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

Andrea Puddu androjoker at hotmail.com
Thu Dec 13 08:49:48 UTC 2007

I've tried to browse other settings but without success.

I need to have G729 codec as first priority :(

Unfortunately I can only take the log from the phone side  ... 

Date: Thu, 13 Dec 2007 10:05:15 +0800
From: baolovebao at gmail.com
To: androjoker at hotmail.com
Subject: Re: [SIPForum-discussion] Is "fmtp" line needed in 200 OK answer?
CC: herve.jourdain at mstarsemi.com; discussion at sipforum.org

#any other settings that you can try with the sip phone? such as the priority of the codec. adjust the g729 codec's priority.
#maybe there are still other reasons cause this issue. Who sends the BYE? the AS or SBC?

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

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
androjoker at hotmail.com; baolovebao at gmail.com
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 DTMF.

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 09:35

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 +0800
baolovebao at gmail.com
To: androjoker at hotmail.com
Subject: Re: [SIPForum-discussion] Is "fmtp" line needed in 200 OK answer?

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,



Prenditi una pausa e sfida i tuoi amici a Ladybird su Messenger! 
Messenger Giochi

This is the SIP Forum discussion mailing list
TO UNSUBSCRIBE, or edit your delivery options, please visit 
Post to the list at 
discussion at sipforum.org


Per questo Natale fai i tuoi auguri con Messenger! 
Windows Live Messenger

Per questo Natale fai i tuoi auguri con Messenger! Windows Live Messenger


Conosci Doretta? Contattala, è ora su Messenger!
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://sipforum.org/pipermail/discussion/attachments/20071213/1615cb37/attachment-0002.html>

More information about the discussion mailing list