Traffic limiting

Hi

I have this one very popular site configured go thru nginx reverse proxy
and its so popular that it takes lots of bandwidth and we are now need
to
limit traffic.

It seems there is limit modules in nginx but examples are per user
limit,
but that does not help us because thats the problem here, several
hundred
thousands users are requesting one particular xml file. So i think
limiting per user limit does not limit bandwidth at all, i need to limit
request to that one file per all users. So any help can i use nginx
limit
zone or req_limit_zone in this case, is there some $session_variable
what
i can use here instead of binary_remote_address?

Terveisin/Regards,
Pekka Panula, Sofor Oy - Jatkuvat palvelut