Cross Country Network Bandwidth
A packet-count estimate for a cross-country key press and its echoed character.
Deutsch, L.P., "Cross Country Network Bandwidth", RFC 567, DOI 10.17487/RFC0567, September 1973, <https://www.rfc-editor.org/info/rfc567>.
@misc{rfc567,
series = {Request for Comments},
number = 567,
howpublished = {RFC 567},
publisher = {RFC Editor},
doi = {10.17487/RFC0567},
url = {https://www.rfc-editor.org/info/rfc567},
author = {L.P. Deutsch},
title = {{Cross Country Network Bandwidth}},
pagetotal = 1,
year = 1973,
month = sep,
} RFC text
Network Working Group L. Peter Deutsch (PARC-MAXC)
Request for Comments: 567 September 6, 1973
NIC #18970
CROSS-COUNTRY NETWORK BANDWIDTH
The following computation of cross-country network bandwidth was
contributed by Butler Lampson of PARC.
Consider what happens when a TIP user on the West Coast, connected to a
full-duplex Host on the East Coast, strikes a key on his terminal.
The TIP sends a one-character message (1 packet).
The destination IMP sends a RFNM (1 packet).
The destination Host sends an ALLocate - this seems to be the strategy
used by TENEX Hosts, at least (1 packet).
Thc TIP sends a RFNM for the ALLocate (1 packet).
The same sequence repeats itself, with roles interchanged, for the echo
character (4 packets).
This constitutes 4 packets or 4OOO bits in each direction. The current
cross-country transmission capability of the ARPANET is 3 5OKb phone
lines; ergo, it can only support 3*50000/4000=37.5 such characters per
second!
It may be that RFNMs are transmitted between IMPs more efficiently; at
best this can only double the network capacity.
This computation may help explain why cross-country TIP users (e.g. the
substantial West Coast community of BBN-TENEX users) experience such
bad echo response, at least in bursts: the network itself may be
experiencing momentary peak loads.
If this argument is correct, the proposed remote echoing facilities of
the new TELNET protocol could have a major effect on network operation.
ISO Plain Language
On this page
§1Messages Triggered by a Keystroke
The note presents a calculation contributed by Butler Lampson. It considers a West Coast user at a Terminal Interface Processor (TIP) connected to an East Coast host.
For a key press, the TIP sends a one-character message, the destination IMP sends a Request for Next Message (RFNM), the destination host sends an allocation message, and the TIP sends an RFNM for that allocation. The same sequence repeats with the roles reversed for the echoed character.
A key press triggers several network messages. The character's echo triggers another exchange in the opposite direction.
§2The Bandwidth Estimate
The authors count the exchange as 4 packets or 4000 bits in each direction. They state that cross-country ARPANET capacity is provided by 3 50Kb phone lines and calculate a capacity of 37.5 such characters per second.
The note says that sending RFNMs between IMPs more efficiently could at best double that capacity.
The estimate compares the bits used by one key-and-echo exchange with the capacity of the cross-country lines. Under those assumptions, the network can handle 37.5 such characters each second.
§3Possible Cause of Slow Echo
The author says the calculation may help explain why cross-country TIP users, including the West Coast community of BBN-TENEX users, experience poor echo response in bursts. The network itself may be undergoing momentary peak loads.
If the argument is correct, the proposed remote-echoing facilities of the new Telnet protocol could have a major effect on network operation.
The note proposes that short periods of heavy network traffic may make typing responses slow. It says a proposed Telnet remote-echo feature could change how much traffic crosses the network.