RTP Control Protocol (RTCP) Extended Report (XR) Block for Burst/Gap Discard Metric Reporting
Extracted elements (21)
For VoIP applications with voice activity detection, burst and gap durations shall be determined as if silence packets had been sent, so that a silence period exceeding Gmin packets terminates a burst condition. This ensures consistent burst/gap accounting regardless of whether silence suppression is active.
The method used to distinguish bursts from gaps shall be equivalent to the method defined in Section 4.7.2 of RFC 3611. The definitions of burst, gap, loss, and discard are consistent with RFC 3611, with discard extended to account for packet duplication per RFC 7002.
This block depends on RFC 7002 (Discard Count Metrics Block) for total packet discard counts and on the Measurement Information Block (RFC 6776) for measurement period context. The block is only applicable to RTP applications with de-jitter buffers that do not use stream repair mechanisms such as FEC (RFC 5109) or retransmission (RFC 4588).
Receivers MUST verify that the Measurement Information Block is received in the same compound RTCP packet as the Burst/Gap Discard Metrics Block. If not, the metrics block MUST be discarded.
Reserved bits in the block MUST be set to zero by senders and ignored by receivers, per RFC 6709 Section 4.2.
Senders MUST NOT use the Interval Metric flag values I=00 or I=01. If a block is received with I=00 or I=01, the receiver MUST discard the block.
Senders MUST send the Burst/Gap Discard Metrics Block in the same compound RTCP packet as the Measurement Information Block (RFC 6776).
The Block Length field MUST be 3 (for this block type); the block MUST be discarded if the block length is set to a different value.
When a 24-bit measured value exceeds 0xFFFFFD, the value 0xFFFFFE MUST be reported to indicate an over-range measurement; when the measurement is unavailable, 0xFFFFFF MUST be reported.
Derived metrics from this block include Burst Discard Rate and Gap Discard Rate (defined in RFC 7004), calculated using the Packets Discarded in Bursts and Total Packets Expected in Bursts fields together with total discard count from RFC 7002.
SDP signaling for this block uses the xr-format value "burst-gap-discard" appended to the rtcp-xr SDP attribute, per RFC 3611. XR blocks MAY be used without prior SDP signaling.
The Burst/Gap Discard Metrics Block (Block Type 20) is an RTCP XR block that reports the number of packets discarded during discard bursts and the total packets expected during bursts, enabling calculation of burst and gap discard rates. It is intended for use in conjunction with RFC 7002 (Discard Count Metrics Block) and the Measurement Information Block (RFC 6776).
This document assigns block type value 20 in the IANA "RTP Control Protocol Extended Reports (RTCP XR) Block Type Registry" to the "Burst/Gap Discard Metrics Block".
This document registers the SDP parameter "burst-gap-discard" in the "RTP Control Protocol Extended Reports (RTCP XR) Session Description Protocol (SDP) Parameters Registry".
This RTCP XR block introduces no new security considerations beyond those described in RFC 3611. Because the block does not provide per-packet statistics, the confidentiality risk documented in RFC 3611 Section 7 paragraph 3 does not apply.
Block Type (BT) is 8 bits and takes the constant value 20 to identify a Burst/Gap Discard Metrics Block.
Packets Discarded in Bursts is a 24-bit unsigned field reporting total packets discarded during discard bursts. If the measured value exceeds 0xFFFFFD, 0xFFFFFE MUST be reported (over-range); if unavailable, 0xFFFFFF MUST be reported.
The Burst/Gap Discard Metrics Block is 4 32-bit words (Block Length = 3). Fields are: BT (8 bits, value 20), I (2 bits), resv (6 bits), Block Length (16 bits), SSRC of Source (32 bits), Threshold (8 bits), Packets Discarded in Bursts (24 bits), Total Packets Expected in Bursts (24 bits), Reserved (8 bits).
The Interval Metric flag (I) is 2 bits: I=10 means Interval Duration, I=11 means Cumulative Duration, I=01 means Sampled Value, and I=00 is reserved. Burst/gap discard metrics can only be measured over definite intervals; sampled values are not valid for this block.
The Threshold field is 8 bits, equivalent to Gmin in RFC 3611: the number of successive packets that must not be discarded before and after a discarded packet for that packet to be considered part of a gap.
Total Packets Expected in Bursts is a 24-bit unsigned field reporting the sum of received and lost packets during discard bursts. If the measured value exceeds 0xFFFFFD, 0xFFFFFE MUST be reported; if unavailable, 0xFFFFFF MUST be reported.