[SIPForum-discussion] udptl as Media Protocol in SDP

Mohit Soni Mohit.Soni at matrixcomsec.com
Tue Aug 16 09:15:40 UTC 2011

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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