Video: WebRTC: Mostly the video bits

Who better to dig below the surface of WebRTC, which delivers sub-second latency, than Sean DuBois, creator of the Pion WebRTC library? This video takes a different look at WebRTC to others that focus on latency or scaling. Rather Sean looks at congestion control and managing the impacts of congestion noting that people remember how bad the video got and not how nice your sign-up page was.

Congestion is inevitable in large ‘unmanaged’ networks such as the internet and on wifi and cellular networks. Sean points out that the use of MPEG codecs which add dependencies between frames magnify the effect of lost packets. With frame-by-frame codecs, dropping a frame and repeating the last one is barely noticeable, but with MPEG, many more could be damaged. WebRTC was implemented over UDP so it could use its own congestion control.

RTP and RTCP are the key to WebRTC’s congestion control. RTP is well known for carrying real-time media as it’s used for AES67 audio, SMPTE ST 2110 and ST 2022-6 to name just a few standards. RTCP is RTP’s sidekick. Whilst RTP does the legwork of carrying the media, the RTP Control Protocol (RTCP) passes messages to control the flow. In this case, Sean explains, the RTCP channel is used to tell the sender that it’s sending too much video or which packets it’s lost. In terms of mitigating congestion, the source can adjust the bitrate directly or change the resolution or the framerate of the video to bring the bitrate down indirectly.



Sean shows a summary diagram of congestion controller flow which is built to handle jitter and out of order packets. Buffers are the normal way of fixing out-of-order packets but they have a big downside of adding latency and exacerbating timing problems. WebRTC has to use the RTCP channel to make sure it can map packet timing with NTP, using Sender Reports, as each packet’s timing information is only relative. When packet loss is spotted NACK (negative acknowledgements) are sent via RTCP or if things are worse, a Picture Loss Indication is sent which request a new keyframe. Fixing any impairments that do occur can be done either with FEC or by concealing the error with some form of masking, nowadays this may be based on machine learning.

The talk finishes with a look at a number of innovative projects which use WebRTC in one way or another, including for file transfer.

Watch now!

Sean DuBois Sean DuBois
Creator, Pion WebRTC
Developer, Apple

Video: I know X, what does WebRTC get me?

WebRTC is now a W3C standard providing sub-second peer-to-peer video and audio streaming with NAT traversal. Widely used for video conferencing, its sub-second latency has also been the focus of video streaming companies such as Millicast and Limelight (to name but two) who aim to deliver this otherwise peer-to-peer technology to thousands or millions of people in under a second enabling interactive video, gamefied streams, auctions and ultra-low-latency sports.

Addressing directly people using other streaming protocols, Pion creator Sean DuBois spoke at SF Video Tech about what WebRTC brings over and above protocols like RTMP, SRT and RIST. At the heart of it, WebRTC, like SRT and RIST, creates a connection over which it can send a variety of data. Whilst we expect media to be sent, actually, file transfer can be easily achieved – let’s not forget the whole of SRT is build upon UDT which is specifically a file delivery utility. Where file transfer can be achieved, so can real-time data & metadata transfer.

Sean quickly summarises WebRTC as a Protocol between (typically) browsers, an peer-to-peer secure connection over which multiple audio & video streams can flow. In common with RIST and other recent protocols, it’s based on many pre-existing
technologies such as SRTP, DTLS, ICE and SDP to deliver signalling, connection management, encryption and communication.



The list of improvements over RTMP is very long. They’re spelt out concisely in the video so we will highlight just a few here. Importantly, low-latency is key. RTMP was low-latency for its time, but not by today’s standards. Google’s Stadia can boast 125ms video latency for a keypress, explains Sean. DTLS and SRTP are essential for security but are well understood, trusted methods of securing your data. DTLS is pretty much exactly the same as the TLS which secures your bank transfers, just moved into UDP instead of TCP. However, WebRTC can work by exchanging ‘fingerprints’ (DTLS-SRTP) instead of the full trusted certificate infrastructure that underpins TLS on the web. Removing the requirement for certs is a big boost for flexibility and agility as long as you are confident you can exchange fingerprints securely ahead of time.

NAT traversal is also a big boon where, even with both endpoints behind a firewall, endpoints can always find a way to communicate although this does mean that ICE servers are needed to facilitate connectivity. Within broadcasting, however, it’s more likely that you’ll have control of one end so this is less needed. Sean highlights the ability to send multiple quality levels within the same stream using the ‘simulcast’ ability of WebRTC.

Sean then looks at SRT and RIST. Both of these are low-latency streaming protocols which can, both, also provide sub-second streaming for good connections with a relatively low RTT. Sean highlights the lack of SRT and RIST to negotiate the codec in use and their optional security. Being focused more on delivering contribution feeds, they tend to have a more static configuration often created after a programme of testing to ensure the quality will be acceptable to the broadcaster/streaming provider.

To finish, Sean highlights a whole series of interesting, innovative uses of WebRTC from informal group streaming to drones to shared online games to file transfers and more.

Watch now!

Sean DuBois Sean DuBois
Developer, Apple
Creator of Pion WebRTC