When a message is received which has one or more unrecognized information elements, the receiving entity shall check whether any are encoded to indicate "comprehension required" (refer to Table 4-3/Q.931 for information element identifiers reserved with this meaning). If any unrecognized information element is encoded to indicate"comprehension required" then the procedures in 5.8.6.1 are followed, i.e.as if a "missing mandatory information element" error condition had occurred.If all unrecognized information elements are not encoded to indicate"comprehension required", then the receiving entity shall proceed as follows:
Action shall be taken on the message and those information elements which are recognized and have valid content. When the received message is other than DISCONNECT, RELEASE or RELEASE COMPLETE, a STATUS message may be returned containing one cause information element. The STATUS message indicates the call state of the receiver after taking action on the message. The Cause information element shall contain cause No. 99,"information element non-existent or not implemented", and the diagnostic field, if present, shall contain the information element identifier for each information element which was unrecognized.
Subsequent actions are determined by the sender of the unrecognized information elements. If a clearing message contains one or more unrecognized information elements, the error is reported to the local user in the following manner:
a) When a DISCONNECT message is received which has one or more unrecognized information elements, a RELEASE message with cause No. 99, "information element non-existent or not implemented", shall be returned. The Cause information element diagnostic field, if present, shall contain the information element identifier for each information element which was unrecognized.
b) When a RELEASE message is received which has one or more unrecognized information elements, a RELEASE COMPLETE message with cause No. 99 "information element non-existent or not implemented", shall be returned. The Cause information element diagnostic field, if present, shall contain the information element identifier for each information element which was unrecognized.
c) When a RELEASE COMPLETE message is received which has one or more unrecognized information elements, no action shall be taken on the unrecognized information.
Note - The diagnostic(s) of cause No. 99 facilitates the decision in selecting an appropriate recovery procedure at the reception of a STATUS message. Therefore, it is recommended to provide cause No. 99 with diagnostic(s) if a layer 3 entity expects the peer to take an appropriate action at the receipt of a STATUS message, although inclusion of diagnostic(s) is optional.