[asterisk-dev] A thread for format work

Matthew Jordan mjordan at digium.com
Mon Apr 28 21:59:53 CDT 2014

Like a normal person, I decided to poke at the format work while watching
tornadoes track North and South of Huntsville (because really, what else
are you going to do?) After looking through Josh's notes on the wiki [1]
and the code in the team branch [2], I decided to tackle bridge_native_rtp
(as it was the first thing that didn't compile). After hacking through it a
bit, I had a few questions. Since this is going to be a recurring theme
while working on this, a thread seemed in order.

So: if you decide to work on this or take a poke at it and have some
questions about things, feel free to respond to this thread.

Anyway, without further ado:

There is a portion of bridge_native_rtp that attempts to check if the
channels are still compatible. If not, it breaks the native bridge so that
the bridging framework can attempt a more suitable mixing technology. Part
of the check looks at the packetization attribute of the formats in use:

    read_ptime0 =
    read_ptime1 =
    write_ptime0 =
    write_ptime1 =

    if (read_ptime0 != write_ptime1 || read_ptime1 != write_ptime0) {
        ast_debug(1, "Packetization differs between RTP streams (%d != %d
or %d != %d). Cannot native bridge in RTP\n",
                read_ptime0, write_ptime1, read_ptime1, write_ptime0);
        return 0;

That is, if the packetization between the read/write of each channel pair
is not the same, break the bridge.

Right now, there aren't compatible calls with this code in the branch.
So... questions!

(1) Where will the packetization of a format on a channel reside? With the
format on the channel? Or with the capability of the channel?

(2) Should we even be bothering with the codec's preferred packetization
here? It doesn't really matter what a codec prefers at this point - if an
SDP negotiated a different packetization rate, that's what we care about...

(3) Are all these questions moot, and should I just go ahead and add the
proposed framing size functions from the wiki:

Framing size

The framing size controls the length of media frames (in milliseconds).
Previously this was stored in a separate structure but has now been rolled
into ast_format_cap. To allow control two API calls will be added.

void ast_format_cap_framing_set(struct ast_format_cap *cap, const struct
ast_format *format, unsigned int framing);
unsigned int ast_format_cap_framing_get(const struct ast_format_cap *cap,
const struct ast_format *format);

[1] https://wiki.asterisk.org/wiki/display/AST/Media+Format+Rewrite

[2] https://origsvn.digium.com/svn/asterisk/team/group/media_formats

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