- From: Peter Thatcher <pthatcher@google.com>
- Date: Thu, 16 Apr 2015 09:58:59 -0700
- To: Justin Uberti <juberti@google.com>
- Cc: Martin Thomson <martin.thomson@gmail.com>, Harald Alvestrand <harald@alvestrand.no>, "public-webrtc@w3.org" <public-webrtc@w3.org>
- Message-ID: <CAJrXDUGCzfJLQVRgm=6PCd678HAN_f-XR+egGThDqA_5JrZ3vA@mail.gmail.com>
On Thu, Apr 16, 2015 at 9:53 AM, Justin Uberti <juberti@google.com> wrote: > > > On Wed, Apr 15, 2015 at 7:48 PM, Peter Thatcher <pthatcher@google.com> > wrote: > >> On Wed, Apr 15, 2015 at 6:57 PM, Justin Uberti <juberti@google.com> >> wrote: >> >>> The two arguments (that I know of) against using mid by itself: >>> 1) addTrack followed by removeTrack followed by addTrack will result in >>> the same mid for the second track, due to the quirks of SDP. This means >>> that sometimes addTrack gives you a new mid, sometimes it won't. This is >>> unfortunate, although if we wanted to go down this path, we could >>> prohibit recycling m= sections without a corresponding mid change. IOW, you >>> could only recycle rejected m= sections, and so the example here would >>> result in two m= lines. >>> >> >> This is effectively the problem I was also expressing. If one does >> addTrack + removeTrack + addTrack, can we use a different MID the second >> time without accumulating dead m-lines over time? If we can, then there is >> no problem. If we can't, then either JS cannot safely do >> addTrack+removeTrack repeatedly too many times (do to the accumulation) or >> we must have multiple RtpSenders with the same MID but at different times. >> I think I'd be OK with the accumulation of dead m-lines. If an app really >> had an accumulation problem, they could get around it by doing one of: >> using replaceTrack, munging SDP, or using the future 1.1 API. >> > > This particular issue won't result in too many dead m-lines; we will still > be able to recycle m=lines that aren't in use by either side. We just won't > be able to recycle "half-dead" m-lines, which is probably OK. > > >> >> >>> 2) without a=msid, there is no way to detect a recyclable m= line >>> (currently we can look for a=msid to determine this). >>> >>> Specifically, if you have a remote description without a=msid in a m= >>> section, and you stop your local track for that m= section, you don't know >>> whether to set port 0 (i.e. dead m= section) or not in subsequent offers, >>> because you can't tell if the remote side is still using that m= section. >>> >>> >> So basically we just need a way to know "I'm a WebRTC endpoint; I know >> what to do if you remove/recycle an m-line"? For that I would suggest two >> possible solutions: >> >> 1. Have an a=removable line. Do one thing and do it well (instead of >> having multiple meaning tied to a=msid). >> >> 2. Don't remove/recycle m-lines. Would it really be that bad if we >> didn't? We don't right now, and no one has complained. And, as I >> mentioned, JS could always overcome an accumulation problem by using >> replaceTrack, munging SDP, or using the future 1.1 API. >> > > I think this will be a real problem for conferences, since the list could > grow without bound. > But then it's up to the conference server to add/remove m-lines, right? The browser doesn't have anything to do with it. So the real question is: how does the conference server know that it's safe to remove m-lines that it's not using any more? Using MSID as an indication for such a thing seems a little strange. > >> >> >> All that said, as much as I prefer the idea of having only 1 ID (MID), I >> would probably be OK with an SDP attribute like a=RtpSenderLabel if that >> ends up being more simple due to the quirks of SDP. >> >> >> >> >> >>> >>> >>> On Wed, Apr 15, 2015 at 5:12 PM, Peter Thatcher <pthatcher@google.com> >>> wrote: >>> >>>> Ah, I see. It's true that if we didn't allow JS to choose the value, >>>> it would be the same as now where the JS cannot choose the track ID. So we >>>> wouldn't be losing anything compared to the status quo. >>>> >>>> But isn't the JS always able to set it anyway via SDP munging? >>>> >>>> On Wed, Apr 15, 2015 at 3:24 PM, Martin Thomson < >>>> martin.thomson@gmail.com> wrote: >>>> >>>>> On 15 April 2015 at 15:19, Peter Thatcher <pthatcher@google.com> >>>>> wrote: >>>>> > And te question isn't whether to allow the JS to choose the MID. >>>>> It's >>>>> > whether to have two IDs/labels (MID + something else), or just one >>>>> (MID). >>>>> > If one will work, I prefer one. >>>>> >>>>> >>>>> Like I said, I am OK with just one and with that one being a=mid; I >>>>> was pushing back at your suggestion that we let the JS set it, that's >>>>> all. >>>>> >>>> >>>> >>> >> >
Received on Thursday, 16 April 2015 17:00:10 UTC