What is SDR server?

[Boikinov]

I try to host game, if it failed to load, the server log always complain about
SteamLog [SDR k_ESteamNetworkingSocketsDebugOutputType_Warning] Failed to accept connection from 127.0.0.1:61121. We don't have cert, and self-signed certs not allowed
And in the end it is
SteamLog [SDR k_ESteamNetworkingSocketsDebugOutputType_Error] SDR network config fetch attempt #3 failed. No response from server. Giving up. SDR functionality will not be available!
And the game client just stuck at loading screen

If I host game successfully, no such message exists in log