The Initial Internetting Concepts

The Internet Society hosts a monograph named A Brief History of the Internet, by Vint Cerf etal.. Quoting directly from that paper (READ IT!):

Ground rules:

• Each distinct network would have to stand on its own and no internal changes could be required to any such network to connect it to the Internet.
• Communications would be on a best effort basis. If a packet didn't make it to the final destination, it would shortly be retransmitted from the source.
• Black boxes would be used to connect the networks; these would later be called gateways and routers. There would be no information retained by the gateways about the individual flows of packets passing through them, thereby keeping them simple and avoiding complicated adaptation and recovery from various failure modes.
• There would be no global control at the operations level.

• Algorithms to prevent lost packets from permanently disabling communications and enabling them to be successfully retransmitted from the source.
• Providing for host-to-host pipelining so that multiple packets could be enroute from source to destination at the discretion of the participating hosts, if the intermediate networks allowed it.
• Gateway functions to allow it to forward packets appropriately. This included interpreting IP headers for routing, handling interfaces, breaking packets into smaller pieces if necessary, etc.
• The need for end-end checksums, reassembly of packets from fragments and detection of duplicates, if any.
• The need for global addressing.
• Techniques for host-to-host flow control.
• Interfacing with the various operating systems.
• There were also other concerns, such as implementation efficiency, internetwork performance, but these were secondary considerations at first.