SRT (Secure Reliable Transport)

showstack / protocols / srt

Open-source low-latency video transport designed to carry contribution-quality streams over unmanaged networks, recovering lost packets with ARQ inside a configurable latency budget. The standard way a live feed leaves a venue over the public internet.

Video transportOpenSRT Alliance / HaivisionReported

Media Large, continuous, on time and in order

Continuous streams that are large enough to be sized properly, and that must arrive in order and on time.

What kills it. Anything that starves the link — usually a management transfer nobody scheduled — and jitter that outruns the receive buffer.

The four flows, and why the model is worth having →

Ports

udp/9000 Common default, but SRT has no fixed port: it is set per stream
Deployment chooses the port; 9000 and 1935 are conventions, not assignments.

What it carries

Resolution: Not fixed by the specification
Frame rate: Not fixed by the specification
Video compression: Whatever the payload carries

Video: SRT does not define a video format at all. It carries an MPEG transport stream, so resolution, frame rate and codec are decided by the encoder feeding it. What SRT contributes is retransmission and a configurable latency buffer over a link you do not control.

What goes wrong

Latency is a setting, and it is a trade against packet recovery. A latency budget set too low on a lossy link produces exactly the artefacts SRT is meant to prevent.

Caller, listener and rendezvous modes decide who initiates. Two ends both configured as caller will never connect, and it is the most common first-time failure.

Encryption is optional and off by default in many implementations. "Secure" is in the name, not necessarily in the configuration.

What speaks SRT (Secure Reliable Transport) (3)

Product	Direction	Notes
OBS Studio OBS Project	bidirectional	SRT output and input, for contribution over an unmanaged network.
vMix StudioCoast	bidirectional	Contribution and distribution.
Wireshark Wireshark Foundation	in	SRT dissector in current builds.

Open source implementations

srt — C++, MPL-2.0
FFmpeg (SRT protocol support) — C, LGPL-2.1-or-later

Typical use

Sending a show feed to a remote broadcaster or streaming platform
Site-to-site contribution over the public internet
Remote production where a dedicated circuit is not available

Sources

SRT Alliance — SRT Alliance primary
Haivision/srt on GitHub — GitHub

Understand the mechanism

Show networks Node graphs and game engines

Something wrong, or missing?

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