353At the time of the first edition of this book, the SIP standard was RFC 2543,
which was replaced in June 2002 by RFC 3261. Numerous other RFCs have
been published since with various extensions to SIP. The related working
groups (such as SIMPLE for IM and presence, SIPPING for applications,
ENUM, AVT, and so on) have used RFC 3261 as a basis for numerous exten-
sions to SIP and various applications.
The growth in IETF standards documents for Internet communications is
rather formidable and reflects the work being done to migrate all real-time
communications to the Internet. This growth is illustrated in Figure 21.1. The
large volume of IETF documents is the price paid for making SIP the univer-
sally accepted standard by most wireline and wireless service providers. Most
IM service providers use SIP and SIMPLE as well. SIP is the common denomi-
nator for all IM companies, since even IM services such as Skype or Google are
using SIP to connect to the rest of the world.
The IETF has not been very consequent with upholding a single standard
for IM as is the case of SIP for voice and multimedia. RFC 3920 and RFC 3921
describe the XMPP protocol for IM, which is not applicable as well, however,
for voice and multimedia, as SIP is.