Subject: [AVT] FW: Discuss Mpeg2TS preamble - breakout session

From: Roni Even <[email-masked]>
Date: Wed, 11 Nov 2009 08:19:21 +0900

Hi,

We had a breakout session on Tuesday afternoon on the MPGEG2TS preamble
drafts.  The participants included: Ali Begen, Jeff Goldberg, Colin Perkins,
Cullen Jennings, Robert Sparks, Zou ZiXuan, Peilin Yang, Stephan Wenger and
Roni Even.

The meeting was led by Colin who send the meeting summary in a previous
email to the list, see bellow.

Based on the discussion I would like to ask the authors of
draft-xia-avt-mpeg2ts-preamble to submit a new revision ASAP that will
clarify their solution based on the responses they gave at the session.

I would like to thank Colin and Stephan for their help with running the
session and capturing the discussion.

Roni Even

AVT co-chair

From: [email-masked] [mailto:[email-masked]] On Behalf Of Colin
Perkins
Sent: Tuesday, November 10, 2009 10:23 PM
To: IETF AVT WG
Subject: Re: [AVT] Discuss Mpeg2TS preamble - meeting room

Roni, all,

Thanks to the authors and all those who attended, we had a productive
meeting to discuss the differences, advantages, and disadvantages of these
two proposals. This is my attempt to summarise the discussion, based on
notes taken by myself and Stephan Wenger. Roni will circulate the attendee
list.

The meeting began by reviewing the structure of an MPEG2-TS and the data
that needs to be conveyed as part of the preamble in order for the rapid
acquisition work to operate. We then reviewed the details of operation for
the two drafts (draft-begen-avt-rtp-mpeg2ts-preamble-03.txt and
draft-xia-avt-mpeg2ts-preamble-00.txt).

The idea expressed in the Begen draft is to extract the data needed for the
preamble from the media stream, cache it on the server, and send it to the
receiver on request in the form of one (or occasionally a small number of)
RTP packets using a new RTP payload format. That RTP payload format is
organised as a set of TLV-encoded blocks, where each block represents a
particular piece of preamble data, extracted from the MPEG2-TS. The use of
TLV-encoded blocks provides extensibility.

The idea expressed in the Xia draft is to extract the MPEG2-TS frames needed
for the preamble from the media stream, cache them on the server, and send
them to the receiver on request in the form of MPEG2-TS frames encapsulated
in an RTP packet representing an RFC 4588 retransmission of a synthetic RFC
2250 RTP packet encapsulating those MPEG2-TS frames (note that only the
frames required for the preamble are sent, not the complete RTP packets that
contain them).

After much discussion, it was understood that both proposals send
essentially the same data as the preamble. The key difference between them
is the way in which that data is encapsulated: the Began draft extracts the
preamble data from the MPEG2-TS and wraps it into a TLV-encoded form for
transmission; the Xia draft extracts the preamble data from the MPEG2-TS and
sends it as a sequence of new MPEG2-TS frames wrapped in an RTP
retransmission packet. 

We discussed compatibility of the two approaches with the RTP standards. The
Began draft is clearly compatible. The Xia draft is not compatible as it
stands, since the preamble is sent as an RTP retransmission packet but
contains a synthetic packet that is not a retransmission. This issue with
the Xia draft can be solved by formatting the preamble packets as RFC 2250
packets, rather than the current approach that uses an RFC 4588
retransmission of an RFC 2250 packet.

We discussed overheads, in terms of the number of packets sent, and
vulnerability to packet loss. The two drafts generally send the same
information in the same number of RTP packets in the preamble, and so have
essentially the same overhead and resilience to loss. Acquisition delays are
also the same, since both send the same information.

The Began draft includes TLV headers which may provide extra flexibility,
but will slightly increase the packet size compared to the Xia draft which
sends raw MPEG2-TS frames.

The meeting concluded with an understanding that the two proposals have
essentially identical overheads and behaviour. The key difference between
then, assuming the Xia proposal is updated to use RFC 2250 packets rather
than RFC 4588 retransmissions of RFC 2250 packets to conform to the RTP
specifications, is that the Begen draft includes additional TLV-headers to
provide additional flexibility and extensibility, at the expense of a
slightly larger packet. 

One issue that was discussed, and remains unclear, is the exact behaviour of
the Xia proposal when dealing with encrypted streams. This will need to be
clarified.

Actions:

*	Authors of the Xia draft to update their draft for clarity of
English and to reflect the discussion, especially with respect to the exact
amount of data that has to be sent in the preamble, and to change to using
the RFC 2250 format
*	Authors of the Xia draft to clarify its behaviour when using an
encrypted MPEG2-TS.

Once an update to the Xia draft, and the Begen draft if the authors have any
changes they wish to make, is available, we expect the working group will be
able to review both proposals to make a decision whether it prefers
extensibility and flexibility (the Begen draft) or a slightly lower overhead
with less flexibility (the Xia draft).

Colin

 

 

-- 

Colin Perkins

http://csperkins.org/