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
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.
Copyright (c) 2013 IETF Trust and the persons identified as the document authors. All rights reserved.
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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.
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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].
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].
"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].
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.
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].
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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.
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.
3.2. 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.
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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.
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.
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.
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].
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.
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.