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{
  "type": "header",
  "name": "RequestHeader",
  // Version 0 of the RequestHeader is only used by v0 of ControlledShutdownRequest.
  //
  // Version 1 is the first version with ClientId.
  //
  // Version 2 is the first flexible version.
  "validVersions": "0-2",
  "flexibleVersions": "2+",
  "fields": [
    { "name": "RequestApiKey", "type": "int16", "versions": "0+",
      "about": "The API key of this request." },
    { "name": "RequestApiVersion", "type": "int16", "versions": "0+",
      "about": "The API version of this request." },
    { "name": "CorrelationId", "type": "int32", "versions": "0+",
      "about": "The correlation ID of this request." },
    ...
    // ----- new optional field ----
    { "name": "PrincipalName", "type": "string", "tag": 0, "taggedVersions": "2+", "ignorable": true,
      "about": "Optional value of the principal name when the request is redirected by a broker." },
    // ----- end new field ---------
  ]
}

New Security Endpoint

To maintain the same level of security going along in the post-ZK world, the broker-controller communication should have extra security guarantee. To make that happen, we will introduce a separate `ControllerEndpoint` for user to configure the exclusive access of forwarding requests to only go through this tunnel. Getting a separate communication channel also helps differentiating whether the request is from admin client or forwarded, which means the forwarding brokers don't have to bump the request version unnecessarily.

This part of the design is dependent on the Controller refactoring effort, and more details shall reveal for subsequent KIPs. It won't block the acceptance for this KIP either, since the forwarding behavior shall be the same. 

Monitoring Metrics

To effectively monitor the admin request forwarding status, we would add two metrics: num-client-forwarding-to-controller-rate and num-client-fowarding-to-controller-count to visualize the progress. They are set up to track the number of clients being on a pending forward request for each broker.

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