RFC7002
Internet Engineering Task Force (IETF) A. Clark Request for Comments: 7002 Telchemy Category: Standards Track G. Zorn ISSN: 2070-1721 Network Zen
Q. Wu
Huawei
September 2013
RTP Control Protocol (RTCP) Extended Report (XR) Block
for Discard Count Metric Reporting
Abstract
This document defines an RTP Control Protocol (RTCP) Extended Report (XR) block that allows the reporting of a simple discard count metric for use in a range of RTP applications.
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/rfc7002.
Copyright Notice
Copyright (c) 2013 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.
3.2. Definition of Fields in the Discard Count Metrics Block . 5
Appendix A. Metrics Represented Using the Template from RFC 6390 11
Contents
Introduction
Discard Count Report Block
This document defines a new block type to augment those defined in RFC3611 for use in a range of RTP applications. The new block type supports the reporting of the number of packets that are received correctly but are never played out, typically because they arrive too late (buffer underflow) or too early (buffer overflow) to be played out. The metric is applicable both to systems that use packet loss repair techniques (such as forward error correction RFC5109 or retransmission RFC4588) and to those that do not.
This metric is useful for identifying the existence, and characterizing the severity, of packet transport problems that may affect users' perceptions of a service delivered over RTP.
This block may be used in conjunction with RFC7003, which provides additional information on the pattern of discarded packets. However, the metric in RFC7003 may be used independently of the metrics in this block.
When a Discard Count Metrics Block is sent together with a Burst/Gap Discard Metrics Block (defined in RFC7003) to the media sender or RTP-based network management system, the information carried in the Discard Count Metrics Block and the Burst/Gap Discard Metrics Block allows systems receiving the blocks to calculate burst/gap summary statistics (e.g., the gap discard rate).
The metric belongs to the class of transport-related end-system metrics defined in RFC6792.
RTCP and RTCP Extended 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 document defines a new Extended Report block for use with RFC3550 and RFC3611.
Performance Metrics Framework
"Guidelines for Considering New Performance Metric Development" RFC6390 provides guidance on the definition and specification of performance metrics. "Guidelines for Use of the RTP Monitoring Framework" RFC6792 provides guidance for reporting block format using RTCP XR. The metrics block described in this document is in accordance with the guidelines in RFC6390 and RFC6792.
Applicability
This metric is believed to be applicable to a large class of RTP applications that use a de-jitter buffer RFC5481.
Discards due to late or early arriving packets affect user experience. The reporting of discards alerts senders and other receivers to the need to adjust their transmission or reception strategies. The reports allow network managers to diagnose these user experience problems.
The ability to detect duplicate packets can be used by managers to detect network layer or sender behavior, which may indicate network or device issues. Based on the reports, these issues may be addressed prior to any impact on user experience.
Terminology
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.
In addition, the following terms are defined:
Received, Lost, and Discarded
A packet shall be regarded as lost if it fails to arrive within an implementation-specific time window. A packet that arrives within this time window but is either too early or too late to be played out or is thrown away before playout due to packet duplication or redundancy shall be regarded as discarded. A packet shall not be regarded as discarded if it arrives within this time window but is dropped during decoding by some higher layer decoder, e.g., due to a decoding error. A packet shall be classified as one of the following: received (or OK), discarded, or lost. The discard count metric counts only discarded packets. The metric "cumulative number of packets lost" defined in RFC3550 reports a count of packets lost from the media stream (single synchronization source (SSRC) within a single RTP session). Similarly, the metric "number of packets discarded" reports a count of packets discarded from the media stream (single SSRC within a single RTP session) arriving at the receiver. Another metric defined in RFC5725 is available to report on packets that are not recovered by any repair techniques that may be in use.
Discard Count Metrics Block
Metrics in this block report on the number of packets discarded in the stream arriving at the RTP end system. The measurement of these metrics is made at the receiving end of the RTP stream. Instances of this metrics block use the SSRC to refer to the separate auxiliary Measurement Information Block RFC6776, which describes measurement periods in use (see RFC6776, Section 4.2). This metrics block relies on the measurement interval in the Measurement Information Block indicating the span of the report and MUST be sent in the same compound RTCP packet as the Measurement Information Block. If the measurement interval is not received in the same compound RTCP packet as this metrics block, this metrics block MUST be discarded.
