Controlling bandwidth usage in WebRTC (and how googSuspendBelowMinBitrate works)

There are cases when we would like to limit the maximum bitrate being transmitted by WebRTC to avoid wasting resources in the user endpoints or save money reducing the bandwidth usage in our servers.   This is because the maximum bitrate by default in Chrome is around 2Mbps and for many use cases a much lower bitrate provides still pretty good quality.   BTW using a lower bitrate can also help with stability of quality in multiparty scenarios by reducing the amount of competition among different streams.

There is no simple API to configure the maximum bitrate in WebRTC (although there is one in ORTC) but there are 3 ways to do this by mangling the SDP.

1. Use the standard b=AS:BITRATE (Chrome) or b=TIAS:BITRATE (Firefox) attributes in the SDP for the audio or video channel[1]
2. Use codec specific attributes (this work at least for opus audio codec with maxaveragebitrate property) [2]
3. Use the proprietary x-max-bitrate attribute in the video channel of the SDP answer.  For example with something like this:

answer.sdp += "a=fmtp:100 x-google-max-bitrate=500\r\n";

My network sucks right now (I blame my neighbors wifis) and the bitrate is not very stable, but you can see how in average it stays under 500kbps.

There are also people interested on changing the minimum bitrate being used (the default is 30kbps in Chrome and increasing it is very dangerous unless you are in a very controlled environment). This can also be done mangling the SDP:

answer.sdp += "a=fmtp:100 x-google-min-bitrate=1000\r\n";

Another requirement is changing the initial bitrate to speed up the initialization and start with a higher bitrate than the default one (300 kbps).   This can also be done mangling the SDP:

answer.sdp += "a=fmtp:100 x-google-start-bitrate=1000\r\n";

In the graph you can see a session where the start bitrate and the minbitrate where set to 1000 kbps.

For the last three years we have seen comments in the mailing lists and issue tracker about something called "suspend below min bitrate" (3).  I was very curious about this feature as you can see in the ticket comments and finally decided to play with it and also  take a look at the code.

The feature is very simple to understand. If you enable it then WebRTC will stop sending video as soon as the bandwidth estimation goes below the minimum bitrate.  Otherwise by default WebRTC insists on sending minBitrate even if it creates congestion in the network.

To enable it you just need to pass an optional proprietary constrain while creating the PeerConnection:

var pc = new RTCPeerConnection({ iceServers: [] }, { optional: [{ "googSuspendBelowMinBitrate": true }] });
With that in place the video stops as soon as you don't have enough bandwidth for the video.  So if the bandwidth estimation is 60kbps and the audio needs 40kbps the video will still be sent until bandwidth estimation is <= 40kbps.

One of the tricky/interesting parts is the algorithm to allocate the bandwidth to different streams (for example for audio y video or for multiple streams in a single peerconnection or for different qualities when using simulcast).  For that there is a class BitrateAllocator ( in Google's webrtc code.

def allocate_bitrates(bitrate):
   sum_max_bitrate = sum(stream.max_bitrate) for stream in streams
   sum_min_bitrate = sum(stream.min_bitrate) for stream in streams
   if bitrate > sum_max_bitrate:    
      # All streams get max_bitrate
   elif bitrate > sum(stream.min_bitrate) for stream in streams:
      # Each stream gets stream.min_bitrate + (bitrate - sum_min_bitrate) / streams.length
      foreach(stream in streams)
         if suspend_below_min_bitrate:
             stream.allocated_bitrate = min(stream.min_bitrate, bitrate)
             stream.allocated_bitrate = stream.min_bitrate
         bitrate -= stream.allocated_bitrate

One of the problems with this functionality is detecting when this feature is activated to change the UI or notify the receiver that the video has been suspended.  The only solution at this point is using getStats to monitor the encoded video bytes but some kind of callback/notification is under consideration [4]

In case you have the same question I had, the video recovers automatically when the network conditions improves again.   You can see a graph with the whole suspension + reactivation I generated by degrading artificially my network conditions to force a bandwidth estimation < 40kbps.



