[asterisk-dev] State of FAX (primarily T.38) in Asterisk trunk (planning for 1.8 release)

Kevin P. Fleming kpfleming at digium.com
Thu Dec 3 16:36:24 CST 2009

asterisk at ntplx.net wrote:

> On receiving a T.38 fax I now get this back from the Cisco:
>   Got SIP response 420 "Bad Extension" back from 204.213.xxx.xxx
> And it fails.

That one doesn't make any sense without a complete log.

> On sending a T.38 fax I now get:
>   udptl.c:1054 ast_udptl_write: (SIP/860xxxxxxx): UDPTL asked to send  
> 23 bytes of IFP when far end only prepared to accept 19 bytes; data  
> loss will occur.You may need to override the T38FaxMaxDatagram value  
> for this endpoint in the channel driver configuration.
> But it works... (may be it's just an old problem but a new warning)

This is a known problem with Cisco media gateways doing T.38; they offer
an insanely small value for T38MaxDatagram. If you do as the message
suggests and override the value to something like 200 bytes in sip.conf
for that endpoint, you'll have more success.

Kevin P. Fleming
Digium, Inc. | Director of Software Technologies
445 Jan Davis Drive NW - Huntsville, AL 35806 - USA
skype: kpfleming | jabber: kpfleming at digium.com
Check us out at www.digium.com & www.asterisk.org

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