[asterisk-users] Advanced Codec Negotiation: Need info and uses cases

George Joseph gjoseph at digium.com
Fri Jun 5 10:42:33 CDT 2020

Greetings All,

We've been working hard on new codec negotiation stuff for Asterisk 18 and
we've got some stuff to run by you.  It's a lot so please read carefully.

To give you some idea of just how difficult a job this is, a simple call
from Alice to Bob currently causes 8 attempts to reconcile codecs between
them in app_dial, chan_pjsip, res_pjsip_session and res_pjsip_sdp_rtp.  If
you've noticed a recent addition of a SCOPE_TRACE facility for Asterisk
being committed, this was the driver.  It was also very enlightening.  So
we're trying to simplify things a little.  Well, a lot.  We've got some
questions that we'd like answered with *real* use cases.

Simple use case, Alice to Bob, no direct media.

1.  Under what conditions would we accept a format on an incoming offer
from a UAC (Alice) that *wasn't* in the UAC's endpoint allow= parameter?
 Does whether we accept formats not on the endpoint need to be
configurable?   Don't just say "yes". :)   We need use cases.   We could
use the offer's list exclusively, use the endpoint's list exclusively,
merge the two together, or use only those in common.  What happens if after
applying that operation, there are no formats in common?  Drop the call?
Transocde? Using what format? It'd have to be one Alice accepts.  We'll
save the process of transcoding for a follow-on discussion.

2.  Under what conditions would we send a codec in an offer to a UAS (Bob)
that *wasn't* in the UAS's endpoint allow= parameter.   Similarly, under
what conditions would we send a format to Bob that *was* in his endpoint
allow= parameter but *wasn't* in the reconciled list we got from Alice via
the core?  Same possible options and questions as above.

3.  OK now whatever we've decided to send to Bob, according to RFC3264 para
6.1, Bob MUST send back an answer that contains a common format OR reject
the stream if there are no formats in common.  It doesn't say whether it's
valid for Bob to send back formats we didn't request *in addition *to ones
we did request.  It wouldn't make sense for him to do that because that
same RFC and paragraph only says we MUST accept media in a format we sent.
It doesn't mention what should happen if we get media in a format we
*didn't* request.  Based on this, unless someone can give us a valid use
case for this, and rules governing when it's acceptable and when it's not,
we do NOT plan on supporting receiving media in a format we didn't
request.  We'd just drop the frames.   If Bob wants to use a format not in
the offer, he should RE-INVITE.

4.  Now we've got Bob's answer and are passing it back to the core so we
need to send an answer back to Alice.  First, unless someone can give us a
valid use case, we will never send Alice a format she didn't request in her
offer so those will get removed.  Based on options specified above though,
the potential answer MAY contain formats NOT in Alice's endpoint allow=
parameter.  Same options and questions as "1".

Now let's talk about format preference order.

On the Alice to Bob side...
1.  On Alice's incoming leg, after reconciling Alice's offer and Alice's
endpoint, we can sort by Alice's preferred order or Alice's endpoint's
preferred order based on configuration and send that order to the core.

2.  On Bob's outgoing leg, after reconciling what came from the core and
Bob's endpoint, we can also sort based on either and send that in the offer.

3.  Bob can re-order the formats in his answer so I guess we need another
option to use the order we sent or the order we received before we send it
back to the core.  Do we care about the order we got *from* the core or on
Bob's endpoint any more?  Hopefully not.

4.  Now we've got a list from the core and we need to send an answer back
to Alice...  Do we need any sort alterations at all here or can we just use
what came from the core?

One more thing to consider...   Alice and Bob may *not* both be using a
pjsip channel.  In this case, the process can only be applied on the call
leg that *is* pjsip.  Let's say that Alice's leg is pjsip and Bob's isn't.
We can make Alice's channel aware that Bob isn't capable of the advanced
codec process but only after Bob's channel has been created so the process
Alice runs when passing the list to the core won't know.  The process that
runs when constructing Alice's answer *will* know by virtue of *not getting
anything* from the core because today there's no feedback at all from the
core.   On the flip side, if Alice is chan_sip and Bob is chan_pjsip, Bob
*will* know whether what came from the core is "advanced" or not.   Now the
question is, given all that's talked about above, do the rules change
depending on whether both channels are pjsip or not?  Of course, if
*neither* is pjsip, none of the above applies and the old process is used.

I know this is a lot to take in but I'd implore you to read thoroughly,
respond with real life scenarios and ask questions if something isn't
clear.   We are NOT going to shove this into 18 without everyone
understanding the implications, and if the process gets too complex, we'll
NEVER put it in because it'll work no better and be no better understood
than the current process.


George Joseph
Asterisk Software Developer
direct/fax +1 256 428 6012
Check us out at www.sangoma.com and www.asterisk.org
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