Networks
Datagrams try to go from here to there. Arrival is not a promise.
Networks (Simple Explanation)
Computers send data to other computers. On the public Internet, a basic way of sending is the Internet Protocol. It is written down in RFC 791.
RFC 791 describes datagrams. A datagram is a packet of data. It has a source. It has a destination. The Internet Protocol tries to send it from the source to the destination.
It does not promise that the datagram will arrive. RFC 791 says the Internet Protocol does not provide a reliable communication facility. There are no acknowledgments at this layer. There are no retransmissions at this layer. Delay can happen. Loss can happen. That is normal at IP.
If a file arrives whole, some other layer did extra work. Do not treat IP as a promise. Treat it as a best-effort send from a source address to a destination address.
This page does not teach you to configure a network. It only names what RFC 791 says IP does, and what it does not do.
Key Ideas
Datagrams: data with a source and a destination. Source: RFC 791.
No reliability at IP: no acknowledgments, no retransmissions at this layer. Source: RFC 791.
Delay and loss are normal at IP. A complete file used extra work above IP.
Next question
How does a computer get facts about the physical world in the first place?
References
IETF (1981). Internet Protocol. RFC 791.
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