NIC NCP experiment

Network Working Group                                    L.P. Deutsch
RFC # 550                                                PARC-MAXC
NIC # 17796                                              August 24, 1973


                           NIC NCP EXPERIMENT


For the past couple of weeks, the NIC NCP has been keeping statistics on
total incoming messages, incoming host-host control opcodes, and size of
outgoing messages. The results have been rather enlightening and, I
think, should be carefully considered by future implementors of NCPs for
servers. The statistics will be presented in a rather qualitative
fashion, since they were reset each time the system came up, but they
represent a total of about 100 hours of uptime, most of it during the
working day.

The total numbers of incoming and outgoing messages were almost
identical. There were about 5% more outgoing. There were slightly over
half as many incoming control opcodes processed as incoming messages; on
the assumption that no incoming control message had more than one
opcode, slightly over half the incoming messages were control messages.

The Opcode statistics were somewhat variable. In all cases the ALL
opcode accounted for the great majority, from a low of about 50% on
weekends to a high of 98% on a busy weekday.  Almost all of the
remainder were NOPs. No other opcode ever accounted for more than 5%.

The output message statistics were taken as log2(message size): this
included 1 word of buffer header, 1 word of IMP header, and l word of
host header. As might be expected, 95% of all outgoing messages had l to
4 PDP-10 words (36-bit) of data. However, if one multiplies the count
for each bucket by the average message site for the bucket, the result
is that only 75% of all outgoing data was in the smallest message size:
the remaining data was spread out fairly evenly between the other
buckets.

I would draw the following conclusions from these statistics. First,
half the messages on the network appear to be ALLs. This suggests that
NCPs should give some thought to processing control messages
efficiently. Second, 95% of the messages are very short. This suggests
that elaborate buffering and queuing schemes are not likely to be
valuable, since the hypothetical gain in efficient use of the IMP is
probably swamped by the overhead within the host. Third, a sufficiently
large fraction of all data is in large messages (presumably file
transfers) that it is also necessary to deal with this situation
efficiently, e.g. a NCP which always sent l-character messages would not
be satisfactory.

The ARPANET has been in vigorous operation for a year or two, and many
NCPs have been written during this time (including a rewrite of the
TENEX NCP, which probably handles more traffic than all other NCPs
combined); to my knowledge, no one has bothered to gather these
statistics before.  The total time invested in putting these
measurements into the NIC system was about half an hour (10
instructions).  I find it regrettable that even those of us presumably
engaged in "computer science" have not found it necessary to confirm our
hypotheses about network operation by experiment an to improve our
theories on the basis of evidence.










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