[AVT] Update (-12) of Timed Text draft

["Rey Jose" <Rey at panasonic.de>]

Hi all,

A new revision is out.  Until it goes through the ID manager queue it is
available at:
http://www.prdcg.panasonic.de/ietf/docs/draft-ietf-avt-rtp-3gpp-timed-te
xt-12.txt

In short, the changes reflect the results of the offline discussions as
posted in a previous email to the list :
http://www1.ietf.org/mail-archive/web/avt/current/msg05106.html


In detail:

In Section 4 RTP Payload header definitions changed from:

"TYPE 5 units do not make use of the timestamp, but instead become
active upon reception and not at the time instant indicated by the
timestamp.  Therefore, if RTP packets contain only one TYPE 5 unit or
only (several) TYPE 5 unit(s), the RTP timestamp SHALL be set to the
current value of RTP timestamp plus one.  Adding one (1) unit, as
opposed to just using the current timestamp, further allows the use of
the timestamp for identifying RTP packets that carry fragments of the
same text sample.
"
To 

"TYPE 5 units receive their timestamp from the first non-TYPE 5 unit
following them in the payload or from the RTP packet header itself, if
there are only TYPE 5 unit(s) or if one or several TYPE 5 units follow a
sample of unknown duration (see Section 4.1.2, SDUR definition).  If
there are no non-TYPE 5 units that follow, the timestamp of the sample
description is calculated in the usual way, i.e. by adding sample
duration and timestamp value of the last unit encountered (see case a)
in Figure 10).  Finally, note that for TYPE 5 units, the timestamp
actually does not represent the instant when they are played out, but
instead the instant at which they become available for use.  " 

And in Section 4.5 on aggregate payloads, the following :

"o	TYPE 5 units MAY be placed anywhere in the aggregate and they
SHALL NOT be regarded for calculating the timestamp of the subsequent
units.  This is because they usually do not belong to any text sample in
particular, but may apply to several.  For timestamp calculations, TYPE
5 units MUST simply be ignored, i.e. by jumping to the next unit.  For
setting the timestamp in packets containing only TYPE 5 units refer to
Section 4, timestamp definition."

Changed to:

"o	Regarding sample descriptions in aggregate payloads: since TYPE
5 units may potentially apply to several units in the stream, a sender
shall ensure that a copy of the sample description is received before
the affected text sample is due.  Therefore, a copy of the sample
description SHOULD be placed in the payload before the text sample it
applies to.  Of course, if several text samples in a payload use the
same sample description, once per payload is enough.  A sender MAY
choose to omit the sample description if it knows by some other means,
such as payload specific feedback messages [21], that the sample
description has arrived at the receiver or if it employs additional
transport resiliency measures (Section 5), for example. "


Cheers,
---------------------------------------------
Jose Rey              mailto:rey at panasonic.de
P R D C G - Panasonic R&D Center Germany GmbH
Phone/Fax: +49(0)6103766-134/166
--------------------------------------------- 


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