Valid/Invalid headers

Greetings!
I am trying to identify valid HTTP headers from
invalid headers. Can someone point me to  appropriate
link/spec, where I can know more about it. 

To rephrase my question, is there any combination of
characters or rules that different valid and invalid
characters; or can I get the exhaustive list of valid
headers. 

Thanks in advance
~Krishna


__________________________________________________
Do You Yahoo!?
Great stuff seeking new owners in Yahoo! Auctions! 
http://auctions.yahoo.com