dotnet · gewarren · Aug 30, 2021 · Aug 12, 2021 · Aug 12, 2021 · Aug 12, 2021
@@ -14,13 +14,14 @@ If you're migrating an app to .NET 6, the breaking changes listed here might aff
 
 ## ASP.NET Core
 
-| Title | Preview introduced |
+| Title | Introduced |
 | - | - |
 | [AddDataAnnotationsValidation method made obsolete](aspnet-core/6.0/adddataannotationsvalidation-obsolete.md) |  |
 | [Assemblies removed from Microsoft.AspNetCore.App shared framework](aspnet-core/6.0/assemblies-removed-from-shared-framework.md) |  |
 | [Blazor: Parameter name changed in RequestImageFileAsync method](aspnet-core/6.0/blazor-parameter-name-changed-in-method.md) | Preview 1 |
 | [Blazor: WebEventDescriptor.EventArgsType property replaced](aspnet-core/6.0/blazor-eventargstype-property-replaced.md) |  |
 | [Blazor: Byte array interop](aspnet-core/6.0/byte-array-interop.md) | Preview 6 |
+| [Blazor server: Disable long polling fallback transport](aspnet-core/6.0/blazor-long-polling-fallback.md) | RC 1 |
 | [Changed MessagePack library in @microsoft/signalr-protocol-msgpack](aspnet-core/6.0/messagepack-library-change.md) |  |
 | [ClientCertificate property doesn't trigger renegotiation for HttpSys](aspnet-core/6.0/clientcertificate-doesnt-trigger-renegotiation.md) |  |
 | [Kestrel: Log message attributes changed](aspnet-core/6.0/kestrel-log-message-attributes-changed.md) |  |
@@ -37,7 +38,7 @@ If you're migrating an app to .NET 6, the breaking changes listed here might aff
 
 ## Core .NET libraries
 
-| Title | Preview introduced |
+| Title | Introduced |
 | - | - |
 | [API obsoletions with non-default diagnostic IDs](core-libraries/6.0/obsolete-apis-with-custom-diagnostics.md) | Preview 1 |
 | [Changes to nullable reference type annotations](core-libraries/6.0/nullable-ref-type-annotation-changes.md) | Preview 1-2 |
@@ -54,25 +55,25 @@ If you're migrating an app to .NET 6, the breaking changes listed here might aff
 
 ## Globalization
 
-| Title | Preview introduced |
+| Title | Introduced |
 | - | - |
 | [Culture creation and case mapping in globalization-invariant mode](globalization/6.0/culture-creation-invariant-mode.md) | Preview 7 |
 
 ## JIT compiler
 
-| Title | Preview introduced |
+| Title | Introduced |
 | - | - |
 | [Coerce call arguments according to ECMA-335](jit/6.0/coerce-call-arguments-ecma-335.md) | Preview 1 |
 
 ## Networking
 
-| Title | Preview introduced |
+| Title | Introduced |
 | - | - |
 | [WebRequest, WebClient, and ServicePoint are obsolete](networking/6.0/webrequest-deprecated.md) | Preview 1 |
 
 ## SDK
 
-| Title | Preview introduced |
+| Title | Introduced |
 | - | - |
 | [`-p` option for `dotnet run` is deprecated](sdk/6.0/deprecate-p-option-dotnet-run.md) | Preview 6 |
 | [C# code in templates not supported by earlier versions](sdk/6.0/csharp-template-code.md) | Preview 7 |
@@ -83,7 +84,7 @@ If you're migrating an app to .NET 6, the breaking changes listed here might aff
 
 ## Windows Forms
 
-| Title | Preview introduced |
+| Title | Introduced |
 | - | - |
 | [Selected TableLayoutSettings properties throw InvalidEnumArgumentException](windows-forms/6.0/tablelayoutsettings-apis-throw-invalidenumargumentexception.md) | Preview 1 |
 | [DataGridView-related APIs now throw InvalidOperationException](windows-forms/6.0/null-owner-causes-invalidoperationexception.md) | Preview 4 |

