tentatively called “SDPng” for Next Generation [13]. This new protocol will
have more advanced media negotiation and description capabilities. It is likely
that support of SDP will remain in the base SIP specification, with successors
to SDP being optional to support.Session Modification
Once a session has been established using the INVITE/200/ACKsequence, it
can be modified by another INVITE/200/ACKsequence, sometimes referred
to as a re-INVITE. Since there can only be one pending SIP request at a time, a
re-INVITEcannot be sent until the initial INVITEhas been completed with an
ACK. The re-INVITEcan be done by either party and uses the same To, From
(including tags), and Call-IDas the INVITE. However, the SDP in the re-
INVITEis assumed to replace the initial INVITESDP, if the re-INVITEis suc-
cessful. If the re-INVITEfails in any way or is refused, the original SDP and
the original media session will continue until a BYEis sent by either party.Table 6.5 SDP Response Example with Line-by-Line Description
LINE DESCRIPTION
v=0 Version – Current version number of SDP
( 0 ) — not used by SIP.
o=userb 2890844342 2890844543 Origin — Not used by SIP.
IN IP4 client.example.net
s=- Subject.
c=IN IP4 16.22.3.1 Connection — Network (INfor Internet),
address type (IP4for IP version 4) and
address (16.22.3.1).
t=0 0 Time — Start and stop time — not used
by SIP.
m=video 0 RTP/AVP 34 98 Media — media type (video), port number
is set to zero, which indicates that the video
session has been declined.
m=audio 6002 RTP/AVP 97 Media — Media type (audio), port number
( 6002 ), type (RTP/AVPprofile), and
number (profile 4). By specifying a nonzero
port number, the audio session has been
accepted.
a=rtpmap:97 iLBC/8000 Attribute — rtpmaplists attributes of
RTP/AVP audio profile 97 , including codec
(iLBC) and sampling rate ( 8000 Hz).114 Chapter 6