the postman sitting in the center of the United States received letters
Francisco, he would find that letters from San Francisco arriving
from channels to the west would come in with later cancellation dates
if such letters had arrived in a roundabout manner from the east.
Each letter carries an implicit indication of its length of transmission
path. The astute postman can then deduce that the best channel to
send a message to San Francisco is probably the link associated with
dates of messages from San Francisco.
Distributed Communications, Volume I, 1964.