[asterisk-dev] "cRTP, anyone?" and more crackpot ideas

[Mr. James W. Laferriere]

 	Hello John ,

On Tue, 16 Dec 2008, John Todd wrote:
> I was in a conversation today with someone who does a lot of
> international VoIP traffic.  They have Cisco gear at both ends of
> their links, and the comment was that Asterisk didn't support cRTP,
> which is "compressed RTP".  I'd heard of CRTP, but honestly I've never
> heard it come up before as something people were using extensively,
> though it makes perfect sense for traffic that transits across high-
> cost, low-bandwidth IP links.
>
> This seems to be a niche solution.  Why?  Because cRTP doesn't work
> outside of layer 2 networks, from everything I've read.  If you have a
> T1 (or frame relay, or ATM, or MPLS, or whatever L2 protocol) between
> two endpoints, you apply cRTP on each interface, but each device (hop-
> by-hop) must support it.  You can't compress headers on RTP across a
> network of uncontrolled hops.  So on the internet, this would not
> work.  It would be useful only if your Asterisk systems were doing IP
> routing (point-to-point or frame relay) and could see each other
> directly on some layer 2 link.
 	What would be the difficulty with creating , say a GRE tunnel between 
two devices that support cRTP ?  I am not sure if cRTP is a device function or 
an interface function .

 	Twyl ,  JimL

> Has anyone looked at CRTP implementation for Asterisk?  This is one of
> those questions that might be already solved, but I've just never
> talked to the right person.  It seems like anything with this high a
> dollar value on it would have been programmed by someone, somewhere.
>
> Or am I wrong?  Is it possible to do cRTP or something similar in a
> way that is not hop-by-hop?  Please let me know, especially if you
> have code to back it up.
>
> Many (most?) media sessions are actually between two servers that have
> more than one RTP session active between each other at once.  It seems
> completely bizarre that a method of RTP pipelining does not exist,
> like there is for IAX2 in trunk mode.  Pushing several RTP payloads in
> the same UDP datagram seems like a HUGE bandwidth win - why hasn't
> anyone done it?  For that matter, why haven't we done it in Asterisk
> and created a new RTP hybrid?  Seems like it would be easy to throw an
> additional identifier in the SDP that would offer RTP in a pipelined
> mode so that two systems could know what protocol to use.  Asterisk
> systems with multi-channel SIP RTP sessions would "just work"
> automagically and potentially cut bandwidth usage by 60% in some
> circumstances, but other non-compatible systems wouldn't change
> behaviors.  Your patches are welcome!  ;-)
>
> Anyway... cRTP and related RFCs below.
>
> http://www.faqs.org/rfcs/rfc2508.html
> http://www.faqs.org/rfcs/rfc1144.html
>
> JT
>
>
> ---
> John Todd                       email:jtodd at digium.com
> Digium, Inc. | Asterisk Open Source Community Director
> 445 Jan Davis Drive NW -  Huntsville AL 35806  -   USA
> direct: +1-256-428-6083         http://www.digium.com/
>
>
>
>
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| James   W.   Laferriere | System    Techniques | Give me VMS     |
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