[1] RFC 3427 - Change Process for the Session Initiation Protocol (SIP) (2018-04-08 17:59:02 +09:00) https://tools.ietf.org/html/rfc3427

[2] RFC 6709 - Design Considerations for Protocol Extensions (2018-04-05 17:00:08 +09:00) https://tools.ietf.org/html/rfc6709#section-3.2.1

[3] RFC 5727 - Change Process for the Session Initiation Protocol (SIP) and the Real-time Applications and Infrastructure Area (2018-04-08 17:59:09 +09:00) https://tools.ietf.org/html/rfc5727#section-4