The fundamental notion behind the end-to-end principle is that for two processes communicating with each other via some communication means, the "reliability" obtained from that means cannot be expected to be perfectly aligned with the reliability requirements of the processes. In particular, meeting or exceeding very high-reliability requirements of communicating processes separated by networks of nontrivial size is more costly than obtaining the required degree of reliability by positive end-to-end acknowledgments and retransmissions (referred to as PAR or ARQ). Put differently, it is far easier to obtain reliability beyond a certain margin by mechanisms in the "end hosts" of a network rather than in the "intermediary nodes", especially when the latter are beyond the control of, and not accountable to, the former. Positive end-to-end acknowledgments with infinite retries can obtain arbitrarily high reliability from any network with a higher than zero probability of successfully transmitting data from one end to another.