Satellite Considerations
A warning that satellite paths could add delay to interactive ARPANET sessions.
Postel, J., "Satellite Considerations", RFC 346, DOI 10.17487/RFC0346, May 1972, <https://www.rfc-editor.org/info/rfc346>.
@misc{rfc346,
series = {Request for Comments},
number = 346,
howpublished = {RFC 346},
publisher = {RFC Editor},
doi = {10.17487/RFC0346},
url = {https://www.rfc-editor.org/info/rfc346},
author = {J. Postel},
title = {{Satellite Considerations}},
pagetotal = 1,
year = 1972,
month = may,
} RFC text
Network Working Group Jon Postel
Request for Comments: 346 Computer Science
UCLA-NMC
NIC : 10425 30 May 72
Categories : Echo Plex, Satellite
References : RFC's 1, 5, 51
Satellite Considerations
The consideration of using space satellite transmission links in the
ARPANET should be cause for some thought by the parties making use of
the network. The satellite transmission path will not necessarily affect
the transmission rate but it will affect the delay. The change in the
delay characteristics can be approximated by the change in path length.
Thus if the satellite is in synchronous orbit about 22,000 miles above
the earth, the path length is about 44,000 miles compared to (worst
case) 3,000 miles or about a 15 to 1 increase in path length and delay.
(The time for light to travel 3,000 miles is .016 seconds, to travel
44,000 miles is .236 seconds.)
In the current (surface) ARPANET delays are such that interactive
servers with character-at-a-time remote echo are only marginally useful.
While I believe that this delay (unmeasured) is largely due to the host
systems, adding a half second transmission delay will cause these
marginal systems to become unusable.
Thought should also be given to buffer allocations. If a receiving
system allows only one line of text to be buffered at a time and
refreshes the allocation as each line is output to a human user there
will be at least a half second delay between the arrival of each line at
the receiving system. This need not be a problem until the speed of the
output device is above about 150 characters/second. This "small buffer"
problem can be expected to occur even with lower speed devices since
host delays are estimated to be in the range 0.1 second to 1.0 second.
I suggest that it is appropriate to resume a discussion of measures to
circumvent the difficulties brought about by these large delay
characteristics. Some areas of discussion could be: buffer sizes in
servers and users, echo plex techniques, moving part of the input
processing to the user system. If it is decided to move the echo plex
functions to the user system, it would be wise to try for a "standard"
package, thus reducing a M times N problem to a M plus N problem.
Please dig out and read RFC's #1 Crocker, #5 Rulifson, #51 Elie to see
some previous thinking about this type of problem.
[ This RFC was put into machine readable form for entry ]
[ into the online RFC archives by BBN Corp. under the ]
[ direction of Alex McKenzie. 12/96 ]
ISO Plain Language
On this page
§1Path Length and Delay
RFC 346 says a satellite path would affect delay but not necessarily transmission rate. It compares a path of about 44,000 miles through a synchronous-orbit satellite with a worst-case surface path of 3,000 miles, and estimates light travel times for both.
The author uses the change in path length to approximate the change in delay.
A longer route means more time before a reply arrives. A satellite path can be much longer than a route along the surface.
§2Interactive Echo and Buffers
Postel says interactive servers that echo each typed character remotely were already only marginally useful on the surface ARPANET. Adding a half-second of transmission delay, he argues, could make those systems unusable.
He also discusses systems that buffer one line at a time. Host delays estimated in the RFC could make the time between lines noticeable even with lower-speed output devices.
When a user waits for each typed character to come back, added delay makes the session feel slow. A small buffer can also leave long gaps between lines.
§3Topics for Further Discussion
The author asks the network community to resume discussion of ways to reduce the effects of large delays. Possible topics include buffer sizes at both ends, echo techniques, and moving some input processing to the user's system.
If echo functions move to the user system, he recommends trying to develop a standard package.
The RFC does not prescribe one fix. It asks people to discuss several approaches and consider a shared way to handle echo locally.