End-to-end Principle

The end-to-end principle is a classic design principle of computer networking, first explicitly articulated in a 1981 conference paper by Saltzer, Reed, and Clark.

The end-to-end principle states that application-specific functions ought to reside in the end hosts of a network rather than in intermediary nodes – provided they can be implemented "completely and correctly" in the end hosts. Going back to Baran's work on obtaining reliability from unreliable parts in the early 1960s, the basic intuition behind the original principle is that the payoffs from adding functions to the network quickly diminish, especially in those cases where the end hosts will have to re-implement functions for reasons of "completeness and correctness" anyway (regardless of the efforts of the network). Moreover, there is an unfair performance penalty paid by all network clients when application functions of just a few clients are pushed into the intermediate nodes of a network.

The canonical example for the end-to-end principle is that of arbitrarily reliable file transfer between two communication end points in a distributed network of nontrivial size. The only way two end points can obtain perfect reliability for this file transfer is by positive acknowledgment of end-to-end checksums over the final file in the destination storage locations on the destination machine. In such a system, lesser checksum and acknowledgment (ACK/NACK) protocols are justified only as a performance optimization, useful to the vast majority of clients, but are incapable of anticipating the reliability requirement of the transfer application itself (because said requirements may be arbitrarily high).

In debates about network neutrality a common interpretation of the end-to-end principle is that it implies a neutral or "dumb" network.