RFC 6776 PROPOSED STANDARD RTP · RTCP · MEASUREMENT
RTCP measurement ID: Measurement Identity and Information Reporting Using a Source Description (SDES) Item and an RTCP Extended Report (XR) Block
A way to label which stretch of a call or stream an RTCP quality report is talking about.
RFC EDITOR
Clark, A. and Q. Wu, "Measurement Identity and Information Reporting Using a Source Description (SDES) Item and an RTCP Extended Report (XR) Block", RFC 6776, DOI 10.17487/RFC6776, October 2012, <https://www.rfc-editor.org/info/rfc6776>.
BIBTEX
@misc{rfc6776,
series = {Request for Comments},
number = 6776,
howpublished = {RFC 6776},
publisher = {RFC Editor},
doi = {10.17487/RFC6776},
url = {https://www.rfc-editor.org/info/rfc6776},
author = {A. Clark and Q. Wu},
title = {{Measurement Identity and Information Reporting Using a Source Description (SDES) Item and an RTCP Extended Report (XR) Block}},
pagetotal = 9,
year = 2012,
month = oct,
} RFC text
Contents
Internet Engineering Task Force (IETF) A. Clark
Request for Comments: 6776 Telchemy
Category: Standards Track Q. Wu
ISSN: 2070-1721 Huawei
October 2012
Measurement Identity and Information Reporting Using a
Source Description (SDES) Item and an RTCP Extended Report (XR) Block
Abstract
This document defines an RTP Control Protocol (RTCP) Source
Description (SDES) item and an RTCP Extended Report (XR) block
carrying parameters that identify and describe a measurement period
to which one or more other RTCP XR blocks may refer.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6776.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. RTCP and RTCP XRs . . . . . . . . . . . . . . . . . . . . . 3
1.2. Performance Metrics Framework . . . . . . . . . . . . . . . 3
1.3. Applicability . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Standards Language . . . . . . . . . . . . . . . . . . . . 4
3. Measurement Identity SDES Item . . . . . . . . . . . . . . . . 4
3.1. APSI: Application-Specific Identifier SDES Item . . . . . . 4
4. Measurement Information XR Block . . . . . . . . . . . . . . . 5
4.1. Report Block Structure . . . . . . . . . . . . . . . . . . 5
4.2. Definition of Fields in Measurement Information Report
Block . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
5.1. New RTCP SDES Item Type Value . . . . . . . . . . . . . . . 7
5.2. New RTCP XR Block Type Value . . . . . . . . . . . . . . . 7
5.3. Contact Information for Registrations . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1. Normative References . . . . . . . . . . . . . . . . . . . 8
7.2. Informative References . . . . . . . . . . . . . . . . . . 8
1. Introduction
This document defines one new RTP Control Protocol (RTCP) Source
Description (SDES) [RFC3550] item and one new Extended Report (XR)
block carrying parameters that identify and describe a measurement
period to which one or more other RTCP XR blocks may refer.
The SDES item provides a field for an application-specific auxiliary
identifier. This identifier may be used to correlate data in XR
blocks within an RTP session with data from a non-RTP session.
An RTCP Measurement Identity SDES packet may be associated with a set
of RTCP XR metrics blocks that share the same application-specific
measurement identifier.
The XR block does not contain any measurement results (metrics).
Instead, it provides information relevant to a measurement reported
in one or more other block types, including:
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).
The calculation method of the extended RTP sequence number is
provided in the Real-time Transport Protocol (RTP) [RFC3550].
The RTCP XR block containing the measurement information is intended
to provide a single copy of the information necessary to relate
measurement data in the RTCP XR blocks to the stream and measurement
period to which they refer. Commonly, multiple other small metric
blocks contain measurement data for the same stream and period, and
overhead would be large if all of these metric blocks carried
duplicated data for measurement identification.
The RTCP XR block may be associated with a set of RTCP XR metrics
blocks that share the same information relevant to a reported
measurement. There may be several such sets in an RTCP packet, in
which each set shares the same information relevant to a reported
measurement. There may also be RTCP XR blocks in the packet that are
not associated with a Measurement Information block, for example,
blocks that were defined before the Measurement Identity and
information mechanism were introduced by this document.
