Re: [AVTCORE] Header Compression and RFC5285bis

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

Steve,
This is the full paragraph in the current version.
It looks OK to me

"Use of RTP header extensions will reduce the efficiency of RTP header
   compression, since the header extension will be sent uncompressed
   unless the RTP header compression module is updated to recognize the
   extension header.  If header extensions are present in some packets,
   but not in others, this can also reduce compression efficiency by
   requiring an update to the fixed header to be conveyed when header
   extensions start or stop being sent.  The interactions of the RTP
   header extension and header compression is explored further in
   [RFC2508] and [RFC3095]."

Roni

> -----Original Message-----
> From: Stephen Casner [mailto:[e-mail]]
> Sent: Wednesday, May 11, 2016 8:49 AM
> To: Roni Even
> Cc: 'Magnus Westerlund'; 'IETF AVTCore WG'
> Subject: RE: [AVTCORE] Header Compression and RFC5285bis
> 
> Roni,
> 
> I think it would be appropriate to include another sentence before the one
> you propose:  "The use of RTP header extensions may reduce the
> effectiveness of header compression."
> 
>                                                         -- Steve
> 
> On Tue, 10 May 2016, Roni Even wrote:
> 
> > Hi Magnus,
> > I am updating the document based on other comments from Colin.
> > The current text says "The interactions of the RTP   header extension
and
> > header compression is explored further in   [RFC2508] and [RFC3095]."
> > Do you think we need more text here? Any suggestions Roni
> >
> > > -----Original Message-----
> > > From: avt [mailto:[e-mail]] On Behalf Of Magnus
> > > Westerlund
> > > Sent: Saturday, April 02, 2016 6:47 PM
> > > To: IETF AVTCore WG; Stephen Casner
> > > Subject: [AVTCORE] Header Compression and RFC5285bis
> > >
> > > Hi,
> > >
> > > Stephen Casner raised a private comment on the
> > > draft-ietf-avtext-sdes-hdr-ext-05 when he did the IANA expert review.
> > > This was that both
> > > draft-ietf-avtext-sdes-hdr-ext-05 and RFC5285bis doesn't discuss
> > > impact of header compression due to the presence of RTP header
> extensions.
> > >
> > > I have to agree that this is something that at least RFC5285bis
> > > should
> > have a
> > > discussion on. There are some impact.
> > >
> > > In the case of Robust Header Compression (ROHC) (RFC 5795 and RFC
> > > 5225) the impact to my understanding is the following. RFC5225 and
> > > the RTP profile don't compress the actual header extension at all.
> > > The X bit in
> > the
> > > header is compressed and it is classified in Section A.5 as Rarely
> > changing.
> > > The assumption here is that what value is set on a previous packet
> > > will be maintained on the next.
> > >
> > > Looking at the emerging usages that is mostly right, but there will
> > > be
> > periods
> > > where the x bit may be flipping its value. The usage I am
> > > considering is
> > these
> > > initial inclusion of the MID, CNAME etc. However for example a video
> > > stream, it is sufficient to include the header extension in a single
> > packet out
> > > potentially many for an particular video image/frame. Thus one might
> > > have
> > a
> > > on, off, next TS, on, then off on subsequent packets.
> > >
> > > The impact of the classification as Rarely changing is that when a
> > > change
> > is
> > > needed a larger header than otherwise needed will be sent.
> > > Thus the impact will be that each on and off behaviour will result
> > > in one
> > and
> > > maybe two larger ROHC headers to get the change across compared to
> > > if no RTP header extensions would have been present.
> > >
> > > I haven't looked at CRTP, but I would guess similar impact.
> > >
> > > Cheers
> > >
> > > Magnus Westerlund
> > >
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