Allow non-compliant T.38 endpoints to be supportable via configuration option.

Many T.38 endpoints incorrectly send the maximum IFP frame size they can accept
as the T38FaxMaxDatagram value in their SDP, when in fact this value is
supposed to be the maximum UDPTL payload size (datagram size) they can accept.
If the value they supply is small enough (a commonly supplied value is '72'),
T.38 UDPTL transmissions will likely fail completely because the UDPTL packets
will not have enough room for a primary IFP frame and the redundancy used for
error correction. If this occurs, the Asterisk UDPTL stack will emit log messages
warning that data loss may occur, and that the value may need to be overridden.

This patch extends the 't38pt_udptl' configuration option in sip.conf to allow
the administrator to override the value supplied by the remote endpoint and
supply a value that allows T.38 FAX transmissions to be successful with that
endpoint. In addition, in any SIP call where the override takes effect, a debug
message will be printed to that effect. This patch also removes the
T38FaxMaxDatagram configuration option from udptl.conf.sample, since it has not
actually had any effect for a number of releases.

In addition, this patch cleans up the T.38 documentation in sip.conf.sample
(which incorrectly documented that T.38 support was passthrough only).

(issue #15586)
Reported by: globalnetinc


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@222110 65c4cc65-6c06-0410-ace0-fbb531ad65f3

1 files changed, 18 insertions, 8 deletions

diff --git a/configs/sip.conf.sample b/configs/sip.conf.sample
index 23c16979c..f32d60915 100644
--- a/configs/sip.conf.sample
+++ b/configs/sip.conf.sample
@@ -496,22 +496,32 @@ srvlookup=yes                   ; Enable DNS SRV lookups on outbound calls
 ;callcounter = yes              ; Enable call counters on devices. This can be set per
                                 ; device too.
 
-;----------------------------------------- T.38 FAX PASSTHROUGH SUPPORT -----------------------
+;----------------------------------------- T.38 FAX SUPPORT ----------------------------------
 ;
 ; This setting is available in the [general] section as well as in device configurations.
-; Setting this to yes, enables T.38 fax (UDPTL) passthrough on SIP to SIP calls, provided
-; both parties have T38 support enabled in their Asterisk configuration
-; This has to be enabled in the general section for all devices to work. You can then
-; disable it on a per device basis.
-;
-; T.38 faxing only works in SIP to SIP calls. It defaults to off.
+; Setting this to yes enables T.38 FAX (UDPTL) on SIP calls; it defaults to off.
 ;
 ; t38pt_udptl = yes            ; Enables T.38 with FEC error correction.
 ; t38pt_udptl = yes,fec        ; Enables T.38 with FEC error correction.
 ; t38pt_udptl = yes,redundancy ; Enables T.38 with redundancy error correction.
 ; t38pt_udptl = yes,none       ; Enables T.38 with no error correction.
 ;
-; Faxs Detect will cause the SIP channel to jump to the 'fax' extension (if it exists)
+; In some cases, T.38 endpoints will provide a T38FaxMaxDatagram value (during T.38 setup) that
+; is based on an incorrect interpretation of the T.38 recommendation, and results in failures
+; because Asterisk does not believe it can send T.38 packets of a reasonable size to that
+; endpoint (Cisco media gateways are one example of this situation). In these cases, during a
+; T.38 call you will see warning messages on the console/in the logs from the Asterisk UDPTL
+; stack complaining about lack of buffer space to send T.38 FAX packets. If this occurs, you
+; can set an override (globally, or on a per-device basis) to make Asterisk ignore the
+; T38FaxMaxDatagram value specified by the other endpoint, and use a configured value instead.
+; This can be done by appending 'maxdatagram=<value>' to the t38pt_udptl configuration option,
+; like this:
+;
+; t38pt_udptl = yes,fec,maxdatagram=400 ; Enables T.38 with FEC error correction and overrides
+;                                       ; the other endpoint's provided value to assume we can
+;                                       ; send 400 byte T.38 FAX packets to it.
+;
+; FAX detection will cause the SIP channel to jump to the 'fax' extension (if it exists)
 ; after T.38 is successfully negotiated.
 ;
 ; faxdetect = yes              ; Default false