[asterisk-dev] Bridges, T.38, and other good times

Mon Dec 7 11:59:44 CST 2015

On 12/06/2015 07:57 PM, Matthew Jordan wrote:
> Hello all -
>
> One of the efforts that a number of developers in the community here 
> at Digium have been at work at are cleaning up test failures exposed 
> by Jenkins [1]. One of these, in particular, has been rather difficult 
> to resolve - namely, fax/pjsip/directmedia_reinvite_t38 [2]. This 
> e-mail goes over what has been accomplished, and asks some questions 
> on how we might try and fix Asterisk under this scenario.
>
> The directmedia_reinvite_t38 test attempts to do the following:
>  (1) UAC1 calls UAC2 through Asterisk, with audio as the media. The 
> dial is performed using the 'g' flag, such that UAC2 will continue on 
> if UAC1 hangs up.
>  (2) UAC1 and UAC2 are configured for direct media. Asterisk sends a 
> re-INVITE to UAC1 and UAC2 to initiate direct media.
>  (3) After responding with a 200 OK to the direct media requests, UAC1 
> sends a re-INVITE offering T.38.
>  (4) Asterisk sends an INVITE with T.38 to UAC2
>  (5) UAC2 sends back a 200 OK for T.38; Asterisk sends that to UAC1. 
> Asterisk switches out of a direct media bridge to a core bridge.
>  (6) UAC1 hangs up. Asterisk sends a re-INVITE to UAC2 for audio back 
> to Asterisk. UAC2 responds with a 200 OK for the audio.
>  (7) Asterisk ejects UAC2 back to the dialplan.
>
> It's important to note that this test never should have passed - an 
> update to the test suite "fixed" the test erroneously passing, which 
> led to us investigating why the scenario was failing. This test was 
> copied over from an identical chan_sip test, which passes.
>
> The PJSIP stack has two issues which make life difficult for it in 
> this scenario:
> (1) The T.38 logic is implemented in res_pjsip_t38. While that is 
> _mostly_ a very good thing - as it keeps all the fax state logic 
> outside of the channel driver - we are also a layer removed from 
> interactions that occur in the channel driver. That makes it 
> challenging to influence direct media checks and other 
> Asterisk/channel interactions.
> (2) Being very asynchronous, requests may be serviced that influence 
> T.38 state while other interactions are occurring in the core. 
> Informing the core of what has occurred can have more race conditions 
> than what occurs in chan_sip, which is single threaded.
>
> The first bug discovered when the test was investigated was an issue 
> in step (2). We never actually initiated a direct media re-INVITE. 
> This was due to res_pjsip_t38 using a frame hook, and not implementing 
> the .consume_cb callback. That callback allows a framehook to inform 
> the core (and also the bridging framework) of the types of frames that 
> a framehook wants to consume. If a framehook needs audio, a direct 
> media bridge will be explicitly denied, and - by default - the 
> bridging framework assumes that framehooks will want all frames. 
> Another bug that was discovered occurred in step (6). When UAC1 sends 
> a BYE request, nothing informed UAC2 that the fax had ended - instead, 
> it was merely ejected from the bridge. This meant that it kept its 
> T.38 session going, and Asterisk never sent a re-INVITE to UAC2. Both 
> of these bugs were fixed by 726ee873a6.
>
> Except, unfortunately, the second bug wasn't really fixed.
>
> 726ee873a6 did the "right" thing by intercepting the BYE request sent 
> by UAC1, and queueing up a control frame of type 
> AST_CONTROL_T38_PARAMETERS with a new state of AST_T38_TERMINATED. 
> This is supposed to be passed on to UAC2, informing it that the T.38 
> fax has ended, and that it should have its media re-negotiated back to 
> the last known state (audio) but also back to Asterisk (since we 
> aren't going to be in a bridge any longer). Unfortunately, this code 
> was insufficient.
>
> A race condition exists in this case. On the one hand, we've just 
> queued up a frame on UAC1's channel to be passed into the bridge, 
> which should get tossed onto UAC2's channel. On the other hand, we've 
> just told the bridging framework to kill UAC1's channel with extreme 
> prejudice, thereby also terminating the bridge and ejecting UAC2 off 
> into the dialplan. In the first case, this is an asynchronous, message 
> passing mechanism; in the second case, the bridging framework inspects 
> the channel to see if it should be hung up on *every frame* and 
> *immediately* starts the hangup/shutdown procedure if it knows the 
