End-to-end principle

The end-to-end principle is a design framework in computer networking. In networks designed according to this principle, application-specific features reside in the communicating end nodes of the network, rather than in intermediary nodes, such as gateways and routers, that exist to establish the network. The end-to-end principle is a design framework in computer networking. In networks designed according to this principle, application-specific features reside in the communicating end nodes of the network, rather than in intermediary nodes, such as gateways and routers, that exist to establish the network. The essence of what would later be called the end-to-end principle was contained in the work of Paul Baran and Donald Davies on packet-switched networks in the 1960s. Louis Pouzin pioneered the use of the end-to-end strategy in the CYCLADES network in the 1970s. The principle was first articulated explicitly in 1981 by Saltzer, Reed, and Clark. The meaning of the end-to-end principle has been continuously reinterpreted ever since its initial articulation. Also, noteworthy formulations of the end-to-end principle can be found before the seminal 1981 Saltzer, Reed, and Clark paper. A basic premise of the principle is that the payoffs from adding features to a simple network quickly diminish, especially in cases in which the end hosts have to implement those functions only for reasons of conformance, i.e. completeness and correctness based on a specification. Implementing a specific function incurs some resource penalties regardless of whether the function is used or not, and implementing a specific function in the network distributes these penalties among all clients. The end-to-end principle is closely related, and sometimes seen as a direct precursor, to the principle of net neutrality. 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 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 acknowledgements 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. In a 2001 paper, Blumenthal and Clark note: '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.':80 In the 1960s, Paul Baran and Donald Davies, in their pre-ARPANET elaborations of networking, made brief comments about reliability that capture the essence of the later end-to-end principle. To quote from a 1964 Baran paper, 'Reliability and raw error rates are secondary. The network must be built with the expectation of heavy damage anyway. Powerful error removal methods exist.':5 Similarly, Davies notes on end-to-end error control, 'It is thought that all users of the network will provide themselves with some kind of error control and that without difficulty this could be made to show up a missing packet. Because of this, loss of packets, if it is sufficiently rare, can be tolerated.':2.3 The French CYCLADES network was the first to make the hosts responsible for the reliable delivery of data, rather than this being a centralized service of the network itself. The ARPANET was the first large-scale general-purpose packet switching network – implementing several of the basic notions previously touched on by Baran and Davies, and demonstrating several important aspects of the end-to-end principle: