Subject: [SIP] questions for clarification on SDP negociation

From: Francois-Xavier Guitton <[email-masked]>
Date: Thu, 04 May 2000 09:17:11 +0200

All,

When I read RFC2543, Annex B - Usage of SDP p 136, I am a little bit
confused. Could you answer the following questions :

1/ RFC 2327 (SDP) defines 3 attributes a=recvonly, a=sendrcv and
a=sendonly. But, in SIP, do we consider a media is by default
bi-directional ? The example p 137 seems to tell that, but does someone
know where that is written ?

2/ Is it possible for a user agent to add new media in the SDP part of
the answer to a received INVITE?

For example A INVITE B with  media:
m=audio 49170 RTP/AVP 0
a=rtpmap:0 PCMU/8000

Does it make sense B answers 200 OK with:
m=audio 4910 RTP/AVP 0
a=rtpmap:0 PCMU/8000
m=video 5300 RTP/AVP 32
a=rtpmap:32 MPV/90000

The underlying question being: What type of media are listed in the SDP
part : the media supported by the User Agent ? or the media the Human
user wishes to use in the SIP session ? In that case, A could wish an
audio session but B would prefer an audio-video session. If we suppose A
supports also video capabilities, what will answer A : ACK with audio +
video media description ??? Or is a new INVITE from A to B required ?

3/ Where is defined the list of all the types of rtpmap (e.g.: PCMU,
H261, MPV ...) supported by SIP in the SDP part ?

Regards,

Fxg

PS: What is the status of draft-ietf-sip-rfc2543bis ? Is it now the
reference instead of RFC 2543 ?

./.

--
Francois-Xavier Guitton (SWD/ULC)
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Tel: +33 (0)1 69 63 47 52
Fax: +33 (0)1 69 63 17 89

---

From: Itamar Gilad <[email-masked]>
Date: Thu, 4 May 2000 10:59:49 +0300

Francois, see comments below marked [Itamar Gilad].
 
  Itamar

-----Original Message-----
From: Francois-Xavier Guitton [mailto:[email-masked]]
Sent: Thu, May 04, 2000 9:17 AM
To: SIP
Subject: [SIP] questions for clarification on SDP negociation

All, 

1/ RFC 2327 (SDP) defines 3 attributes a=recvonly, a=sendrcv and a=sendonly.
But, in SIP, do we consider a media is by default bi-directional ? The
example p 137 seems to tell that, but does someone know where that is
written ? 

[Itamar Gilad] I'm not sure where it's written, but the general idea as I
understand it is that by default the sender of the INVITE informs of the
media it is willing to send AND receive.  If you wish to specify asymmetric
send/receive capabilities you need to set up two m= lines, one with
a=sendonly and one with a=recvonly. 

2/ Is it possible for a user agent to add new media in the SDP part of the
answer to a received INVITE? 

The underlying question being: What type of media are listed in the SDP part
: the media supported by the User Agent ? or the media the Human user wishes
to use in the SIP session ? In that case, A could wish an audio session but
B would prefer an audio-video session. If we suppose A supports also video
capabilities, what will answer A : ACK with audio + video media description
??? Or is a new INVITE from A to B required ? 

[Itamar Gilad] B can't add new media which A did not specify in the original
INVITE. It can either accept a subset or whole of the the media streams and
codecs suggested by A or decline the invitation. Once a session has been
established either side can attempt adding new media streams by sending a
re-INVITE with an appropriate SDP body (see B.5 in RFC2543-bis).  

3/ Where is defined the list of all the types of rtpmap (e.g.: PCMU, H261,
MPV ...) supported by SIP in the SDP part ? 

[Itamar Gilad] As far as I know SIP does not set any limit on the types of
media describable in the SDP body.  See also RFC1889 and RFC1890.  

Regards, 

Fxg 

./. 

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