1.1. RTCP and RTCP XRs
The use of RTCP for reporting is defined in [RFC3550]. [RFC3611]
defines an extensible structure for reporting by using an RTCP XR.
[RFC3611] also defines the use of XR blocks. This document defines a
new Extended Report block.
1.2. Performance Metrics Framework
The Performance Metrics Framework [RFC6390] provides guidance on the
definition and specification of performance metrics. The RTP
Monitoring Architectures [MONARCH] provides guidelines for reporting
block format using RTCP XR. The SDES item and XR block described in
this document are in accordance with [RFC6390] and [MONARCH].
1.3. Applicability
The RTCP SDES item and the RTCP XR block defined in this document
provide information relevant to the measurements for members of a
family of RTCP XR metrics blocks that are designed to use it. To use
the mechanism defined here, the RTCP XR block containing measurement
information is not required to be in the same RTCP packet as the SDES
item containing measurement identity.
2. Terminology
2.1. Standards Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
3. Measurement Identity SDES Item
This section defines the format of the Measurement Identity SDES
item. The SDES item is carried in the RTCP SDES packet. The packet
format for the RTCP SDES is defined in Section 6.5 of [RFC3550].
Each SDES packet is composed of a header with fixed-length fields for
version, source count, packet type (PT), and length, followed by zero
or more SDES items. In the SDES packet, the PT field is set to SDES
(202).
3.1. APSI: Application-Specific Identifier SDES Item
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| APSI=10 | length |application-specific identifier
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ....
+-+-+-+-+-+-+-+-+
The application-specific identifier is an additional identifier that
is useful in the context of a specific application, e.g., an MPEG-2
transport identifier [MPEG2]. This item MUST be ignored by
applications that are not configured to make use of it. The
identifier is variable length. Its length is described by the length
field. The value of the length field does not include the two-octet
SDES item header.
4. Measurement Information XR Block
4.1. Report Block Structure
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=14 | Reserved | block length = 7 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of stream source |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | first sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| extended first sequence number of interval |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| extended last sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Measurement Duration (Interval) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Measurement Duration (Cumulative) - Seconds (bit 0-31) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Measurement Duration (Cumulative) - Fraction (bit 0-31) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Report Block Structure
4.2. Definition of Fields in Measurement Information Report Block
Block type (BT): 8 bits
A Measurement Information Block is identified by the constant 14.
Reserved: 8 bits
These bits are reserved. They MUST be set to zero by senders and
ignored by receivers.
Block Length: 16 bits
The length of this report block in 32-bit words minus one. For
the Measurement Information Block, the block length is equal to 7.
SSRC of source: 32 bits
As defined in Section 4.1 of [RFC3611].
Reserved: 16 bits
These bits are reserved. They MUST be set to zero by senders and
ignored by receivers.
First sequence number: 16 bits
The RTP sequence number of the first received RTP packet of the
session, used to determine the number of packets contributing to
cumulative measurements.
Extended first sequence number of interval: 32 bits
The extended RTP sequence number of the first received RTP packet
of the current measurement interval. The extended sequence number
is expressed as the low 16-bit value containing the sequence
number received in an RTP data packet and the most significant
16-bit value containing the corresponding count of sequence number
cycles. For additional information on extended sequence numbers,
see the "extended highest sequence number received" definition in
Section 6.4.1 of RFC 3550 and Appendix A.1 of RFC 3550.
Extended last sequence number: 32 bits
The extended RTP sequence number of the last received RTP packet
that contributed to this measurement. The extended sequence
number is expressed as the low 16-bit value containing the
sequence number received in an RTP data packet and the most
significant 16-bit value containing the corresponding count of
sequence number cycles. For additional information on extended
sequence numbers, see the "extended highest sequence number
received" definition in Section 6.4.1 of RFC 3550 and Appendix A.1
of RFC 3550.
Measurement Duration (Interval): 32 bits
The duration, expressed in units of 1/65536 seconds, of the
reporting interval applicable to Interval reports that use this
Measurement Information Block. The value of this field can be
calculated by the receiver of the RTP media stream, for example,
based on received RTP media packets or using the RTCP method
described in [RFC3550].
