From: James Sandford <james.sandford@bbc.co.uk>
To: Magnus Westerlund <magnus.westerlund@ericsson.com>, "iesg@ietf.org"
 <iesg@ietf.org>
CC: "avtcore-chairs@ietf.org" <avtcore-chairs@ietf.org>,
 "draft-ietf-payload-rtp-ttml@ietf.org"
 <draft-ietf-payload-rtp-ttml@ietf.org>, "avt@ietf.org" <avt@ietf.org>
Thread-Topic: [AVTCORE] Magnus Westerlund's Discuss on
 draft-ietf-payload-rtp-ttml-03: (with DISCUSS and COMMENT)
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Date: Tue, 29 Oct 2019 13:52:05 +0000
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Subject: Re: [AVTCORE] Magnus Westerlund's Discuss on
 draft-ietf-payload-rtp-ttml-03: (with DISCUSS and COMMENT)
Precedence: list

Thank you for your further comments.=0A=
=0A=
I will make the suggested changes to Section 8 and Section 11.1.=0A=
=0A=
With regards to further clarifying the fragmentation of documents, I propos=
e the following:=0A=
=0A=
Section 6 OLD:=0A=
  If a TTML document is assessed to be invalid then it MUST be discarded. W=
hen processing a valid document, the following requirements apply.=0A=
=0A=
Section 6 NEW:=0A=
  If a TTML document is assessed to be invalid then it MUST be discarded. T=
his includes empty documents, i.e. those of zero length. When processing a =
valid document, the following requirements apply.=0A=
=0A=
Section 8 ADDITIONAL PARAGRAPH:=0A=
  As described in Section 6, only zero or one TTML document may be active a=
t any point in time.  As such, there MUST only be one document transmitted =
for a given RTP Timestamp.  Furthermore, as stated in Section 4.1, the Mark=
er Bit MUST be set for a packet containing the last fragment of a document.=
  A packet following one where the Marker Bit is set contains the first fra=
gment of a new document.  The first fragment might also be the last.=0A=
=0A=
=0A=
Regards,=0A=
James=0A=
=0A=
=0A=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=0A=
James Sandford=0A=
R&D Project Engineer=0A=
=0A=
BBC Research and Development=0A=
5th Floor=0A=
Dock House=0A=
MediaCityUK=0A=
Salford=0A=
M50 2LH=0A=
=0A=
Tel: 030304 (09549)=0A=
Web: http://www.bbc.co.uk/rd=0A=
=0A=
________________________________________=0A=
From: Magnus Westerlund [magnus.westerlund@ericsson.com]=0A=
Sent: 29 October 2019 11:47=0A=
To: James Sandford; iesg@ietf.org=0A=
Cc: avtcore-chairs@ietf.org; draft-ietf-payload-rtp-ttml@ietf.org; avt@ietf=
.org=0A=
Subject: RE: [AVTCORE] Magnus Westerlund's Discuss on draft-ietf-payload-rt=
p-ttml-03: (with DISCUSS and COMMENT)=0A=
=0A=
Hi James,=0A=
=0A=
Thanks for the  many updates in -04. However, I think there are a couple of=
=0A=
adjustments still needed.=0A=
=0A=
Section 8:=0A=
=0A=
When a document spans more than one RTP packet, the entire document=0A=
is obtained by concatenating User Data Words from each contributing=0A=
packet in ascending order of Sequence Number.=0A=
=0A=
I think this can be further clarified by adding "consecutive"=0A=
=0A=
When a document spans more than one RTP packet, the entire document=0A=
is obtained by concatenating User Data Words from each consecutive=0A=
contributing=0A=
packet in ascending order of Sequence Number.=0A=
=0A=
What I think is unclear is what is considered contributing packets. It is=
=0A=
quite common that one determine fragments based on timestamp and that may b=
e=0A=
assumed by some. I don't know if that is a dangerous assumption here. To my=
=0A=
understanding one can determine the set of fragments by looking at the=0A=
marker bit for the packets. From first 0 after a 1, until and including the=
=0A=
packet with a m=3D1. If that is your intention for how one should do it, so=
=0A=
that it works for multiple documents to share epoch and thus RTP timestamp=
=0A=
documents I think this needs to be made explicit.=0A=
=0A=
In section 11.1 it says:=0A=
=0A=
In these situations, it is RECOMMENDED that streams use=0A=
the same Synchronization Source and Clock Rate as the related media.=0A=
=0A=
You do need to insert "Time" before Synchronization source to not be=0A=
misinterpret to mean SSRC. Or maybe better is to say "clock source".=0A=
=0A=
Cheers=0A=
=0A=
Magnus Westerlund=0A=