Internet Protocol - Research by Kirils Solovjovs

End-to-end principle 102
End-to-end principle
The end-to-end principle is a classic design principle of computer networking, [1] first explicitly articulated in a
[2] [3]
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). [4] 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 [5] . 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).
As an example of the end-to-end principle in practice, to reach Six Sigma reliability, file servers made by NetApp
were forced to implement end-to-end checksums (making the disk drive checksums redundant) because the
checksums used by certain disk makers were insufficient to meet a guarantee of Six Sigma reliability.
In debates about network neutrality a common interpretation of the end-to-end principle is that it implies a neutral or
"dumb" network.
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). [6] 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, [7] especially when the latter are beyond the control of and
accountability to the former. [8] 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. [9]
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 [5] )
Blumenthal and Clark in a 2001 paper note: [10]
[F]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)