> channel should die. This is not asynchronous in any way. As a result, 
> UAC1 may be hung up and the bridge dissolved before UAC2 ever gets its 
> control frame from UAC1.
>
> There were a couple of solutions to this problem that were tried:
> (1) First, I tried to make sure that enqueued control frames were 
> flushed out of a channel and passed over the bridge when a hangup was 
> detected. In practice, this was incredibly cumbersome - some control 
> frames should get tossed, others need to be preserved. What was worse 
> was the sheer number of places the bridge dissolution can be 
> triggered. While it wasn't hard to make sure we flushed frames off an 
> ejected channel into a bridge, it was nigh impossible to ensure that 
> this occurred every single time before the other channels were 
> ejected. Again, the bridging framework is ridiculously - perhaps 
> ludicrously - aggressive in tossing channels out of a bridge once it 
> has decided the bridge should be dissolved.
> (2) Second, I tried to make the bridge ejection process asynchronous. 
> This was done by enqueuing another control frame onto the channel 
> being ejected; when it leaves, it flushes its control frames into the 
> bridge. When the 'ejection' control frame gets passed into the 
> bridging core, that causes the bridge to dissolve. This worked well in 
> some scenarios, and it also guaranteed that the T.38 control frame 
> would be delivered. Unfortunately, in other cases, it caused all of 
> the channels to hang out in the bridge ... permanently. Again, there's 
> a lot of edge cases in the bridging code that deal with channels being 
> kicked out of a bridge, and the bridge dissolving... and it was more 
> than I could chew on.
>
> The long and short of it is: while Asterisk 12+ has a nice bridging 
> framework that hides or eliminates a lot of the horrendous 
> masquerade/transfer code, as well as the 'triple infinite loop' in 
> features/channel that existed in Asterisk 11-, it is still 
> ridiculously complex and prone to breaking spectacularly in subtle 
> ways. Not to mention both (1) and (2) end up being massive changes to 
> the design that are risky in an LTS (no one likes it when a channel 
> can't be hung up.)
>
> So those ideas were scratched.
>
> The next solution was to try a bridge mixing technology that 
> specifically managed the T.38 state. This worked ... really well. 
> Incredibly well, in fact. It avoided all of the previous problems 
> because, unlike external modules or even certain places in the 
> bridging core, a bridge technology is guaranteed by the core to be 
> called in a synchronized fashion when any of the following occurs:
> (1) When a bridge technology is chosen
> (2) When that technology is started
> (3) When that bridge has a channel added
> (4) When that bridge has a channel removed
> (5) When that technology is stopped
> All of which covers the necessary places to know when a channel has 
> hung up, and gives us a place where we can safely inform the other 
> channels before the bridging framework starts doing mean things. 
> bridge_t38 was the result [3]. It managed a bit of T.38 state for the 
> two channels in a core bridge that were in a T.38 fax, and, when one 
> of them leaves, it informed the other channel that it should end its 
> T.38 fax.
>
> Problem solved.
>
> \o/
>
> Not quite.
>
> After merging [3] in f42d22d3a1, we noticed that the masquerade test 
> [4] started to fail. That's a really, really bad sign. The masquerade 
> 'super test' was originally tested to stress test masquerades in 
> Asterisk 1.8 and 11. It constructs a chain of 300 Local channels, then 
> optimizes them all down to a single pair of 'real' channels. In 
> Asterisk 12+, masquerades were eliminated in this scenario, but we 
> instead have a series of incredibly complex Local channel 
> optimization-caused bridge/swaps/merges that kick off as the Local 
> channels collapse and merge their bridges down to one. It's a great 
> "canary in the coal mine" test, as when it fails, it almost certainly 
> means you've introduced a dead lock into one of the more complex 
> operations in Asterisk - regardless of the versions.
>
> And lo and behold, we had.
>
> Local channels are weird. One of the 'fun things' they do is 'help' 
> T.38 along by passing along a channel query option for T.38 state. 
> This lets us do ridiculous things like make sure a T.38 fax works 
