Subject: [AVTCORE] Discuss on draft-ietf-avt-forward-shifted-red-06

From: Magnus Westerlund <[email-masked]>
Date: Thu, 19 May 2011 09:30:32 +0200

WG,

Peter Saint-Andre has one part of his discuss which I believe we need WG
input on.

> 1) It is not clear to me why this document is registering 2 new MIME types,
> instead of just registering a new optional parameter "forwardshift" for the
> already registered audio/red MIME type. Handling of these 2 new MIME types
> without the "forwardshift" parameter seems to be identical to the handling of
> audio/red.

I have discussed this discuss with Peter and clarified that there is one
actual difference between the two approaches.

Where using new mime types results in that one handles this as two
independent media formats. Thus requiring two different payload types
and negotiating them independently. And these two payload types is in
fact resulting to a doubling of the number of payload types required in
relation to the combination of primary secondary. This as each payload
type only can carry one specific combination of redundancy.

Using a optional parameter in the existing types has the result that one
uses only one payload type and in a negotiation context, like SDP
offer/answer one will use forward redundancy if both peers acknowledge
the optional parameter, and if the other doesn't understand one gets
automatic downgrade to RFC 2198 behavior. If this is the desired
behavior my small concern is how well the implementations handle new
optional parameters. They should but I have no knowledge how well people
implements that handling.

So the question to the WG is what behavior is really desired here?

Please provide any feedback before the 2nd of June.

Cheers

Magnus Westerlund

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From: Colin Perkins <[email-masked]>
Date: Thu, 19 May 2011 08:45:59 +0100

On 19 May 2011, at 08:30, Magnus Westerlund wrote:

> WG,
> 
> Peter Saint-Andre has one part of his discuss which I believe we need WG input on.
> 
>> 1) It is not clear to me why this document is registering 2 new MIME types, instead of just registering a new optional parameter "forwardshift" for the already registered audio/red MIME type. Handling of these 2 new MIME types without the "forwardshift" parameter seems to be identical to the handling of audio/red.
> 
> I have discussed this discuss with Peter and clarified that there is one actual difference between the two approaches.
> 
> Where using new mime types results in that one handles this as two independent media formats. Thus requiring two different payload types and negotiating them independently. And these two payload types is in fact resulting to a doubling of the number of payload types required in relation to the combination of primary secondary. This as each payload type only can carry one specific combination of redundancy.
> 
> Using a optional parameter in the existing types has the result that one uses only one payload type and in a negotiation context, like SDP offer/answer one will use forward redundancy if both peers acknowledge the optional parameter, and if the other doesn't understand one gets automatic downgrade to RFC 2198 behavior. If this is the desired behavior my small concern is how well the implementations handle new optional parameters. They should but I have no knowledge how well people implements that handling.
> 

The problem with using an optional parameter is that the extension changes the semantics and is not backwards compatible. This would break older implementations that ignore the new parameter. A new payload type will not be understood by old implementations, but at least it won't be misinterpreted. Because of this, I think we want new media types.

-- 
Colin Perkins
http://csperkins.org/

---

From: Magnus Westerlund <[email-masked]>
Date: Thu, 19 May 2011 10:13:18 +0200

On 2011-05-19 09:45, Colin Perkins wrote:
>>> 1) It is not clear to me why this document is registering 2 new
>>> MIME types, instead of just registering a new optional parameter
>>> "forwardshift" for the already registered audio/red MIME type.
>>> Handling of these 2 new MIME types without the "forwardshift"
>>> parameter seems to be identical to the handling of audio/red.
>> 
>> I have discussed this discuss with Peter and clarified that there
>> is one actual difference between the two approaches.
>> 
>> independent media formats. Thus requiring two different payload
>> types and negotiating them independently. And these two payload
>> types is in fact resulting to a doubling of the number of payload
>> types required in relation to the combination of primary secondary.
>> This as each payload type only can carry one specific combination
>> of redundancy.
>> 
>> Using a optional parameter in the existing types has the result
>> that one uses only one payload type and in a negotiation context,
>> like SDP offer/answer one will use forward redundancy if both peers
>> acknowledge the optional parameter, and if the other doesn't
>> understand one gets automatic downgrade to RFC 2198 behavior. If
>> this is the desired behavior my small concern is how well the
>> implementations handle new optional parameters. They should but I
>> have no knowledge how well people implements that handling.
>> 
> The problem with using an optional parameter is that the extension
> changes the semantics and is not backwards compatible. This would
> break older implementations that ignore the new parameter. A new
> payload type will not be understood by old implementations, but at
> least it won't be misinterpreted. Because of this, I think we want
> new media types.
> 

I agree this is an issue for declarative usages when an end-point using
a declarative SDP will think it understands the media format but it
doesn't. But in negotiated usages this issue would not be present as the
introducing part can only use the new semantics if the peer confirmed
support.

Cheers

Magnus Westerlund

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From: Qiaobing Xie <[email-masked]>
Date: Thu, 19 May 2011 14:49:59 -0500

Magnus,

I will be fine with either approach. From an implementation and documentation maintenance point of view, the "new optional parameter" approach is simpler. As for your concern "how well the implementations handle new optional parameters", I think the question actually applies to *any* new optional parameter introduced here. We must have tons of experiences for that from the field already. I would be shocked if this would be the first time a new optional parameter is introduced.

-Q