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Re: [AVT] I-D Action:draft-ietf-avt-rtp-svc-07.txt
Colin Perkins writes
> > A URL for this Internet-Draft is:
> > http://www.ietf.org/internet-drafts/draft-ietf-avt-rtp-svc-07.txt
>
> I had a couple of comments on section 8.1.1. This section is much
> improved from the previous version, but is still unclear on what
> timestamp should be used for various stages of the process. In
> particular, the bullets in part two need clarification:
>
> o NAL units with the same timestamp are grouped in decoding order
> to Operation Point representations in each RTP stream.
>
> Since this refers to order within an RTP stream, I assume the RTP
> timestamp of the NAL units is used?
>
> o Operation Point representations with the same timestamp SHALL
> be
> grouped to access units in order of the RTP session dependency
> from lowest to highest Enhancement RTP session, where the highest
> Enhancement RTP session is the one which no other RTP session
> depends on and the lowest Enhancement RTP session may be the Base
> RTP session.
>
> Since this is ordering data across RTP sessions, it presumably maps
> the RTP timestamp to an NTP format wall-clock value using the
> information contained in RTCP SR packets, and then uses those NTP
> format timestamps for ordering?
Your assumption is correct. As far as I understand RFC3550, NTP timestamps
SHOULD be used for inter session synchronization. Since random RTP timestamp
offsets SHOUDLD be used per RTP stream, the usage of RTP timestamps for any
inter session processing is not possible.
Actually 8.1.1. describes the usage of session multiplexing for a layered
video codec with RFC3550. Besides the requirement of using the SDP
dependency signaling instead of standard SDP RFC4566, 8.1.1. does only rely
on mechanisms defined in RFC3550. Thus, I still believe that this mode
should be always allowed, whatever the exact gain using any other method may
be.
Thomas Schierl
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