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 Tel: +1-678-954-0670 Fax: +1-404-961-1892 Web: http://www.evaristesys.com/