Benjamin Fitzgerald
LETS Corporation
(925) 235-1154
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On Fri, Jan 8, 2016 at 12:08 PM, Alex Balashov <abalashov@evaristesys.com> wrote:
Hi Benjamin,
To some extent, this is just a perennial, existential problem of using a proxy, so part of the answer is going to be that you need fundamentally reliable signalling, speaking from the vantage point of something which operates are a signalling relay (i.e. Kamailio).
However, I understand that reality does not mirror expectations. As the purveyor of a SIP service delivery platform based entirely on Kamailio, we run into this problem all the time, particularly since our system generates accounting records with billing involvement. There are some well-established and canonical solutions:
1. You make it sound like the Asterisk channel stays up indefinitely in such a situation. Why is that?
The normal behaviour is for Asterisk to hang up the call after some number of seconds without incoming RTP.
It's likely that tuning the RTP timeout setting to something conservative[1] would solve a lot of your problems off the bat.
2. The Kamailio 'dialog' module can spoof a BYE toward both endpoints based on an absolute dialog timeout (regardless of whether both dialog peers are still actively engaged), which can be set globally or on a per-dialog basis:
http://kamailio.org/docs/modules/4.3.x/modules/dialog.html#timeout-avp-id
http://kamailio.org/docs/modules/4.3.x/modules/dialog.html#default-timeout-id
http://www.kamailio.org/wiki/cookbooks/4.3.x/pseudovariables#dlg_ctx_attr
3. The 'dialog' module also has a dead peer detection / keepalive scheme based on sequential OPTIONS pings:
http://kamailio.org/docs/modules/4.3.x/modules/dialog.html#idp1898328
If one or both of the peers don't respond to these, the dialog will be timed out, and if you've set $dlg_ctx(timeout_bye) = 1, this will result in a spoofed BYE toward both peers as well.
4. There are various other signalling-oriented UA-side mechanisms intended to solve this problem as well, such as SIP Session Timers (RFC 4028).
...
Of course, all this depends on the maintenance of dialog state in Kamailio, which is an additional complication and a potential wrinkle if that data were to be lost.
So, it's a bit hard to say whether Kamailio is the _best_ place to solve this problem. The first line of defence really should be at the endpoint level on both sides of the proxy. Beyond that, Kamailio does offer some pragmatic solutions.
-- Alex
[1] Notwithstanding RTP interruptions due to VAD, hold, etc.
--
Alex Balashov | Principal | Evariste Systems LLC
303 Perimeter Center North, Suite 300
Atlanta, GA 30346
United States
Tel: +1-800-250-5920 (toll-free) / +1-678-954-0671 (direct)
Web: http://www.evaristesys.com/, http://www.csrpswitch.com/
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