As per RFC3550 p.11:
An implementation is not required to run the Network Time Protocol in
order to use RTP. Other time sources, or none at all, may be used
(see the description of the NTP timestamp field in Section 6.4.1).
However, running NTP may be useful for synchronizing streams
transmitted from separate hosts.
p.36:
NTP timestamp: 64 bits
Indicates the wallclock time (see Section 4) when this report was
sent so that it may be used in combination with timestamps
returned in reception reports from other receivers to measure
round-trip propagation to those receivers. Receivers should
expect that the measurement accuracy of the timestamp may be
limited to far less than the resolution of the NTP timestamp. The
measurement uncertainty of the timestamp is not indicated as it
may not be known. On a system that has no notion of wallclock
time but does have some system-specific clock such as "system
uptime", a sender MAY use that clock as a reference to calculate
relative NTP timestamps. It is important to choose a commonly
used clock so that if separate implementations are used to produce
the individual streams of a multimedia session, all
implementations will use the same clock. Until the year 2036,
relative and absolute timestamps will differ in the high bit so
(invalid) comparisons will show a large difference; by then one
hopes relative timestamps will no longer be needed. A sender that
has no notion of wallclock or elapsed time MAY set the NTP
timestamp to zero.
So the RFC3550 does not prescribe compulsory usage of NTP time for RTP purposes.
In the Avaya G450 branch gateways these time stamps are arbitrary so they do not use a particluar time base and are only used to calculate RTT so that packet loss and jitter can be determined. These time stamps are based on the SSRC (Synchronization Source identifier) of the RTP packet .
