[VoiceOps] SIP calls that aren't torn down

Alex Balashov abalashov at evaristesys.com
Tue Jun 7 17:00:20 EDT 2011

On 06/07/2011 04:55 PM, anorexicpoodle wrote:

> Im not familliar with Broadsofts method, but Metaswitch does something
> similar, and instead of using session timers, uses regular re-invites to
> detect if all involved endpoints are still alive.
> We found this a little heavy handed with their stock timers, so we
> fiddled with it until we were happy.
> I suspect the rationale here is that using RFC 4028 depends on both far
> ends supporting that RFC, and implementing it properly, which takes the
> control out of the softswitch's hands. Using a re-invite as a polling
> mechanism guarantees the polling works with even older (or cheaper) SIP
> endpoints that dont support session timers.

What?  RFC 4028 only requires that one endpoint support timers.

If the far end does not support them, the softswitch UA will take on the 
refresher role and send the reinvites, and the other endpoint need not 
support SSTs or know what they're for, but just to answer them.

If both endpoints support SSTs, they'll negotiate the refresher role 
amongst themselves.  But that's optional.  In practice, the essence of 
the standard _is_ the sending of periodic reinvites as a signaling-only 
method of dead peer detection, not ping-pong between UAs.

 From 4028 section 7.1:

   "They are not needed, since the extension works even when only the
    UAC supports the extension."

Alex Balashov - Principal
Evariste Systems LLC
260 Peachtree Street NW
Suite 2200
Atlanta, GA 30303
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Web: http://www.evaristesys.com/

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