Alan Clark
Telchemy
Robert Cole
AT&T Labs
Kaynam Hedayat
Brix Networks
Internet Draft
Document: draft-clark-avt-rtcpvoip-01
July 2002
Expires: January 2003
RTCP Extensions for Voice over IP Metric Reporting
Status of this Memo
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Abstract
This document specifies an extension to the Real-time Transport Control
Protocol (RTCP) to support reporting of Voice over IP metrics from end-
points. The proposed extension is useful for supporting both mid-call
and end-of-call reporting of metrics for management and active control
applications.
1. Conventions and Acronyms
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD
NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this document,
are to be interpreted as described in RFC 2119.
2. Introduction
The Real-time Transport Protocol (RTP) [1] provides a real-time
transport mechanism suitable for unicast or Internet Standard multicast
communication between multimedia applications. Typical uses are for
real-time or near real-time group communication via audio and video data
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streams. An important component of the RTP protocol is the control
channel, defined as the Real-time Control Protocol (RTCP). RTCP involves
the periodic transmission of control packets between group members in a
session, enabling the distribution or calculation of session specific
information such as packet loss and round trip time to other hosts, and
group size estimation. An additional advantage of providing a control
channel for a session is that a third-party session monitor can listen
to the traffic to establish network conditions and to diagnose faults
based on receiver locations.
Multimedia services, for example Voice over IP, are very sensitive to
short term variations in network impairments such as packet loss and
jitter. The use of adaptive jitter buffers and other impairment
mitigation techniques can improve performance however make it very
difficult to infer the behavior of the end to end connection from
observations of the packet stream or from RTCP reported statistics.
The extensions to RTCP described in this document provide concise but
useful metrics for Voice over IP calls.
3. Basic Operation
This draft proposes a concise set of metrics that provide sufficient
information to characterize a Voice over IP connection. The set of
metrics includes:
3.1 Packet loss metrics
It has been shown [2] that the distribution of packet loss on the
Internet can be reasonably approximated by a Markov Model. In the case
of a Voice over IP connection it is necessary to consider both packets
lost within the IP network and those discarded by a jitter buffer due
to late arrival, overrun or underrun.
(i) Packet loss rate - a measure of the proportion of packets lost
within the IP network
(ii) Packet discard rate - a measure of the proportion of packets
discarded by the jitter buffer
Assume a Gilbert-Elliott model of the IP "channel" in which a "burst"
is a period of time during which the number of packets received between
two successive lost or discarded packets is less than some minimum Gmin
[3]. This approach works well for Voice over IP as it treats isolated
lost packets as distinct from periods of high packet loss - reflecting
the effectiveness of packet loss concealment.
(iii) Mean burst duration (mS)- the average length of a burst
(iv) Mean burst density - the proportion of packets lost within a burst
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(v) Mean gap duration (mS) - the average time between bursts
(vi) Mean gap density - the proportion of packets lost within a gap
3.2 Delay
Delay is critical in interactive multimedia sessions. In general this
comprises the transmission delay and the end-system delay. Users are
generally not aware of asymmetry in delay and hence it is sufficient to
report round trip delay.
(vii) Round trip delay (mS) - as measured by RTCP
(viii) End-system delay (mS) û round trip delay including jitter buffer
and encoding/ decoding delay
3.3 Analog metrics
Analog metrics may be available in some systems
(ix) Voice signal relative power (dB)
(x) Echo level
(xi) Noise level
(xii) Distortion level
3.4 Voice Quality metrics
Computing voice quality metrics in the end-system has the advantage that
all the essential information is available and that time correlation is
preserved.
(xiii) Voice quality metric - Voice over IP segment (R factor)
(xiv) Voice quality metric - Voice over IP segment (MOS score)
(xv) Voice quality metric - External network (R factor)
4. Extended Report Blocks
4.1 RTCP Report Format
Metrics will be carried in an RTCP extended report block [4].
An extended report block has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BT | type-specific | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: type-specific data :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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block type (BT): 8 bits = TBD
Identifies the specific block format.
type-specific: 8 bits
The use of these bits is defined by the particular block type.
length: 16 bits
The length of this report block in 32-bit words minus one, including
the header.
type-specific data: variable length
This MUST be a multiple of 32 bits long. It MAY be zero bits long.
