Monitoring QoS for Real-Time Communications
The concern with ensuring adequate voice quality within domains has led
telephony-minded network operators to deploy probes that monitor packet
loss, delay, and even voice-packet-like performance in various locations in the
network. This is actually monitoring network performance for one specific
application: telephony. Such equipment is sometimes deployed in addition to
the traffic-monitoring capabilities of most types of IP routers.
The similar and fractal nature of Internet traffic makes it, however, unlikely
that observations from a limited number of probes distributed in a network
domain can accurately describe the voice quality perceived by any specific
user, at a specific time for a specific endpoint. Recent IETF work that has had
the benefit of many inputs from the industry and several prestandard imple-
mentations has been aimed at standardizing a better approach:
■■ To obtain quality reports that are closest to the actual user experience,
the only logical placement is in the SIP UA application.
■■ The extended RTCP reports (RTCP-XR) are used to report the burst
error packet loss for one of for all endpoints in the session.
■■ If the QoS falls below a predetermined threshold, a real-time alarm can
be provided for the network administrator.
■■ If no QoS threshold alarms occur, the QoS data can be stored in the end-
point(s) and periodically reported to a third party, such as a server in
the network.
■■ Fat network pipes in the core with lots of aggregated flows are the only
place in the network where probing is meaningful. Here, the similar
nature of the traffic fades away and the utilization of probes provides a
useful measurement of quality. However, this measurement of quality is
mostly an indicator for required capacity increase or rerouting.
The techniques to support this procedure are described in detail in the IETF
standards track document on the SIP event package for reporting RTCP-XR
events [18]. Note the approach here is not based on network management
techniques but on monitoring the quality metrics for applications using SIP
events. This technique is also applicable for monitoring other real-time appli-
cations running in SIP endpoints. The event package can be used either with
the SUBSCRIBE/NOTIFYmethods or the PUBLISHmethod using the Voice
Quality Syntax expressed in BNF. The most recent software implementations
have shown the image to be small enough to fit in SIP phones or PC-based
SIP UAs.314 Chapter 18