The IMS: IP Multimedia Concepts And Services, Second Edition

13.1. SDP message contents

An SDP message contains three levels of information:

• Session-level description – this includes the session identifier and other session-level parameters, such as the IP address, subject, contact info about the session and/or creator.

• Timing description – start and stop times, repeat times, one or more media-level descriptions.

• Media type and format – transport protocol and port number, other media-level parameters. Note that the media address may be different from the signalling address.

The three levels of information must appear in the order described above. The SDP message is a collection of SDP lines.

13.1.1. Session description

Table 13.1 lists all the session-level description lines and indicates their mandate and the letter used as the line name.

Table 13.1. Session-level description SDP lines.

Field	Description	Mandate
v	Protocol version	m
o	Origin and session ID	m
s	Session name	m
i	Session information	o
u	URI for sesssion	o
e	Email address	o
p	Phone number	o
c	Connection information	m[]
b	Bandwidth information	m
z	Time zone correction	o
k	Encryption key	o
a	Attribute lines	o
[]

^[] Not required if present in every media line.

13.1.2. Time description

Table 13.2 lists all the time description lines and indicates their mandate and the letter used as the line name.

Table 13.2. Time-level description SDP lines.

Field	Description	Mandate
t	Time session is active	m
r	Repeat times	o

13.1.3. Media description

Table 13.3 lists all the media-level description lines and indicates their mandate and the ...