The encoding of the VoIP performance metrics consists of a six 32 bit
words encoded using the following format.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BT | type-specific | length=6 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Loss Rate | Discard rate | Burst duration |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Burst density | Gap duration | Gap density |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Round trip delay | End system delay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sig power | Echo level | Noise level | Distortion |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| R Factor | R external | MOS-LQ | MOS-CQ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Gmin | Imp Spec 1 | Imp Spec 2 | Imp Spec 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4.2 Packet Loss and Discard Statistics
Packet loss rate
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| loss rate |
+-+-+-+-+-+-+-+-+
The packet loss rate is defined as the fraction of packets detected as
lost at the receiving RTCP instance during the entire RTP session or
VoIP call expressed as a floating point value. This packet loss rate
is calculated with both duplicated packets and packets arriving outside
the jitter buffer window (i.e. discarded packets) excluded. The fraction
is to be calculated by dividing the total number of packets lost by the
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total number of packets expected and expressing the result as a fixed
point number with the binary point at the left hand side of the field
(this is equivalent to multiplying the result of the division by 256
and taking the integer part).
Packet discard rate
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| discard rate |
+-+-+-+-+-+-+-+-+
The packet discard rate is defined as the fraction of packets discarded
due to late or early arrival, under-run or overflow at the receiving
jitter buffer during the entire RTP session or VoIP call expressed as a
floating point number. The fraction is to be calculated by dividing the
total number of packets discarded (excluding duplicate packet discards)
by the total number of packets expected and expressing the result as a
fixed point number with the binary point at the left hand side of the
field (this is equivalent to multiplying the result of the division by
256 and taking the integer part).
Mean burst duration (mS)
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| burst duration |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The mean burst duration is defined as the average period of time spent
in a high loss state during the entire RTP session or VoIP call. A high
loss state is defined as a period of time bounded by loss or discard
events during which the number of consecutive received packets between
lost or discarded packets is less than some maximum value Gmin (default
value 16). This parameter is expressed in milliseconds as these units
are commonly used when describing such events.
Mean burst density
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| burst density |
+-+-+-+-+-+-+-+-+
Burst density is defined as the fraction of packets lost or discarded
whilst in the burst state expressed as a floating point number. The
fraction is to be calculated by dividing the total number of packets
discarded or lost whilst in the burst state (as defined above) by the
total number of packets expected whilst in the burst state and
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expressing the result as a fixed point number with the binary point at
the left hand side of the field (this is equivalent to multiplying the
result of the division by 256 and taking the integer part).
Mean gap duration (mS)
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| gap duration |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The mean gap duration is defined as the average period of time spent in
a low loss state during the entire RTP session or VoIP call. A low loss
state is defined as a period of time bounded by received packets during
which the number of consecutive received packets between lost or
discarded packets is greater than some maximum value Gmin (default value
16). This parameter is expressed in milliseconds as these units are
commonly used when describing such events.
Mean gap density
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| gap density |
+-+-+-+-+-+-+-+-+
Gap density is defined as the fraction of packets lost or discarded
whilst in the gap state expressed as a floating point number. The
fraction is to be calculated by dividing the total number of packets
discarded or lost whilst in the gap state (as defined above) by the
total number of packets expected whilst in the gap state and expressing
the result as a fixed point number with the binary point at the left
hand side of the field (this is equivalent to multiplying the result
of the division by 256 and taking the integer part).
Minimum gap size parameter û Gmin [Default 16]
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| Gmin |
+-+-+-+-+-+-+-+-+
This parameter is used to determine the packet loss density corres-
ponding to a burst. The default value of 16 will result in the
identification and measurement of bursts that have a packet loss
rate of at least 6 percent
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4.3 Delay metrics
Round trip delay (mS)
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTD |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Round trip delay is defined as the two-way delay measured between
RTCP end points expressed in milliseconds. This may, for example, be
calculated from RTCP SR/RR reports.
End-system delay (mS)
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ESD -end system delay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
End system delay is defined as the total encoding, decoding and jitter
buffer delay determined at the reporting end point. This is defined as
the time delay that would result from an arriving RTP frame being
buffered, decoded, converted to "analog" form, being looped back at the
local "analog" interface, encoded and made available for transmission as
an RTP frame.
