## Proxy implementation

In its initial implementations, the ztunnel was actually implemented in 3 different ways: a bespoke Rust implementation, a bespoke Go implementation, and in Envoy.

In the end, [after evaluation](https://docs.google.com/document/d/1c2123cKuYsBDpIon9FFdctWTUIMFweSjgwG7r8l3818/edit), the decision was to move forward with a Rust implementation.

environment-specific details. This means that the test code can (and should, where possible) be written in an
environment-agnostic manner, so that they can be run against any Istio implementation.

For example, the following code creates and then interacts with a Galley and Pilot component:

```go
func TestMyLogic(t *testing.T) {
    framework.
        NewTest(t).

	RootCertFilePath string
}

// Client interface defines the clients need to implement to talk to CA for CSR.
// The Agent will create a key pair and a CSR, and use an implementation of this
// interface to get back a signed certificate. There is no guarantee that the SAN
// in the request will be returned - server may replace it.
type Client interface {

messages:
  - name: "InternalError"
    code: IST0001
    level: Error
    description: "There was an internal error in the toolchain. This is almost always a bug in the implementation."
    template: "Internal error: %v"
    args:
      - name: detail
        type: string

  - name: "Deprecated"
    code: IST0002
    level: Warning

	// queryable and should be preserved when modifying objects.
	Annotations map[string]string `json:"annotations,omitempty"`

	// ResourceVersion is an opaque identifier for tracking updates to the config registry.
	// The implementation may use a change index or a commit log for the revision.
	// The config client should not make any assumptions about revisions and rely only on
	// exact equality to implement optimistic concurrency of read-write operations.
	//

	"istio.io/istio/pkg/config/resource"
)

var (
	// InternalError defines a diag.MessageType for message "InternalError".
	// Description: There was an internal error in the toolchain. This is almost always a bug in the implementation.
	InternalError = diag.NewMessageType(diag.Error, "IST0001", "Internal error: %v")

	// Deprecated defines a diag.MessageType for message "Deprecated".

| HOST_PROBE_SNAT_IPV6 | "fd16:9254:7127:1337:ffff:ffff:ffff:ffff" | IPv6 link local ranges are designed to be collision-resistant by default, and so this probably never needs to be overridden |

## Sidecar Mode Implementation Details

Istio CNI injection is currently based on the same Pod annotations used in init-container/inject mode.

### Selection API

- plugin config "exclude namespaces" applies first
- ambient is enabled if:

func NewCollection[I, O any](c Collection[I], hf TransformationSingle[I, O], opts ...CollectionOption) Collection[O] {
	// For implementation simplicity, represent TransformationSingle as a TransformationMulti so we can share an implementation.
	hm := func(ctx HandlerContext, i I) []O {
		res := hf(ctx, i)
		if res == nil {
			return nil
		}
		return []O{*res}
	}

)

// DeploymentController implements a controller that materializes a Gateway into an in cluster gateway proxy
// to serve requests from. This is implemented with a Deployment and Service today.
// The implementation makes a few non-obvious choices - namely using Server Side Apply from go templates
// and not using controller-runtime.
//
// controller-runtime has a number of constraints that make it inappropriate for usage here, despite this

	// DisableAutomountSAToken indicates to opt out of auto mounting ServiceAccount's API credentials
	DisableAutomountSAToken bool

	// Ports for this application. Port numbers may or may not be used, depending
	// on the implementation.
	Ports Ports

	// ServiceAnnotations is annotations on service object.
	ServiceAnnotations map[string]string

	// ServiceLabels is the labels on service object.
	ServiceLabels map[string]string