Update index.md

Author: jakebennert

geps/gep-3171/index.md

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+# GEP-3171: Percentage-based Request Mirroring
 
+* Issue: [#3171](https://github.com/kubernetes-sigs/gateway-api/issues/3171)
+* Status: **Provisional**
+
+(See status definitions [here](/geps/overview/#gep-states).)
+
+## TLDR
+
+Enhance the existing [Request Mirroring](https://gateway-api.sigs.k8s.io/guides/http-request-mirroring/) feature by allowing users to specify a percentage of requests they'd like mirrored.
+
+## Goals
+
+Successfully implement the feature.
+
+## Introduction
+
+[Request Mirroring](https://gateway-api.sigs.k8s.io/guides/http-request-mirroring/) is a feature that allows a user to mirror requests going to some backend A along to some other specified backend B. Right now Request Mirroring is an all or nothing feature – either 100% of request are mirrored, or 0% are. Percentage-based Request Mirroring will allow users to specify a percentage of requests they'd like mirrored as opposed to every single request.   
+
+This feature is already [supported by Envoy](https://www.envoyproxy.io/docs/envoy/latest/api-v3/config/route/v3/route_components.proto#envoy-v3-api-msg-config-route-v3-routeaction-requestmirrorpolicy), so adding it for the Gateway API would enable better integration between the two products. There's also an existing user desire for this feature on the [HAProxy side](https://www.haproxy.com/blog/haproxy-traffic-mirroring-for-real-world-testing) and [NGINX side](https://alex.dzyoba.com/blog/nginx-mirror/). Since Request Mirroring is already supported by the Gateway API, Percentage-based Request Mirroring would a clear improvement on this pre-existing feature.
+
+## API
+
+This GEP proposes the following API changes:
+
+* Add utility type `Fraction` to [v1/shared_types.go](https://github.com/kubernetes-sigs/gateway-api/blob/cb5bf1541fa70f0692aebde8c64bba434cf331b6/apis/v1/shared_types.go) and include the equivalent type `Percentage`:
+
+
+```go
+type Fraction struct {
+        // +optional
+        // +kubebuilder:validation:Minimum=0
+        Numerator int32 `json:"numerator,omitempty"`
+
+        // +optional
+        // +kubebuilder:default=100
+        // +kubebuilder:validation:Minimum=1
+        Denominator int32 `json:"denominator,omitempty"`
+}
+
+type Percentage Fraction
+```
+
+
+* Update the `HTTPRequestMirrorFilter` struct to include a `MirrorPercent` field of type `Percentage`:
+
+
+```go
+// HTTPRequestMirrorFilter defines configuration for the RequestMirror filter.
+type HTTPRequestMirrorFilter struct {
+        // BackendRef references a resource where mirrored requests are sent.
+        //
+        // Mirrored requests must be sent only to a single destination endpoint
+        // within this BackendRef, irrespective of how many endpoints are present
+        // within this BackendRef.
+        //
+        // If the referent cannot be found, this BackendRef is invalid and must be
+        // dropped from the Gateway. The controller must ensure the "ResolvedRefs"
+        // condition on the Route status is set to `status: False` and not configure
+        // this backend in the underlying implementation.
+        //
+        // If there is a cross-namespace reference to an *existing* object
+        // that is not allowed by a ReferenceGrant, the controller must ensure the
+        // "ResolvedRefs"  condition on the Route is set to `status: False`,
+        // with the "RefNotPermitted" reason and not configure this backend in the
+        // underlying implementation.
+        //
+        // In either error case, the Message of the `ResolvedRefs` Condition
+        // should be used to provide more detail about the problem.
+        //
+        // Support: Extended for Kubernetes Service
+        //
+        // Support: Implementation-specific for any other resource
+        BackendRef BackendObjectReference `json:"backendRef"`
+      
+        // MirrorPercent represents the fraction of requests that should be
+        // mirrored to BackendRef.
+        //
+        // If MirrorPercent is not specified, then 100% of requests will be
+        // mirrored.
+        //
+        // +optional
+        MirrorPercent Percentage `json:"mirrorPercent,omitempty"`
+}
+```