TL;DR – YES.
Do I need a media server for a one-to-many WebRTC broadcast?
That’s the question I was asked on my chat widget this week. The answer was simple enough – yes.
Decided you need a media server? Here are a few questions to ask yourself when selecting an open source media server alternative.
Then I received a follow up question that I didn’t expect:
That caught me off-guard. Not because I don’t know the answer. Because I didn’t know how to explain it in a single sentence that fits nicely in the chat widget. I guess it isn’t such a simple question either.
The simple answer is a limit in resources, along with the fact that we don’t control most of these resources.
First Things First
Before I go into an explanation, you need to understand that there are 4 types of WebRTC servers:
- Application server
- Signaling server
- NAT traversal server (STUN & TURN)
- Media server
This question is specifically focused on (4) Media server. The other 3 servers are needed here no matter if this is 1:1 or 1:many session.
The Hard Upper Limit
Whenever we want to connect one browser to another with a direct stream, we need to create and use a peer connection.
Chrome 65 includes an upper limit to that which is used for garbage collection purposes. Chrome is not going to allow more than 500 concurrent peer connections to exist.
500 is a really large number. If you plan on more than 10 concurrent peer connections, you should be one of those who know what they are doing (and don’t need this blog). Going above 50 seems like a bad idea for all use cases that I can remember taking part of.
Understand that resources are limited. Free and implemented in the browser doesn’t mean that there aren’t any costs associated with it or a need for you to implement stuff and sweat while doing so.
Bitrates, Speeds and Feeds
This is probably the main reason why you can’t broadcast with WebRTC, or with any other technology.
We are looking at a challenging domain with WebRTC. Media processing is hard. Real time media processing is harder.
Assume we want to broadcast a video at a low VGA resolution. We checked and decided that 500kbps of bitrate offers good results for our needs.
What happens if we want to broadcast our stream to 10 people?
Broadcasting our stream to 10 people requires bitrate of 5mbps uplink.
If we’re on an ADSL connection, then we can find ourselves with 1-3mbps uplink only, so we won’t be able to broadcast the stream to our 10 viewers.
For the most part, we don’t control where our broadcasters are going to be. Over ADSL? WiFi? 3G network with poor connectivity? The moment we start dealing with broadcast we will need to make such assumptions.
That’s for 10 viewers. What if we’re looking for 100 viewers? A 1,000? A million?
With a media server, we decide the network connectivity, the machine type of the server, etc. We can decide to cascade media servers to grow our scale of the broadcast. We have more control over the situation.
Broadcasting a WebRTC stream requires a media server.
I see this one a lot in the context of a mesh group call, but it is just as relevant towards broadcast.
When we use WebRTC for a broadcast type of a service, a lot of decisions end up taking place in the media server. If a viewer has a bad network, this will result with packet loss being reported to the media server. What should the media server do in such a case?
While there’s no simple answer to this question, the alternatives here include:
- Asking the broadcaster to send a new I-frame, which will affect all viewers and increase bandwidth use for the near future (you don’t want to do it too much as a media server)
- Asking the broadcaster to reduce bitrate and media quality to accomodate for the packet losses, affecting all viewers and not only the one on the bad network
- Ignoring the issue of packet loss, sacrificing the user for the “greater good” of the other viewers
- Using Simulcast or SVC, and move the viewer to a lower “layer” with lower media quality, without affecting other users
You can’t do most of these in a browser. The browser will tend to use the same single encoded stream as is to send to all others, and it won’t do a good job at estimating bandwidth properly in front of multiple users. It is just not designed or implemented to do that.
You Need a Media Server
In most scenarios, you will need a media server in your implementation at some point.
If you are broadcasting, then a media server is mandatory. And no. Google doesn’t offer such a free service or even open source code that is geared towards that use case.
It doesn’t mean it is impossible – just that you’ll need to work harder to get there.
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