Re: [AVTCORE] Header Compression and RFC5285bis

"Roni Even" <ron.even.tlv@gmail.com> Wed, 11 May 2016 11:06 UTC

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From: "Roni Even" <ron.even.tlv@gmail.com>
To: "'Stephen Casner'" <casner@acm.org>
References: <56FFE98E.2070406@ericsson.com> <071a01d1aa8e$9e4c1800$dae44800$@gmail.com> <alpine.OSX.2.20.1605110646350.94339@auge.local>
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Date: Wed, 11 May 2016 14:06:14 +0300
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Cc: 'Magnus Westerlund' <magnus.westerlund@ericsson.com>, 'IETF AVTCore WG' <avt@ietf.org>
Subject: Re: [AVTCORE] Header Compression and RFC5285bis
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Steve,
Thanks, will add this sentence
Roni

> -----Original Message-----
> From: Stephen Casner [mailto:casner@acm.org]
> 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:avt-bounces@ietf.org] 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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