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

Steve Underwood steveu at coppice.org
Thu Dec 3 19:01:16 CST 2009

On 12/04/2009 06:36 AM, Kevin P. Fleming wrote:
> 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.
Its not just Cisco. T38MaxDatagram is so messed up in real world 
implementations of T.38, it is entirely useless. Just use a sane 
override, and hope for the best.


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