> across a Local channel chain (and is covered by the 
> fax/sip/local_channel_t38_queryoption test). Unfortunately, the 
> bridge_t38 module had to query for T.38 state in its compatible 
> callback - this allowed it to determine the current state of T.38 on 
> the channels in the bridge to see if it needed to be activated. 
> Unfortunately, in a 300 Local channel chain, that means reaching 
> across 300 bridges - simultaneously - locking bridges, 
> bridge_channels, channels, PVTs, and the entire world in the process. 
> Since the bridge lock was already held in the compatible callback, 
> this caused a locking inversion (no surprise there), deadlocking the 
> whole thing.
>
> This is not a trivial locking situation to resolve. Even if we unlock 
> the bridge, we're still liable to deadlock merely by trying to lock 
> 300 bridges simultaneously. (There may even be another bug in here, 
> but it is hardly worth trying to find or fix at this point.) And we 
> can't remove the query option code in chan_local, as T.38 faxes will 
> no longer work across Local channels.
>
> As an aside, if there's a lesson in all this, it is that synchronous 
> code in a heavily multi-threaded environment is bad. Message passing 
> may be harder to write, but it is far easier to maintain.
>
> Anyway, as a result, I've reverted the bridge_t38 module in 75c800eb28.
>
> So what do we do now?
>
> The crux of this problem is that the bridging framework does not have 
> a standard way of informing a channel when it has joined or - more 
> importantly - left a bridge. Direct media has its own mechanism 
> managed by the RTP engine - so it works around this. However, we have 
> a number of scenarios where "things happen" in a bridge that involves 
> state on a channel and - right now - we don't have a unified way of 
> handling it. In addition to T.38, we also have channels being put on 
> hold, DTMF traversing a channel, and more. Often, the channel driver 
> has this state - but instead, we have a lot of 'clean up' logic being 
> added to the bridging core to handle these situations.
>
> As I see it, we really only have two options here:
> (1) Add code to the bridging framework to clean up T.38 on a channel 
> when it leaves. This is kind of annoying, as it will happen on every 
> channel when it leaves, regardless of whether or not the channel even 
> supports T.38.
> (2) Add a new channel technology callback that a bridge can use to 
> inform a channel driver that it is being ejected from a bridge. This 
> would give us a single place to put cleanup logic that has to happen 
> in a channel driver when it is no longer bridged.
>
> I'm not sure those two options will work, exactly, but it's the best 
> options that I can think of after exhausting lots of other code 
> changes in the bridging core. If someone has other suggestions, I'd be 
> more than happy to entertain them.
>
> Matt
>
>
> [1] https://jenkins.asterisk.org/
> [2] 
> https://jenkins.asterisk.org/jenkins/job/periodic-asterisk-master/75/testReport/junit/%28root%29/AsteriskTestSuite/tests_fax_pjsip_directmedia_reinvite_t38/
> [3] https://gerrit.asterisk.org/#/c/1761/
> [4] 
> https://jenkins.asterisk.org/jenkins/job/periodic-asterisk-master/80/testReport/junit/%28root%29/AsteriskTestSuite/tests_masquerade/
>
> -- 
> Matthew Jordan
> Digium, Inc. | Director of Technology
> 445 Jan Davis Drive NW - Huntsville, AL 35806 - USA
> Check us out at: http://digium.com & http://asterisk.org
>

Hi Matt,

Of the two ideas you propose, the channel technology callback sounds 
like the better option. That way, only relevant channel drivers will 
need to bother implementing the callback, limiting the scope of the work.

Something else to consider is that the T.38 bridge technology worked 
really well except for when chains of local channels were involved. The 
main issue was the method by which the local channels proxied 
information. Would there be some non-earth-shattering change that could 
be made to make it so that local channels store the proxied information 
locally when the proxied information is first set? That way, when the 
bridge technology queries the local channel, there is no complex 
reaching across bridges to get the information; the local channel 
already has that knowledge stored on it. I'm not 100% sure of the 
mechanism for getting this state stored on the local channel, but I was 
curious more if the idea had crossed your mind to explore that avenue.