[asterisk-dev] Native Bridging: terminology

[Kevin P. Fleming]

Mark Michelson wrote:

> The term "native bridge" is a bit overloaded in Asterisk. On the surface, it 
> simply means that the two legs of a call have the same channel->tech->bridge 
> function.

Right. The channel driver can then choose how it wants to implement
'native bridging' and even offer multiple options.

> I can't speak for DAHDI, but within SIP, native bridging has two subcategories. 
> One, typically referred to as "SIP native bridging" is used when reINVITEs are 
> enabled. The endpoints send their media directly to one another. The other 
> subcategory is called "Packet 2 Packet" or "P2P" bridging. If reINVITEs are not 
> enabled, but there are also no features that require the Asterisk core to be in 
> the voice path, then the bridging will be done at the RTP layer of Asterisk.

For chan_dahdi, native bridging means the channels are bridged in DAHDI,
and the audio never passes through Asterisk. For cases where the two
channels involved live on the same DAHDI-driven card, and the card
supports direct timeslot bridging (interchange), then the audio won't
even pass across the system bus at all. If that can't be done, then the
bridging will be done in the DAHDI core in the kernel.

For chan_iax2, native bridging means a media-only transfer (similar to a
 SIP reinvite that only changes the media destination addresses/ports)
will be attempted to keep the media from going through Asterisk at all.

For chan_sip, the native bridging is actually (mostly) handled by the
RTP engine being used, and thus it would be possible to 'native bridge'
between any two channel drivers that use the same RTP engine... but in
practice, the non-SIP channel drivers that use RTP don't have any way to
tell endpoints to redirect their media to a different address, so any
native bridging involving those channel drivers is limited to
Packet2Packet mode.

-- 
Kevin P. Fleming
Digium, Inc. | Director of Software Technologies
445 Jan Davis Drive NW - Huntsville, AL 35806 - USA
skype: kpfleming | jabber: kfleming at digium.com
Check us out at www.digium.com & www.asterisk.org