Proxies, how to determine response body length

After reading RFC2616 I'am not sure how HTTP1.1 proxy server can 
determine response body length on client-proxy side connection 
when this length was determined by the server closing the connection
on proxy-originserver side connection. I'am not sure that proxy server
can determine it by closing the connection, because for some 
particular client-proxy connection proxy can have many persistent 
connections (to different origin servers) opened so determining 
response body length by closing client-proxy connection would cause 
loss of remaining proxy-originserver persistent connections I think. 
Thanks for answers j.n. 
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