Add SSL section to Snapshot Debugger troubleshooting

Author: pharring

articles/azure-monitor/app/snapshot-debugger-troubleshoot.md

@@ -28,6 +28,31 @@ If that doesn't solve the problem, then refer to the following manual troublesho
 
 Make sure you're using the correct instrumentation key in your published application. Usually, the instrumentation key is read from the ApplicationInsights.config file. Verify the value is the same as the instrumentation key for the Application Insights resource that you see in the portal.
 
+## Check SSL client settings (ASP.NET)
+
+If you have an ASP.NET application hosted in Azure App Service or in IIS on a virtual machine, your application could fail to connect to the Snapshot Debugger service due to a missing SSL security protocol.
+[The Snapshot Debugger endpoint requires TLS version 1.2](snapshot-debugger-upgrade.md?toc=/azure/azure-monitor/toc.json). The set of SSL security protocols is one of the quirks enabled by the httpRuntime targetFramework value in the system.web section of web.config.
+If the httpRuntime targetFramework is 4.5.2 or lower, then TLS 1.2 isn't included by default.
+
+> [!NOTE]
+> The httpRuntime targetFramework value is independent of the target framework used when building your application.
+
+To check the setting, open your web.config file and find the system.web section. Ensure that the `targetFramework` for `httpRuntime` is set to 4.6 or above.
+
+   ```xml
+   <system.web>
+      ...
+      <httpRuntime targetFramework="4.7.2" />
+      ...
+   </system.web>
+   ```
+
+> [!NOTE]
+> Modifying the httpRuntime targetFramework value changes the runtime quirks applied to your application and can cause other, subtle behavior changes. Be sure to test your application thoroughly after making this change. For a full list of compatibility changes, please see https://docs.microsoft.com/dotnet/framework/migration-guide/application-compatibility#retargeting-changes
+
+> [!NOTE]
+> If the targetFramework is 4.7 or above then Windows determines the available protocols. In Azure App Service, TLS 1.2 is available. However, if you are using your own virtual machine, you may need to enable TLS 1.2 in the OS.
+
 ## Preview Versions of .NET Core
 If the application uses a preview version of .NET Core, and Snapshot Debugger was enabled through the [Application Insights pane](snapshot-debugger-appservice.md?toc=/azure/azure-monitor/toc.json) in the portal, then Snapshot Debugger may not start. Follow the instructions at [Enable Snapshot Debugger for other environments](snapshot-debugger-vm.md?toc=/azure/azure-monitor/toc.json) first to include the [Microsoft.ApplicationInsights.SnapshotCollector](https://www.nuget.org/packages/Microsoft.ApplicationInsights.SnapshotCollector) NuGet package with the application ***in addition*** to enabling through the [Application Insights pane](snapshot-debugger-appservice.md?toc=/azure/azure-monitor/toc.json).