http window size zero

I have implemented a web server that sends out a page via CGI. It works fine the first time. But the second time the browser tries to connect to the web server, the web server sends out zero window size in the connection handshake.


browser->rabbit: SYN win=5808
rabbit->browser SYN ACK win=0

The first time around, the browser host sent the same SYN packet, and the rabbit responded with win=2048.

What would cause the protocol stack to send a SYN ACK with window size 0?