Measurement Duration (Cumulative): 64 bits
The duration of the reporting interval applicable to Cumulative
reports that use this Measurement Information Block. The value of
this field is represented using a 64-bit NTP-format timestamp as
defined in [RFC5905], which is a 64-bit unsigned fixed-point
number with the integer part in the first 32 bits and the
fractional part in the last 32 bits. It can be calculated by the
receiver of the RTP media stream, for example, based on received
RTP media packets or using the RTCP method described in [RFC3550].
5. IANA Considerations
A new SDES item type for RTCP SDES and a new XR block type for RTCP
XR have been registered with IANA. For general guidelines on IANA
considerations, refer to [RFC3550] for RTCP SDES and [RFC3611] for
RTCP XR.
5.1. New RTCP SDES Item Type Value
This document adds the Measurement Identity SDES item to the IANA
"RTP SDES item types" registry as follows:
abbrev. name value
APSI Application-Specific Identifier 10
5.2. New RTCP XR Block Type Value
This document assigns the block type value 14 in the IANA "RTCP XR
Block Type Registry" to the "Measurement Information Block".
5.3. Contact Information for Registrations
The contact information for the registrations is:
Qin Wu (sunseawq@huawei.com)
101 Software Avenue, Yuhua District
Nanjing, Jiangsu 210012
China
6. Security Considerations
RTCP reports can contain sensitive information, including information
about the nature and duration of a session established between two or
more endpoints. Therefore, the use of security mechanisms with RTP,
as documented in Section 9 of [RFC3550], applies.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC3611] Friedman, T., Ed., Caceres, R., Ed., and A. Clark, Ed.,
"RTP Control Protocol Extended Reports (RTCP XR)",
RFC 3611, November 2003.
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, June 2010.
7.2. Informative References
[MONARCH] Wu, Q., Hunt, G., and P. Arden, "Monitoring Architectures
for RTP", Work in Progress, September 2012.
[MPEG2] ISO/IEC, "Standard 13818-1, Information technology --
Generic coding of moving pictures and associated audio
information: Systems", October 2007.
[RFC6390] Clark, A. and B. Claise, "Guidelines for Considering New
Performance Metric Development", BCP 170, RFC 6390,
October 2011.
Authors' Addresses
Alan Clark
Telchemy Incorporated
2905 Premiere Parkway, Suite 280
Duluth, GA 30097
United States
EMail: alan.d.clark@telchemy.com
Qin Wu
Huawei
101 Software Avenue, Yuhua District
Nanjing, Jiangsu 210012
China
EMail: sunseawq@huawei.com
AI ELI5
On this page
PROBLEMSeveral small RTCP XR metric blocks often describe the same stream and the same measurement period, and repeating the identifying details in each one would waste space.
MECHANISMThe RFC defines one Measurement Information XR block (block type 14) that carries sequence numbers and durations once, plus an APSI SDES item (type 10) for an application-specific identifier.
WHY IT MATTERSOther XR blocks can refer to one shared description of the measurement period instead of each carrying its own copy.
§1What the Document Adds
RFC 6776 defines two small additions to the RTP Control Protocol (RTCP): one new Source Description (SDES) item and one new Extended Report (XR) block. Both carry parameters that identify and describe a measurement period, which one or more other RTCP XR blocks may refer to.
The XR block contains no measurement results. It only provides context for results reported in other block types, such as the sequence number of the first packet of the RTP session, the extended sequence numbers that bound the current measurement interval, the duration of the most recent interval and the duration of the interval used for cumulative measurements.
Imagine many report cards for the same school term. Instead of writing the term dates on every card, you write them once on a cover sheet.
§2Why One Shared Block
Commonly, several small metric blocks hold measurement data for the same stream and period. The RFC points out that the overhead would be large if each of them carried duplicate identification data. The Measurement Information block is meant to provide a single copy of what is needed to relate the measurements to the stream and period they refer to.
A block may be shared by a set of metric blocks, and an RTCP packet may hold several such sets. Blocks defined before this mechanism existed can sit in the same packet without being tied to a Measurement Information block. The XR block does not have to be in the same RTCP packet as the SDES item that carries the measurement identity.
The design follows the Performance Metrics Framework (RFC 6390) and builds on RTCP as defined in RFC 3550 and the XR structure from RFC 3611.
Writing the same dates on every card wastes paper. One cover sheet that every card points to is neater.
