[asterisk-dev] Issue 10417 - T.38 with devices behind NAT does not work in all circumstances

Joshua Colp jcolp at digium.com
Thu May 22 07:51:02 CDT 2008

----- "Raj Jain" <rj2807 at gmail.com> wrote:

> This sounds more like a band-aid than an actual solution.

That was my feeling as well.
> There is a big assumption involved here that the NAT binding
> established at the time of audio will indeed be valid when the media
> switches over to fax. This means that the device behind NAT must not
> change it's own listening IP and port when the switchover of media
> occurs. While this may be true for the scenario and device reported
> in
> the bug report, it seems like a very big assumption that will
> probably
> not hold true in other environments or when the media change happens
> for other reasons (change to MoH source, for instance).

Correct. There is also the assumption that you forwarded the right ports on your router to the T38 capable device.
> That said, if this is how some T.38 devices behave (that they wait
> for
> incoming packets before sending outbound packets after media
> switchover and they don't change their IP/port), then MAYBE this is
> worth considering as a solution. Of course, any configurable
> parameter
> defined here should only be applied to the very specific case of
> media
> switching from audio to image.

I agree. Which makes me wonder whether the device in question has the ability to specify external IP address for the SDP... if that was possible then changes in Asterisk to band-aid it would not be needed as we would just use that IP address.

Joshua Colp
Software Developer
Digium, Inc.

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