[SIPForum-discussion] Is "fmtp" line needed in 200 OK answer?

Andrea Puddu androjoker at hotmail.com
Wed Dec 12 10:38:15 UTC 2007

Thanks for your analysis.

By chance, I've just found a parameter on the phone that can help me  -> "Full SDP Answer" .... I've switched it to OFF.
Now the phone sends only one codec in the 200 OK response (g729), but without the "fmtp" line.
So the phone starts talking in G729 but AS (or SBC) sends a BYE to the phone!!!!!

I'm starting to think that SBC does not like the lack of "fmtp" filed in the response .....


From: herve.jourdain at mstarsemi.com
To: androjoker at hotmail.com; baolovebao at gmail.com
CC: discussion at sipforum.org
Subject: RE: [SIPForum-discussion] Is "fmtp" line needed in 200 OK answer?
Date: Wed, 12 Dec 2007 11:25:55 +0100



As far as I remember of
RFC 3264, the devices should be prepared to receive ANY of the codecs it sends
in its offer/response…

So basically, if you
respond with 3 codecs, you should be able to receive at any time on any of
those 3 codecs, and switch between them…

That’s why it seems
customary on several phones to answer with only one codec…


But even if it’s
stated this way in the specs, I still do think that before switching to another
codec, the UAs should at least use the codec that was agreed for in the negotiation
at first (the “highest priority” one, in your case G729).

Experience shows,
unfortunately, that even if it’s usually the case, it’s NOT ALWAYS
the case…

I think you might have an
example here, and I know I’ve met several devices – phones or
gateways usually – that would also behave in this way…


So maybe try with only 1
codec in the response, to see if it works better…


But if you do so, please
switch to outband DTMF (RFC 2833 or RFC 4733), because once you give only G729
in the answer there should not be any possibility to switch to G711 for in-band

And your SDP suggests
outband DTMF is not activated.






discussion-bounces at sipforum.org [mailto:discussion-bounces at sipforum.org] On Behalf Of Andrea Puddu

Sent: mercredi 12 décembre 2007

To: Donald Lee

Cc: discussion at sipforum.org

Subject: Re: [SIPForum-discussion]
Is "fmtp" line needed in 200 OK answer?


I agree with your analysis. I've noticed that for example
Cisco and Linksys phone sends only one codec in the 200 OK SDP envelope.

But... Can the sending of multiple codec in the 200 OK response be an issue?

I cannot complain with phone provider about this behaviour. 

Anyway why does the phone talk with G729 and AS talk with G711?  I'm
wondering if the AS performs a check on  fmtp field in the 200 OK response



Date: Wed, 12 Dec 2007 16:24:05

From: baolovebao at gmail.com

To: androjoker at hotmail.com

Subject: Re: [SIPForum-discussion] Is "fmtp" line needed in 200 OK

CC: discussion at sipforum.org

# the "g" and "G" have no difference.
and the offer indicate that it's not G729B codec.

# I don't think the lack of fmtp in answer is the reason
for issue. Maybe the ip phone should reply only one codec in its SDP. Maybe the
multi codec in answer make the offer confused.


On 12/11/07, Andrea Puddu <androjoker at hotmail.com> wrote: 

Hello guys,

I'm facing an issue when a mobile (common mobile 3G phone) tries to call an
internal SIP phone through its geographical number.

The issue is that when the SBC sends the INVITE to the IP phone,  in the
SDP  enclosure it  suggests these codecs: 

m=audio 19570 RTP/AVP 18 8 0

a=rtpmap:18 G729/8000

a=fmtp:18 annexb=no

a=rtpmap:8 PCMA/8000

a=rtpmap:0 PCMU/8000

The 200 OK of the phone (SDP part only) is:

m=audio 57956 RTP/AVP 18 8 0

a=rtpmap:18 g729/8000

a=rtpmap:8 pcma/8000

a=rtpmap:0 pcmu/8000


So the IP phone start to talk with G729 and SBC replies with G711A!!! So I
can't hear the voice coming from mobile phone!!!!

I have two questions:

- Can the difference between the offer (G729) and the answer (g729) be
significant? I mean the difference because the "g" is not capital in
the answer

- Can the lack of the "fmtp" line in the answer cause troubles? 

Thanks 1000,




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