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Re: [AVTCORE] Framemarking in video packets

Sergio Garcia Murillo <sergio.garcia.murillo@gmail.com> Sat, 20 August 2022 02:34 UTC

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From: Sergio Garcia Murillo <sergio.garcia.murillo@gmail.com>
Date: Sat, 20 Aug 2022 09:34:10 +0700
Message-ID: <CA+ag07bCES8xZf3e0MEJ4ZqN-D0OaP5EzKXrOFK=fFAgR7S9bA@mail.gmail.com>
To: "Dale R. Worley" <worley@ariadne.com>
Cc: "shihang (C)" <shihang9@huawei.com>, IETF AVTCore WG <avt@ietf.org>
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Subject: Re: [AVTCORE] Framemarking in video packets
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I don't even think that the overall mechanism would be  viable, but in
order to reduce the potential compatibility issues of a fixed extension id,
it could be possible to use the 4 bits in the appbits of the two header
bits extensions and fix the extension id to 256.

I am not aware of anyone having used them and implemented it properly (even
libsrtp had a bug about handling the appbits).

Best regards
Sergio



El sáb, 20 ago 2022 8:22, Dale R. Worley <worley@ariadne.com> escribió:

> "shihang (C)" <shihang9@huawei.com> writes:
> > Got it.
> >
> >    For "one-byte" header extensions:
> >    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
> >    |       0xBE    |    0xDE       |           length              |
> >    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
> >    |flag ID| L=2   |     flag data                                 |
> >    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
> >    | ID    | L     | framemarking data ...                         |
> >    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
> >    For "two-byte" header extensions:
> >    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
> >    |       0x100           |appbits|           length              |
> >    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
> >    | flag ID       |     L=1       |     flag data                 |
> >    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
> >    | ID            | L             | framemarking data ...         |
> >    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
>
> > From the extension format you give, the 32 bit also contains ID, length
> > and value, just like an extension header. The difference is that the ID
> > is a fixed global value instead of a local value negotiated by SDP. It
> > looks like creating a special purpose header extension entry which
> > identify and locate the framemarking extension.
> >
> > I wonder if 16 bit is enough unless you want to put some data into the
> > flag data field such as the offset of the framemarking extension
> > header.
>
> The proposal is that the frammarking header extension would immediately
> follow the flag header extension, so there is no need to record the
> offset.  Thus the "flag data" would be a fixed value.  That would allow
> the four bytes (flag ID, L, flag data) to all have fixed values.
>
> > The router can recognize framemarking with a fixed global ID by
> > iterating the list of header extensions until a match is found. In
> > this way, there will not be false positive. To reduce the iteration
> > overhead, we can force the framemarking extension to appear on the
> > first one.
>
> The difficulty with that strategy is that it requies that the router be
> able to determine which packets contain RTP and which do not.  But
> there's no reliable way to detect that.
>
> Dale
>
> _______________________________________________
> Audio/Video Transport Core Maintenance
> avt@ietf.org
> https://www.ietf.org/mailman/listinfo/avt
>

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