[asterisk-dev] [IAX] Separate QoS settings for signaling, voice and video packets

[Matthew Jordan]

On Thu, Mar 5, 2015 at 5:27 AM, Yousf Ateya <y.ateya at starkbits.com> wrote:

Asterisk provides a way to set tos and cos fields
> <https://wiki.asterisk.org/wiki/display/AST/IP+Quality+of+Service> in IP
> headers. But as stated in documentation:
>
> Since IAX connections combine signalling, audio, and video into one UDP
> stream, it is not possible to set the TOS separately for the different
> types of traffic
>
> So the packet priority is assigned to signaling, voice and video; which
> minimizes the benefit of having packet priority.
>
> If there are separate settings for signaling and voice, users can increase
> priority of signaling; to ensure that control packets get better chance
> than voice packets.
>

The issue isn't that the settings for signalling and voice are combined, it
is that signalling and voice information are multiplexed over a single
socket. These quality settings are set on the socket that services both
signalling and media; changing them on the fly between frames on a single
socket would probably have "interesting" effects. Keep in mind that the
settings determine the priority of packets being sent from the socket.
Given how often signalling and media can change on a single socket for a
number of chan_iax2 channels, you would defeat the purpose of setting
TOS/COS if you changed them frequently.

The only way you could overcome that would be to break apart the signalling
and media onto different sockets. That has several drawbacks:
(1) One of IAX2's strengths is its ability to pass media and signalling
over a single port, which helps with NAT/firewalls
(2) It would make chan_iax2 no longer compliant with its own RFC
(3) It would certainly be a non-trivial amount of work. Since chan_iax2 can
really only be tested against itself, it makes verifying that such a change
does not inject a large amount of risk into the driver rather difficult.

-- 
Matthew Jordan
Digium, Inc. | Director of Technology
445 Jan Davis Drive NW - Huntsville, AL 35806 - USA
Check us out at: http://digium.com & http://asterisk.org
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