Re: [AVTCORE] Comments on draft-ietf-avtcore-rfc5285bis-03

["Roni Even" <[e-mail]>]

Hi Magnus and Dave,
My question here is that if you have the extmap attribute in SDP it means
that you support RFC5285 so does it imply support for two bytes extension?
On the other hand the document mandates the support of one byte by the
receivers and does not say anything about support of two bytes.

So it looks to me that like what Magnus said about the negotiation phase
that only the a=extmap-allow-mixed by both sides implies full support of two
bytes extensions may be correct?

Is this your view also?

Do you think that we should emphasis this in the text?

Roni


> -----Original Message-----
> From: Magnus Westerlund [mailto:[e-mail]]
> Sent: Tuesday, October 18, 2016 4:52 PM
> To: David Singer
> Cc: Roni Even; [e-mail]; IETF AVTCore WG
> Subject: Re: [AVTCORE] Comments on draft-ietf-avtcore-rfc5285bis-03
> 
> Den 2016-10-18 kl. 12:07, skrev David Singer:
> >
> >> On Oct 18, 2016, at 16:53 , Magnus Westerlund
> >> <[e-mail]> wrote:
> >>
> >> I think there is a negotiation mechanism, in that if one include
> >> extmap mappings for values above 15, then you indicate that you
> >> support the two-byte format, and if you accept you can clearly
> >> indicate this. Maybe this should be made more explicit, as a method
> >> of determining the operations mode. To me we do now end up with four
> >> different cases:
> >>
> >> A. One-byte only: only ID's in range 1-14. The state of
> >> a=extmap-allow-mixed has no meaning, only indication if one want to
> >> renegotiate.
> >>
> >> B. two-byte only: only ID's above 15. The state of
> >> a=extmap-allow-mixed has no meaning, only indication if one want to
> >> renegotiate.
> >
> > sorry, I don’t get this.  Two-byte headers get you full-byte length
> > fields and thus longer payloads. You could use these with small local
> > IDs.  Why say you have to use IDs outside 1-14???
> 
> Yes, you are correct. I made a mistake in my thoughts, and forgot about
> these possibilities. I would like to correct myself to say that only at
least one
> ID above 15 implies and no a=extmap-allow-mixed that you can use two-
> byte headers and may do that for some RTP streams.
> 
> But the other implication of your comment is that one can't draw the
> conclusion that IDs only in 1-14 range implies only one-byte headers.
> 
> So to make this more complete, the additional criteria that can be used to
> determine at signaling time if one will use one or two-byte headers is the
> inclusion and acceptance of an RTP header extension that requires a length
> of data that is more than 16 bytes. But, for extension that doesn't have
that
> given, like the CNAME that could be of quite different lengths both above
> and below the range one can't make that determination.
> 
> I was trying to arrive here that one could determine support for two-byte
> headers based on what was signalled, even without the a=extmap-allow-
> mixed SDP attribute.
> 
> So a=extmap-allow-mixed provides not only indication that you can switch,
> but also that one supports both one and two-byte headers.
> 
> But for the legacy case, one could negotiate using low IDs (1-14) range
only,
> and then realize at transmission start time that one have to use the
two-byte
> headers. Then one could end up in an interoperability failure as the old
> receiver is not required to support the two-bytes header.
> 
> So based on the above implications, one could write text that gives
> recommendations that for any signaling case where one knows that one will
> have to use two-byte to assign that extension an ID above 15 to indicate
this.
> But, I guess for the one where one don't know one simply have to hope that
> it works, and indicate that this issue is resolved if one follows this
> specification and indicates a=extmap-allow-mixed.
> 
> Cheers
> 
> Magnus Westerlund
> 
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