Subject: [AVT] RFC 2833 questions: Intervals, durations, and lost packets.

From: "Lubbs, Stephen" <[email-masked]>
Date: Mon, 28 Oct 2002 11:18:10 -0800

Hi Folks,
I have a few questions related to RFC 2833, to wit:

1.	Is the intent of the spec that RFC 2833 packets should be
transmitted every 50 ms or is it that they should be transmitted at the
interval represented by the duration of audio packets sent prior to RFC 2833
events? 

2.	If both audio packets and RFC 2833 packets are transmitted
concurrently, is it allowable for the duration of each RFC packet be
different from that of the packet interval of the audio packets. Is there a
recommendation with respect to this? 

3.	If both audio packets and RFC 2833 packets are transmitted
concurrently and an RFC 2833 packet is missing, is the intent that the last
RFC 2833 packet received should be extended or should the audio packet be
played out in its stead? Should the ability of the codec to represent the
event be considered? 

4.	If both audio packets and RFC 2833 packets are transmitted
concurrently and an RFC 2833 packet is missing, assuming that the last RFC
2833 packet should be extended for a period that would represent up to 3
lost packets, which should be used to calculate the duration of the
extension: the duration of the previously received RFC 2833 packet or the
packet interval of the previously received audio packets? 

5.	If both audio packets and RFC 2833 packets are transmitted
concurrently which takes precedence? There seem to be contradictory
statements in the RFC.

Regards,
Steve Lubbs

---

From: Henning Schulzrinne <[email-masked]>
Date: Mon, 28 Oct 2002 14:32:46 -0500

Please read the updated version (draft-ietf-avt-rfc2833bis, issued a few 
days ago) and see if it answers your questions.

Lubbs, Stephen wrote:
>    1. Is the intent of the spec that RFC 2833 packets should
>       be transmitted every 50 ms or is it that they should be
>       transmitted at the interval represented by the duration of audio
>       packets sent prior to RFC 2833 events?

See the draft.

>       concurrently, is it allowable for the duration of each RFC packet
>       be different from that of the packet interval of the audio
>       packets. Is there a recommendation with respect to this?

The spec can't spell all allowable permutations. If nothing is said, 
normal RTP rules apply. Since every RTP packet even with the same PT 
could, conceivably, cover a different time interval, this is certainly 
possible. I would expect that this would test the robustness of most 
implementations, so maybe we should add this to a torture test...

>       concurrently and an RFC 2833 packet is missing, is the intent that
>       the last RFC 2833 packet received should be extended or should the
>       audio packet be played out in its stead? Should the ability of the
>       codec to represent the event be considered?

This is an implementation decision since it does not affect correct 
interoperation. Consider it product differentiation.

>       concurrently and an RFC 2833 packet is missing, assuming that the
>       last RFC 2833 packet should be extended for a period that would
>       represent up to 3 lost packets, which should be used to calculate
>       the duration of the extension: the duration of the previously
>       received RFC 2833 packet or the packet interval of the previously
>       received audio packets?

See above; you can only guess at what you might have received.

>       concurrently which takes precedence? There seem to be
>       contradictory statements in the RFC.

Please point out contradictory statements by citation in the I-D. In 
general, this is one of million of possible failure cases which should 
only occur if somebody goofs. Depending on the application, you could 
justify any approach, including dropping the packet, since the sender is 
obviously confused and may not mean either.

> Steve Lubbs

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