RTCP XR Report Block for Loss Concealment metric Reporting

This draft defines a new block type to augment those defined in , for use in a range of RTP applications. At any instant, the audio output at a receiver may be classified as either 'normal' or 'concealed'. 'Normal' refers to playout of audio payload received from the remote end, and also includes locally generated signals such as announcements, tones and comfort noise. Concealment refers to playout of locally-generated signals used to mask the impact of network impairments or to reduce the audibility of jitter buffer adaptations. The new block type provides metrics for actions taken by the receiver to mitigate the effect of packet loss and packet discard. Specifically, the first metric (On-Time Playout Duration) reports the duration of normal playout of data which the receiver obtained from the sender's stream. A second metric (Loss Concealment Duration) reports the total time during which the receiver played out media data which was manufactured locally, because the sender's data for these periods was not available due to packet loss or discard. A similar metric (Buffer Adjustment Concealment Duration) reports the duration of playout of locally-manufactured data replacing data which is unavailable due to adaptation of an adaptive de-jitter buffer. Further metrics (Playout Interrupt Count and Mean Playout Interrupt Size) report the number of times normal playout was interrupted, and the mean duration of these interruptions. Loss Concealment Duration and Buffer Adjustment Concealment Duration are reported separately because buffer adjustment is typically arranged to occur in silence periods so may have very little impact on user experience, whilst loss concealment may occur at any time. The metric belongs to the class of transport-related terminal metrics defined in [MONARCH] (work in progress). Instances of this Metrics Block refer by tag to the separate auxiliary Measurement Identity block which contains information such as the SSRC of the measured stream, and RTP sequence numbers and time intervals indicating the span of the report.

The use of RTCP for reporting is defined in . 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 and .

The Performance Metrics Framework 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 .

This metric is primarily applicable to audio applications of RTP. EDITOR'S NOTE: are there metrics for concealment of transport errors for video?

Loss Concealment 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=NLC |I| tag |plc|rsv| block length=4 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | On-time Playout Duration | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Loss Concealment Duration | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Buffer Adjustment Concealment Duration | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Playout Interrupt Count | Mean Playout Interrupt Size | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

block type (BT): 8 bits A Loss Concealment Metrics Report Block is identified by the constant NLC. [Note to RFC Editor: please replace NLC 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 Loss Concealment metric 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 which describes this measurement. The relevant Measurement Identifier block has the same tag value as the Loss Concealment Metrics block. Note that there may be more than one Measurement Identifier block per RTCP packet. Packet Loss Concealment Method (plc): 2 bits This field is used to identify the packet loss concealment method in use at the receiver, according to the following code:

bits 014-015 0 = silence insertion 1 = simple replay, no attenuation 2 = simple replay, with attenuation 3 = enhanced Reserved (rsv): 2 bits These bits are reserved. They SHOULD be set to zero by senders and MUST be ignored by receivers. block length: 16 bits The length of this report block in 32-bit words, minus one. For the Loss Concealment Metrics block, the block length is equal to 4. On-time Playout Duration (ms): 32 bits 'On-time' playout is the uninterrupted, in-sequence playout of valid decoded audio information originating from the remote endpoint. This includes comfort noise during periods of remote talker silence, if VAD is used, and locally generated or regenerated tones and announcements. An equivalent definition is that on-time playout is playout of any signal other than those used for concealment. On-time playout duration MUST include both speech and silence intervals, whether VAD is used or not. This duration is reported in millisecond units. If the measured value exceeds 0xFFFFFFFD, the value 0xFFFFFFFE SHOULD be reported to indicate an over-range measurement. If the measurement is unavailable, the value 0xFFFFFFFF SHOULD be reported. Loss Concealment Duration (ms): 32 bits The duration, in milliseconds, of audio playout corresponding to Loss-type concealment. Loss-type concealment is reactive insertion or deletion of samples in the audio playout stream due to effective frame loss at the audio decoder. "Effective frame loss" is the event in which a frame of coded audio is simply not present at the audio decoder when required. In this case, substitute audio samples are generally formed, at the decoder or elsewhere, to reduce audible impairment. If the measured value exceeds 0xFFFFFFFD, the value 0xFFFFFFFE SHOULD be reported to indicate an over-range measurement. If the measurement is unavailable, the value 0xFFFFFFFF SHOULD be reported. Buffer Adjustment Concealment Duration (ms): 32 bits The duration, in milliseconds, of audio playout corresponding to Buffer Adjustment-type concealment, if known. If the measured value exceeds 0xFFFFFFFD, the value 0xFFFFFFFE SHOULD be reported to indicate an over-range measurement. If the measurement is unavailable, the value 0xFFFFFFFF SHOULD be reported. Buffer Adjustment-type concealment is proactive or controlled insertion or deletion of samples in the audio playout stream due to jitter buffer adaptation, re-sizing or re-centering decisions within the endpoint. Because this insertion is controlled, rather than occurring randomly in response to losses, it is typically less audible than loss-type concealment. For example, jitter buffer adaptation events may be constrained to occur during periods of talker silence, in which case only silence duration is affected, or sophisticated time-stretching methods for insertion/deletion during favorable periods in active speech may be employed. Concealment events which cannot be classified as Buffer Adjustment- type MUST be classified as Loss-type. Playout Interrupt Count: 16 bits The number of interruptions to normal playout which occurred during the reporting period. If the measured value exceeds 0xFFFD, the value 0xFFFE SHOULD be reported to indicate an over-range measurement. If the measurement is unavailable, the value 0xFFFF SHOULD be reported. Mean Playout Interrupt Size (ms): 16 bits The mean duration, in ms, of interruptions to normal playout which occurred during the reporting period. If the measured value exceeds 0xFFFD, the value 0xFFFE SHOULD be reported to indicate an over-range measurement. If the measurement is unavailable, the value 0xFFFF SHOULD be reported.

defines the use of SDP (Session Description Protocol) for signaling the use of XR blocks. XR blocks MAY be used without prior signaling. This section augments the SDP attribute "rtcp-xr" defined in 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 ) xr-format = xr-format / xr-conceal-block xr-conceal-block = "loss-conceal"

New block types for RTCP XR are subject to IANA registration. For general guidelines on IANA considerations for RTCP XR, refer to [RFC3611].

This document assigns the block type value NLC in the IANA "RTCP XR Block Type Registry" to the "Loss Concealment Metrics Block". [Note to RFC Editor: please replace NLC with the IANA provided RTCP XR block type for this block.]

This document also registers a new parameter "loss-conceal" in the "RTCP XR SDP Parameters Registry".

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

It is believed that this proposed RTCP XR report block introduces no new security considerations beyond those described in . This block does not provide per-packet statistics so the risk to confidentiality documented in Section 7, paragraph 3 of does not apply.

The authors gratefully acknowledge the comments and contributions made by Bruce Adams, Philip Arden, Amit Arora, Bob Biskner, Kevin Connor, Claus Dahm, Randy Ethier, Roni Even, Jim Frauenthal, Albert Higashi, Tom Hock, Shane Holthaus, Paul Jones, Rajesh Kumar, Keith Lantz, Mohamed Mostafa, Amy Pendleton, Colin Perkins, Mike Ramalho, Ravi Raviraj, Albrecht Schwarz, Tom Taylor, and Hideaki Yamada.

Expanded and clarified IANA Considerations section Changed SDP tag for block to "loss-conceal"