HI
Firewall controlls network traffic on a machine for Security.These will not allow any connections from outside network to be established.As we all know , port 80 is open for web traffic.So in this case, this port can be used for client server communications.Sending a request an Http request on port 80 is know as Http Tunneling through firewalls.
This is different from servlet chaining.
In Serlet Chaining , the output from one servlet is sent to another servlet which takes place on the server side.
An example of Http Tunneling is an
Applet downloaded on a client machine which is behind a firewall.
Then the applet can't directly communicate with the webserver in which case you have to use HttpTunneling