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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:
avt-bounces at ietf.org [mailto:avt-bounces at ietf.org] On Behalf Of Colin
Perkins 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:
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 |
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