Network Working Group Jerry Cole, UCLA
Request for Comments: 32 February 5, 1970
SOME THOUGHTS ON SRI'S PROPOSED REAL TIME CLOCK
Re: NWG/RFC's #28 and 29.
The addition of a clock in one or more of the network HOST's seems to be
very desireable since it (or they) would allow user-oriented message
delay measurements. Our present network measurement facilities do not
include any internal HOST delays, and these delays may be an appreciable
portion of the total delay encountered by a HOST-to-HOST message
transmission. We may find that an extension of our "Trace" capabilities
to include internal HOST delays would be an appropriate mechanism for
utilizing such a clock. Such usage would require a clock at both the
source and the destination of the message, although such clocks would
not have to be particularly accurate nor synchronized. Other tests,
such as the absolute overall message delay from HOST A to HOST B would
require synchronization of the two clocks.
A reasonable specification for the SRI real-time clock would seem to
include a resolution of about 1 msec., an accuracy of about 1 part in
10E7 (so that two such clocks could maintain reasonable relative
accuracies over periods of many hours), and a range of about 24 hours.
A crystal controlled clock should easily meet these requirements at a
moderate cost.
The choice of the mechanism by which the HOST can read the clock appears
to be of concern also. The 1 msec. resolution may require that the
clock be entirely hardware (as opposed to a core location which would be
incremented at each clock pulse), and therefore the clock may require
some rather compli- cated interface circuitry.
At UCLA, we presently have two clocks on the Sigma 7, and one of these
has a resolution of about 2 msec. which might be usable for some
internal HOST measurements. However, it does not have the long term
accuracy for the absolute measurements mentioned above.
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