SIP is flexible and provides a lot of opportunity for operators looking to simplify their
networks while enabling interoperability and security. This is the spirit of the 3GPP
specifications for the IMS, and why SIP was chosen for session control. The structure
is also simple, and it allows for extensions to be defined by organizations looking to
enhance the capabilities of the SIP protocol.
Indeed the 3GPP has done just exactly that by submitting protocol extensions to the
IETF to support the various functions defined by the 3GPP for supporting the IMS.
Charging in the IMS is a good example of protocol extensions. Security is another
good example where the protocol has been expanded through protocol extensions. All
of these extensions are discussed throughout the book when we discuss the various
procedures used throughout the IMS.
SIP Methods
SIP methods can be thought of as message types. They identify the request being made
by the user device (or the network entity in some cases). As we will discuss later, these
requests are used both by the destinations to determine what action is being requested
and by the intermediary network elements (such as the Call Session Control Function)
to determine how the message should be routed through the network, and in some
cases how the message should be treated prior to delivery to the end destination.
Pages:
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147