Windows Communication Foundation (WCF) relays (formerly Service Bus Relays) uses .NET Framework and WCF to establish a connection and send a message. The on-premises application uses WCF Relay bindings, which creates WCF channels that integrate with the Azure Service Bus.
For more information on how to use Azure Relay WCF relays with .NET, you can refer to the following article: https://docs.microsoft.com/en-us/azure/service-bus-relay/service-bus-relay-tutorial.
In the next section, we are going to cover Azure Notification Hubs.