Editing End-to-end principle (section)

==Concept==
[[File:End-to-end Concept-en.svg|thumb|According to the end-to-end principle, the network is only responsible for providing the terminals with best-effort connections. Features such as reliability and security must be provided by mechanisms and protocols located at the terminals.]]
The fundamental notion behind the end-to-end principle is that for two [[Process (computing)|processes]] communicating with each other via some communication means, the ''[[Reliability (computer networking)|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 [[Positive acknowledgment with re-transmission|PAR]] or [[Automatic repeat request|ARQ]]).{{efn|In fact, even in [[local area network]]s there is a non-zero probability of communication failure{{snd}} "attention to reliability at higher levels is required regardless of the control strategy of the network".<ref name="Clark1978" />}} 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'',{{efn|Put in economics terms, the [[marginal cost]] of additional reliability in the network exceeds the marginal cost of obtaining the same additional reliability by measures in the end hosts. The economically efficient level of reliability improvement inside the network depends on the specific circumstances; however, it is certainly nowhere near zero:<ref name="SRC1984" /> "Clearly, some effort at the lower levels to improve network reliability can have a significant effect on application performance. (p. 281)."}} especially when the latter are beyond the control of, and not accountable to, the former.{{efn|The possibility of enforceable contractual remedies notwithstanding, it is impossible for any network in which intermediary resources are shared in a non-deterministic fashion to guarantee perfect reliability. At most, it may quote statistical performance averages.}} 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.{{efn|More precisely:<ref name="Sunshine1975" /> "THM 1: A correctly functioning PAR protocol with infinite retry count never fails to deliver, loses, or duplicates messages. COR 1A: A correctly functioning PAR protocol with finite retry count never loses or duplicates messages, and the probability of failing to deliver a message can be made arbitrarily small by the sender." (p. 3).}}

The end-to-end principle does not extend to functions beyond end-to-end error control and correction, and security. E.g., no straightforward end-to-end arguments can be made for communication parameters such as [[Latency (engineering)|latency]] and [[throughput]]. In a 2001 paper, Blumenthal and Clark note: "[F]rom the beginning, the end-to-end arguments revolved around requirements that could be implemented correctly at the endpoints; 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."<ref name="BC2001" />{{rp|80}}

The end-to-end principle is closely related, and sometimes seen as a direct precursor, to the principle of [[net neutrality]].<ref>{{cite web|url=https://www.theatlantic.com/technology/archive/2014/04/the-best-writing-on-net-neutrality/361237/|title=Net Neutrality: A Guide to (and History of) a Contested Idea|publisher=The Atlantic|author1=Alexis C. Madrigal |author2=Adrienne LaFrance |name-list-style=amp|date=25 Apr 2014|access-date=5 Jun 2014|quote=This idea of net neutrality...[Lawrence Lessig] used to call the principle e2e, for end to end}}</ref>