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.

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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.

[ This RFC was put into machine readable form for entry ]

       [ into the online RFC archives by Alex McKenzie with    ]
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