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: http://svn.digium.com/svn/asterisk/trunk@222110 f38db490-d61c-443f-a65b-d21fe96a405b

1 files changed, 8 insertions, 0 deletions

diff --git a/UPGRADE.txt b/UPGRADE.txt
index 1c41d90fe..61d4ec148 100644
--- a/UPGRADE.txt
+++ b/UPGRADE.txt
@@ -67,6 +67,14 @@ From 1.6.1 to 1.6.2:
   instead, it is configured on a per-peer (or global) basis in sip.conf, with
   the same default as was present in udptl.conf.sample.
 
+* T.38 FAX maximum datagram size can no longer be configured in updtl.conf;
+  instead, it is either supplied by the application servicing the T.38 channel
+  (for a FAX send or receive) or calculated from the bridged endpoint's
+  maximum datagram size (for a T.38 FAX passthrough call). In addition, sip.conf
+  allows for overriding the value supplied by a remote endpoint, which is useful
+  when T.38 connections are made to gateways that supply incorrectly-calculated
+  maximum datagram sizes.
+
 * There have been some changes to the IAX2 protocol to address the security
   concerns documented in the security advisory AST-2009-006.  Please see the
   IAX2 security document, doc/IAX2-security.pdf, for information regarding