Kurento is a low-level platform to create WebRTC applications from scratch. You will be responsible of managing STUN/TURN servers, networking, scalability, etc. If you are new to WebRTC, we recommend using OpenVidu instead.

OpenVidu is an easier to use, higher-level, Open Source platform based on Kurento.


This is a glossary of terms that often appear in discussion about multimedia transmissions. Some of the terms are specific to GStreamer or Kurento, and most of them are described and linked to their RFC, W3C or Wikipedia documents.

Agnostic media

One of the big problems of media is that the number of variants of video and audio codecs, formats and variants quickly creates high complexity in heterogeneous applications. So Kurento developed the concept of an automatic converter of media formats that enables development of agnostic elements. Whenever a media element’s source is connected to another media element’s sink, the Kurento framework verifies if media adaption and transcoding is necessary and, if needed, it transparently incorporates the appropriate transformations making possible the chaining of the two elements into the resulting Pipeline.


Audio Video Interleaved, known by its initials AVI, is a multimedia container format introduced by Microsoft in November 1992 as part of its Video for Windows technology. AVI files can contain both audio and video data in a file container that allows synchronous audio-with-video playback. AVI is a derivative of the Resource Interchange File Format (RIFF).


Bower is a package manager for the web. It offers a generic solution to the problem of front-end package management, while exposing the package dependency model via an API that can be consumed by a build stack.

Builder Pattern

The builder pattern is an object creation software design pattern whose intention is to find a solution to the telescoping constructor anti-pattern. The telescoping constructor anti-pattern occurs when the increase of object constructor parameter combination leads to an exponential list of constructors. Instead of using numerous constructors, the builder pattern uses another object, a builder, that receives each initialization parameter step by step and then returns the resulting constructed object at once.


Cross-origin resource sharing is a mechanism that allows JavaScript code on a web page to make XMLHttpRequests to different domains than the one the JavaScript originated from. It works by adding new HTTP headers that allow servers to serve resources to permitted origin domains. Browsers support these headers and enforce the restrictions they establish.

See also

Wikipedia: Cross-origin resource sharing


Information on the relevance of CORS and how and when to enable it.


Document Object Model is a cross-platform and language-independent convention for representing and interacting with objects in HTML, XHTML and XML documents.


End Of Stream is an event that occurs when playback of some media source has finished. In Kurento, some elements will raise an EndOfStream event.


GStreamer is a pipeline-based multimedia framework written in the C programming language.


A Video Compression Format. The H.264 standard can be viewed as a “family of standards” composed of a number of profiles. Each specific decoder deals with at least one such profiles, but not necessarily all.

See also

Wikipedia: H.264/MPEG-4 AVC

RFC 6184

RTP Payload Format for H.264 Video (This RFC obsoletes RFC 3984).


The Hypertext Transfer Protocol (HTTP) is an application protocol for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide Web.

See also

Wikipedia: Hypertext Transfer Protocol

RFC 2616

Hypertext Transfer Protocol – HTTP/1.1


Interactive Connectivity Establishment (ICE) is a protocol used for NAT Traversal. It defines a technique that allows communication between two endpoints when one is inside a NAT and the other is outside of it. The net effect of the ICE process is that the NAT will be left with all needed ports open for communication, and both endpoints will have complete information about the IP address and ports where the other endpoint can be contacted.

ICE doesn’t work standalone: it uses a couple of helper protocols called STUN and TURN.

See also

Wikipedia: Interactive Connectivity Establishment

RFC 5245

Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols.


IP Multimedia Subsystem (IMS) is the 3GPP’s Mobile Architectural Framework for delivering IP Multimedia Services in 3G (and beyond) Mobile Networks.

See also

Wikipedia: IP Multimedia Subsystem

Wikipedia: 3GPP

RFC 3574

Transition Scenarios for 3GPP Networks.


jQuery is a cross-platform JavaScript library designed to simplify the client-side scripting of HTML.


JSON (JavaScript Object Notation) is a lightweight data-interchange format. It is designed to be easy to understand and write for humans and easy to parse for machines.


JSON-RPC is a simple remote procedure call protocol encoded in JSON. JSON-RPC allows for notifications and for multiple calls to be sent to the server which may be answered out of order.


