BT's eXtended Network Quality RTP Control Protocol Extended Reports (RTCP XR XNQ)

A set of metrics of packet-transport quality has been defined by BT for pre-standards use in its network. These metrics are known as "XNQ" for "eXtended Network Quality". This document defines an RTCP-XR Report Block using the extension mechanism defined in . The new Report Block provides transport of the XNQ measures from an RTP end system to its peer. The metrics are described in the following text. However, some additional explanation is required for the metrics vmaxdiff, vrange, vsum, and c, which measure aspects of packet delay variation. The metrics are based on the measure known as IP Packet Delay Variation (IPDV) defined in . The IPDV of a packet is the amount by which the packet was delayed in the network, minus the amount a reference packet was delayed in the network. The reference packet is usually the first packet of the connection. IPDV is a signed quantity. The metric vrange is the difference (longest minus shortest) between the longest and shortest network packet delays seen over the duration of the connection to date. The metric vrange is usually a positive quantity, but may be zero if the packet delay is exactly constant over the lifetime of the connection to date. The metric vmaxdiff is found as follows. For each RTCP measurement cycle, find the difference (longest minus shortest) between the longest and shortest network packet delays within that measurement cycle. These differences are usually all positive quantities, but a difference may be zero if the packet delay is exactly constant throughout the measurement cycle. Take the set of these differences and find the maximum, which is vmaxdiff. The metric vmaxdiff is also usually a positive quantity, but will be zero if all the members of the set of per-cycle differences are zero. The metric vsum is simply the sum of the per-RTCP-cycle differences, which were obtained to find vmaxdiff as described above. The metric c is the number of per-RTCP-cycle differences, that is, the cardinality of the set of differences. The two metrics vsum and c allow calculation of vsum/c, the average IPDV per RTCP measurement cycle. The format of the report is as shown in Figure 1.

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=8 | reserved | block length = 8 | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | begin_seq | end_seq | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | vmaxdiff | vrange | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | vsum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | c | jbevents | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | reserved | tdegnet | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | reserved | tdegjit | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | reserved | es | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | reserved | ses | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ The report consists of an RTCP-XR block header and a single 8-word sub-block. block type (BT): 8 bits An XNQ Metrics Report Block is identified by the constant 8. reserved: 8 bits These fields are reserved for future definition. In the absence of such a definition, the bits in these fields MUST be set to zero and MUST be ignored by the receiver. block length: 16 bits Defined in Section 3 of . begin_seq: 16 bits As defined in Section 4.1 of . end_seq: 16 bits As defined in Section 4.1 of . vmaxdiff: 16 bits unsigned Largest IPDV difference seen to date within a single RTCP measurement cycle, measured in RTP timestamp units. If the measured value exceeds 0xFFFE, the value 0xFFFF should be reported to indicate an over-range measurement. vrange: 16 bits unsigned Largest IPDV difference over the lifetime of the RTP flow to date, measured in RTP timestamp units. If the measured value exceeds 0xFFFE, the value 0xFFFF should be reported to indicate an over-range measurement. vsum: 32 bits unsigned Sum of the peak IPDV difference values within each RTCP cycle, summed over RTCP cycles over the lifetime of the RTP flow to date. If the measured value exceeds 0xFFFFFFFE, the value 0xFFFFFFFF should be reported to indicate an over-range measurement. c: 16 bits unsigned Number of RTCP cycles over which vsum was accumulated. If the measured value exceeds 0xFFFE, the value 0xFFFF should be reported to indicate an over-range measurement. jbevents: 16 bits unsigned Cumulative number of jitter buffer adaptation events over the lifetime of the RTP flow to date. If the measured value exceeds 0xFFFE, the value 0xFFFF should be reported to indicate an over-range measurement. tdegnet: 24 bits unsigned The total time in sample periods affected either by packets unavailable due to network loss, or late delivery of packets, since the start of transmission. If the measured value exceeds 0xFFFFFE, the value 0xFFFFFF should be reported to indicate an over-range measurement. tdegjit: 24 bits unsigned The total time in sample periods degraded by jitter buffer adaptation events, e.g., where the jitter buffer either plays out a sample sequence not originating at the transmitter, repeats samples, or chooses not to play out a sample sequence that was sent by the transmitter. If the measured value exceeds 0xFFFFFE, the value 0xFFFFFF should be reported to indicate an over-range measurement. es: 24 bits unsigned cumulative seconds affected by "unavailable packet" events over the lifetime of this ephemeral, to date. If the measured value exceeds 0xFFFFFE, the value 0xFFFFFF should be reported to indicate an over-range measurement. ses: 24 bits unsigned cumulative seconds affected by severe "unavailable packet" events over the lifetime of this ephemeral, to date. If the measured value exceeds 0xFFFFFE, the value 0xFFFFFF should be reported to indicate an over-range measurement.