ask help for nginx and tomcat

[niedh]

Hi everybody,
  I'm new to nginx. Recently I use comet in my project. I choose tomcat 
for the java server. and enable the Http11NioProtocol connector.
  client send a ajax request,and the server side suspend it, and set a 
timeout(5 seconds),when timeout reach,the tomcat end the response
  in tomat server.xml
    <Connector port="9090" 
protocol="org.apache.coyote.http11.Http11NioProtocol"
               connectionTimeout="20000"
               redirectPort="8443" />

  when I test with tomcat directly,  means http://localhost:8080/xxxxx, 
it works as what I expect,
  but when I use nginx to proxy, when the timeout reach,the tomcat 
server end the response,but the request still there,
  nginx wait for its own "request timeout"(I don't what to call it).
  in nginx.conf,I set the follow proxy-relative config
        proxy_buffering          off;
        proxy_redirect          off;
        proxy_set_header        Host            $host;
        proxy_set_header        X-Real-IP       $remote_addr;
        proxy_set_header        X-Forwarded-For $proxy_add_x_forwarded_for;
 
is there something i missed ? Any suggestion will be    appreciate.. thanks