Web server implementation details related to HTTP requests and responses are defined in interfaces. These interfaces are used by server implementations and middleware to create and modify the application's hosting pipeline.
ASP.NET Core defines a number of HTTP feature interfaces in `Microsoft.AspNetCore.Http.Features` which are used by servers to identify the features they support. The following feature interfaces handle requests and return responses:
Defines support for [HTTP Upgrades](https://tools.ietf.org/html/rfc2616.html#section-14.42), which allow the client to specify which additional protocols it would like to use if the server wishes to switch protocols.
The `Features` property of `HttpContext` provides an interface for getting and setting the available HTTP features for the current request. Since the feature collection is mutable even within the context of a request, middleware can be used to modify the collection and add support for additional features.
While servers are responsible for creating the feature collection, middleware can both add to this collection and consume features from the collection. For example, the `StaticFileMiddleware` accesses the `IHttpSendFileFeature` feature. If the feature exists, it's used to send the requested static file from its physical path. Otherwise, a slower alternative method is used to send the file. When available, the `IHttpSendFileFeature` allows the operating
system to open the file and perform a direct kernel mode copy to the network card.
Additionally, middleware can add to the feature collection established by the server. Existing features can even be replaced by middleware, allowing the middleware to augment the functionality of the server. Features added to the collection are available immediately to other middleware or the underlying application itself later in the request pipeline.
By combining custom server implementations and specific middleware enhancements, the precise set of features an application requires can be constructed. This allows missing features to be added without requiring a change in server, and ensures only the minimal amount of features are exposed, thus limiting attack surface area and improving performance.
## Summary
Feature interfaces define specific HTTP features that a given request may support. Servers define collections of features, and the initial set of features supported by that server, but middleware can be used to enhance these features.
