[Sip-implementors] SDP negotiation

[isshed]

Hi Everyone,

I am implementing a sip client. I have a basic doubt on SDP negotation. When
client A and B are connected. Basically A wants to send FAx to B. So first
voice call is connected. now they are switching to fax. In this process the
SDPs as negotiated as follows. A supports t38 and VBD both but B supports
only t38 (does not support VBD). In reinvite A sends dynamic codec 104.
Since B does not support should it remove 104 from media line? It already
removes the a:gpmd:104 vbd=yes line. All the 4 SDPs are listed below please
help me understand it.

UAC(A)----------------------------------------------------------------------------------------------------------------UAS(B)

Initial Invite from A has SDP as follows

m=audio 23508 RTP/AVP 0 8 101 13
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-15
a=rtpmap:13 CN/8000

B responds to A with SDP as follows

m=audio 11986 RTP/AVP 0 8 101 13
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-15
a=rtpmap:13 CN/8000

now it switches on FAX

A sends reinvite with SDP

a=pmft:T38
m=audio 23508 RTP/AVP 0 8 99 104 103
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:99 RED/8000
a=rtpmap:104 PCMA/8000
a=rtpmap:103 t38/8000
a=fmtp:103
T38FaxVersion=0;T38MaxBitrate=14400;T38FaxMaxDatagram=173;T38FaxRateManagement=transferredTCF
a=fmtp:99 104/104
a=fmtp:99 103/103
a=gpmd:104 vbd=yes


And B responds with SDP

a=pmft:T38
m=audio 11986 RTP/AVP 0 8 99 104 103
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:99 RED/8000
a=rtpmap:104 PCMA/8000
a=rtpmap:103 t38/8000
a=fmtp:103
T38FaxVersion=0;T38MaxBitrate=14400;T38FaxMaxDatagram=173;T38FaxRateManagement=transferredTCF
a=fmtp:99 104/104
a=fmtp:99 103/103


Thanks,
Isshed.

[Schwarz, Albrecht (Albrecht)]

Hello Isshed,

the problem may be solved by negotiating already the preferred configurations at call establishment, - and T.38 would be then negotiated as latent configuration.
The re-INVITE would then just trigger the switch from the audio configuration to the T.38 configuration.


-Albrecht

PS
SDP for FoIP T.38 is defined in Annex D of T.38 (09/2010).
These procedures are normative for T.38.

SDP for VBDoIP V.152 is defined in V.152 (09/2010).

Both Recommendations provide examples with configurations which contain audio VoIP, FoIP T.38 and VBD.

-----Original Message-----
From: sip-implementors-bounces at lists.cs.columbia.edu
[mailto:sip-implementors-bounces at lists.cs.columbia.edu] On
Behalf Of isshed
Sent: Dienstag, 19. Juli 2011 12:23
To: sip-implementors
Subject: [Sip-implementors] SDP negotiation
Hi Everyone,
I am implementing a sip client. I have a basic doubt on SDP
negotation. When client A and B are connected. Basically A
wants to send FAx to B. So first voice call is connected. now
they are switching to fax. In this process the SDPs as
negotiated as follows. A supports t38 and VBD both but B
supports only t38 (does not support VBD). In reinvite A sends
dynamic codec 104.
Since B does not support should it remove 104 from media
line? It already removes the a:gpmd:104 vbd=yes line. All the
4 SDPs are listed below please help me understand it.
UAC(A)--------------------------------------------------------
--------------------------------------------------------UAS(B)
Initial Invite from A has SDP as follows
m=audio 23508 RTP/AVP 0 8 101 13
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-15
a=rtpmap:13 CN/8000
B responds to A with SDP as follows
m=audio 11986 RTP/AVP 0 8 101 13
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-15
a=rtpmap:13 CN/8000
now it switches on FAX
A sends reinvite with SDP
a=pmft:T38
m=audio 23508 RTP/AVP 0 8 99 104 103
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:99 RED/8000
a=rtpmap:104 PCMA/8000
a=rtpmap:103 t38/8000
a=fmtp:103
T38FaxVersion=0;T38MaxBitrate=14400;T38FaxMaxDatagram=173;T38F

axRateManagement=transferredTCF

a=fmtp:99 104/104
a=fmtp:99 103/103
a=gpmd:104 vbd=yes
And B responds with SDP
a=pmft:T38
m=audio 11986 RTP/AVP 0 8 99 104 103
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:99 RED/8000
a=rtpmap:104 PCMA/8000
a=rtpmap:103 t38/8000
a=fmtp:103
T38FaxVersion=0;T38MaxBitrate=14400;T38FaxMaxDatagram=173;T38F

axRateManagement=transferredTCF

a=fmtp:99 104/104
a=fmtp:99 103/103
Thanks,
Isshed.
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