@@ -0,0 +1,65 @@
+---
+title: "Breaking change: Blazor server: Disable LongPolling fallback transport"
+description: "Learn about the breaking change in ASP.NET Core 6.0 where LongPolling is not used as a fallback transport when WebSockets aren't available."
+ms.date: 08/12/2021
+no-loc: [ Blazor, WebSocket, LongPolling ]
+---
+# Blazor server: Disable long polling fallback transport
+
+Prior to .NET 6, *LongPolling* was a fallback transport utilized when WebSockets weren't available. The LongPolling fallback can lead to a degraded user experience, so it has been removed. Both the client and server now support only WebSockets by default.
+
+## Version introduced
+
+ASP.NET Core 6.0 RC 1
+
+## Old behavior
+
+If WebSockets are unavailable for a circuit (for example, due to network issues or browser incompatibility), LongPolling is used instead.
+
+## New behavior
+
+The following table shows the error message you'll receive for each combination of client and server configurations.
+
+| Client | Server | Message |
+|---|---|---|
+| WS (without browser WS support) | WebSockets | `Unable to connect, please ensure you are using an updated browser that supports WebSockets.` |
+| WS (with WS connection being rejected) | WebSockets | `Unable to connect, please ensure WebSockets are available. A VPN or proxy may be blocking the connection.` |
+| WS | LongPolling | `An unhandled error has occurred.` Console Error: `Unable to initiate a SignalR connection to the server. This might be because the server is not configured to support WebSockets. To troubleshoot this, visit https://aka.ms/blazor-server-websockets-error.` |
+| LongPolling | WebSockets | `An unhandled error has occurred.` |
+
+## Reason for change
+
+This change was made to improve the overall end-user experience by enforcing WebSocket use.
+
+## Recommended action
+
+Ensure WebSockets are functioning as expected with your application. If you must use LongPolling, you can enable it by making the following client and server side changes.
+
+### Server side
+
+In `Startup.cs`, replace `endpoints.MapBlazorHub()` with:
+
+```c#
+endpoints.MapBlazorHub(configureOptions: options => 
+{ 
+    options.Transports = HttpTransportType.WebSockets | HttpTransportType.LongPolling; 
+});
+```
+
+### Client side
+
+Add the following script to `_Layout.cshtml` before the closing `</body>` tag. The supported <xref:Microsoft.AspNetCore.Http.Connections.HttpTransportType> fields are <xref:Microsoft.AspNetCore.Http.Connections.HttpTransportType.WebSockets?displayProperty=nameWithType> (`1`) and <xref:Microsoft.AspNetCore.Http.Connections.HttpTransportType.LongPolling?displayProperty=nameWithType> (`4`).
+
+```html
+<script>
+    (function start() {
+        Blazor.start({
+            configureSignalR: builder => builder.withUrl('_blazor', 1 | 4) // WebSockets and LongPolling
+        });
+    })()
+</script>
+```
+
+## Affected APIs
+
+None.
@@ -35,6 +35,8 @@ items:
           href: aspnet-core/6.0/blazor-eventargstype-property-replaced.md
         - name: "Blazor: Byte-array interop"
           href: aspnet-core/6.0/byte-array-interop.md
+        - name: "Blazor server: Disable long-polling fallback"
+          href: aspnet-core/6.0/blazor-long-polling-fallback.md
         - name: ClientCertificate doesn't trigger renegotiation
           href: aspnet-core/6.0/clientcertificate-doesnt-trigger-renegotiation.md
         - name: "Kestrel: Log message attributes changed"
@@ -413,6 +415,8 @@ items:
           href: aspnet-core/6.0/blazor-eventargstype-property-replaced.md
         - name: "Blazor: Byte-array interop"
           href: aspnet-core/6.0/byte-array-interop.md
+        - name: "Blazor server: Disable long-polling fallback"
+          href: aspnet-core/6.0/blazor-long-polling-fallback.md
         - name: ClientCertificate doesn't trigger renegotiation
           href: aspnet-core/6.0/clientcertificate-doesnt-trigger-renegotiation.md
         - name: "Kestrel: Log message attributes changed"