Measurement Identity and Information Reporting Using a Source Description (SDES) Item and an RTCP Extended Report (XR) Block
Extracted elements (16)
A single Measurement Information XR Block is used to carry identification and duration data shared by multiple metric blocks, avoiding duplication of this overhead across every individual small metric block that reports on the same stream and measurement period.
The RTCP XR block containing measurement information is not required to be in the same RTCP packet as the SDES item containing measurement identity, providing deployment flexibility.
Reserved bits in the Measurement Information XR Block MUST be set to zero by senders and ignored by receivers.
The APSI SDES item MUST be ignored by applications that are not configured to make use of it.
The APSI (Application-Specific Identifier) SDES item (type value 10) carries a variable-length application-specific identifier in an RTCP SDES packet. It enables correlation of XR block data within an RTP session with data from a non-RTP session.
The 'extended first sequence number of interval' field (32 bits) is the extended RTP sequence number of the first received packet of the current measurement interval, with the low 16 bits holding the sequence number and the high 16 bits holding the cycle count.
The 'extended last sequence number' field (32 bits) is the extended RTP sequence number of the last received RTP packet contributing to the measurement, encoded the same way as the extended first sequence number of interval.
The 'first sequence number' field (16 bits) records the RTP sequence number of the first received RTP packet of the session, used to determine the number of packets contributing to cumulative measurements.
The 'Measurement Duration (Cumulative)' field (64 bits) expresses the cumulative reporting duration as a 64-bit NTP-format fixed-point timestamp (integer part in bits 0–31, fractional part in bits 32–63), as defined in RFC 5905.
The 'Measurement Duration (Interval)' field (32 bits) expresses the duration of the reporting interval in units of 1/65536 seconds, applicable to interval reports referencing this block.
The Measurement Information XR Block (BT=14) is a fixed 7-word (32-bit) block that carries measurement period metadata—first sequence number, interval and cumulative sequence number bounds, and measurement durations—to which other XR metric blocks may refer.
This document registers APSI (Application-Specific Identifier) with value 10 in the IANA 'RTP SDES item types' registry.
This document registers the 'Measurement Information Block' with block type value 14 in the IANA 'RTCP XR Block Type Registry'.
RTCP reports can contain sensitive information about the nature and duration of sessions between endpoints. The security mechanisms for RTP defined in Section 9 of RFC 3550 apply to this document.
The APSI SDES item begins with a 1-octet type field (APSI=10), followed by a 1-octet length field (excluding the 2-octet header), followed by the variable-length application-specific identifier value.
The Measurement Information XR Block is 32 bytes (block length = 7, in 32-bit words minus one): BT=14 (8 bits), Reserved (8 bits), block length (16 bits), SSRC of source (32 bits), Reserved (16 bits), first sequence number (16 bits), extended first sequence number of interval (32 bits), extended last sequence number (32 bits), Measurement Duration Interval (32 bits), Measurement Duration Cumulative (64 bits as NTP timestamp).