One way symmetric voice path delay
This may be calculated from the round trip and end system delays as
follows. If the round trip delay is denoted RTD and the end system
delays associated with the two endpoints are ESD(A) and ESD(B) then:-
one way symmetric voice path delay = ( RTD + ESD(A) + ESD(B) ) / 2
4.4 Analog signal metrics
Voice signal relative power (dBm)
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| Sig Power |
+-+-+-+-+-+-+-+-+
The voice signal relative power is defined as the ratio of the peak
signal power to overflow signal power expressed in dB as a signed 8 bit
integer number. A value of +127dB, which would be encoded as binary
1111110, indicates that this parameter is not available.
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Echo level
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| echo level |
+-+-+-+-+-+-+-+-+
The echo level is defined as the ratio of far end echo to transmit level
expressed in dB as a signed 8 bit integer number. A value of +127dB,
which would be encoded as binary 1111110, indicates that this parameter
is not available.
Noise level
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| Noise level |
+-+-+-+-+-+-+-+-+
The noise level is defined as the ratio of the silent period background
noise level to overflow signal power expressed in dB as a signed 8 bit
integer number. A value of +127dB, which would be encoded as binary
1111110, indicates that this parameter is not available.
Distortion level
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| distortion |
+-+-+-+-+-+-+-+-+
The distortion level is defined as the mean ratio of the signal
distortion power to signal power expressed in dB as a signed 8 bit
integer number. A value of +127dB, which would be encoded as binary
1111110, indicates that this parameter is not available.
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4.5 Voice quality metrics
Voice quality metric - Voice over IP segment R Factor
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| R Factor |
+-+-+-+-+-+-+-+-+
Voice quality metric expressed as an R +Factor. An R Factor is an
integer number in the range 0 to 100 with a value of 94 corresponding
to "toll quality" and values of 50 or less being regarded as unusable.
This metric is defined as including the effects of delay, consistent
with ITU G.107 and ETSI TS101329-5. A value of +127, which would be
encoded as binary 1111110, indicates that this parameter is not
available.
Voice quality metric - External network R Factor
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| R(ext) Factor |
+-+-+-+-+-+-+-+-+
Voice quality metric related to an external network segment, for example
a cellular network. This metric is defined as including the effects of
delay, consistent with ITU G.107 and ETSI TS101329-5. A value of +127,
which would be encoded as binary 1111110, indicates that this parameter
is not available.
An overall R factor may be estimated from the Voice over IP segment R
Factor and the External Network R Factor.
R total (estimated) = R + R(ext) û 94
Voice quality metric - Voice over IP segment Listening Quality MOS
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| MOS-LQ |
+-+-+-+-+-+-+-+-+
Voice quality metric expressed as an estimated Mean Opinion Score (MOS).
This metric is defined as not including the effects of delay and can be
compared to MOS scores obtained from listening quality (ACR) tests. This
is a score ranging from 1 to 5 in which 5 represents excellent and 1
represents unacceptable. The metric is represented as MOS x 10, for
example a value of 35 would correspond to an estimated MOS score of 3.5.
A value of +127, which would be encoded as binary 1111110, indicates
that this parameter is not available.
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Voice quality metric - Voice over IP segment Conversational MOS
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| MOS-CQ |
+-+-+-+-+-+-+-+-+
Voice quality metric expressed as an estimated Mean Opinion Score (MOS).
This metric is defined as including the effects of delay and other
effects that would affect conversational quality. The metric may be
calculated by converting an R Factor determined according to G.107 or
TS 101 329-5 into an estimated MOS using the equation specified in G.107
The metric is represented as MOS x 10, for example a value of 35 would
correspond to an estimated MOS score of 3.5. A value of +127, which
would be encoded as binary 1111110, indicates that this parameter is
not available.
