Browser and proxy implementors at Netscape proposed a clever workaround to the blind relay problem that didn’t require all web applications to support advanced version of HTTP. The workaround introduced a new header called Proxy-Connection and solved the problem of a single blind relay interposed directly after the client— but not all other situations. Proxy-Connection is implemented by modern browsers when proxies are explicitly configured and is understood by many proxies.
The idea is that dumb proxies get into trouble because they blindly forward hop-byhop headers such as Connection: Keep-Alive. Hop-by-hop headers are relevant only for that single, particular connection and must not be forwarded. This causes trouble when the forwarded headers are misinterpreted by downstream servers as requests from the proxy itself to control its connection.
In the Netscape workaround, browsers send nonstandard Proxy-Connection extension headers to proxies, instead of officially supported and well-known Connection headers. If the proxy is a blind relay, it relays the nonsense Proxy-Connection header to the web server, which harmlessly ignores the header. But if the proxy is a smart proxy (capable of understanding persistent connection handshaking), it replaces the nonsense Proxy-Connection header with a Connection header, which is then sent to the server, having the desired effect.
From the below image, d shows how a blind relay harmlessly forwards Proxy-Connection headers to the web server, which ignores the header, causing no keep-alive connection to be established between the client and proxy or the proxy and server.
The smart proxy, from the image-h understands the Proxy-Connection header as a request to speak keep-alive, and it sends out its own Connection: Keep-Alive headers to establish keep-alive connections.
This scheme works around situations where there is only one proxy between the client and server. But if there is a smart proxy on either side of the dumb proxy, the problem will rear its ugly head again, as shown in the below image.
Furthermore, it is becoming quite common for “invisible” proxies to appear in networks, either as firewalls, intercepting caches, or reverse proxy server accelerators. Because these devices are invisible to the browser, the browser will not send them Proxy-Connection headers. It is critical that transparent web applications implement persistent connections correctly.
