[MMUSIC] Comments on draft-ietf-mmusic-msid-08
Magnus Westerlund <magnus.westerlund@ericsson.com> Mon, 16 March 2015 08:59 UTC
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Date: Mon, 16 Mar 2015 09:59:03 +0100
From: Magnus Westerlund <magnus.westerlund@ericsson.com>
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Subject: [MMUSIC] Comments on draft-ietf-mmusic-msid-08
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Hi, I have reviewed draft-ietf-mmusic-msid-08 and have some comments. 1. Section 3: ABNF: I think usage of " " for a space is easy to result in errors, and considering that the ABNF in section 2 uses SP I think this one can also use SP for space. OLD: msid-semantic-value = msid-semantic msid-list msid-semantic = token ; see RFC 4566 msid-list = *(" " msid-id) / " *" NEW: msid-semantic-value = msid-semantic msid-list msid-semantic = token ; see RFC 4566 msid-list = *(SP msid-id) / SP "*" 2. Section 3: ABNF Considering how good you have on limiting the length of the MSID and MSID appdata to 64 characters, shouldn't the semtantics token also be limited? I could see one change OLD: msid-semantic = token ; see RFC 4566 NEW: msid-semantic = 1*64token-char ; see RFC 4566 3. Section 4: This section fails to discuss a=msid completely. 4. Section 4: What are the rules for directionality for the two attributes? Is that given by the semantics? The issue I have is that I am not certain that if the identifier and application date for an MSID is limited in scope to the offer or answer only, and thus only label the media description, or if it actually are depending on direction. Thus, having different interpretation depending on if the media description is a sendrecv, sendonly, recvonly? To me it appears reasonable that it is actually semantics defined, otherwise any future user of this mechanism will run into the issue that it is the wrong direction. Because I can clearly see that some have a need for something that applies to the receiving direction whiles other need to work with the send direction. I note that the offer for anything in the sending direction is actually making a shot in the dark. The offerer can't know what the answer will say and if that is compatible with the offered MSID-semantics. 5. Section 5. " The value of the "identifier" field in the msid consists of the "id" attribute of a MediaStream, as defined in its WebIDL specification. The value of the "appdata" field in the msid consists of the "id" attribute of a MediaStreamTrack, as defined in its WebIDL specification." First, it is not clear what WebIDL specification is, missing reference. However, the point I like to make it is in relation to the previous issue. To my understanding the MediaStream and MediaStreamTrack id is generated on the sender side, thus the values in this case are for the send direction. Thus, it only makes sense to add them for media descriptions where directionality is sendrecv or sendonly from the point of the agent who is generating the SDP to send, be it an offer or answer. 6. Section 5.1: Entities that do not use the WMS semantic will not send "msid- semantic:WMS". This means that there will be some incoming RTP packets that the recipient has no predefined MediaStream id value for. I think this needs to state the assumption that the SDP is interpreted in a context where one expect to receive WMS semantics. If one does not expect I don't see the point in following these and applying them. 7. Section 5.1: "Handling will depend on whether or not the msid-semantic:WMS attribute is present." Is present where, in the Offer or Answer? Is there actually a difference here depending of if this is in the answer or offer? If it is answer to an offer that included WMS semantics, then it is clearer that you are in a SDP session context where WMS is expected. I wonder if we will run into issues around this when we get "smarter" gateways that attempt to provide a view that the endpoint actually is closer to a WebRTC browser than not. 8. Section 5.2: Should there be an actual discussion of what conclusions one can draw from actually seeing the semantics in both offer and answer. This is not without meaning? Also, is there a point of allowing declaration of the semantics even if one has no msid identifiers to group, simply for capability declaration or indicate desire to use? Cheers Magnus Westerlund ---------------------------------------------------------------------- Services, Media and Network features, Ericsson Research EAB/TXM ---------------------------------------------------------------------- Ericsson AB | Phone +46 10 7148287 Färögatan 6 | Mobile +46 73 0949079 SE-164 80 Stockholm, Sweden | mailto: magnus.westerlund@ericsson.com ----------------------------------------------------------------------
- [MMUSIC] Comments on draft-ietf-mmusic-msid-08 Magnus Westerlund