I read from here that sockets are uniquely identified by an end-to-end protocol. What does "end-to-end" mean, generally, in networks?
An end-to-end protocol is responsible for delivering messages to one or multiple network endpoints.
Referring to the ISO/OSI model, end-to-end protocols work on the Transport Layer (Layer 4), i.e. UDP or TCP.
IMHO, the end-to-end principle basically is about the communication between two or more processes in the network.