Tweak the meaning of the ECN convar.

Now, -1 means "auto" and 0 means "don't set anything".
Add g_nSendECNAuto
Add ResolveECNSendGlobal

P4:9166210,9168032
(cherry picked from commit fcc19e1)

Author: zpostfacto

include/steam/steamnetworkingtypes.h

@@ -1693,12 +1693,20 @@ enum ESteamNetworkingConfigValue
 	k_ESteamNetworkingConfig_LogLevel_P2PRendezvous = 17, // [connection int32] P2P rendezvous messages
 	k_ESteamNetworkingConfig_LogLevel_SDRRelayPings = 18, // [global int32] Ping relays
 
-	// Experimental.  Set the ECN header field on all outbound UDP packets
-	// -1 = the default, and means "don't set anything".
-	// 0..3 = set that value.  (Even though 0 is the default UDP ECN value, a 0 here means "explicitly set a 0".)
+//
+// Experimental values.  These are subject to be deleted or change at any time,
+// do not set them, except as a result of an explicit and advanced user opt-in,
+// and do not write anything that depends on them existing, or have any particular
+// behaviour.
+//
+	// [global int32] ECN value to send in every packet.
+	// -1 = The default, and means "auto".  We will set ECN=1, if it appears that the local internet connection appears to understand it and there may be some benefit.
+	// 0..2 = use that value.
 	k_ESteamNetworkingConfig_ECN = 999,
 
-	// Deleted, do not use
+//
+// Deleted, do not use
+//
 	k_ESteamNetworkingConfig_DELETED_EnumerateDevVars = 35,
 
 	k_ESteamNetworkingConfigValue__Force32Bit = 0x7fffffff

src/steamnetworkingsockets/clientlib/steamnetworkingsockets_lowlevel.cpp

@@ -96,6 +96,10 @@ static void FlushSystemSpew();
 
 int g_cbUDPSocketBufferSize = 256*1024;
 
+#if PlatformCanSendECN()
+int g_nSendECNAuto = -1;
+#endif
+
 /// Global lock for all local data structures
 static Lock<RecursiveTimedMutexImpl> s_mutexGlobalLock( "global", 0, LockDebugInfo::k_nOrder_Global );
 
@@ -1098,8 +1102,8 @@ class CRawUDPSocketImpl final : public IRawUDPSocket
 
 			// Check if we need to send ECN
 			if ( ecn < 0 )
-				ecn = GlobalConfig::ECN.Get();
-			if ( ecn >= 0 )
+				ecn = ResolveECNSendGlobal();
+			if ( ecn > 0 ) // We assume that if we don't explicit specify an ECN, that zero will be used
 			{
 				wsaMsg.Control.len = sizeof(control);
 				wsaMsg.Control.buf = control;

src/steamnetworkingsockets/steamnetworkingsockets_internal.h

@@ -1102,7 +1102,19 @@ inline uint8 IPv4_TOS_make( uint8 dscp, uint8 ecn )
 {
 	Assert( dscp < 0x40 );
 	Assert( ecn < 0x4 );
-	return ( (dscp<<2) | ecn );
+	return ( (dscp<<2U) | ecn );
+}
+
+// Extract ECN from TOS
+inline uint8 IPv4_TOS_ECN( uint8 tos )
+{
+	return tos & IPv4_TOS_ECN_bits;
+}
+
+// Extract DSCP from TOS
+inline uint8 IPv4_TOS_DSCP( uint8 tos )
+{
+	return tos>>2U;
 }
 
 // FIXME POSIX supports this!  Need to check console support and implement all supported platforms,
@@ -1113,6 +1125,23 @@ inline uint8 IPv4_TOS_make( uint8 dscp, uint8 ecn )
 	#define PlatformCanSendECN() false
 #endif
 
+#if PlatformCanSendECN()
+	// ECN value to send, if "auto" option is specified.
+	// -1 means that we have not completed detection, and so 0 will be used
+	extern int g_nSendECNAuto;
+
+	// Get the ECN value we will actually use for sending
+	inline uint8 ResolveECNSendGlobal()
+	{
+		int ecn = GlobalConfig::ECN.Get();
+		if ( ecn < 0 )
+			ecn = std::max( 0, g_nSendECNAuto );
+		return (uint8)ecn;
+	}
+#else
+	inline uint8 ResolveECNSendGlobal() { return 0; }
+#endif
+
 } // namespace SteamNetworkingSocketsLib
 
 #include <tier0/memdbgon.h>