[AVTCORE] Header Compression and RFC5285bis

[Magnus Westerlund <[e-mail]>]

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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