End-to-end Principle - Basic Content of The Principle

Basic Content of The Principle

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 acknowledgements and retransmissions (referred to as PAR or ARQ). Put differently, it is far easier and more tractable 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 accountability to the former. An end-to-end PAR protocol 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.

The end-to-end principle does not trivially extend to functions beyond end-to-end error control and correction. E.g., no straightforward end-to-end arguments can be made for communication parameters such as latency and throughput. Based on a personal communication with Saltzer (lead author of the original end-to-end paper) Blumenthal and Clark in a 2001 paper note:

rom the beginning, the end-to-end arguments revolved around requirements that could be implemented correctly at the end-points; if implementation inside the network is the only way to accomplish the requirement, then an end-to-end argument isn't appropriate in the first place. (p. 80)