Report Block Structure
The structure of the Discard Count Metrics Block is as follows.
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=24 | I |DT | resv | Block Length = 2 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SSRC of Source | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Discard Count | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Report Block Structure
Definition of Fields in the Discard Count Metrics Block
Block Type (BT): 8 bits
A Discard Count Metrics Block is identified by the constant 24.
Interval Metric flag (I): 2 bits
This field indicates whether the reported metric is an Interval, Cumulative, or Sampled metric RFC6792:
I=10: Interval Duration - the reported value applies to the
most recent measurement interval duration between successive
metrics reports.
I=11: Cumulative Duration - the reported value applies to the
accumulation period characteristic of cumulative measurements.
I=01: Sampled Value - the reported value is a sampled
instantaneous value.
In this document, the discard count metric can only be measured over definite intervals and cannot be sampled. Accordingly, the value I=01, indicating a sampled value, MUST NOT be sent, and MUST be discarded when received. In addition, the value I=00 is reserved and also MUST NOT be sent, and MUST be discarded when received.
Discard Type (DT): 2 bits
This field is used to identify the discard type used in this report block. The discard type is defined as follows:
00: Report packet discarded or being thrown away before playout
due to packet duplication.
01: Report packet discarded due to too early to be played out.
10: Report packet discarded due to too late to be played out.
The value DT=11 is reserved for future definition and MUST NOT be sent, and MUST be discarded when received.
An endpoint MAY report any combination of discard types in each reporting interval by including several Discard Count Metrics Blocks in a single RTCP XR packet.
Some systems send duplicate RTP packets for robustness or error resilience. This is NOT RECOMMENDED since it breaks RTCP packet statistics. If duplication is desired for error resilience, the mechanism described in [RTPDUP] can be used, since this will not cause breakage of RTP streams or RTCP statistics.
Reserved (resv): 4 bits
These bits are reserved. They MUST be set to zero by senders and ignored by receivers (see RFC6709, Section 4.2).
Block Length: 16 bits
The length of this report block in 32-bit words, minus one, in accordance with the definition in RFC3611. This field MUST be set to 2 to match the fixed length of the report block. The block MUST be discarded if the block length is set to a different value.
SSRC of Source: 32 bits
As defined in Section 4.1 of RFC3611.
Discard Count
Number of packets discarded over the period (Interval or Cumulative) covered by this report.
The measured value is an unsigned value. If the measured value exceeds 0xFFFFFFFD, the value 0xFFFFFFFE MUST be reported to indicate an over-range measurement. If the measurement is unavailable, the value 0xFFFFFFFF MUST be reported.
Note that the number of packets expected in the period associated with this metric (whether Interval or Cumulative) is available from the difference between a pair of extended sequence numbers in the Measurement Information Block RFC6776, so it need not be repeated in this block.
SDP Signaling
RFC3611 defines the use of the Session Description Protocol (SDP) RFC4566 for signaling the use of XR blocks. However, XR blocks MAY be used without prior signaling (see Section 5 of RFC 3611).
SDP rtcp-xr Attribute Extension
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. The ABNF RFC5234 syntax is as follows.
xr-format =/ xr-pdc-block xr-pdc-block = "pkt-discard-count"
Offer/Answer Usage
When SDP is used in Offer/Answer context, the SDP Offer/Answer usage defined in RFC3611 for unilateral "rtcp-xr" attribute parameters applies. For detailed usage of Offer/Answer for unilateral parameters, refer to Section 5.2 of RFC3611.
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.
New RTCP XR Block Type Value
This document assigns the block type value 24 in the IANA "RTP Control Protocol Extended Reports (RTCP XR) Block Type Registry" to the "Discard Count Metrics Block".
New RTCP XR SDP Parameter
This document also registers a new parameter "pkt-discard-count" in the "RTP Control Protocol Extended Reports (RTCP XR) Session Description Protocol (SDP) Parameters Registry".
Contact Information for Registrations
The following contact information is provided for all registrations in this document:
Qin Wu ([email protected]) 101 Software Avenue, Yuhua District Nanjing, Jiangsu 210012 China
Security Considerations
In some situations, returning very detailed error information (e.g., over-range measurement or measurement unavailable) using this report block can provide an attacker with insight into the security processing. Where this is a concern, the implementation should apply encryption and authentication to this report block. For example, this can be achieved by using the Audio-Visual Profile with Feedback (AVPF) profile together with the Secure RTP profile, as defined in RFC3711; an appropriate combination of those two profiles ("SAVPF") is specified in RFC5124.
