fastcgi msg_id always 1?

[Shaun savage]

I was looking at the fastcgi, I found out that all fcgi request have a
msg_id of 1? 

What happens when more than 1 fcgi requests comes in and first one is
still being handled. how does the fcgi server know  the the difference
in requests?  Usually each request has it own connection, but if
keepalive is on the then many request can come in on the same connection. 

I am use "ab" to test the nginx+ fastcgi server response time.