Figure 6.3 Successful session establishment example using INVITESDP was initially developed in the framework of the Internet multimedia
architecture as a sort of “TV Guide” for multicast multimedia sessions over the
Internet. Some of the capabilities of SDP are, therefore, not used in SIP, such as
advertising the origin of the session advertisement, the subject of the session
and the scheduling function based on starting and end time, with repeat fea-
tures (the t=... line).
The negotiation is an offer answer model defined in RFC 3264 in which one
user agent proposes one or more media types, and the other user agent either
accepts or declines each media session in a response. Referencing Figure 6.3,
usually, the offer is made in the initial INVITEby the caller, and the response
is carried in the 200 OK. However, the caller can allow the called party to select
the media session type by not sending SDP in the INVITE. In this case, the
called party makes the offer in the 200 OK(or in a reliable provisional
response), and the caller responds in the ACK. In the SDP body attached to the
SIP header, the user agents specify the media type, codec, IP address, and port
number for each media stream. More than one codec can be specified for each
media type. Once an offered codec has been accepted, user agents must be pre-
pared to receive media with that codec for the duration of the session. For
examples of offer/answer SDP exchanges, see RFC 4317.
The example SDP offer shown in Table 6.4 contains two media lines: one for
video and one for audio. Each media line has two possible alternative codecs
that the calling user agent supports.SIP User Agent1 INVITE
Successful SIP
session establishment
involves an INVITE,
optional provisional
responses, a 200 OK
final response, then
an ACK.SIP User Agent5 ACKMedia Session2 100 Trying3 180 Ringing4 200 OK112 Chapter 6