Re: [MMUSIC] Offer/Answer PT Questions

Paul,

I agree with Christer, this needs to be fixed. We are doing interop testing with other carriers for VoIP interconnection, and this is  a major issue.

Fixing this would help accelerate VoIP interconnection.

Regards,

Martin

-----Original Message-----
From: mmusic [mailto:mmusic-bounces@ietf.org] On Behalf Of Christer Holmberg
Sent: Monday, February 22, 2016 3:42 PM
To: Paul Kyzivat <pkyzivat@alum.mit.edu>; mmusic@ietf.org
Subject: Re: [MMUSIC] Offer/Answer PT Questions

Hi,

>> One thing that bothered me here is that PT cannot be reused for the 
>> duration of the session. It is probably safe to reuse the PT after 
>> session modification if PT is no longer used. I always felt that 
>> dynamic PT reuse criteria were much stricter then realistically possible or needed.
>...
>> I think the most important criteria here is that there should be no 
>> ambiguity regarding how an RTP packet with particular PT should be 
>> decoded. If it is guaranteed that there are no packets sent between 
>> Alice and Bob with PT 111 over some reasonable time interval (couple 
>> of network round trip times), then PT 111 can be safely reused. If, 
>> in this scenario Bob's end point knows that it was not sending 
>> anything with payload 111 recently, then it can safely reuse this payload.
>> Alice could not be sending anything with payload 111 since Bob did 
>> not accept it previously. On the other hand, Bob must not reuse PT 
>> 100 for, let's say, CN, since there are packets with this payload in 
>> flight and this will create decoding ambiguity. To conclude, an end 
>> point should be able to safely reuse any PT that it is not currently 
>> accepting or was not sending or accepting for at least a few network round trip intervals.
>
> I agree that this is how it *should* be. I don't know if we can change 
> this safely or not. It seems likely to me that some implementations might be freaked out by this. It is worth discussion.
>
> There are cases where it really isn't feasible to meet the current 
> requirement. In particular, when a transfer is done via 3pcc the transferee won't know the history of past PT usage. So I expect in cases such cases the rule is commonly broken.
>
> Coming up with clear rules for when PTs can be recycled could be a challenge.

Well, I think we have to try, because these kind of issues keep causing problems.

The fact that something may freak about because we fix it is not a reason for not fixing it :)

Regards,

Christer

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