Kurento is a platform for the development of multimedia-enabled applications. Kurento is the Esperanto term for the English word ‘stream’. We chose this name because we believe the Esperanto principles are inspiring for what the multimedia community needs: simplicity, openness and universality. Some components of Kurento are the Kurento Media Server, the Kurento API, the Kurento Protocol, and the Kurento Client.

Kurento API

An object oriented API to create media pipelines to control media. It can be seen as and interface to Kurento Media Server. It can be used from the Kurento Protocol or from Kurento Clients.

Kurento Client

A programming library (Java or JavaScript) used to control an instance of Kurento Media Server from an application. For example, with this library, any developer can create a web application that uses Kurento Media Server to receive audio and video from the user web browser, process it and send it back again over Internet. The Kurento Client libraries expose the Kurento API to application developers.

Kurento Protocol

Communication between KMS and clients by means of JSON-RPC messages. It is based on WebSocket that uses JSON-RPC v2.0 messages for making requests and sending responses.

Kurento Media Server

Kurento Media Server is the core element of Kurento since it responsible for media transmission, processing, loading and recording.


Maven is a build automation tool used primarily for Java projects.

Media Element

A Media Element is a module that encapsulates a specific media capability. For example RecorderEndpoint, PlayerEndpoint, etc.

Media Pipeline

A Media Pipeline is a chain of media elements, where the output stream generated by one element (source) is fed into one or more other elements input streams (sinks). Hence, the pipeline represents a “machine” capable of performing a sequence of operations over a stream.

Media Plane

In a traditional IP Multimedia Subsystem, the handling of media is conceptually splitted in two layers. The layer that handles the media itself -with functionalities such as media transport, encoding/decoding, and processing- is called Media Plane.


MPEG-4 Part 14 or MP4 is a digital multimedia format most commonly used to store video and audio, but can also be used to store other data such as subtitles and still images.


Multimedia is concerned with the computer controlled integration of text, graphics, video, animation, audio, and any other media where information can be represented, stored, transmitted and processed digitally. There is a temporal relationship between many forms of media, for instance audio, video and animations. There 2 are forms of problems involved in

  • Sequencing within the media, i.e. playing frames in correct order or time frame.

  • Synchronization, i.e. inter-media scheduling. For example, keeping video and audio synchronized or displaying captions or subtitles in the required intervals.

Multimedia container format

Container or wrapper formats are meta-file formats whose specification describes how different data elements and metadata coexist in a computer file. Simpler multimedia container formats can contain different types of audio formats, while more advanced container formats can support multiple audio and video streams, subtitles, chapter-information, and meta-data, along with the synchronization information needed to play back the various streams together. In most cases, the file header, most of the metadata and the synchro chunks are specified by the container format.

Network Address Translation

Network Address Translation (NAT) is a mechanism that hides from the public access the private IP addresses of machines inside a network. The NAT mechanism is typically found in all types of network devices, ranging from home routers to full-fledged corporate firewalls. In all cases the effect is the same: machines inside the NAT cannot be freely accessed from outside.

NAT introduces a lot of problems for WebRTC communications: machines inside the network will be able to send data to the outside, but they won’t be able to receive data from remote participants that are outside the network. In order to allow for this, NAT devices typically allow to configure NAT bindings to let data come in from the outside part of the network; creating these NAT bindings is what is called NAT Traversal, also commonly referred as “opening ports”.

See also

Wikipedia: Network address translation

NAT Types and NAT Traversal

Entry in our Knowledge Base.

How Network Address Translation Works (archive)

A comprehensive description of NAT and its mechanics.

NAT Traversal

NAT Traversal is a general term for techniques that establish and maintain Internet protocol connections traversing network address translation (NAT) gateways, which break end-to-end connectivity. Intercepting and modifying traffic can only be performed transparently in the absence of secure encryption and authentication.

See also

NAT Types and NAT Traversal

Entry in our Knowledge Base.


Node.js is a cross-platform runtime environment for server-side and networking applications. Node.js applications are written in JavaScript, and can be run within the Node.js runtime on OS X, Microsoft Windows and Linux with no changes.


NPM is the official package manager for Node.js.


OpenCV (Open Source Computer Vision Library) is a BSD-licensed Open Source computer vision and machine learning software library. OpenCV aims to provide a common infrastructure for computer vision applications and to accelerate the use of machine perception.

