RFC32 - Some Thoughts on SRIs Proposed Real Time Clock

　　 
　　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 aclearcase/" target="_blank" >ccurate 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.

[ This RFCwas put into machine readable form for entry ]
[ into the online RFCarchives by Richard Ames 1/98 ]

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