This interface represents an Extension SIP header that was not defined at the
baseline of this specification. Extension Headers can be added as
required by extending this interface assuming other endpoints understand the Header.
Any Header that extends this class must define a "NAME" String constant identifying the name
of the extension Header. A server must ignore Headers that it does not
understand. A proxy must not remove or modify Headers that it does not
understand. Implementation note : The implementation of any new headers
added since version 1.1 of this specification
implement ExtensionHeader for backwards compatibility (even if these
headers are defined in the current version of this specification).
Headers that are not part of the current version of this specification
- BEA Systems, NIST