Need help: websocket proxy stops working after a while

I got nginx websocket proxy working but the socket would close
in a minute or so. So I add
proxy_read_timeout 180s;
and it works within 3 minutes but closed the websocket after 3
minutes, but it works OK before that. Why is that?
BTW, I am using Nginx 1.4 now.
Thanks,

- Alder
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