Set the *high*- and *low*-water limits for write flow control.

These two values control when call the protocol's

These two values (measured in number of

bytes) control when the protocol's

:meth:`pause_writing` and :meth:`resume_writing` methods are called.

If specified, the low-water limit must be less than or equal to the

high-water limit. Neither *high* nor *low* can be negative.

:meth:`pause_writing` is called when the buffer size becomes greater

than or equal to the *high* value. If writing has been paused,

:meth:`resume_writing` is called when the buffer size becomes less

than or equal to the *low* value.

The defaults are implementation-specific. If only the

high-water limit is given, the low-water limit defaults to an

implementation-specific value less than or equal to the