Kurento is an Open Source Software WebRTC media server

Kurento 6.7: Moving Forward

It's been a while since Kurento was acqui-hired by Twilio, more than one year ago. Since then, the project lost almost all of its momentum, and it entered a "minimal maintenance" mode. Understandably, some people were wondering whether the Kurento Media Server would end up becoming abandonware.

Fear not, work hasn't stopped behind the curtains. On the contrary: a new team has been formed, and lots of necessary infrastructure updates have been introduced. We know that the rhythm has been slow, but believe us when we say that we are ready to start rocking streaming again!

So let's talk about the new team behind Kurento, what is the current status of the project, the roadmap for the future, and a new complementary project started by this team: OpenVidu. Last but not least, let us introduce a new official release, which marks the start of this new development cycle for the project: Kurento 6.7.


Development team

The Kurento development team is formed under the CodeURJC Research Group, which belongs to the spanish Rey Juan Carlos University, located in Madrid. This team is financed by the University and by NaevaTec.



While Kurento implements a powerful engine, able to manage a multitude of use cases and complex scenarios, we found that a lot of users were interested in the more basic and simpler idea (but still technically difficult to accomplish) of typical videoconference rooms.

With this in mind, we started working on a complementary solution that builds upon Kurento, to provide an easy to use and effective way of creating and managing videoconference rooms, adding video calls to web or mobile applications. OpenVidu acts as a wrapper around a standard installation of Kurento Media Server, and takes care of all the complexities inherent to WebRTC video calls such as SDP negotiation, ICE connectivity checks, video/audio codec compatibility, etc.

Also, a very important planned feature of OpenVidu will be to provide seamless scaling of Kurento resources over multiple servers, providing in one single and compact package what is currently being done manually by Kurento users in their own applications. Check out the OpenVidu project page, which contains all the relevant information: http://openvidu.io/

Besides this, we have migrated some parts of our infrastructure to be hosted under the OpenVidu name (such as the Debian package repositories, which now are under the openvidu.io domain) and it is possible that more migrations happen over time.


Kurento Status and Roadmap

We want Kurento to become the reference media server for streaming and WebRTC communications. The bulk of work done in the last year has been focused on rebuilding our Continuous Integration infrastructure, fixing bugs in Kurento Media Server, and adding some small features that had been asked for by some customers.

Our immediate next steps will be to redirect our attention back to the greater community: both the mailing list and the issue tracker are full of topics that require our consideration. We don't have a strict roadmap with specific dates, but we do have a very clear idea of what are our intended next steps in terms of feature development for Kurento and OpenVidu:

Kurento Media Server:

  • Support Safari browser (preliminary support already available in Kurento 6.7).
  • Support Microsoft Edge browser.
  • Update GStreamer and several other underlying support libraries to their latest versions.
  • Review the documentation. Unify and merge all bits of information that have been scattered over time.
  • Review all tutorials. Ensure that everything is working properly.
  • Review and clean up all open issues in the bug tracker.


  • Improve features and stability of videoconference recording.
  • Enable peer-to-peer calls, without an intermediate media server.
  • Add a moderator role.
  • Start planning the intended auto-scaling capabilities.


Kurento Media Server 6.7

This has been a busy year for the Kurento team, although sadly the results of it aren't very colorful to show off in a blog post. The latest release of Kurento is a minor point release, which means that some features have been added but this server is still backwards-compatible with previous client applications.


Debian packages

There are some changes in the packaging of KMS: previously, all packages (as installed with apt-get install) included the suffix -6.0 in their name (for example: kms-core-6.0). We have changed the way new KMS releases will be distributed, so now there is no need to have the explicit major version in the package names. Starting from version 6.7, all package names will consist of only the actual name, without any suffix. For example, kurento-media-server-6.0 becomes kurento-media-server.

The ability to install old releases of KMS had been broken for a while, so we rebuilt the CI scripts from the ground up, to recover this feature. Starting with this release, each independent new release of KMS will be available to install in its own package repository, and users will be able to manually choose their preferred versions by changing the later part of the URL that is written in the file /etc/apt/sources.list.d/kurento.list:

deb http://ubuntu.openvidu.io/6.7.1 xenial kms6


New documentation

The whole documentation of Kurento has been fully reviewed, from back to front, in an attempt to unify all corrections and random annotations that we had noticed over the time but never got to edit into the actual text. Also, the structure of the documents has been reworked in an attempt to provide for a more natural reading flow.

Some highlights of the new documentation would be the Getting Started and Installation guides, and the new Developer Guide, which until now had been sitting in a shared Google Docs document (not the best option for discoverability!).

Head to the documentation main page to see what is up with Kurento: https://doc-kurento.readthedocs.io/en/stable/


Safari preliminary support

We've integrated all required changes that were needed to make Kurento play nicely with Safari browsers. These changes are still considered to be in a preview state, while more exhaustive testing is taking place.

If you want to help with testing Safari in different Apple devices & Safari versions, we opened a thread in our Google Groups mailing list, where any user who wants to collaborate is able to do so: https://groups.google.com/forum/#!topic/kurento/rWZroM3SBWk

At the time of this writing, we have a test application running at https://apple.openvidu.io/ where everybody could test the Safari support and provide some feedback. We are currently working on an updated version of that demo, and will announce appropriately when it is done. Also, the discussion and feedback will be moved to a GitHub issue, for better ability to track updates.


