GloballyUniqueID duplicates

Douglas Clowes dclowes at OZEMAIL.COM.AU
Tue Jun 16 20:50:58 EDT 1998

On reading the text in H.225.0 describing the construction of a globally
unique identifier, it seems to me that duplicates are possible if the clock
has not advanced since the last identifier was generated.

This may occur if the clock ticks at a slow rate (say ten milliseconds), if
the CPU is sufficiently fast, or there is more than one CPU.

In the case of a slow clock, say ten milliseconds, this can be compensated
for with the low order bits of the time field being cleared and used as a
sequence number for identifiers generated in the same clock tick.

In what has been called a "Carrier Class Gateway," with tens of thousands
of SCN ports and more than one CPU, it may be possible to generate a
duplicate identifier, for a CallIdentifier, within the same 100-nanosecond

In this event, a possible action is to increment the clock sequence if the
time "has not advanced" rather than "has gone backward." Thus, if the
current UTC is less that *or equal* to the saved value, or the saved value
was lost, the clock sequence is incremented.

Douglas Clowes
OzEmail Interline

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