Pad, Media

A Media Pad is is an element’s interface with the outside world. Data streams from the MediaSource pad to another element’s MediaSink pad.

See also

GStreamer Pad

Definition of the Pad structure in GStreamer.


QR code (Quick Response Code) is a type of two-dimensional barcode. that became popular in the mobile phone industry due to its fast readability and greater storage capacity compared to standard UPC barcodes.


Receiver Estimated Maximum Bitrate (REMB) is a type of RTCP feedback message that a RTP receiver can use to inform the sender about what is the estimated reception bandwidth currently available for the stream itself. Upon reception of this message, the RTP sender will be able to adjust its own video bitrate to the conditions of the network. This message is a crucial part of the Google Congestion Control (GCC) algorithm, which provides any RTP session with the ability to adapt in cases of network congestion.

The GCC algorithm is one of several proposed algorithms that have been proposed by an IETF Working Group named RTP Media Congestion Avoidance Techniques (RMCAT).

See also

What is RMCAT congestion control, and how will it affect WebRTC? (archive)


RTCP message for Receiver Estimated Maximum Bitrate.


A Google Congestion Control Algorithm for Real-Time Communication.


Representational state transfer (REST) is an architectural style consisting of a coordinated set of constraints applied to components, connectors, and data elements, within a distributed hypermedia system. The term representational state transfer was introduced and defined in 2000 by Roy Fielding in his doctoral dissertation.


The RTP Control Protocol (RTCP) is a sister protocol of the RTP, that provides out-of-band statistics and control information for an RTP flow.

See also

Wikipedia: RTP Control Protocol

RFC 3605

Real Time Control Protocol (RTCP) attribute in Session Description Protocol (SDP).


Real-time Transport Protocol (RTP) is a standard packet format designed for transmitting audio and video streams on IP networks. It is used in conjunction with the RTP Control Protocol. Transmissions using the RTP audio/video profile (RTP/AVP) typically use SDP to describe the technical parameters of the media streams.

See also

Wikipedia: Real-time Transport Protocol

Wikipedia: RTP audio video profile

RFC 3550

RTP: A Transport Protocol for Real-Time Applications.

Same-origin policy

The “same-origin policy” is a web application security model. The policy permits scripts running on pages originating from the same domain to access each other’s DOM with no specific restrictions, but prevents access to DOM on different domains.

Session Description Protocol
SDP Offer/Answer

The Session Description Protocol (SDP) is a text document that describes the parameters of a streaming media session. It is commonly used to describe the characteristics of RTP streams (and related protocols such as RTSP).

The SDP Offer/Answer model is a negotiation between two peers of a unicast stream, by which the sender and the receiver share the set of media streams and codecs they wish to use, along with the IP addresses and ports they would like to use to receive the media.

This is an example SDP Offer/Answer negotiation. First, there must be a peer that wishes to initiate the negotiation; we’ll call it the offerer. It composes the following SDP Offer and sends it to the other peer -which we’ll call the answerer-:

o=- 0 0 IN IP4
s=Example sender
c=IN IP4
t=0 0
m=audio 5006 RTP/AVP 96
a=rtpmap:96 opus/48000/2
m=video 5004 RTP/AVP 103
a=rtpmap:103 H264/90000

Upon receiving that Offer, the answerer studies the parameters requested by the offerer, decides if they can be satisfied, and composes an appropriate SDP Answer that is sent back to the offerer:

o=- 3696336115 3696336115 IN IP4
s=Example receiver
c=IN IP4
t=0 0
m=audio 0 RTP/AVP 96
a=rtpmap:96 opus/48000/2
m=video 31278 RTP/AVP 103
a=rtpmap:103 H264/90000

The SDP Answer is the final step of the SDP Offer/Answer Model. With it, the answerer agrees to some of the parameter requested by the offerer, but not all.

In this example, audio 0 means that the answerer rejects the audio stream that the offerer intended to send; also, it accepts the video stream, and the a=recvonly acknowledges that the answerer will exclusively act as a receiver, and won’t send any stream back to the other peer.

See also

Wikipedia: Session Description Protocol

Anatomy of a WebRTC SDP

RFC 4566

SDP: Session Description Protocol.

RFC 4568

Session Description Protocol (SDP) Security Descriptions for Media Streams.

