You need mandatory ACKs in both directions if you want to implement a reliable stream over HTTP requests. Otherwise, data can get lost in several scenarios, in

You need mandatory ACKs in both directions if you want to implement a reliable stream over HTTP requests. Otherwise, data can get lost in several scenarios, including: server responds to a long-poll request, connection breaks before client receives it, server assumes the data arrived, user never sees the message. Instead of having an ACK for every message in both C2S and S2C directions, socket.io implements some kind of bizarre optional-per-message ACK functionality that you manage with callbacks. There are libraries/protocols that do this right, including Closure Library's BrowserChannel.