m=audio 49170 RTP/AVP 97
a=rtpmap:97 iLBC/8000
[Second-Answer]
v=0
o=bob 2808844564 2808844565 IN IP4 host.biloxi.example.com
s=
c=IN IP4 host.biloxi.example.com
t=0 0
m=audio 49172 RTP/AVP 97
a=rtpmap:97 iLBC/8000
5.2. Hold and Unhold 2
The first offer from Alice contains the connection address 0.0.0.0
and a random port number, which means that Bob can not send media to
Alice (the media stream is "black holed" or "bh"). Bob accepts with
normal SDP. In the second exchange, Alice changes the connection
address, Bob accepts, and a media session is established.
[Offer]
v=0
o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
s=
c=IN IP4 0.0.0.0
t=0 0
m=audio 23442 RTP/AVP 97
a=rtpmap:97 iLBC/8000
[Answer]
v=0
o=bob 2808844564 2808844564 IN IP4 host.biloxi.example.com
s=
c=IN IP4 host.biloxi.example.com
t=0 0
m=audio 49170 RTP/AVP 97
a=rtpmap:97 iLBC/8000
[Second-Offer]
v=0
o=alice 2890844526 2890844527 IN IP4 host.atlanta.example.com
s=
c=IN IP4 host.atlanta.example.com
t=0 0
m=audio 49170 RTP/AVP 97
a=rtpmap:97 iLBC/8000
[Second-Answer]
v=0
o=bob 2808844564 2808844564 IN IP4 host.biloxi.example.com
s=
c=IN IP4 host.biloxi.example.com
t=0 0
m=audio 49170 RTP/AVP 97
a=rtpmap:97 iLBC/8000
5.3. Hold and Unhold 3
The first offer from Alice contains an audio stream, but the answer
from Bob contains the connection address 0.0.0.0 and a random port
number, which means that Alice can not send media to Bob (the media
stream is "black holed" or "bh"). In the second exchange, Bob
changes the connection address, Alice accepts, and a media session is
established.
[Offer]
v=0
o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
s=
c=IN IP4 host.atlanta.example.com
t=0 0
m=audio 49170 RTP/AVP 97
a=rtpmap:97 iLBC/8000
[Answer]
v=0
o=bob 2808844564 2808844564 IN IP4 host.biloxi.example.com
s=
c=IN IP4 0.0.0.0
t=0 0
m=audio 9322 RTP/AVP 97
a=rtpmap:97 iLBC/8000
[Second-Offer]
v=0
o=bob 2808844564 2808844565 IN IP4 host.biloxi.example.com
s=
c=IN IP4 host.biloxi.example.com
t=0 0
m=audio 49172 RTP/AVP 97
a=rtpmap:97 iLBC/8000
[Second-Answer]
v=0
o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
s=
c=IN IP4 host.atlanta.example.com
t=0 0
m=audio 49170 RTP/AVP 97
a=rtpmap:97 iLBC/8000
6. Security Considerations
SDP offer and answer messages can contain private information about
addresses and sessions to be established between parties. If this
information needs to be kept private, some security mechanism in the
protocol used to carry the offers and answers must be used. For SIP,
this means using TLS transport and/or S/MIME encryption of the SDP
message body.
It is important that SDP offer and answer messages be properly
authenticated and authorized before they are used to establish a
media session. Examples of SIP mechanisms include SIP Digest, certs,
and cryptographically-verified SIP identity.
7. Informative References
[1] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002.
[2] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[3] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
[4] Schulzrinne, H. and S. Petrack, "RTP Payload for DTMF Digits,
Telephony Tones and Telephony Signals", RFC 2833, May 2000.
[5] Rosenberg, J., Peterson, J., Schulzrinne, H., and G. Camarillo,
"Best Current Practices for Third Party Call Control (3pcc) in
the Session Initiation Protocol (SIP)", BCP 85, RFC 3725,
April 2004.
[6] Duric, A. and S. Andersen, "Real-time Transport Protocol (RTP)
Payload Format for internet Low Bit Rate Codec (iLBC) Speech",
RFC 3952, December 2004.
Authors’ Addresses
Alan Johnston
Tello Corporation
999 Baker Way, Suite 250
San Mateo, CA 94404
EMail: ajohnston@tello.com
Robert J. Sparks
Estacado Systems
EMail: rjsparks@estacado.net
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