  1. thanks Gustavo!

    this article reminded me about the one I'm linking below, where @fippo described how to restrict the bandwidth usage and an open issue in Chrome for removing the limit:

    1. The development of artificial intelligence (AI) has propelled more programming architects, information scientists, and different experts to investigate the plausibility of a vocation in machine learning. Notwithstanding, a few newcomers will in general spotlight a lot on hypothesis and insufficient on commonsense application. machine learning projects for final year In case you will succeed, you have to begin building machine learning projects in the near future.

      Projects assist you with improving your applied ML skills rapidly while allowing you to investigate an intriguing point. Furthermore, you can include projects into your portfolio, making it simpler to get a vocation, discover cool profession openings, and Final Year Project Centers in Chennai even arrange a more significant compensation.

      Data analytics is the study of dissecting crude data so as to make decisions about that data. Data analytics advances and procedures are generally utilized in business ventures to empower associations to settle on progressively Python Training in Chennai educated business choices. In the present worldwide commercial center, it isn't sufficient to assemble data and do the math; you should realize how to apply that data to genuine situations such that will affect conduct. In the program you will initially gain proficiency with the specialized skills, including R and Python dialects most usually utilized in data analytics programming and usage; Python Training in Chennai at that point center around the commonsense application, in view of genuine business issues in a scope of industry segments, for example, wellbeing, promoting and account.

  2. Thanks for sharing your honest experience. When I first took a look at my head shots,
    I wasn’t too thrilled with mine but you’ve given me a new perspective!

    Virtual Edge

  3. Today's colleges and universities are changing with the times. Programs change as do the ways that students are taught. However there is one growing program on almost every public college campus that administrators are not proud to point out to distinguished visitors -- developmental education.

  4. x-google-min-bitrate and googSuspendBelowMinBitrate not working for me. Video never get suspended, when by speed goes below specified Bitrate. Is it still working for you?

  5. Thanks for sharing,
    now, i know how to use it...


  6. Terribly attention-grabbing, sensible job and thanks for sharing such a decent journal. Your article is thus convincing that I ne'er stop myself to mention one thing regarding it. You’re doing a good job. Keep it up

  7. Thanks for sharing this informations.
    data science course in coimbatore

    data science training in coimbatore

    android training institutes in coimbatore

    ios training in coimbatore

    aws training in coimbatore

    amazon web services training in coimbatore

    big data training in coimbatore

  8. This comment has been removed by the author.

  9. This article is really helpful for me. I am regular visitor to this blog. Share such kind of article more in future. Personally i like this article a lot and you can have a look at my services also: I was seriously search for a Salesforce training institutes in ameerpet which offer job assistance and Salesforce training institutes in Hyderabad who are providing certification material. It's worth to join Salesforce training institutes in India because of their real time projects material and 24x7 support from customer desk. You can easily find the best Salesforce training institutes in kukatpally kphb which are also a part of Pega training institutes in hyderabad. This is amazing to join Data science training institutes in ameerpet who are quire popular with Selenium training institutes in ameerpet and trending coureses like Java training institutes in ameerpet and data science related programming coures python training institutes in ameerpet If you want HCM course then this workday training institutes in ameerpet is best for you to get job on workday.

  10. Allowing poor performers to hang out too long simply reinforces poor performance. If no accountability is in place, then why should the poor sales performer step up and make an effort? Empty promises and hollow threats from the sales manager don't mean a thing. Salesforce training in Chennai

  11. Great Article Image Processing Projects

    Deep Learning Projects for Final Year

    JavaScript Training in Chennai

    JavaScript Training in Chennai

    The Angular Training covers a wide range of topics including Components, Angular Directives, Angular Services, Pipes, security fundamentals, Routing, and Angular programmability. The new Angular TRaining will lay the foundation you need to specialise in Single Page Application developer. Angular Training

  12. Thanks for sharing this informative content, Great work.

    To crack Scrum master interview: Scrum master interview questions

  13. Check out the maths quiz questions with answers to become effective and increase your level. They provide the best and effective questionnaires to develop yourself.


Post a Comment

Popular posts from this blog

Bandwidth Estimation in WebRTC (and the new Sender Side BWE)

Improving Real Time Communications with Machine Learning