METRODATA NetTESTER FAMILY USER MANUAL
76-02-107 Rev.A
Page 51 of 133
12 IN-SERVICE PERFORMANCE MONITORING
In-Service Performance Monitoring (normally referred to simply as performance
monitoring, or PM) is the process of ensuring that a service continues to meet the
performance requirements (SLA) once it has started to carry live traffic.
Performance monitoring operates by sending and receiving special protocol
messages that can be used to measure the key performance metrics: packet loss,
delay and delay variation (FLR, FTD and IFDV). The protocol packets are small and
are sent at a low rate, so that their bandwidth usage is negligible.
As with service activation testing, units generally operate in pairs, with one at either
end of the service being monitored. Unlike service activation testing, however, either
or both devices may be acting as 'tester' (ie. collecting and reporting performance
data).
The term peer is again used to refer to the other device in the pair. Another
difference from service activation testing is that the unit may have peer relationships
with more than one device and may be collecting PM data from these concurrently.
12.1 TWAMP
Two Way Active Measurement Protocol (TWAMP) is a Layer 3 performance
monitoring tool defined in RFC5357 [10]. Strictly speaking, the unit supports TWAMP
Light, a simplified version described in an appendix to the document.
TWAMP uses UDP/IP to carry protocol packets between a generator/checker unit
(the sender) and a loopback unit (the reflector). A single message type is used to
measure both packet delay/variation (FTD/IFDV) and synthetic packet loss (FLR).
The TWAMP implementation uses the same hardware-based timestamping
mechanism SAM testing, providing high-accuracy delay/variation measurements.
A UDP port number is configured globally. (This can only be changed by using the
Advance Mode user interface.) All TWAMP communication by the unit will use this
port (as both source and destination). All other devices with which TWAMP
communications are to be made must use the same port number. The default is port
900.
Provided the appropriate management interface settings are in place to allow IP
communication with the desired remote unit (peer), all that is required to get each
PM session running is the peer IP address.
The unit automatically acts as a reflector (loopback unit) for any TWAMP messages
it receives from other sender devices (provided they use the correct UDP port). This
is done whether or not the unit is also acting as a sender.
