Re: [AVTCORE] RFC5285bis open issues

[Magnus Westerlund <[e-mail]>]

Hi,

Personal opinions, not responding as WG chair and doc shepherd.

Den 2017-02-19 kl. 16:06, skrev Roni Even:
> Hi,
>
> Based on the email discussion I see some issues that need conclusion
>
>
>
> 1.       For the one-byte RTP header extension if id=0 and len>0 what
> action to take since there is no current text
>
> a.       This is also padding
>
> b.      This is an error so stop interpreting following header
> extensions – similar to id=15
>

 From my perspective, RFC 5285 is very clear that padding is a byte with 
value 0, i.e. all 8  bits are zero. Also, sending for example 0x04 is 
very undefined and a sender could expect b) option response from the 
receiver. Without agreed interpretation I rather send a clear message, 
don't use this. Thus, I would favour option b).


> 2.       Can we conclude from RFC5285 that if only IDs between 1-14 are
> offered it means that only one-byte RTP header extension are supported?
>

I think I have been involved in discussion about this previously. The 
conclusion I remember is that it is not possible. It is allowed to 
negotiate IDs below 16 and still use the two-byte header format for a 
particular RTP stream. So, it is not a conclusion we can draw.

However, the reality was that until we created the SDES header extension 
we didn't have any header extension that had the possibility to be more 
than 16 bytes.

At least if I remember correctly how the definition of the various 
registered header extensions.
http://www.iana.org/assignments/rtp-parameters/rtp-parameters.xhtml#rtp-parameters-10

But, there could exist proprietary header extensions that are longer.

So, not guaranteed, but a fairly safe bet assuming no header extensions 
that are known to allow longer than 16 bytes. But, I think one has to 
make it clear that there are no guarantees, but one could write a 
guideline for a case that is likely to work.

Cheers

Magnus Westerlund

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