Figure 21.1 The growth of SIP-related IETF standards (http://rfc3261.net).The core SIP protocol is probably more than 95 percent fully developed and
stable, and we may expect only minor extensions in the future that will not
affect the SIP core protocol.
What is the SIP core protocol? For simplicity, and for lack of a widely
accepted definition, you may assume that the SIP core protocol includes every-
thing described in this book and in the quoted references. If it’s not in the
quoted references, we assume that it is probably not part of the SIP core.
Many SIP extensions have been proposed and accepted in IETF informa-
tional RFCs, to meet the business needs of certain types of service providers
(such as cable, telephone companies, and especially 3G mobile providers).
Such extensions are part of the so-called P- extensions, where P- stands for
“preliminary,” “private,” or “proprietary.” RFC 2427 specifies that, for P-
extensions, “it is valid to allow for the development of SIP extensions that...
are private or proprietary in nature, because a characteristic motivating them
is usage that is known not to fit the Internet architecture for SIP.” A large num-
ber of the IETF documents counted in Figure 21.1 are of this nature.10009008007006005004003002001000
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IPTEL354 Chapter 21