[asterisk-dev] Attended transfers and MOH

Mark Michelson mmichelson at digium.com
Fri Feb 14 08:26:54 CST 2014

> Kevin Harwell <mailto:kharwell at digium.com>
> Thursday, February 13, 2014 5:43 PM
> Greetings,
> There is a bug currently in Asterisk that essentially boils down to MOH
> not being [re]started for an attended transfered call when the
> transferor was listening to it.
> For example, here is a specific scenario (issue ASTERISK-19499): Alice
> calls Bob, Bob starts a SIP attended transfer into a confbridge that is
> set to play music when empty. Alice hears music since she is now on
> hold and Bob hears music from the confbridge. Bob then completes the
> transfer and then Alice enters the confbridge, but hears no music.
> This situation occurs because during the transfer and channel masquerade
> the application has no way to know the channel was swapped out. There
> are a couple of ways to approach a fix for this:
> 1) Use a channel fixup in the applications that are affected. A small
> change to the "do masquerade" function has to be made to get this to
> work too.
> 2) In the relevant channel drivers when an attended transfer occurs
> check to see if MOH is currently playing on the transferor. If so, once
> the transfer occurs start MOH on the transferee. This idea was put
> forth by Olle Johansson (currently implemented in his
> rana-moh-queue-transfer-1.8 team branch). Albeit a few changes will
> have to be made in order to use it in the main branches, but the idea
> will essentially remain.
> I like option 2 as it seems to be a bit less intrusive (no changes to
> masquerades and such). Also in 12+ the logic can be put into the bridge
> transfer code reducing the changes further.
> Thoughts? Ideas? Something I have missed or need to be aware of?
> I'll start to move forward with #2 soon and put it up for review.
> Comments and feedback always welcome.
> Thanks!
> ------------------------------------------------------------------------
I was thinking about this, and one of the things that the masquerade 
code currently does is to determine if the original channel in a 
masquerade has a "visible indication" on it. A visible indication is 
something that is indicated by a control frame and which the user of the 
channel can actually tell is being indicated. For instance, if an 
AST_CONTROL_RINGING frame is indicated to a channel, the user can hear 
ringing. It is a visible indication. If the ringing channel  is going to 
be part of a masquerade, the masquerade detects that the party has a 
visible indication of AST_CONTROL_RINGING on it. After performing all of 
the rest of the masquerade operation, any visible indication is then 
re-indicated to the masqueraded channel. So in the AST_CONTROL_RINGING 
case, the control frame is re-indicated to the channel after the 
masquerade so that the channel will still hear ringing.

Since masquerades already have code in them to attempt to detect 
user-perceivable state on the channel, I think you should essentially do 
option 2, but do it in the masquerade code instead of in channel 
drivers. Here's a summary of my idea for how to do this:
1) Around the same area where the visible indication on the original is 
checked, set a variable if AST_FLAG_MOH is set on original.
2) Around the same area where the visible indication is re-indicated on 
the original, if the variable you created for AST_FLAG_MOH is set, then 
call ast_moh_start() on original to get music playing on the channel again.

The advantage to doing it in the masquerade code is that it will work 
for situations other than attended transfers (e.g. AMI redirects) and 
you won't have to put the same code in multiple places. Since masquerade 
code already has a blueprint for how to do this sort of thing, there 
shouldn't be much risk in expanding on it.
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