Request for Comments: 567
September 6, 1973
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.