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This document provides an overview of the architecture and design decisions around Ztunnel, the node-proxy component in ambient mode.

## Background and motivation

Motivations to implement ztunnel generally came from two areas.

First, and most importantly, it serves as a means to implement the real goal: waypoints.

## WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
## See the License for the specific language governing permissions and
## limitations under the License.

# Our build tools, post jammy, breaks old versions of docker.
# These old versions are surprisingly common, so build in a check here
define warning
Docker version is too old, please upgrade to a newer version.
endef
ifneq ($(findstring google,$(HOSTNAME)),)

  // line breaks. The message must not contain line breaks.
  // If unset, the message is "failed rule: {Rule}".
  // e.g. "must be a URL with the host matching spec.host"
  // If the Expression contains line breaks. Message is required.
  // The message must not contain line breaks.
  // If unset, the message is "failed Expression: {Expression}".
  // +optional

See the [krt README](../../pkg/kube/krt/README.md) for more information.

*krt is currently experimental; please ask maintainers before utilizing it in new areas.*

## Writing Controllers

The `controllers` package offers a variety of helpers for writing controllers.

See the
[design doc](https://docs.google.com/document/d/11j9ZtYWNWnxQYnZy8ayZav1FMwTH6F6z6fkDYZ7V298/edit#heading=h.qex63c29z2to)
for a more complete design description. The operator code is divided roughly into five areas:

1. [IstioOperatorSpec API](#istiooperatorspec-api) and related infrastructure, which is expressed as a
[proto](https://github.com/istio/api/blob/master/operator/v1alpha1/operator.proto) and
compiled to [Go