* [Out-of-Process](xref:host-and-deploy/iis/index#out-of-process-hosting-model). In Out-of-Process, IIS responds to the client using HTTP/3, but the reverse proxy connection to the [Kestrel server](xref:fundamentals/servers/kestrel) uses HTTP/1.1.
For more information on the in-process and out-of-process hosting models, see <xref:host-and-deploy/aspnet-core-module>.
* The [EnableHttp3 registry key](https://techcommunity.microsoft.com/t5/networking-blog/enabling-http-3-support-on-windows-server-2022/ba-p/2676880) is set.
For an in-process deployment when an HTTP/3 connection is established, [`HttpRequest.Protocol`](xref:Microsoft.AspNetCore.Http.HttpRequest.Protocol*) reports `HTTP/3`. For an out-of-process deployment when an HTTP/3 connection is established, [`HttpRequest.Protocol`](xref:Microsoft.AspNetCore.Http.HttpRequest.Protocol*) reports `HTTP/1.1` because that is how IIS proxies the requests to Kestrel.
## Alt-Svc
HTTP/3 is discovered as an upgrade from HTTP/1.1 or HTTP/2 via the [`alt-svc`](https://developer.mozilla.org/docs/Web/HTTP/Headers/Alt-Svc) header. That means the first request will normally use HTTP/1.1 or HTTP/2 before switching to HTTP/3. IIS doesn't automatically add the `alt-svc` header, it must be added by the application. The following code is a middleware example that adds the `alt-svc` response header.
```C#
app.Use((context, next) =>
{
context.Response.Headers.AltSvc = "h3=\":443\"";
return next(context);
});
```
Place the preceding code early in the request pipeline.
IIS also supports sending an AltSvc HTTP/2 protocol message rather than a response header to notify the client that HTTP/3 is available. See the [EnableAltSvc registry key](https://techcommunity.microsoft.com/t5/networking-blog/enabling-http-3-support-on-windows-server-2022/ba-p/2676880). Note this requires netsh sslcert bindings that use host names rather than IP addresses.
:::moniker-end
:::moniker range="= aspnetcore-6.0"
[HTTP/3](https://datatracker.ietf.org/doc/rfc9114/) is supported with ASP.NET Core in the following IIS deployment scenarios:
* In-process
* [Out-of-Process](xref:host-and-deploy/iis/index#out-of-process-hosting-model). In Out-of-Process, IIS responds to the client using HTTP/3, but the reverse proxy connection to the [Kestrel server](xref:fundamentals/servers/kestrel) uses HTTP/1.1.
For more information on the in-process and out-of-process hosting models, see <xref:host-and-deploy/aspnet-core-module>.
The following requirements also need to be met:
* Windows Server 2022 / Windows 11 or later
* An `https` url binding is used.
* The [EnableHttp3 registry key](https://techcommunity.microsoft.com/t5/networking-blog/enabling-http-3-support-on-windows-server-2022/ba-p/2676880) is set.
For an in-process deployment when an HTTP/3 connection is established, [`HttpRequest.Protocol`](xref:Microsoft.AspNetCore.Http.HttpRequest.Protocol*) reports `HTTP/3`. For an out-of-process deployment when an HTTP/3 connection is established, [`HttpRequest.Protocol`](xref:Microsoft.AspNetCore.Http.HttpRequest.Protocol*) reports `HTTP/1.1` because that is how IIS proxies the requests to Kestrel.
HTTP/3 is discovered as an upgrade from HTTP/1.1 or HTTP/2 via the `alt-svc` header. That means the first request will normally use HTTP/1.1 or HTTP/2 before switching to HTTP/3. IIS doesn't automatically add the `alt-svc` header, it must be added by the application. The following code is a middleware example that adds the `alt-svc` response header.
```C#
app.Use((context, next) =>
{
context.Response.Headers.AltSvc = "h3=\":443\"";
return next(context);
});
```
Place the preceding code early in the request pipeline.
IIS also supports sending an AltSvc HTTP/2 protocol message rather than a response header to notify the client that HTTP/3 is available. See the [EnableAltSvc registry key](https://techcommunity.microsoft.com/t5/networking-blog/enabling-http-3-support-on-windows-server-2022/ba-p/2676880). Note this requires netsh sslcert bindings that use host names rather than IP addresses.
