[rtcweb] Review, RTP recommendations - draft-ietf-rtcweb-rtp-usage-03
Harald Alvestrand <harald@alvestrand.no> Sun, 17 June 2012 15:57 UTC
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Date: Sun, 17 Jun 2012 17:57:41 +0200
From: Harald Alvestrand <harald@alvestrand.no>
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Subject: [rtcweb] Review, RTP recommendations - draft-ietf-rtcweb-rtp-usage-03
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At the interim, I promised to make more precise my objections to some of the language of this I-D. I'm taking the opportunity to read through the whole document and give commentary. I hope this is helpful. - Section 2, rationale: Should add that an important driver for the profile is to rule things OUT of scope - that features not mentioned here are not required for successful RTCWEB interoperation. - Section 3, terminology: This should reference the RTP document and the rtcweb-overview document. There are lots of terms that come from there. In particular, the term "media stream" needs to be called out, since a WebRTC "MediaStream" is not the same as an RTP "media stream", and both are used in this document. I suggest consistently using the terms "WebRTC MediaStream" (prefix, no space) and "RTP media stream" (prefix, with space) for the two concepts, and explicitly never using the term "media stream" without a prefix. - Section 4.7 Symmetric RTP/RTCP: Between -02 and -03, I see that support for symmetric RTP/RTCP has been changed from Using Symmetric RTP and RTCP [RFC4961] is REQUIRED. to implementations are REQUIRED to implement Symmetric RTP [RFC4961] Reading RFC 4961, I see that it states: A device supports symmetric RTP if it selects, communicates, and uses IP addresses and port numbers such that, when receiving a bidirectional RTP media stream on UDP port "A" and IP address "a", it also transmits RTP media for that stream from the same source UDP port "A" and IP address "a". So it's impossible to support RFC 4961 without using it, and the formulations are equivalent. But that's such a novel concept among the usual universe of RTP extensions that it's worth calling out explicitly. It might even be beneficial to say that it's OK to not interoperate with devices that don't do RFC 4961. - Section 4.8 RTCP CNAME generation - as discussed at the interim, it might be good to state that support for receiving RFC 6222-style CNAMEs, as well as RFC 3550-style names, is required. - Section 5.1 states that RTP Transport Translators "are expected to be supported". RTP Transport Translators require that call setup be coordinated across all endpoints in the session to avoid clashing payload types, as discussed in the multiplex draft. I'm OK with a formulation that says that "this RTP profile should be able to support all these topologies, if supported by appropriate signallling", but I'm not OK with a formulation that absolutely requires signalling support for RTP Transport Translators; I'm not at all confident that we can successfully negotiate RTP Transport Translator-based applications for all cases. - Section 5.1.5 TMMBR - should we say explicitly that the sender is expected to honor the constraint? - Section 7.4 Legacy Interop Limitations (for congestion) seems like a work in progress. We'll discuss this more in the congestion context, but the "it's been suggested ... send 64 Kbits/sec" at the bottom should definitely not be stated like this in the final document. - Section 11.1 "However, one SSRC will identify a media stream and its timing. " - I think in this case, "WebRTC MediaStream" is the intended usage. - Section 12.1 and 12.2 - in this section, I think all the uses of "media stream" refer to "RTP media stream". I can't parse the grammar of "In addition the above discussed criteria to support multiple media types in one single RTP session results that also an end-point that has one audio and one video source still need two transmit using two SSRCs concurrently" - I think it's trying to say that when audio and video go into the same RTP session, they use different SSRCs. - Section 12.3 is not very clear to me. I'm not sure I would agree with it if it was clear; the "relay" described here is probably an RTP Transport Translator, but if it is, it should be called that. This is a quick pass, where I have focused on sections 1-7. There are many formulation issues, and some of them may hide technical disagreements. But on the whole, I think this document is very close to saying most of the things that need to be said. Harald
- [rtcweb] Review, RTP recommendations - draft-ietf… Harald Alvestrand