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List:       sip-implementors
Subject:    Re: [Sip-implementors] T.38 UDPTL FAX when call is made using SRTP
From:       "Kevin P. Fleming" <kpfleming () digium ! com>
Date:       2012-06-07 15:46:44
Message-ID: 4FD0CCE4.5000602 () digium ! com
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On 05/07/2012 06:56 AM, Mohit Soni wrote:
> Hi all,
>
> I want to explore the case where a FAX call is made using the SRTP.
>
> The case is like this,
>
> UA-1                                                                   UA-2
>     ------------------INVITE (SDP = SRTP)----------------------->         The
> UA-1 generates INVITE with desired SRTP credentials
>
>    <----------------200 OK (SDP = SRTP)-------------------------        The
> UA-2 responds with 200 OK with negotiated SRTP credentials
>
>    --------------------------------ACK------------------------------------>
>
>    ---------------------SRTP with FAX tones---------------------->         FAX
> tones in secure RTP packets are exchanged between UA-1 and UA-2
>    <-------------------SRTP with FAX tones------------------------
>
>    <-------------------INVITE (T.38 UDPTL)-------------------------
> UA-2 identifies the FAX tone and determines that the call is FAX call and
> sends re-INVTIE with T.38 UDPTL as it is configured as FAX option in it
>
>    ---------------------200 OK (T.38 UDPTL)---------------------->
> UA-1 responds with 200 OK to complete the FAX transmission over UDPTL
>
>    <-------------------------------ACK------------------------------------
>
>    <-------------non secure T.38 UDPTL packets--------------        UA-1 and
> UA-2 stops secure RTP and exchanges the FAX over non secure UDPTL packets.
>    ---------------non secure T.38 UDPTL packets----------->
>
>
> Is this a valid scenario?

What is your definition of 'valid'? It is certainly compliant with all 
the relevant RFCs and ITU-T recommendations, if that's what you mean.

> Is there any device/UA who behaves like this?

Yes, I suspect there are many devices that operate like this, if they 
support both SRTP and T.38. This would be a very common occurrence if 
the user of the device has enabled SRTP.

> Is it practical to update the session of secure RTP with non secure UDPTL
> (or even normal RTP) using re-INVITE at the middle of the session?

Can you explain what you mean by 'practical'?

-- 
Kevin P. Fleming
Digium, Inc. | Director of Software Technologies
Jabber: kfleming@digium.com | SIP: kpfleming@digium.com | Skype: kpfleming
445 Jan Davis Drive NW - Huntsville, AL 35806 - USA
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