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
On 10 Nov 2009, at 09:44, Roni Even wrote: Hi, We have a room for the meeting thanks to David Oran, It is the IAB room which is the Sirius room on the 7th floor So we will meet there from 16:00 Roni Even Hi, We will have an offline meeting to talk about MPEG2TS preamble at 16:00 today. We can meet at the registration desk on 3rd floor. I will try to get us a room. Regards Roni Even AVT co-chair
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