WebSocket Protocol
https://tools.ietf.org/html/rfc6455

ws:// or wss://

HTTP is strictly a client-server protocol

How to do bidirectional communication?
traditional: "long polling"

2 phases: 

(1) HTTP upgrade

GET /chat HTTP/1.1
Host: server.example.com
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==
Sec-WebSocket-Protocol: chat, superchat
Sec-WebSocket-Version: 13

HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo=
Sec-WebSocket-Protocol: chat

(2) arbitrary, bidirectional data exchange
exchange "messages", split in "frames"


Network considerations
======================

NAT (Network Address Translation)
(most) clients have private IP addresses (192.168.x.y, 10.x.y.z, etc.)
cannot connect directly to client

Proxy (control outbound traffic, may be caching)