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Carrying instructions between calculation machines and early computers was done by human users. In September 1940 George Stibitz used a teletype machine to send instructions for a problem set from his Model K at Dartmouth College in New Hampshire to his Complex Number Calculator in New York and received results back by the same means.
Ealry phone regulations prohibited any electrical connection to phone system, acoustically coupled modems (see picture) avoided direct connection.
Baran developed the concept of packet switching during his research for the US Air Force into survivable communications networks, first published as RAND Paper P-2626 in 1962[1], and then including and expanding somewhat within a series of eleven papers titled On Distributed Communications in 1964 [2]. Baran's P-2626 paper described a general architecture for a large-scale, distributed, survivable communications network. The paper focuses on three key ideas: first, use of a decentralized network with multiple paths between any two points; and second, dividing complete user messages into what he called message blocks (later called packets); then third, delivery of these messages by store and forward switching. Baran's study made its way to Robert Taylor (computer scientist) and J.C.R. Licklider at the Information Processing Technology Office, both wide-area network evangelists, and it helped influence Lawrence Roberts to adopt the technology when Taylor put him in charge of development of the ARPANET. Baran's packet switching work was similar to the research performed independently by Donald Davies at the National Physical Laboratory, UK. In 1965, Davies developed the concept of packet switched networks and proposed development of a UK wide network. He gave a talk on the proposal in 1966, after which a person from the Ministry of Defense told him about Baran's work. Davies met Lawrence Roberts at the 1967 ACM Symposium on Operating System Principles, bringing the two groups together.
RFC 1149: A Standard for the Transmission of IP Datagrams on Avian Carriers
Viewing layers as providing or consuming a service is a method of abstraction to isolate upper layer protocols from the nitty gritty detail of transmitting bits over, say, Ethernet and collision detection while the lower layers avoid having to know the details of each and every application and its protocol. This abstraction also allows upper layers to provide services that the lower layers cannot, or choose not, to provide. For example, IP is designed to not be reliable and is a best effort delivery protocol. This means that all transport layers must choose whether or not to provide reliability and to what degree. UDP provides data integrity (via a checksum) but does not guarantee delivery; TCP provides both data integrity and delivery guarantee (by retransmitting until the receiver receives the packet).
"The WorldWideWeb (WWW) project aims to allow links to be made to any information anywhere. [...] The WWW project was started to allow high energy physicists to share data, news, and documentation. We are very interested in spreading the web to other areas, and having gateway servers for other data. Collaborators welcome!" —from Tim Berners-Lee's first message
TCP turns packets into a reliable byte stream