https://app.knowbase.one/vault/rfc/docid/6315
RFC6809: Mechanism to Indicate Support of Features and Capabilities in the Session Initiation Protocol (SIP)
Internet Engineering Task Force (IETF) C. Holmberg Request for Comments: 6809 I. Sedlacek Category: Standards Track Ericsson ISSN: 2070-1721 H. Kaplan ...