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RFC 29 UNKNOWN ARPANET · TIMING · MEASUREMENTS

Response to RFC 28

A response explaining the IMP's hardware and software clocks.

On rfc-editor.org ↗
PUBLISHED
January 1970
AUTHORS
R.E. Kahn
STREAM
Legacy
HARDWARE CLOCK
16-bit; increments every 100 microseconds
SOFTWARE CLOCK FOR MEASUREMENTS
25.6 ms
DOI
10.17487/RFC0029

RFC text

Original text · RFC Editor View on rfc-editor.org ↗ 
Network Working Group                                   Robert Kahn
RFC-29                                                  BBN
                                                        19 January 1970





This note is in response to Bill English's Request for Comments:  28.

A "millisecond" clock should be satisfactory for most network measurements.
Round-trip message transit times typically should be at least on the
order of tens to hundreds of milliseconds.  The IMP contains a 16-bit
hardware clock which is incremented every 100 microseconds to allow for
timing of internal events within the IMP, as for example, during tracing.
However, most measurements are made using a 25.6 ms. software clock.














DISTRIBUTION:

A. Bhushan, MIT
S. Carr. Utah
G. Cole, SDS
S. Crocker, UCLA
K. Fry, MITRE
J. Heafner, RAND
B. Kahn, BBN
T. O'Sullivan, Raytheon
L. Roberts, ARPA
P. Rovner, LL
R. Stoughton, UCSB

ISO Plain Language

On this page
PROBLEMNetwork measurements need timing, but most do not require the IMP's finest internal timing resolution.
MECHANISMThe note describes a 16-bit hardware clock that increments every 100 microseconds and a software clock of 25.6 milliseconds used for most measurements.
TIMING APPROACHThe response distinguishes internal event timing from the coarser timing used for most network measurements.

§1Timing for Network Measurements

The note responds to Bill English's RFC 28. Robert Kahn says a millisecond clock should be satisfactory for most network measurements because round-trip message transit times are typically tens to hundreds of milliseconds.

Most network measurements do not need extremely fine timing. The response says a millisecond clock should be enough for them.

§1Hardware Clock for Internal Events

The IMP contains a 16-bit hardware clock that increments every 100 microseconds. The note says this clock allows timing of internal IMP events, such as events during tracing.

The IMP's hardware clock updates every 100 microseconds. It can time events inside the IMP, including events during tracing.

§1Software Clock for Most Measurements

Although the hardware clock provides finer timing, the note says most measurements are made using a 25.6 ms software clock. Kahn considers a millisecond-scale clock satisfactory for most measurements.

Most measurements use the slower software clock rather than the hardware clock. The response says this level of timing is adequate for those measurements.