What's the packet-level difference between an XMLHttpRequest and a regular HTTP request?

Question

I'm wondering: if I were a a router, packet inspector, firewall, or other packet-sniffing device (which I'm glad I'm not) would I be able to tell the difference between a traditional HTTP request and an XMLHttpRequest? Less theoretically, is it possible that some ISP or (let's say) cell phone data provider could restrict XMLHttpRequest traffic without interrupting HTTP service?

Thanks.

Answer 1

There's nothing at the packet level to distinguish them because and XMLHttpRequest is an HTTP request. The XML bit refers to the fact that if the response is of an xml Content-Type then the responseXML method will return a DOM Object.

Answer 2

To the best of my knowledge, there is no fundamental difference - so from the point of view of a router etc. you can't tell in general.

But I do believe that most popular Javascript toolkits will add an HTTP header to their XMLHttpRequests to identify them as such. I forget the name, though...

EDIT: Here's an example (top Google hit for "jquery xmlhttprequest header", no quotes) that shows that jQuery apparently sets X-Requested-With to "XMLHttpRequest".

Answer 3

at packet, network, session levels: no.

at application level, that is with an HTTP-specific device like a filtering proxy, maybe.

i'd check the HTTP request headers. they might (just might) have some differences. but i'm sure any difference there would be very browser-specific, and quite probably the right JavaScript code could insert the appropriate headers to make it totally indistinguishable.

in short: check the HTTP headers; but don't expect it to be general, much less useful.