chunked-encoding disable range request

[shuxinyang]

Hi, There:

    I find that range-request is disabled by Nginx proxy if backend 
server responses with chunked encoding.
I verify my observation by:
    1) manually edit the cache by replacing chunked-encoding header with 
content-length header,  then
      range-request is enabled.

    2) examine the header-filter of range filter, which indicates range 
filter is disabled when content-length
       is not valid.

   Question:
    a). Is my observation and verification approach correct?
    b). If so, is it Nginx's limitation or is the behavior defined by RFC?
         I tried to find the answer from RFCs, but in vain.

Thanks!
Shuxin