RIST Streaming Protocol Publicly Available

The low latency RIST (Reliable Internet Streaming Transport) Protocol is now publicly available for developers to start implementing in projects, products, platforms and services.

This follows announcement that the first publicly available version of RIST, called libRIST 2.1.2, is now ready for general evaluation and deployment. The RIST Forum is dedicated to promoting adoption of the protocol, while Florida-based media software developer SipRadius LLC has been sponsoring the project.

This first generally available version comprises a library and sample calling programs implementing the RIST Protocol, as defined in the Video Services Forum VSF-TR-06 series of specifications. The RIST Simple Profile (TR-06-1) and the more comprehensive forthcoming Main Profile (TR-06-2) are supported, which is crucial as only the latter will be really competitive against the main alternative contenting low latency protocol, SRT (Secure Reliable Transport).

The open source licensing for libRIST allows free redistribution of source and binaries when accompanied by copyright and disclaimer notices. The project includes command-line applications and also documentation with a suggested test regimen.

“LibRIST provides a fast-start for developers wishing to incorporate a production quality implementation of the new standard in their own projects,” said Sergio Ammirata, Managing Director of SipRadius, and primary author of the code. “It also provides a platform which they can test or measure their own work against.”

RIST Forum President, Ciro Noronha, suggested the announcement was timely given the current high demand for efficient and high performance streaming during the covid-19 crisis. “With the current climate of uncertainty and with many people suddenly seeking to transmit reliable, high-quality video from homes and remote facilities, internet delivery has become more important than ever. Many of the more advanced features of the protocol, such as encryption, are supported. By providing so much code and functionality up front, we hope to speed the adoption of RIST in general.”

Advanced features of RIST, such as encryption, are now supported, said Ciro Noronha, RIST Forum President.

Advanced features of RIST, such as encryption, are now supported, said Ciro Noronha, RIST Forum President.

However, RIST and the alternative SRT are less suitable for the interactive applications like video conferencing that have witnessed a huge surge in demand with the swing to homeworking. For those applications, demanding sub-second latencies, WebRTC is more suitable. Conversely, the on demand content distribution required by SVoD services like Netflix that have also experienced soaring demand, are less sensitive to latency. The real sweet spot for protocols such as RIST and SRT are live streaming services, particularly sports, which are in abeyance during the global lockdown. It may then be that in fact demand for RIST and SRT picks up after the crisis is over.

Both RIST and SRT have a common technological foundation based on UDP (User Datagram Protocol), the version of IP transport that is connectionless with no end-to-end path and no inherent error correction. This lack of error correction has meant that the alternative TCP (Transport Control Protocol) was widely adopted for on-demand video delivery over the internet with its support for retransmission of IP packets dropped, corrupted, or lost, in transmission. TCP ensured that video could be delivered over unmanaged IP networks including the internet at acceptable quality, but did not work for live services because the delays incurred by packet retransmission impose unacceptable latency.

Mechanisms based on UDP to emulate the packet retransmission of TCP but do it much faster using more advanced forms of acknowledgement back from the destination to the source have evolved to address live video in particular. SRT came along as the most efficient version yet. That might have been the end of the journey towards low latency streaming, but some in the field were concerned that SRT had been developed initially by a vendor, Haivision, rather than being based on neutral industry standards. SRT was accused of being ad hoc and certainly the biggest difference between the two protocols lies in their provenance rather than their technology.

SRT has been made available open source by Haivision and has now been backed by a large impressive array of vendors such as Microsoft, Harmonic and Tencent Cloud, as well as influential operators like Comcast. It can hardly be said to be proprietary anymore. The RIST Forum in fact has fewer members, but does stick to standards emanating from the IETF and other bodies, having evolved entirely inside the body.

While both SRT and RIST use forms of the packet retransmission to cope with errors, like the original TCP, WebRTC has succeeded in cutting latency to the bone by eliminating any resending of data at all. Instead WebRTC uses a version of Forward Error Correction (FEC), where extra data bits are added to enable packets that are lost or corrupted be reconstructed at the receiving end. This increases the amount of data sent, so that WebRTC consumes more network bandwidth, while being more limited than SRT or RIST in its ability to withstand packet loss. WebRTC therefore relies on a reasonably stable network and is confined to lower qualities, at most 1080p HD, not yet 4K. This makes it suitable for those applications like web conferencing, and also some others like in-sports betting, where low latency trumps quality. 

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