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Mean round-trip times in the ARPANET (RFC0619) Disclosure Number: IPCOM000003691D
Original Publication Date: 1974-Mar-01
Included in the Prior Art Database: 2019-Feb-12
Document File: 14 page(s) / 17K

Publishing Venue

Internet Society Requests For Comment (RFCs)

Related People

W. Naylor: AUTHOR [+1]

Related Documents

10.17487/RFC0619: DOI


Actual measurements of round-trip times.

This text was extracted from a PDF file.
This is the abbreviated version, containing approximately 12% of the total text.

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 [Page 1]

RFC 619 Mean Round-Trip Times in the ARPANET March 1974

T(6): The RFNM arrives at the source IMP

T(7): The RFNM is accepted by the source HOST

The time intervals T(i)-T(i-1) are mainly due to the following delays and waiting times:

T(2)-T(1): -HOST processing delay -HOST-IMP transmission delay for the 32-bit leader -Waiting time for a message number to become free (only four messages can simultaneously be transmitted between any pair of source IMP - destination IMP) -Waiting time for a buffer to become free (there must be more than three buffers on the "free buffer list") -HOST-IMP transmission delay for the first packet -Waiting time for an entry in the PPT or PLT to become available (there are eight entries in the PPT and twelve in the PLT table)

T(3)-T(2): -Waiting time for a store-and-forward (S/F) buffer to become free (the maximum number of S/F-buffers is 20). -Waiting time for a logical ACK-channel to become free (there are 8 logical ACK-channels for each physical channel). -For multiple packet messages, waiting time until the ALLOCATE is received (unless an allocation from a previous multiple-...