REQUEST_UPDATE — MoQT draft-16 §9.11.

The sender of a request (SUBSCRIBE, PUBLISH, FETCH, TRACK_STATUS, PUBLISH_NAMESPACE, SUBSCRIBE_NAMESPACE) can later send a REQUEST_UPDATE to modify it. A subscriber can also send REQUEST_UPDATE to modify parameters of a subscription established with PUBLISH.

The receiver of a REQUEST_UPDATE MUST respond with exactly one REQUEST_OK or REQUEST_ERROR message indicating if the update was successful.

If a parameter previously set on the request is not present in REQUEST_UPDATE, its value remains unchanged.

There is no mechanism to remove a parameter from a request.

The format of REQUEST_UPDATE is as follows:

REQUEST_UPDATE Message {
  Type (i) = 0x2,
  Length (16),
  Request ID (i),
  Existing Request ID (i),
  Number of Parameters (i),
  Parameters (..) ...
}

Figure 13: MOQT REQUEST_UPDATE Message

Request ID: See Section 9.1.
Existing Request ID: The Request ID of the request this message is updating. This MUST match the Request ID of an existing request. The receiver MUST close the session with PROTOCOL_VIOLATION if the sender specifies an invalid Existing Request ID, or if the parameters included in the REQUEST_UPDATE are invalid for the type of request being modified.
Parameters: The parameters are defined in Section 9.2.2.

9.11.1. Updating Subscriptions

When a subscriber decreases the Start Location of the Subscription Filter (see Section 5.1.2), the Start Location can be smaller than the Track's Largest Location, similar to a new Subscription. FETCH can be used to retrieve any necessary Objects smaller than the current Largest Location.

When a subscriber increases the End Location, the Largest Object at the publisher might already be larger than the previous End Location. This will create a gap in the subscription. The REQUEST_OK in response to the REQUEST_UPDATE will include the LARGEST_OBJECT parameter, and the subscriber can issue a FETCH to retrieve the omitted Objects, if any.

When a subscriber narrows their subscription (increase the Start Location and/or decrease the End Group), it might still receive Objects outside the new range if the publisher sent them before the update was processed.

When a subscription update is unsuccessful, the publisher MUST also terminate the subscription with PUBLISH_DONE with error code UPDATE_FAILED.