Re: [AVTCORE] Header Compression and RFC5285bis

Stephen Casner <casner@acm.org> Wed, 11 May 2016 19:22 UTC

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From: Stephen Casner <casner@acm.org>
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Date: Wed, 11 May 2016 20:22:16 +0100
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References: <56FFE98E.2070406@ericsson.com> <071a01d1aa8e$9e4c1800$dae44800$@gmail.com> <alpine.OSX.2.20.1605110646350.94339@auge.local> <082301d1aba6$bc375a00$34a60e00$@gmail.com>
To: Roni Even <ron.even.tlv@gmail.com>
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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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Yes, that's fine. Thanks.

-- Steve

> On May 11, 2016, at 6:01 PM, Roni Even <ron.even.tlv@gmail.com> wrote:
> 
> 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: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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> 
>