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.

## Reference

### Design details

Broadly, `istio-cni` accomplishes ambient redirection by instructing ztunnel to set up sockets within the application pod network namespace, where:

- one end of the socket is in the application pod

- [node selector](https://kubernetes.io/docs/concepts/configuration/assign-pod-node/#nodeselector)
- [toleration](https://kubernetes.io/docs/concepts/configuration/taint-and-toleration/)
- [affinity and anti-affinity](https://kubernetes.io/docs/concepts/configuration/assign-pod-node/#affinity-and-anti-affinity)
- [deployment strategy](https://kubernetes.io/docs/concepts/workloads/controllers/deployment/)

  // controller. If a Job is suspended after creation (i.e. the flag goes from
  // false to true), the Job controller will delete all active Pods associated
  // with this Job. Users must design their workload to gracefully handle this.
  // Suspending a Job will reset the StartTime field of the Job, effectively
  // resetting the ActiveDeadlineSeconds timer too. Defaults to false.
  //
  // +optional

# Istio operator code overview

## Introduction

This document covers primarily the code, with some background on how the design maps to it.
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

  // +optional
  optional ParamKind paramKind = 1;

  // MatchConstraints specifies what resources this policy is designed to validate.
  // The AdmissionPolicy cares about a request if it matches _all_ Constraints.
  // However, in order to prevent clusters from being put into an unstable state that cannot be recovered from via the API