“…SIP uses HTTP messages in Uniform Resource Identifiers (URIs) requests that are initiated between SIP peers, while SMTP is used in the SIP headers. SIP security mechanisms were described in the following RFCs: RFC 3261 (Rosenberg, et al, 2003), RFC 3329 (Camarillo, 2004), RFC 4474 (Peterson et al, 2006), and RFC 7057 (Winter et al, 2013). These mechanisms are related to HTTP Digest authentication, S/MIME, TLS, and IPSec models.…”