Same as &buffer, but instead includes the round-trip-time
&buffer2=500is the same as
&buffer, but instead also tells the system to include the round-trip-time in the buffer delay calculation. This way 500-ms of buffer on a connection that has a 200ms ping time will result in a smaller 300-ms buffer, leading to an end-to-end playout delay of ~500ms.
It's not super precise, but on a stable connection maybe within 20-ms of flux?