Reverse HTTP is an experimental protocol which takes advantage of the HTTP/1.1 Upgrade: header to turn one HTTP socket around. When a client makes a request to a server with the Upgrade: PTTH/0.9 header, the server may respond with an Upgrade: PTTH/1.0 header, after which point the server starts using the socket as a client, and the client starts using the socket as a server. There is also a COMET
![Reverse HTTP](https://cdn-ak-scissors.b.st-hatena.com/image/square/7a524265d125bae414cc3219b1923d371e3ed6bf/height=288;version=1;width=512/https%3A%2F%2Fll-pwiki-assets-production.s3.amazonaws.com%2Flinden%2Fsl_logo_2021_1x.png)