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

Herve Jourdain herve.jourdain at mstarsemi.com
Wed Dec 12 10:25:55 UTC 2007



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


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.







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

Thanks 1000,





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