4.5 Type specific data
Implementation specific octets
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| Imp - 1 |
+-+-+-+-+-+-+-+-+
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| Imp - 2 |
+-+-+-+-+-+-+-+-+
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| Imp - 3 |
+-+-+-+-+-+-+-+-+
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5. Discussion and Applications
6. Intellectual Property
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
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7. References
[1] H. Schulzrinne, S. Casner, R. Frederick and V. Jacobson, "RTP:
A transport protocol for real-time applications", RFC 1889, IETF,
February 1996
[2] J. Bolot, S. Fosse-Parisis, D. Towsley "Adaptive FEC-Based Error
Control for Interactive Audio in the Internet", IEEE Infocom 1999
[3] QoS Measurement for Voice over IP, ETSI TIPHON TS 101 329-5,
December 2000
[4] T. Freidman, R. Caceres, K. Almeroth, K. Sarac, "RTCP Reporting
Extensions", draft-friedman-avt-rtcp-report-extns-02.txt
8 Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
This document and translations of it may be copied and furnished to
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or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
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9. Authors' Addresses
Alan Clark
Telchemy Incorporated
3360 Martins Farm Road, Suite 200
Suwanee, GA 30024
Tel: +1-770-614-6944
Fax: +1-770-614-3951
Email: alan@telchemy.com
Robert Cole
AT&T Labs
330 Saint Johns Street,
2nd Floor
Havre de Grace, MD, USA 21078
Tel: +1 410 939-8732
Fax: +1 410 939-8732
E-mail: rgcole@att.com
Kaynam Hedayat
Brix Networks
285 Mill Road
Chelmsford, MA 01824
Tel: +1-978-367-5600
Fax: +1-978-367-5700
Email: khedayat@brixnet.com
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Annex A Example Algorithm for Determining Burst and Gap Metrics
The channel is assumed to have high packet loss (burst) and low packet
loss (gap) conditions. During the Voice over IP call packet loss
events and inter-loss gaps are counted. During the call or the end of
the call the transition probabilities of the Markov model may be
determined and used to compute a voice quality metric for the call.
A gap is defined with respect to a parameter Gmin which represents the
criteria that at least Gmin successive packets must be received between
two lost packets in order for the channel to be in the gap state/
condition. If the number of packets received between two successive
lost packets is less than the minimum value Gmin then the sequence of
the two lost packets and the intervening received packets are regarded
as part of a burst.
The Markov model is defined as having the following states and
associated transitions:
State 1 - gap condition û packet not lost
p11 - packet received
p13 - packet loss (start of burst)
p14 - isolated packet loss
State 2 - burst condition û packet not lost
p22 - packet received within burst
p23 - packet lost within burst
State 3 - burst condition - packet lost
p31 - packet received (end of burst)
p32 - packet received within burst
p33 - packet lost
State 4 - gap condition û isolated packet lost
p41 - packet received
This model can be constructed either by accumulating packet loss
information during fixed sampling intervals or at packet loss events.
An example of a computationally efficient method for determining the
parameters of the Markov model is given below.
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Assume a counter 'pkt' tracks the number of consecutively received
packets since the last packet loss event, 'lost' tracks the number of
lost packets in a burst, 'gmin' is the minimum gap size, and that an
event can be generated if a packet loss is detected:
Initial condition:
lost = 0
c11, c13, c14, c22, c23, c33 = 0
Packet loss event (pkt)->
if pkt >= gmin then
if lost = 1 then
c14 = c14 + 1
else
c13 = c13 + 1
lost = 1
c11 = c11 + pkt
else
lost = lost + 1
if pkt = 0 then
c33 = c33 + 1
else
c23 = c23 + 1
c22 = c22 + pkt û 1
pkt = 0
The series of counters c11 to c14 are used to determine the corres-
ponding Markov model transition probabilities (i.e c11 is used to
calculate p11). Counter c5 is used to measure the delay since the
last "significant" burst of lost packets. Parameter gmin, the minimum
gap size, is typically 16.
The key metrics needed for determining application performance are:-
c31 = c13
c32 = c23
c11 = c11 + c14 (for simplicity - combine states 4 and 1)
p11 = c11 / (c11 + c13)
p13 = 1 - p11
p31 = c31 / (c31 + c32 + c33)
p32 = c32 / (c31 + c32 + c33)
p33 = 1 - p31 - p32
p22 = c22 / (c22 + c23)
p23 = 1 - p22
d = (p23 p31 + p13 p32 + p13 p23)
p1 = p31 p23 / d
p2 = p13 p32 / d
p3 = p13 p23 / d
frame size F = frame size (in seconds)
average packet loss rate L = 100 p3 percent
gap length g = F / (1 - p11) seconds
gap loss density Dg= 100 c14 / c11 percent
burst length b = F (1 - p1) / (p1 p13) seconds
burst loss density Db = 100 p23 / (p23+p32) percent
delay since last burst y = F c5 seconds
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