XML Firewall

[wuggy77 () msn com]

I have been around the security block, but obviously I have not been exposed to everything because I was asked to 
research an XML Firewall.  I have never heard of an XML Firewall and so I wanted to start by getting some input on what 
an XML Firewall is supposed to do.  I am not opposed to doing some reading so if anyone has some good links to sites 
that explain these devices I will be happy to look it up.  Secondly I would like to recieve some recommendation as to 
what would be considered good XML firewall.