[SIPForum-discussion] udptl as Media Protocol in SDP

Maciej Wasiel Maciej.Wasiel at dialogic.com
Fri Aug 19 00:45:24 UTC 2011

I don't know of anyone that supports T.38 over RTP. RTP was added to the t.38 spec to support security. Redundancy and FEC are already supported by UDPTL.

Maciej Wasiel
Software Engineer, Dialogic Research Inc | Address: 15 Crawford Street, Needham, MA 02494 | Phone: (781) 433 9771 | Mobile: (508) 740-2641 | Fax: (781) 433 9255
Email: maciej.wasiel at dialogic.com

From: discussion-bounces at sipforum.org [mailto:discussion-bounces at sipforum.org] On Behalf Of HM Kias
Sent: Thursday, August 18, 2011 2:33 AM
To: Mohit Soni
Cc: discussion at sipforum.org
Subject: Re: [SIPForum-discussion] udptl as Media Protocol in SDP

Faxes can be transported using ulaw or another noncompressed codec, although delays and latency can cause problems and failures. Sending faxes directly over IP networks can be done using the T.38 fax over IP. The MIME type image/t38
 has been defined for the transport of faxes over UDP, using the UDP Transport Layer (UDPTL). While this approach works, it is better to transport faxes over RTP. This allows faxes to utilize various RTP extensions including security, redundancy, and so fourth. Faxes over RTP use the audio/t38 MIME type.
On Tue, Aug 16, 2011 at 2:45 PM, Mohit Soni <Mohit.Soni at matrixcomsec.com<mailto:Mohit.Soni at matrixcomsec.com>> 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).

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

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.

Mohit Soni

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HM Kias
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