Semantic Versioning

Semantic Versioning is a formal convention for specifying compatibility using a three-part version number: major version; minor version; and patch.

Signaling Plane

It is the layer of a media system in charge of the information exchanges concerning the establishment and control of the different media circuits and the management of the network, in contrast to the transfer of media, done by the Media Plane. Functions such as media negotiation, QoS parametrization, call establishment, user registration, user presence, etc. as managed in this plane.

Sink, Media

A Media Sink is a MediaPad that outputs a Media Stream. Data streams from a MediaSource pad to another element’s MediaSink pad.


Session Initiation Protocol (SIP) is a Signaling Plane protocol widely used for controlling multimedia communication sessions such as voice and video calls over Internet Protocol (IP) networks. SIP works in conjunction with several other application layer protocols:

  • SDP for media identification and negotiation.

  • RTP, SRTP or WebRTC for the transmission of media streams.

  • A TLS layer may be used for secure transmission of SIP messages.

Source, Media

A Media Source is a Media Pad that generates a Media Stream.

Single-Page Application

A single-page application is a web application that fits on a single web page with the goal of providing a more fluid user experience akin to a desktop application.


Sphinx is a documentation generation system. Text is first written using reStructuredText markup language, which then is transformed by Sphinx into different formats such as PDF or HTML. This is the documentation tool of choice for the Kurento project.

Spring Boot

Spring Boot is Spring’s convention-over-configuration solution for creating stand-alone, production-grade Spring based applications that can you can “just run”. It embeds Tomcat or Jetty directly and so there is no need to deploy WAR files in order to run web applications.


SRTCP provides the same security-related features to RTCP, as the ones provided by SRTP to RTP. Encryption, message authentication and integrity, and replay protection are the features added by SRTCP to RTCP.

See also



Secure RTP is a profile of RTP (Real-time Transport Protocol), intended to provide encryption, message authentication and integrity, and replay protection to the RTP data in both unicast and multicast applications. Similarly to how RTP has a sister RTCP protocol, SRTP also has a sister protocol, called Secure RTCP (or SRTCP).

See also

Wikipedia: Secure Real-time Transport Protocol

RFC 3711

The Secure Real-time Transport Protocol (SRTP).


Secure Socket Layer. See TLS.


Session Traversal Utilities for NAT (STUN) is a protocol that complements ICE in the task of solving the NAT Traversal issue. It can be used by any endpoints to determine the IP address and port allocated to it by a NAT. It can also be used to check connectivity between two endpoints, and as a keep-alive protocol to maintain NAT bindings. STUN works with many existing types of NAT, and does not require any special behavior from them.

Trickle ICE

Extension to the ICE protocol that allows ICE agents to send and receive candidates incrementally rather than exchanging complete lists. With such incremental provisioning, ICE agents can begin connectivity checks while they are still gathering candidates and considerably shorten the time necessary for ICE processing to complete.

See also


Trickle ICE: Incremental Provisioning of Candidates for the Interactive Connectivity Establishment (ICE) Protocol.


Transport Layer Security (TLS) and its predecessor Secure Socket Layer (SSL).

See also

Wikipedia: Transport Layer Security

RFC 5246

The Transport Layer Security (TLS) Protocol Version 1.2.


Traversal Using Relays around NAT (TURN) is an extension of STUN, used where the NAT security policies are too strict and the needed NAT bindings cannot be successfully created to achieve NAT Traversal. In these situations, it is necessary for the host to use the services of a TURN server that acts as a communication relay.


You don’t need to configure both STUN and TURN, because TURN already includes STUN functionality.


VP8 is a video compression format created by On2 Technologies as a successor to VP7. Its patents rights are owned by Google, who made an irrevocable patent promise on its patents for implementing it and released a specification under the Creative Commons Attribution 3.0 license.

See also

Wikipedia: VP8

RFC 6386

VP8 Data Format and Decoding Guide.


WebM is an open media file format designed for the web. WebM files consist of video streams compressed with the VP8 video codec and audio streams compressed with the Vorbis audio codec. The WebM file structure is based on the Matroska media container.


WebRTC is a set of protocols, mechanisms and APIs that provide browsers and mobile applications with Real-Time Communications (RTC) capabilities over peer-to-peer connections.


WebSocket specification (developed as part of the HTML5 initiative) defines a full-duplex single socket connection over which messages can be sent between client and server.