This allows the host to provide a 32-bit integer that will be sent
in the data of the ENet connect event, similar to X-SS-Ping-Payload
for video and audio.
The host can use this data to uniquely identify a client when IP
addresses are not stable across the various separate connections,
such as when the client is behind a Carrier-Grade NAT.
We currently scale bitrate based on both remote vs local, SDR vs HDR, and H.264 vs
HEVC vs AV1. This has led to a lot of user confusion wondering why the bitrate
doesn't seem to match their selection in some configurations.
In H.264 local streams, we will currently overshoot the selected bitrate by about
20% due to FEC, while remote streams will be right around the selected bitrate due
to remote-specific FEC bitrate adjustments.
HEVC and AV1 streams (as configured by most clients) basically behave similarly
between local and remote, since the codec bitrate adjustment factor of 75% is nearly
the same as the FEC bitrate adjustment factor of 80%. However, this adjustment was
only performed for SDR streams so local HDR streams would overshoot like H.264.
This change cleans up all this mess by using a single non-codec-specific video
bitrate adjustment for FEC in all cases. It also allows Sunshine to perform the FEC
adjustment on its end if the default FEC value of 20% has been overridden by the
user or if we implement dynamic FEC support in the future.
The net result is HEVC and AV1 SDR streams will only see a tiny bitrate increase,
but HDR and H.264 may see noticable 20% bitrate reductions that may require the
user to adjust their bitrate setting to reach the effective value they got before.
However, the new behavior should be more intuitive for users going forward since
changing codecs, using a VPN, or enabling HDR won't cause significant changes to the
video bitrate.
This means we can ensure a consistent local address for our outgoing PING
traffic to keep the UDP flows alive without having to call connect() which breaks
with multi-homed hosts on GFE and Sunshine v0.20 and earlier.
With predictive RFI, we can recover from a lost frame as soon as the next frame
by predicting whether we will have enough FEC data to be be able to recover a
frame based on the sequence numbers of the received packets.
