188.8.131.52.9 Handling of Callbacks
Method calls declared as callbacks have some additional rules for handling on the network compared to calls without this attribute. A callback on the network is represented as a regular RPC except that the direction of the PDUs is reversed. The server sends one or more request PDUs, and the client responds with one or more response PDUs. The server MUST NOT send request PDUs while in any state other than the Dispatch state, and a client SHOULD NOT accept callbacks in any state other than the Wait For Response state.
Callbacks are allowed recourse to any level that the implementation is willing to support. That is, if a client gets a callback, it SHOULD initiate another RPC method call by sending more request PDUs instead of replying to the previous request. This same rule applies for the server. For the server, if it sends one or more request PDUs during the Dispatch state, the call that is in the Dispatch state is called a nesting or outer call. The callback call that the request PDUs sent from the server is called a nested or inner call. For the client, if it sends one or more request PDUs during the Wait For Response state, the call that is in the Wait For Response state is called a nesting or outer call. The callback call that the request PDUs sent from the client is called a nested or inner call. Callback calls by definition use the presentation and security context of the nesting call and MUST NOT send bind or alter_context PDUs.
The call_id field for all request and response PDUs of a nested callback MUST be the same as the call_id of the request/response PDUs of the nesting callback.