Tunnels are HTTP applications that, after setup, blindly relay raw data between two connections. HTTP tunnels are often used to transport non-HTTP data over one or more HTTP connections, without looking at the data.
One popular use of HTTP tunnels is to carry encrypted Secure Sockets Layer (SSL) traffic through an HTTP connection, allowing SSL traffic through corporate firewalls that permit only web traffic. As sketched in Figure 1-14, an HTTP/SSL tunnel receives an HTTP request to establish an outgoing connection to a destination address and port, then proceeds to tunnel the encrypted SSL traffic over the HTTP channel so that it can be blindly relayed to the destination server.
Figure 1-14. Tunnels forward data across non-HTTP networks (HTTP/SSL tunnel shown)
--摘自HTTP:The Definitive Guide
