RFC 619 (rfc619) - Page 1 of 14

Mean round-trip times in the ARPANET

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Network Working Group                                          W. Naylor
Request for Comment: 619                                    H. Opderbeck
NIC 21990                                                       UCLA-NMC
                                                           March 7, 1974


                  Mean Round-Trip Times in the ARPANET


In one of our current measurement projects we are interested in the
average values of important network parameters.  For this purpose we
collect data on the network activity over seven consecutive days.  This
data collection is only interrupted by down-time or maintenance of
either the net or our collecting facility (the "late" Sigma-7 or, in
future, the 360/91 at CCN).

The insight gained from the analysis of this data has been reported in
Network Measurement Group Note 18 (NIC 20793):

    L.  Kleinrock and W. Naylor
    "On Measured Behavior of the ARPA Network"

This paper will be presented at the NCC '74 in Chicago.

In this RFC we want to report the mean round-trip times (or delays) that
were observed during these week-long measurements since we think these
figures are of general interest to the ARPA community.  Let us first
define the term "round trip time" as it is used by the statistics
gathering program in the IMPs.  When a message is sent from a source
HOST to a destination HOST, the following events, among others, can be
distinguished (T(i) is the time of event i):

  T(1): The message is passed from the user program to the NCP in the
        source HOST

  T(2): The proper entry is made in the pending packet table (PPT) for
        single packet messages or the pending leader table (PLT) for
        multiple packet messages after the first packet is received by
        the source IMP

  T(3): The first packet of the message is put on the proper output
        queue in the source IMP (at this time the input of the second
        packet is initiated)

  T(4): The message is put on the HOST-output queue in the destination
        IMP (at this time the reassembly of the message is complete)

  T(5): The RFNM is sent from the destination IMP to the source IMP



Naylor & Opderbeck
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