§3The APSI SDES Item
The Application-Specific Identifier (APSI) item rides in an RTCP SDES packet, whose PT field is set to SDES (202). The item starts with the type value APSI=10 and a length field, followed by a variable-length identifier. The length does not count the two-octet item header.
The identifier is useful in the context of a specific application. The RFC gives an MPEG-2 transport identifier as an example. It can be used to correlate data in XR blocks within an RTP session with data from a non-RTP session. Applications that are not configured to use the item MUST ignore it.
It is a name tag you can stick on a stream so that a different system can recognise the same thing. Anyone who does not care about the tag just ignores it.
§4The Measurement Information XR Block
The block starts with BT=14, an 8-bit reserved field and a 16-bit block length, which is always 7 (the length in 32-bit words minus one). Next comes the 32-bit SSRC of the stream source, as defined in RFC 3611. The reserved fields MUST be set to zero by senders and ignored by receivers.
The remaining fields are:
- First sequence number (16 bits): the RTP sequence number of the first received packet of the session, used to work out how many packets contribute to cumulative measurements.
- Extended first sequence number of interval (32 bits): the extended sequence number of the first received packet of the current measurement interval.
- Extended last sequence number (32 bits): the extended sequence number of the last received packet that contributed to the measurement.
- Measurement Duration (Interval) (32 bits): the length of the reporting interval, in units of 1/65536 seconds.
- Measurement Duration (Cumulative) (64 bits): the length of the interval for cumulative reports, as a 64-bit NTP-format timestamp with the integer part in the first 32 bits and the fraction in the last 32.
An extended sequence number has the 16-bit sequence number from the RTP packet in the low half and the count of sequence number cycles in the high half. The receiver of the media stream can calculate the durations, for example from received RTP packets or with the RTCP method in RFC 3550.
The block is a short form with boxes for the first and last packet numbers and for how long the timing lasted, once for the latest window and once for the whole call so far.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
BT | BT | 1 byte | 14 | Block type. |
RESERVED-1 | Reserved | 1 byte | Set to zero by senders, ignored by receivers. | |
BLOCK-LENGTH | Block Length | 2 bytes | 7 | Length in 32-bit words minus one. |
SSRC | SSRC of stream source | 4 bytes | The stream the block refers to. | |
RESERVED-2 | Reserved | 2 bytes | Set to zero by senders, ignored by receivers. | |
FIRST-SEQ | First sequence number | 2 bytes | First received RTP packet of the session. | |
EXT-FIRST | Extended first sequence number of interval | 4 bytes | First received packet of the current interval. | |
EXT-LAST | Extended last sequence number | 4 bytes | Last received packet that contributed to the measurement. | |
DUR-INTERVAL | Measurement Duration (Interval) | 4 bytes | Interval length in units of 1/65536 seconds. | |
DUR-CUM-SEC | Measurement Duration (Cumulative), seconds | 4 bytes | Integer part of the cumulative duration. | |
DUR-CUM-FRAC | Measurement Duration (Cumulative), fraction | 4 bytes | Fractional part of the cumulative duration. |
§5IANA Registrations
The document adds the Measurement Identity item to the IANA RTP SDES item types registry as abbreviation APSI, name Application-Specific Identifier, value 10. It also assigns block type value 14 in the IANA RTCP XR Block Type Registry to the Measurement Information Block.
The authors reserved two official numbers, like claiming two locker numbers so nobody else uses them.
§6Security
RTCP reports can contain sensitive information, including the nature and duration of a session between two or more endpoints. For that reason the security mechanisms for RTP described in Section 9 of RFC 3550 apply. The RFC defines no security machinery of its own.
Reports can reveal who talked and for how long, so you should protect them the same way you protect the call itself.
§7References
[RFC3550]RFC 3550Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, July 2003.July 2003[RFC3611]RFC 3611Friedman, T., Ed., Caceres, R., Ed., and A. Clark, Ed., "RTP Control Protocol Extended Reports (RTCP XR)", RFC 3611, November 2003.November 2003[RFC5905]RFC 5905Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch, "Network Time Protocol Version 4: Protocol and Algorithms Specification", RFC 5905, June 2010.June 2010[RFC6390]RFC 6390Clark, A. and B. Claise, "Guidelines for Considering New Performance Metric Development", BCP 170, RFC 6390, October 2011.October 2011