded as 0x7FFD), the value 0x7FFE SHOULD be reported to
indicate an over-range positive measurement. If the measurement
is unavailable, the value 0x7FFF SHOULD be reported.
Positive PDV Percentile: 16 bit, 8:8 format
The percentage of packets on the call for which individual packet
delays were less than the Positive Threshold PDV.
If the measurement is unavailable, the value 0xFFFF SHOULD be
reported.
Negative Threshold/Peak PDV: 16 bit, S11:4 format
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The PDV associated with the Negative PDV percentile expressed in
milliseconds. The term Negative is associated with packets
arriving earlier than the expected time.
If the measured value is more negative than -2047.9375 (the value
which would be coded as 0x8001), the value 0x8000 SHOULD be
reported to indicate an over-range negative measurement. If the
measured value is more positive than +2047.8125 (the value which
would be coded as 0x7FFD), the value 0x7FFE SHOULD be reported to
indicate an over-range positive measurement. If the measurement
is unavailable, the value 0x7FFF SHOULD be reported.
Negative PDV Percentile: 16 bit, 8:8 format
The percentage of packets on the call for which individual packet
delays were more than the Negative Threshold PDV.
If the measurement is unavailable, the value 0xFFFF SHOULD be
reported.
If the PDV Type indicated is IPDV and the Positive and Negative PDV
Percentiles are set to 100.0 then the Positive and Negative
Threshold/Peak PDV values are the peak values measured during the
reporting interval (which may be from the start of the call for
cumulative reports). In this case, the difference between the
Positive and Negative Threshold/Peak values defines the range of
IPDV.
Mean PDV: (16 bit, S11:4 format) expressed in milliseconds
For MAPDV this value is generated according to [G.1020]. For
interval reports the MAPDV value is reset at the start of the
interval.
For PPDV the value reported is the value of J(i) calculated
according to [RFC3550] at the time the report is generated.
For IPDV, the value reported is the mean of per-packet IPDV
values. This metric indicates the arrival time of the first media
packet of the session with respect to the mean of the arrival
times of every packet of the session. A single value of the
metric (for a single session) may not be useful by itself, but its
average over a number of sessions may be useful in diagnosing
media delay at session startup. For example, this might occur if
media packets are often delayed behind signalling packets due to
head-of-line blocking.
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If the measured value is more negative than -2047.9375 (the value
which would be coded as 0x8001), the value 0x8000 SHOULD be
reported to indicate an over-range negative measurement. If the
measured value is more positive than +2047.8125 (the value which
would be coded as 0x7FFD), the value 0x7FFE SHOULD be reported to
indicate an over-range positive measurement. If the measurement
is unavailable, the value 0x7FFF SHOULD be reported.
unused: 16 bits
These bits are unused. They SHOULD be set to zero by the sender
and MUST be ignored by the receiver.
2.3. Guidance on use of PDV metrics
This subsection provides informative guidance on when it might be
appropriate to use each of the PDV metric types.
PPDV [RFC3550] allows comparison of results with those from RTP end
systems which support only RTCP as defined in [RFC3550].
MAPDV [G.1020] compares instantaneous (per-packet) delay variation
against a moving average delay variation. This metric could be
useful in determining residual impairment when an RTP end system uses
an adaptive de-jitter buffer which tracks the average delay
variation, provided the MAPDV algorithm and the adaptive de-jitter
buffer have similar averaging behaviour.
IPDV [Y.1540] reports absolute packet delay variation with respect to
the time of arrival of the first packet of the connection. This
metric includes the effect of the frequency offsets of clocks in both
the sender and receiver end systems, so it is useful mainly in
network where synchronisation is distributed. As well as measuring
packet delay variation in such networks, it may be used to ensure
that synchronisation is effective, for example where the network
carries ISDN data traffic over RTP [RFC4040].
2.4. Examples of use
(a) To report PPDV (RFC3550):
Threshold PDV = FFFF (Undefined); PDV Percentile = FFFF (Undefined);
PDV type = 0 (PPDV)
causes PPDV to be reported in the Mean PDV field.
(b) To report MAPDV (G.1020):
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Pos Threshold PDV = 50.0; Pos PDV Percentile = 95.3; Neg Threshold
PDV = 50.0 (note - implies -50ms); Neg PDV Percentile = 98.4; PDV
type = 1 (MAPDV)
causes average MAPDV to be reported in the Mean PDV field.
Note that implementations may either fix the reported percentile and
calculate the associated PDV level OR may fix a threshold PDV level
and calculate the associated percentile. From a practical
implementation perspective it is simpler to use the second of these
approaches (except of course in the extreme case of a 100%
percentile).
IPDV, according to Y.1540 is the difference in delay between the
current packet and the first packet of the stream. If the sending
and receiving clocks are not synchronized, this metric includes the
effect of relative timing drift.
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3. SDP Signaling
[RFC3611] defines the use of SDP (Session Description Protocol)
[RFC4566] for signaling the use of XR blocks. XR blocks MAY be used
without prior signaling.
