When a message is received which has one or more non-mandatory information elements with invalid content, action shall betaken on the message and those information elements which are recognized and have valid content. 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. 100 "invalid information element contents", and the diagnostic field, if present, shall contain the information element identifier for each information element which has invalid contents.
Information elements with a length exceeding the maximum length (given in 3) will be treated as an information element with content error. But for access information elements(e.g. User-user information element, Called party subaddress information element), cause No. 43, "access information discarded" is used instead of cause No. 100 "invalid information element contents". However, in some networks,access information elements may be truncated and processed.
The Call identity information element shall have a special treatment and shall be truncated and processed in the case that it exceeds the maximum length implemented.
Note - As an option of user equipment (e.g. NT2), cause values, location codes, and diagnostics which are not understood by the NT2 may be accepted, or in the case of an NT2, passed on to another entity(e.g. user or NT2) instead of ignoring the cause information element contents and optionally sending a STATUS message with cause No. 100, invalid information element contents. This option is intended to aid user equipment to be compatible with future additions of cause values, location codes and diagnostics to the Recommendation.
If the network cannot interpret the Low layer compatibility or the High layer compatibility information elements, it may accept these information elements without declaring a protocol error.