How to control keepalive connections for upstream before the version of 1.15.3

[yf chu]

The Nginx document said that there are two upstream-related directives introduced in the version of 1.15.3:  "keepalive_requests" and "keepalive_timeout".  But the directive "keepalive" has already been instroduced in the version of 1.1.4. So I want to know how does Nginx handle the keepalive connections for upstream before the version 1.15.3 was released. When will the keepalive connections for upstream be closed?
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