[codec] Draft 07: Chapter 3 Editorial comments
"Christian Hoene" <hoene@uni-tuebingen.de> Mon, 25 July 2011 17:41 UTC
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From: Christian Hoene <hoene@uni-tuebingen.de>
To: codec@ietf.org
Date: Mon, 25 Jul 2011 13:41:43 -0400
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Subject: [codec] Draft 07: Chapter 3 Editorial comments
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Dear Authors, beside my fundamental critics on Section 3 of draft-ietf-codec-opus-07 (see previous email), I have a couple of editorial comments to the chapter. The text is fine till A well-formed Opus packet MUST contain at least one byte with the TOC Information, though the frame(s) within a packet MAY be zero bytes long. The sentence It must also obey various additional rules indicated by "MUST", "MUST NOT", etc., in this section. is superfluous and can be dropped. I would move A receiver MUST NOT process packets which violate these rules as normal Opus packets. They are reserved for future applications, such as in-band headers (containing metadata, etc.) or multichannel support. to an additional subsection that explains on how to extend the codec format for future applications. This section could also include a description of how these extensions might look like. I would also suggest to introduce subsections for each value of the parameter c: Section 3.1 One frame in the packet Section 3.2. Two frames in the packet, each with equal compressed size Section 3.3. Two frames in the packet, with different compressed sizes Section 3.4 An arbitrary number of frames in the packet Currently, the description for these modes is mixed up a bit. For example the text starting at When a packet contains multiple VBR frames, till to allow for repacketization by gateways, conference bridges, or other software. can be moved to Section 3.4. The sentence No length is transmitted for the last frame in a VBR packet, or any of the frames in a CBR packet, as it can be inferred from the total size of the packet and the size of all other data in the packet. does not sound well to my German ears. Maybe one can rephrase it to The last frame in a VBR packet is not preceded by a length field as the length can be inferred from the total size of the packet and the size of all other data in the packet. The remaining or any of the frames in a CBR packet can be moved to Section 3.2. Please add Section 3.1 One frame in the packet in front of For code 0 packets, I find the notion frame and packet quite confusing as packet might be misunderstood as IP or UDP or RTP packet. Other codec standards use subframe and frame. Please add Section 3.3. Two frames in the packet, with different compressed sizes. Here you can add the text from above describing the format N1. For a better understanding, I would paint two figures. In the first N1 format has a size of 1 bytes, in the second figure N1 has a size of two bytes. The length of the first frame, N1, MUST be no larger than the size of the payload remaining after decoding that length for all code 2 packets. Please rephrase as I do not understand: The frame must not be larger than the packet? Some more technical comments on padding: Padding is support because of security reasons as VBR frame size might leaks information. But in the draft, you support padding mainly for VBR having multiple frames? Multiple VBR frames concatenated in one packet do leak even less information than single frames . I just want to note again that padding has not been listed in the requirement draft and thus can be dropped. Do you have any definition of compressed frame or compressed data. These terms are shown in the Figures. That is enough for now. I need a break Christian ---------------------------------------------------------------------------- ----- Dr.-Ing. Christian Hoene, University of Tübingen, Computer Science, Chair of Communication Networks, Research Group Interactive Communication Systems (ICS) Sand 13, 72076 Tübingen, Germany, Tel +49 7071 2970532, www.net.uni-tuebingen.de
- [codec] Draft 07: Chapter 3 Editorial comments Christian Hoene