There is a basic implementation in JavaScript that reads from the Serial Port and outputs the GPS data through UDP. This same code uses a library that permits to establish communication with NTRIP casters (https://github.com/dxhbiz/ntrip-client.git).
According to the Best Practices for NTRIP Developers document, from the RTCM organization (http://www.rtcm.org/), to receive the RTCM corrections from the Casters, it is required to send a GGA sentence data through HTTP. Once that the GGA data is sent, then the client will receive the RTCM corrections. With those, then it is injected directly into the rover (GPS) module and from there, the output from this module will be with the RTK enabled.
To validate this, there are messages that are without the RTK enabled, like so:
$GNGGA,162913.00,4130.99253,N,00221.79242,E,2,12,0.50,109.7,M,49.2,M,,0136*44
$GNGGA,162914.00,4130.99252,N,00221.79243,E,2,12,0.50,109.8,M,49.2,M,,0136*4C
$GNGGA,162915.00,4130.99252,N,00221.79244,E,2,12,0.50,109.8,M,49.2,M,,0136*4A
And here are the messages with RTK enabled:
$GNGGA,163003.00,4130.99013,N,00221.79081,E,5,12,0.56,113.7,M,49.2,M,1.0,0011*67
$GNGGA,163004.00,4130.99013,N,00221.79081,E,5,12,0.56,113.7,M,49.2,M,1.0,0011*60
Notice how in the GGA sentence, the sixth field (the GPS Quality indicator) changes from 2 to 5, indicating that its type is Float RTK.