mustUnderstand

MustUnderstand is an attribute used in SOAP messages to designate header blocks that must be processed by a recipient for the message to be considered valid. It is attached to a header element within the SOAP envelope and signals that the processing of that header is mandatory for the intended recipient or any intermediaries along the path.

In SOAP 1.1, mustUnderstand is expressed as a header attribute with values "1" or "0", where "1" indicates the header must be understood and "0" means it can be ignored if not understood. In SOAP 1.2, the attribute is a boolean value, typically "true" or "false", with the same obligation associated with "true".

If a processor encounters a header marked mustUnderstand and cannot understand or process it, it must generate a fault, commonly a MustUnderstand fault, and the message may be rejected or trigger error handling. The fault typically identifies the header that could not be understood and indicates the processing failure.

MustUnderstand headers are often used for critical extensions such as security, routing, or transaction information. They ensure that essential metadata is not silently ignored during routing through intermediaries and end recipients. Implementations vary in how strictly they enforce mustUnderstand, but interoperability relies on adhering to the SOAP specification.