Every request message includes a Required Request ID Delta field that specifies a dependency on a prior request. The Required Request ID is computed as:
Required Request ID = Request ID - (2 × Required Request ID Delta)
A Required Request ID Delta of 0 indicates no dependency. When a dependency exists, the receiver MUST deliver the referenced request to the application before delivering the dependent request. If the required request does not arrive, the receiver will time out the dependent request.
The delta is scaled by two because request IDs from each endpoint use alternating parity (odd or even), so valid dependencies always differ by a multiple of two. An endpoint MUST close the session with INVALID_REQUIRED_REQUEST_ID if it receives a delta where 2 × Required Request ID Delta exceeds the Request ID.