Streaming replication protocol

Hello,

I'm trying to write a program that speaks the streaming replication
protocol (for logical decoding). I get to the part where I issue a query:

START_REPLICATION SLOT regression_slot LOGICAL 0/0;

And after that, I receive an empty copy_both_response then a copy_data that
has a "Primary keepalive message" in it. Then I'm stuck; I don't know how
to respond to that.

I'm going off the documentation here:

https://www.postgresql.org/docs/9.5/static/protocol-replication.html

Which I find really confusing and hard to understand. Any help would be
appreciated! Thank you.