The Require header field contains a list of option tags. Each option tag defines a SIP extension that MUST be understood to process the request. Frequently, this is used to indicate that a specific set of extension header fields need to be understood. A UAC compliant to this specification MUST only include option tags corresponding to standards-track RFCs.
If a server does not understand the option, it must respond by returning a BAD_EXTENSION Response and list those options it does not understand in the UnsupportedHeader.
Proxy and redirect servers must ignore features that are not understood. If a particular extension requires that intermediate devices support it, the extension must be tagged in the ProxyRequireHeader as well.