Secure Reliable Transport (SRT)

The open-source protocol for low-latency, secure, and reliable video streaming over unpredictable networks. Built for the public internet.

The Challenge: Video Streaming on the Public Internet

Transmitting high-quality video over the public internet is inherently difficult. Network conditions are unpredictable, leading to:

  • Packet Loss: Data packets get dropped due to congestion.
  • Jitter: Packets arrive at inconsistent intervals.
  • Bandwidth Fluctuations: Available network capacity changes constantly.
  • High Latency: Especially with protocols like TCP that retransmit lost packets.

Traditional protocols like RTMP (TCP-based) struggle with latency, while raw UDP offers speed but no reliability. SRT was created to solve this dilemma.

What is SRT?

Secure Reliable Transport (SRT) is an open-source video transport protocol and technology stack that optimizes streaming performance across unpredictable networks.

Developed by Haivision and managed by the SRT Alliance, it's designed to deliver the best quality of service (QoS) for low-latency live video streaming over the public internet.

SRT is built on the UDT (UDP-based Data Transfer) protocol but adds critical enhancements for real-time media, making it the ideal choice for professional video applications.

SRT's Three Pillars

SRT is defined by three core principles that set it apart from other protocols:

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Security

End-to-end AES encryption (128 or 256-bit) protects your video content from interception, ensuring privacy and integrity.

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Reliability

Advanced error correction (ARQ, FEC) and packet recovery ensure a stable, uninterrupted stream even in poor network conditions.

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Low Latency

UDP-based transport with intelligent recovery mechanisms keeps end-to-end latency as low as possible, often under 1 second on the public internet.

How SRT Works: Key Technologies

SRT combines several advanced techniques to achieve its performance:

1. UDP as the Foundation

SRT uses UDP instead of TCP. This avoids TCP's inherent latency caused by its retransmission mechanism and "head-of-line blocking," enabling much lower transmission delays.

2. Adaptive Recovery: ARQ & FEC

SRT doesn't just rely on UDP's speed; it adds smart recovery:

  • ARQ (Automatic Repeat reQuest): The receiver detects lost packets and requests retransmission. SRT's ARQ is optimized to minimize overhead.
  • FEC (Forward Error Correction): Extra data is sent alongside the stream, allowing the receiver to reconstruct lost packets without retransmission, ideal for high-loss scenarios.

You can choose the recovery method (ARQ, FEC, or both) based on your network and application needs.

3. Dynamic Congestion Control

SRT continuously monitors network conditions (latency, bandwidth, loss) and dynamically adjusts its transmission rate to avoid congestion and maximize bandwidth utilization without overwhelming the network.

4. Stream Encryption

SRT supports AES-128 and AES-256 encryption, providing robust security for sensitive video streams.

SRT vs. Traditional Protocols

How does SRT compare to common alternatives?

  • vs RTMP: RTMP uses TCP, leading to higher latency (3-30s). SRT offers sub-second latency with better reliability over unstable networks.
  • vs Raw UDP: Raw UDP is fast but unreliable. SRT adds the reliability layer (ARQ/FEC) on top of UDP's speed.
  • vs RTSP/RTP: While RTSP/RTP can use UDP, they lack SRT's sophisticated adaptive recovery and congestion control, making SRT superior for long-distance internet transmission.

Typical Use Cases

SRT excels in scenarios requiring high-quality, low-latency video over the public internet:

  • Live contribution from remote locations (news, events).
  • Secure video transmission for surveillance and public safety.
  • Cloud-based video production and live streaming.
  • Remote production (REMI) workflows.
  • Any application where network conditions are unpredictable.
Limitation: SRT is designed for point-to-point (one-to-one) connections. It is not a content distribution network (CDN) protocol and is not suitable for one-to-many broadcasting to thousands of viewers.

Why Choose SRT?

  • Open Source & Free: No licensing fees, fostering widespread adoption and innovation.
  • High Quality: Maximizes use of available bandwidth, enabling higher bitrate streams.
  • High Bandwidth Efficiency: ARQ is more bandwidth-efficient than TCP's retransmission under loss.
  • Interoperability: The SRT Alliance ensures compatibility across different vendors' implementations.