There will be no human being in the loop, and both endpoints are autonomous Node.js applications operating as independent services.
Endpoint A is responsible for contacting Endpoint B via secure web socket, and maintaining that connection 24/7/365.
Both endpoints will initiate messages independently (without human intervention), and both endpoints will have an API (RESTful or otherwise) to receive and process messages. You might say that each endpoint is both a client of, and a server to, the other endpoint.
I am considering frameworks like Sails.js and LoopBack (implemented on both endpoints), as well as simply passing JSON messages over ws, but remain unclear what the most idiomatic approach would be.