This section augments the SDP [RFC4566] attribute "rtcp-xr" defined
in [RFC3611] by providing an additional value of "xr-format" to
signal the use of the report block defined in this document.
rtcp-xr-attrib = "a=" "rtcp-xr" ":" [xr-format *(SP xr-format)] CRLF
(defined in [RFC3611])
xr-format = xr-format / xr-pdv-block
xr-pdv-block = "pkt-dly-var" [ "," pdvtype ] [ "," nspec "," pspec ]
pdvtype = "pdv=" 0 ; ppdv RFC 3550
/ 1 ; mapdv ITU-T G.1020
/ 2 ; ipdv ITU-T Y.1540
nspec = "nthr=" fixpoint ; negative threshold PDV (ms)
/ "npc=" fixpoint ; negative PDV percentile
pspec = "pthr=" fixpoint ; positive threshold PDV (ms)
/ "ppc=" fixpoint ; positive PDV percentile
fixpoint = 1*DIGIT "." 1*DIGIT ; fixed point decimal
DIGIT = %x30-39
When SDP is used in offer-answer, a system sending SDP may request a
specific type of PDV measurement. In addition, they may state a
specific percentile or threshold value, and expect to receive the
corresponding threshold or percentile metric, respectively. The
system receiving the SDP SHOULD send the PDV metrics requested, but
if the metric is not available, the system receiving the SDP SHOULD
send the flag value indicating that the metric is unavailable.
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4. IANA Considerations
New block types for RTCP XR are subject to IANA registration. For
general guidelines on IANA considerations for RTCP XR, refer to
[RFC3611].
4.1. New RTCP XR Block Type value
This document assigns the block type value NPDV in the IANA "RTCP XR
Block Type Registry" to the "Packet Delay Variation Metrics Block".
[Note to RFC Editor: please replace NPDV with the IANA provided RTCP
XR block type for this block.]
4.2. New RTCP XR SDP Parameter
This document also registers a new parameter "pkt-dly-var" in the
"RTCP XR SDP Parameters Registry".
4.3. Contact information for registrations
The contact information for the registrations is:
Geoff Hunt (geoff.hunt@bt.com)
Orion 2 PP3, Adastral Park, Martlesham Heath, Ipswich IP5 3RE, United
Kingdom
4.4. New registry of PDV types
This document creates a new registry to be called "RTCP XR PDV block
- PDV type" as a sub-registry of the "RTP Control Protocol Extended
Reports (RTCP XR) Block Type Registry". Policies for this new
registry are as follows:
o The information required to support an assignment is an
unambiguous definition of the new metric, covering the base
measurements and how they are processed to generate the reported
metric. This should include the units of measurement, how values
of the metric are reported in the three 16-bit fields "Pos
Threshold/Peak PDV", "Neg Threshold/Peak PDV" and "Mean PDV"
within the report block, and how the metric uses the two 16-bit
fields "Pos PDV Percentile" and "Neg PDV Percentile".
o The review process for the registry is "Specification Required" as
described in Section 4.1 of [RFC5226].
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o Entries in the registry are integers. The valid range is 0 to 15
corresponding to the 4-bit field "pdvtyp" in the block. Values
are to be recorded in decimal.
o Initial assignments are as follows:
0: PPDV according to [RFC3550],
1: MAPDV according to [G.1020],
2: IPDV according to [Y.1540]
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5. Security Considerations
It is believed that this proposed RTCP XR report block introduces no
new security considerations beyond those described in [RFC3611].
This block does not provide per-packet statistics so the risk to
confidentiality documented in Section 7, paragraph 3 of [RFC3611]
does not apply.
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6. Changes from previous version
Changed IANA considerations section to improve readability and
explicitly to define new registry, addressing Colin Perkins' comment
of 15-Nov-2008.
Added section 2.3 with guidance on usage, addressing Colin Perkins'
comment of 15-Nov-2008.
Changed SDP tag for block to more readable "pkt-dly-var".
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7. References
7.1. Normative References
[G.1020] ITU-T, "ITU-T Rec. G.1020, Performance parameter
definitions for quality of speech and other voiceband
applications utilizing IP networks", November 2003.
[MEASIDENT]
Hunt, G., "RTCP XR Measurement Identifier Block",
ID draft-ietf-avt-rtcp-xr-measid-01, February 2009.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, BCP 14, March 1997.
[RFC3550] Schulzrinne, H., "RTP: A Transport Protocol for Real-Time
Applications", RFC 3550, July 2003.
[RFC3611] Friedman, T., "RTP Control Protocol Extended Reports (RTCP
XR)", RFC 3611, November 2003.
[RFC4040] Kreuter, R., "RTP Payload Format for a 64 kbit/s
Transparent Call", RFC 4040, April 2005.
[RFC4566] Handley, M., "SDP: Session Description Protocol",
RFC 4566, July 2006.
[RFC5226] Narten, T., "Guidelines for Writing an IANA Considerations
Section in RFCs", RFC 5226, BCP 26, May 2008.
[Y.1540] ITU-T, "ITU-T Rec. Y.1540, IP packet transfer and
availability performance parameters", November 2007.
7.2. Informative References
[MONARCH] Hunt, G., "Monitoring Architectures for RTP",
ID draft-hunt-avt-monarch-01, August 2008.
[PMOLFRAME]
Clark, A., "Framework for Performance Metric Development",
ID draft-ietf-pmol-metrics-framework-00, July 2008.
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Authors' Addresses
Geoff Hunt
BT
Orion 2 PP3
Adastral Park
Martlesham Heath
Ipswich, Suffolk IP4 2TH
United Kingdom
Phone: +44 1473 651704
Email: geoff.hunt@bt.com
Alan Clark
Telchemy Incorporated
2905 Premiere Parkway, Suite 280
Duluth, GA 30097
USA
Email: alan.d.clark@telchemy.com
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