4.7.3. Delay Metrics

4.7.3. Delay Metrics

For the purpose of the following definitions, the RTP interface is the interface between the RTP instance and the voice application (i.e., FEC, de-interleaving, de-multiplexing, jitter buffer). For example, the time delay due to RTP payload multiplexing would be considered part of the voice application or end-system delay, whereas delay due to multiplexing RTP frames within a UDP frame would be considered part of the RTP reported delay. This distinction is consistent with the use of RTCP for delay measurements.

round trip delay: 16 bits The most recently calculated round trip time between RTP interfaces, expressed in milliseconds. This value MAY be measured using RTCP, the DLRR method defined in Section 4.5 of this document, where it is necessary to convert the units of measurement from NTP timestamp values to milliseconds, or other approaches. If RTCP is used, then the reported delay value is the time of receipt of the most recent RTCP packet from source SSRC, minus the LSR (last SR) time reported in its SR (Sender Report), minus the DLSR (delay since last SR) reported in its SR. A non-zero LSR value is required in order to calculate round trip delay. A value of 0 is permissible; however, this field MUST be populated as soon as a delay estimate is available.

end system delay: 16 bits The most recently estimated end system delay, expressed in milliseconds. End system delay is defined as the sum of the total sample accumulation and encoding delay associated with the sending direction and the jitter buffer, decoding, and playout buffer delay associated with the receiving direction. This delay MAY be estimated or measured. This value SHOULD be provided in all VoIP metrics reports. If an implementation is unable to provide the data, the value 0 MUST be used.

Note that the one way symmetric VoIP segment delay may be calculated from the round trip and end system delays is as follows; if the round trip delay is denoted, RTD and the end system delays associated with the two endpoints are ESD(A) and ESD(B) then:

one way symmetric voice path delay = (RTD + ESD(A) + ESD(B)) / 2