Besides this, it is believed that this RTCP XR 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.
Contributors
Geoff Hunt wrote the initial draft of this document.
Acknowledgments
The authors gratefully acknowledge reviews and feedback provided by Bruce Adams, Philip Arden, Amit Arora, Claire Bi, Bob Biskner, Gonzalo Camarillo, Benoit Claise, Kevin Connor, Claus Dahm, Spencer Dawkins, Randy Ethier, Roni Even, Stephen Farrel, Jim Frauenthal, Kevin Gross, Albert Higashi, Tom Hock, Shane Holthaus, Paul Jones, Rajesh Kumar, Keith Lantz, Jonathan Lennox, Mohamed Mostafa, Amy Pendleton, Colin Perkins, Mike Ramalho, Ravi Raviraj, Dan Romascanu, Albrecht Schwarz, Varun Singh, Tom Taylor, Dan Wing, and Hideaki Yamada.
References
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., Caceres, R., and A. Clark, "RTP Control
Protocol Extended Reports (RTCP XR)", RFC 3611, November 2003.
RFC3711 Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
RFC4566 Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
RFC5124 Ott, J. and E. Carrara, "Extended Secure RTP Profile for
Real-time Transport Control Protocol (RTCP)-Based Feedback
(RTP/SAVPF)", RFC 5124, February 2008.
RFC5234 Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
RFC6709 Carpenter, B., Aboba, B., and S. Cheshire, "Design
Considerations for Protocol Extensions", RFC 6709, September 2012.
RFC6776 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, October 2012.
Informative References
RFC4588 Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
Hakenberg, "RTP Retransmission Payload Format", RFC 4588, July 2006.
RFC5109 Li, A., "RTP Payload Format for Generic Forward Error
Correction", RFC 5109, December 2007.
RFC5481 Morton, A. and B. Claise, "Packet Delay Variation
Applicability Statement", RFC 5481, March 2009.
RFC5725 Begen, A., Hsu, D., and M. Lague, "Post-Repair Loss RLE
Report Block Type for RTP Control Protocol (RTCP) Extended
Reports (XRs)", RFC 5725, February 2010.
RFC6390 Clark, A. and B. Claise, "Guidelines for Considering New
Performance Metric Development", BCP 170, RFC 6390, October 2011.
RFC6792 Wu, Q., Hunt, G., and P. Arden, "Guidelines for Use of the
RTP Monitoring Framework", RFC 6792, November 2012.
RFC7003 Clark, A., Huang, R., and Q. Wu, Ed., "RTP Control
Protocol(RTCP) Extended Report (XR) Block for Burst/Gap
Discard Metric Reporting", RFC 7003, September 2013.
[RTPDUP] Begen, A. and C. Perkins, "Duplicating RTP Streams", Work
in Progress, March 2013.
Appendix A. Metrics Represented Using the Template from RFC 6390
a. Number of Packets Discarded Metric
* Metric Name: Number of RTP packets discarded.
* Metric Description: Number of RTP packets discarded over the
period covered by this report.
* Method of Measurement or Calculation: See Section 3.2, Discard
Count definition.
* Units of Measurement: See Section 3.2, Discard Count
definition.
* Measurement Point(s) with Potential Measurement Domain: See
Section 3, 1st paragraph.
* Measurement Timing: See Section 3, 1st paragraph for
measurement timing and Section 3.2 for Interval Metric flag.
* Use and Applications: See Section 1.4.
* Reporting Model: See RFC 3611.
Authors' Addresses
Alan Clark Telchemy Incorporated 2905 Premiere Parkway, Suite 280 Duluth, GA 30097 USA
EMail: [email protected]
Glen Zorn Network Zen 227/358 Thanon Sanphawut Bang Na, Bangkok 10260 Thailand
Phone: +66 (0) 8-1000-4155 EMail: [email protected]
Qin Wu Huawei 101 Software Avenue, Yuhua District Nanjing, Jiangsu 210012 China
EMail: [email protected]