RTP Congestion Control

Prior to KMS 6.7, only WebRTC connections made use of the REMB mechanism, which allows to reduce the sender video bitrate in situations where the network congestion doesn't allow high bitrates to be sent. Now, REMB can also enabled for basic RTP streams.

This means that any REMB-enabled RTP sender will be able to leverage the benefits that this protocol provides for network congestion control.


Security hardening

Hardening is a set of mechanisms that can be activated by turning on several compiler flags, and are commonly used to protect resulting programs against memory corruption attacks. These mechanisms have been standard practice since at least 2011, when the Debian project set on the goal of releasing all their packages with the security hardening build flags enabled (ref1). Ubuntu has also followed the same policy regarding their own build procedures (ref2).

Kurento Media Server had been lagging in this respect, and old releases only implemented the standard Debian hardening options that are applied by the dpkg-buildflags tool by default:

  • Format string checks (-Wformat -Werror=format-security).
  • Fortify Source (-D_FORTIFY_SOURCE=2).
  • Stack protector (-fstack-protector-strong).
  • Read-Only Relocations (RELRO) (-Wl,-z,relro).

Starting from version 6.7, KMS also implements these extra hardening measurements:

  • Position Independent Code (-fPIC) / Position Independent Executable (-fPIE -pie).
  • Immediate Binding (-Wl,-z,now).

The PIC/PIE options allow taking advantage of the Address Space Layout Randomization (ASLR) protection offered by the Kernel. This protects against Return-Oriented Programming (ROP) attacks. However, this technique has an important caveat: it is an all-or-nothing switch, so all shared objects must be compiled as position-independent code, and non-PIC plugins will need to be rebuilt with -fPIC in order to link with Kurento libraries. This means that all custom application modules that were linked against Kurento will need to be rebuilt.

For more information about the hardening goals and each of the protection mechanisms that are implemented, check the corresponding section of the Kurento documentation: Security Hardening.


New event: MediaTranscodingStateChangeEvent

All Endpoint objects in Kurento Media Server embed a custom-made GStreamer element called agnosticbin. This element is used to provide seamless interconnection of components in the MediaPipeline, regardless of the format and codec configuration of the input and output media streams.

When media starts flowing through any MediaElement-derived object, an internal dynamic configuration is done in order to match the incoming media format with the requested output media format. If both input and output formats are compatible (at the codec level), then the media can be transferred directly without any extra processing. However, if the input and output media formats are not compatible, the internal transcoding module will get enabled to convert the input media format to be compatible with the required output.

For example, if a WebRtcEndpoint receives a VP8 video stream from a Chrome browser, and then has to send the stream to a Safari browser which only accepts H.264, then the media will need to be transcoded.

This has big implication regarding the CPU usage of the media server: typically, application writers would like to avoid triggering the transcoding feature altogether, in order to ensure the lowest possible usage of processor resources. To aid this objective, we have added a new event to the Kurento API: MediaTranscodingStateChangeEvent allows KMS to communicate with the client application and inform it whether the transcoding is being enabled or not at any time.


New sample application: RTP Sender

During the last year we have given some attention to the pure RTP capabilities of Kurento, i.e. the RtpEndpoint component; while doing so, we ended up writing multiple variations of client applications that made use of this Endpoint, and for this release this became a full-blown demo application that has been added to the tutorials section: Kurento Java Tutorial - RTP Receiver.

This new demo application showcases how to configure KMS to receive an RTP stream, and also shows several related variations over this basic case:

  • It allows to choose between receiving RTP or SRTP streams.
  • It is compatible with Google Congestion Control algorithm, namely REMB.
  • It allows the RTP sender to use Symmetric RTP, so no ICE process is required. This provides for some cheap NAT traversal, at least for the most common cases of NAT.


H.264 support by default

The package openh264-gst-plugins-bad-1.5 is what provides Kurento Media Server with support for the video codec H.264. However, this package was optional, so it wasn't included in the standard installation of KMS. This was nothing but a source of confusion and support requests, given that most users didn't even know that they needed this package to be installed before being able to use H.264.

The Expects at least 4 fields error message will also ring a bell for some people:

DOMException: Failed to parse SessionDescription: Expects at least 4 fields

All this was caused by the same reason: Kurento was unable to satisfy the requested stream parameters during an SDP Offer/Answer negotiation. We have noticed that this issue started to appear more frequently since the first experimental versions of Safari support were added to Kurento; the reason being that Safari doesn't support the VP8 codec (which by the way is mandated by the WebRTC standard, which makes Safari's WebRTC implementation a half-baked one... but that's another story and shall be told another time).

As a solution to all these problems, we decided to mark the package openh264-gst-plugins-bad-1.5 as requirement of KMS, so any installation will have this package already included.


Support services

Kurento is formed by a small team of people. This means that our task pipeline is quite restricted, and most feature or support requests end up being stored in the backlog for a long time. We advance as fast as we can, but time and resources are limited and at the end of the day there is so much that we can do.

If you have some needs that require urgent attention, or want to help with funding development on the Kurento project, we offer consultancy and support services on demand. Please contact us at: openvidu@gmail.com and let us know about your project.