[SIPForum-discussion] udptl as Media Protocol in SDP

Mohit Soni Mohit.Soni at matrixcomsec.com
Thu Aug 18 04:15:54 UTC 2011

I had attached the PCAP trace of the scenario.
You can check the same SDP in it.

What actually I was trying to ask here was, as per my knowledge, the Media
subtype udptl is used only when the Media type is image. But in this case
the Media subtype udptl is used when the Media type is audio.

As you had mentioned, there should be a separate line for udio/udptl that
means udptl can be used as Media subtype when Media type is audio.

So, can you tell me what should be the answer of such offer. Is this
mentioned in any RFC or recommendation or any draft?

Mohit Soni

-----Original Message-----
From: discussion-bounces at sipforum.org
[mailto:discussion-bounces at sipforum.org] On Behalf Of Steve Underwood
Sent: Wednesday, August 17, 2011 6:41 AM
To: discussion at sipforum.org
Subject: Re: [SIPForum-discussion] udptl as Media Protocol in SDP

On 08/16/2011 05:15 PM, Mohit Soni wrote:
> Hi friends,
> I have encountered a method of performing FAX over IP during testing 
> my UA with a service provider's gateway.
> When I had tried to send FAX from my UA, the service provider gateway 
> sends re-INVITE on my UA by setting Media Protocol as "udptl" when 
> Media Type is "audio" in SDP body (shown with bold in following trace).
> v=0
> o=CiscoSystemsSIP-GW-UserAgent 8963 636 IN IP4
> s=SIP Call
> c=IN IP4
> t=0 0
> m=audio 19176 *udptl* 0 97
> c=IN IP4
> a=rtpmap:0 PCMU/8000
> a=rtpmap:97 telephone-event/8000
> a=fmtp:97 0-16
> a=ptime:20
> This cause my UA to send 488 as the Media Protocol udptl is not 
> supported by my UA.
> Normally when the Media Type is audio, the Media Protocol is set as 
> RTP/AVP but here, it is udptl.
> Can any one tell me that how this method of FAX works and what kind of 
> method is this, i.e., passthrough, T.38 UDPTL or T.38 RTP?
> If there any RFC or draft or specification for performing FAX over IP 
> in such a way then please tell me.
UDPTL is used for T.38, and only T.38. Recent versions of the T.38 spec 
also define T.38 over RTP, but you will find very few systems which 
support this. Your SDP looks messed up, though. Packet types 97 
(telephony events) and 0 (u-Law) cannot be carried over UDPTL. You 
should have an audio/RTP line for these, and an audio/UDPTL line for T.38.


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