[asterisk-dev] Potentially naughty patch

[Mark Michelson]

On 10/29/2013 04:01 AM, Olle E. Johansson wrote:
> Friends,
>
> I did something that may cause issues today, but solved a lot of 
> issues. I need your feedback on this.
>
> I have a branch that makes it possible to add custom mappings to the 
> isdn cause code to SIP codes table in chan_sip - and the opposite ones.
>
> In order to a fine grained control I may want a cause code to send a 
> very specific SIP message, maybe even a custome one. And in some cases 
> I want to translate a specific SIP code to the same without affecting 
> other calls in sip2sip situations.
>
> My solution was to add five custom codes to the cause table - custom1 
> to custom5 in the range 115-119 that is not used as far as I can tell.
>
> This solved the problem we worked on and also added the benefit of 
> being able to run "hangup(118)" on a SIP call to generate "498 IAX2 
> not supported" in the SIP channel.
>
> The questions are:
> - is this too dangerous, to grab some ISDN cause codes?

I think that if those values have not been allocated by now, they aren't 
going to be. It's probably fine to make use of them for your own benefit.

> - will it have side effects?

As long as you're within the valid range for a Q.850 cause code (which I 
believe is 7 bits), then there shouldn't be side effects directly from 
stealing those values.

> - what will other channels do if they get a hangup(119) ?

This is the interesting question. I did a brief search of channel 
drivers and I found two different behaviors:
1) The Q.850 cause code is sent as-is
2) The Q.850 cause code is translated into a channel-specific value.

For case 1, you could have issues with far-end equipment if they do not 
understand the cause code. I, however, have no direct knowledge of what 
sort of equipment might have such issues. For case 2, usually channel 
drivers will have some default to use if the given cause code is unknown.

>
> All of this is of course a bit dangerous and have to be used only by 
> experienced asterisk admins, but is something that have been asked for 
> for a long time. There is nothing stopping you from redefining any 
> cause code to 401, 180 or something else, like 302. But that will hurt...
>

In a controlled environment where you are aware of the equipment that 
you are communicating with, the change you have made would probably not 
cause trouble. I would recommend performing tests against public ISDN 
networks and other equipment to be sure that sending unallocated cause 
codes does not cause a problem for them though. I would not recommend 
this code change as a general improvement to open source Asterisk, 
though, but you probably already knew that :)

> Cheers,
> /O
>
>


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