Internet Communications Using SIP : Delivering VoIP and Multimedia Services With Session Initiation Protocol {2Nd Ed.}

In the example of Figure 6.4, a call is set up between two user agents using
the media description sdp1, carried in the initial INVITE and 200 OK
response. The called party tries to change the session parameters by sending
another INVITEwith a new message body sdp2’. However, this is not
acceptable to the other party, and the re-INVITE fails with a 405 Not
Acceptableresponse in Message 6. The media session continues using the
initial media parameters. The called party tries one more time and this time
the re-INVITEsucceeds, and the old media session is terminated and a new
one using sdp2’’and sdp1’’is established with different values in each
direction. Note that the re-INVITEs do not usually generate provisional
responses (such as 180 Ringing), since the two parties are already communi-
cating with each other.
Note that a re-INVITEmay change any of the media characteristics, includ-
ing the session type, codec used, even the source IP addresses and port number.

Figure 6.4 Session modification example using INVITE

SIP User Agent 1

1 INVITE sdp1

The failure of the

re-INVITE does not

cause the initial

Media Session to Fail

The success of the

second re-INVITE

establishes a New

Media Session which

replaces the old

session.

SIP User Agent 2

5 INVITE sdp2^1

7 ACK

10 ACK

8 INVITE sdp2^11

6 405 Not Acceptable

9 200 OK 5 sdp1^11

Media Session

2 180 Ringing

3 200 OK sdp3

4 ACK

New Media Session

SIP Overview 115