60 Chapter 3
Basically, the status code can be provisional (in progress), successful, redirect, or one
of three failure response categories. Within each of these categories there are a number
of predefined codes.
There may also be headers containing other parameters below the status line. The
headers follow directly after the status line and the start line, and before the message
body. All SIP messages are in text format (rather than binary coded); therefore, they
are very simple to decode. Likewise, the various headers and parameters are also very
easy to understand.
The message body can contain content, or it can contain details about the bearer traffic
being sent in a different message (and protocol such as Real Time Protocol, or RTP).
The message body for a voice call, for example, would contain the Session Description
Protocol (SDP), describing the voice content itself (such as the encoding used and any
other special requirements).
Note that a SIP message does not necessarily have to have session description content
contained within the message body. The message body can be used to transport anything,
in addition to a session description. For example, text messaging is carried through the
network using the message body of a SIP message. This simplifies the network requirements
to support messaging significantly (and is also analogous with SS7, where text
messages are often carried in the Transaction Capabilities Application Part, or TCAP).
Pages:
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146