Subject: [Sipping] Question regarding SDP 'M' Line

From: "Jayantheesh Srimushnam" <[email-masked]>
Date: Wed, 16 Apr 2008 15:28:40 +0530

Hi All,

While adding a new media stream is it MUST to add a new 'M' line in the
SDP or existing 'M' line can be replaced with the new one. 

Please go through the below specified example and clarify me which is
the correct way of implementing? 

Example 1:

========

Initial INVITE:

m=audio 7100 RTP/AVP 0 101

a=rtcp:7101

a=sendonly

a=rtpmap:0 PCMU/8000

a=rtpmap:101 telephone-event/8000

Re-Invite:

m=image 20112 udptl t38

a=T38FaxVersion:0

a=T38MaxBitRate:1440

Example 2:

========

Initial INVITE:

m=audio 7100 RTP/AVP 0 101

a=rtcp:7101

a=sendonly

a=rtpmap:0 PCMU/8000

a=rtpmap:101 telephone-event/8000

Re-Invite:

m=audio 7100 RTP/AVP 0 101

a=rtcp:7101

a=sendonly

a=rtpmap:0 PCMU/8000

a=rtpmap:101 telephone-event/8000

m=image 5080 udptl t38

c=IN IP4 10.101.210.30

a=T38FaxVersion:0

Thanks,

Jayantheesh

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---

From: "Harsha. R" <[email-masked]>
Date: Thu, 17 Apr 2008 09:55:20 +0530

It depends on what you want. If you want to retain audio while sending
image, the second example is correct.

The first example. i think violates protocol. If you want to re-use an
existing media slot, you should have set deleted the media stream first(by
setting port to 0; by setting the port to 0 like this, you will loose
audio.) before re-using it.

RFC 3264 section 8.1

*8.1 Adding a Media Stream*

   New media streams are created by new additional media descriptions
   below the existing ones, or by reusing the "slot" used by an old
   media stream which had been disabled by setting its port to zero.

   Reusing its slot means that the new media description replaces the
   old one, but retains its positioning relative to other media
   descriptions in  the SDP

Regards
Harsha

On 16/04/2008, Jayantheesh Srimushnam <[email-masked]> wrote:
> Please go through the below specified example and clarify me which is the
> correct way of implementing?
>
>
>
> Example 1:
>
> ========
>
>
>
> Initial INVITE:
>
>
>