SRT (Secure Reliable Transport) Protocol

SRT was introduced pioneered by Haivision for use in its own encoder/decoder products. In 2017 it became an open source protocol and technology stack for the delivery of live video. It is often seen as the front runner in video transport because it came to market first and has a large membership.
SRT was originally designed as an alternative to the real-time messaging protocol (RTMP) to transport low-latency streams over noisy networks.

Despite advances in internet streaming, live video transport still faces latency challenges due to:

  • Packet loss when sending UDP-based low latency MPEG Transport Streams over the public internet
  • Bandwidth limitations introduced by network congestion control when using the TCP protocol in RTMP
  • Segment-based protocols such as HLS and DASH transmitted over HTTP (TCP)

Generally, overcoming these problems requires the provisioning of high-cost reserved links like MPLS or satellite networks. With the open source implementation of a streaming protocol like SRT, broadcasters can enable the best quality live video over unpredictable.

SRT transport

While RTMP remains the most common protocol for video contribution, the technology wasn’t designed for interactivity. In terms of end-to-end latency and video quality, SRT transport outperforms RTMP as detailed in Haivision’s white paper RTMP vs. SRT: Comparing Latency and Maximum Bandwidth | Haivision.

According to consultants Deloitte, “Legacy protocols such as RTMP, developed over a decade ago to encode video and move it across networks to clients, will likely be displaced by newer solutions such as SRT, designed to further decrease latency and meet the demands of live and on-demand streaming.”

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https://www2.deloitte.com/content/dam/insights/us/
articles/722835_tmt-predictions-2020/DI_TMT-Prediction-2020.pdf

Specifically, SRT transport is designed to manage and solve:

  • Packet loss (packets that the router discards)
  • Jitter (packets that arrive at the wrong time)
  • Latency (delay between when the video is captured and when it plays on viewers’ devices)
  • Changes in bandwidth (fluctuations in the capacity between two points)

In this way, SRT enables a layer of error correction that reduces the need for expensive network infrastructure.

UDP and TCP are both core components of the internet protocol suite, residing in the transport layer. SRT takes some of the best aspects of User Datagram Protocol (UDP), such as low latency, but adds error-checking to match the reliability of Transmission Control Protocol/Internet Protocol (TCP/IP). While TCP/IP handles all data profiles and is optimal for its job, SRT can address high-performance video specifically. By combining the speed of UDP and the reliability of TCP, SRT provides the following benefits.

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SRT benefits

The benefits of SRT as claimed by the SRT Alliance, include:

  • High quality: Provides professional, high-grade video across any network — withstanding up to 10% packet loss without detectable degradation.
  • Resilient: Maintains stream integrity by accounting for packet loss, jitter, and fluctuating bandwidth.
  • Cost-effective: Replaces expensive satellite networks or rigid MPLS networks with affordable and flexible public internet.

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  • Interactive: This allows you to deliver two-way, interactive video from anywhere.
  • Low latency: Enables near real-time IP communications as fast as UDP with the reliability of TCP.
  • Media-agnostic: Acts as a wrapper around your content — whether MPEG-2, H.264 or HEVC.
  • Firewall friendly: Reliably streams from unknown locations and event centers without requiring additional IT resources.
  • Secure: End-to-end 128/256 bit AES encryption makes sure your content is protected from contribution to distribution

One notable limitation of SRT is that it’s not as widely supported for playback as technologies like HTTP Live Streaming (HLS). For that reason, most SRT-based workflows leverage a streaming server or service to repackage content.

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The SRT Alliance

The SRT Alliance of supporting vendors was created in 2017 to promote the industry-wide recognition and adoption of SRT as a common standard for all low-latency internet streaming.

Haivision subsequently released SRT on GitHub to encourage interoperability, as well as a comprehensive technical review that serves as the specification. This makes it simple to verify connectivity without costly hardware or software solutions.

Microsoft announced support for the protocol in 2019, making SRT an ingest option alongside RTMP.

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Since then, Haivision has launched SRTHub a cloud-based service for routing media across the Microsoft Azure backbone.

Net Insight is a member of the SRT Alliance. Joining the SRT Alliance is in line with Net Insight’s strategy to embrace open technology for transporting professional high-quality media content over the Internet.

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SRT Open Source

The SRT Open Source project, managed and maintained by the SRT Alliance, is a collaborative community of industry leaders and developers striving to achieve lower latency internet video transport by continuously improving open-source SRT.

Open source SRT is distributed under the MPL-2.0 license. Haivision chose Mozilla Public License because it strikes a balance between driving adoption for open source SRT while encouraging contributions to improve upon it by the community of adopters. Any third party is free to use the SRT source in a larger work regardless of how that larger work is compiled. Should a third party make source code changes, they would be obliged to make those changes available to the community.

Open source is designed to deliver interoperability and longevity with large-scale adoption.

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Secure Reliable Transport is a video streaming transport protocol and technology stack.

The open source implementation of SRT connects two endpoints for the purposes of delivering low latency video and other media streams across lossy networks.