The extra headers are encoded in the JSON Data Interchange Standard as defined by ECMA-404. All extra headers are optional as far as the format is concerned.
Extra headers must follow these rules:
All extra headers are contained in an anonymous (unnamed) object
All entries are key-value pairs where values can be any valid JSON type
The key value of “FDSN” in the root object is reserved for values defined by the FDSN
Extra headers are specified and documented in JSON Schema.
FDSN Reserved Headers
The “FDSN” key at the root of the extra headers are reserved for definition by the FDSN.
See FDSN Reserved Headers for documentation of these headers.
See A: Extra header examples for an example of FDSN extra headers.
Guidelines for Extension
Network operators, manufacturers, data centers, users and other agencies may wish to define their own extra headers. The following guidelines should be considered, in particular for data that is expected to reside in a public repository:
All headers defined by a group or agency should be contained in a JSON object that is the value of a key in the root container with a clearly identifiable name, i.e. at the same level as “FDSN”.
Creation of a JSON Schema document describing the field(s) is strongly recommended. The schema should be submitted to the FDSN to be made publically available.
Headers that would be generally useful should be submitted to the FDSN for consideration of being added to the reserved headers for general definition and use.
Multiple JSON Schema documents are easily combined for use in validating extra headers that may contain headers defined in multiple schema documents.
See Non-FDSN extra headers for an example of non-FDSN extra headers.