[asterisk-dev] Add attributes to Asterisk T38 invite for MAX TNT Interop?

JR Richardson jmr.richardson at gmail.com
Tue Dec 8 19:31:51 CST 2009

> JR Richardson wrote:
> > Here is what Asterisk sends to the tnt:
> > m=image 20228 udptl t38
> > a=T38FaxVersion:0
> > a=T38MaxBitRate:14400
> > a=T38FaxRateManagement:transferredTCF
> > a=T38FaxMaxBuffer:0
> > a=T38FaxMaxDatagram:0
> > a=T38FaxUdpEC:t38UDPRedundancy
> That is broken; T38FaxMaxDatagram should never be zero. Something is
> wrong there.
> As far as the non-present parameters, they are all optional and it does
> not matter whether they are present or not, unless the answerer intends
> to support that particular option/feature. Most T.38 SDP parameters are
> *not* negotiated, but instead are declarative and indicate what that
> endpoint wants to receive.
> --
> Kevin P. Fleming

Hmm, ok, I think I buy that as well as Steve's commnet on a=T38FaxMaxBuffer:0.

I did enable the three questionable parameters in chan_sip.c,
re-compiled and ran test with same results, so even though all the
proper attribuets were sent in the invite to the TNT, the TNT did not
respond with any udptl packets.

Here is my testing setup:
PSTN PRI><MAX TNT><SIP><Asterisk><NAT><AC MP203 ATA><Fax machine

I send fax from the fax machine out to the PSTN, the t38 session is
started from the fax ATA.
The invite from the ATA includes:

Asterisk ACK the invite back to the ATA and send t38 invite to TNT:
The invite from Asterisk includes:

So I'm guessing Asterisk is not reading or storing the parameters
correctly from the ATA?

I'll poke around here for a bit, if you have any suggestions, let me know.


JR Richardson
Engineering for the Masses

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