The IMS: IP Multimedia Concepts And Services, Second Edition

13.3. Selected SDP lines

13.3.1. Protocol version line

The SDP protocol version is 0 and, therefore, the v-line in an SDP message must always be set to 0:

v=0

13.3.2. Connection information line

The c-line must be either present at the session level or media level. It must be present at the media level if it is not present at the session level. If it is present at both levels, then media-level connection information overrides session-level information:

c=<network type> <address type> <network address>

The c-line has three parameters:

• Network type – the only currently defined network type is the Internet. The value appears as "IN".

• Address type – there are two address types, IP4 or IP6.

• Network address – this parameter identifies the Internet Protocol (IP) address or domain name where media are received.

13.3.3. Media line

The m-line carries information about the media, including transport information. The syntax is as follows:

m=<media> <port> <transport> <format-list>

The m-line has four parameters:

• Media – the type of media (e.g., audio, video, game).

• Port – contains the port number where these media can be received.

• Transport – the transport protocol to use, either the User Datagram Protocol (UDP) or Real-time Transport Protocol Audio and Video Profile (RTP/AVP) (RTP is explained in Chapter 15).

• Format-list – contains more information about the media, usually payload types defined in RTP/AVPs (see Section 15.3 for details).

If the transport is RTP/AVP, then the port number for ...