Introduction to SRT (Secure Reliable Transport)

Introduction

SRT (Secure Reliable Transport) is an open-source video transport protocol developed by Haivision Systems Inc. in 2013. It’s designed to optimize video streaming performance across unpredictable networks, ensuring secure and reliable delivery of high-quality, low-latency streams.
SRT was born to solve the “Live over Internet Protocol” problem. Making it possible to stream high-quality, low-latency video over public networks without compromise.

How SRT Works

Imagine you’re sending a live video stream across a bumpy road, filled with potholes, traffic jams, and detours (aka the public internet). What Secure Reliable Transport does is pave that road in real time, ensuring your video arrives smoothly, securely, and

Key Features of SRT

• End-to-End Encryption

  With AES-128 or 256-bit encryption, video streams remain fully protected, even when transmitted across open and unsecured public networks.

  How It Works:

  a. AES Encryption: SRT uses Advanced Encryption Standard (AES), typically with 128- or 256-bit keys, to encrypt your video streams.
  b. Encryption at the source: The stream is encrypted before it leaves the sender.
  c. Decryption at the destination: Only the receiver with the correct key can decrypt and view the content.
  d. No access in between: Even if someone intercepts the stream mid-transit, they can’t see or use the video without the key.

• Packet Loss Recovery

  Uses ARQ (Automatic Repeat Request) and forward error correction to recover lost packets, which is essential for streaming over Wi-Fi, LTE, or on the open internet.

  Here’s how SRT deals with Packet Loss Recovery:

  a. Selective Retransmission: Instead of resending everything, SRT identifies exactly which video packets are missing and only asks for those to be sent again.
  b. ACK/NACK Feedback: The receiver constantly sends feedback to the sender about which packets it received (ACKs) and which are missing (NACKs).
  c. Fast Recovery: As soon as a packet is detected as missing, SRT quickly triggers a resend to minimize delay and avoid visual glitches or audio dropouts.
  d. Built-in Buffering: A configurable buffer helps absorb jitter and gives time for retransmissions to arrive without stalling the video.

• Dynamic Latency Control

  SRT intelligently adapts to changing network conditions by adjusting latency on the fly. This feature is called Dynamic Latency Control, and it helps strike the right balance between low delay and smooth playback.

  Here’s how SRT deals with Dynamic Latency Control:

  a. SRT monitors the network for jitter, packet loss, and congestion.
  b. When the network gets shaky, SRT intelligently expands its latency buffer just enough to absorb disruptions and keep the stream running smoothly without interruptions.
  c. When the network improves, it lowers the latency again to reduce delay.

• Firewall/NAT (Network Address Translation) Friendly

  SRT uses UDP, but with smart handshake mechanisms that work well across firewalls and NAT (Network Address Translation) devices.

  Here’s how SRT handles it:

  a. Caller/Listener Mode: SRT uses a handshake model where one side (Caller) initiates the connection and the other (Listener) accepts it. This helps traverse firewalls, since most firewalls allow outgoing traffic by default.
  b. Hole Punching Support: In peer-to-peer setups, SRT supports UDP hole punching, which allows two endpoints behind NATs to connect without needing manual port forwarding.
  c. No Special Rules Needed: Unlike protocols that require open inbound ports or fixed IPs, SRT can connect reliably without complex network configurations.

• Low Latency

  Can deliver streams with end-to-end latencies as low as 2–3 seconds, far outperforming HLS/DASH, which typically operate at 10–30 seconds.

  How SRT Achieves Low Latency:

  a. Efficient retransmission: SRT only resends the specific lost packets instead of slowing down the whole stream.
  b. Adaptive buffering: It uses just enough buffer to smooth out jitter without adding unnecessary delay, helping keep latency low.
  c. Customizable latency window: Developers can set latency as low as a few milliseconds, depending on the use case and network conditions.
  d. UDP-based: Built on UDP, SRT skips the overhead of protocols like TCP, which adds delays due to strict ordering and reliability.

Let’s break down how it works:

• Smart Packet Recovery: If a chunk of video goes missing during transmission, SRT instantly detects the loss and requests it again, much like asking a friend to repeat something you didn’t catch the first time.
• Jitter Buffering: When network speed fluctuates, SRT uses a smart buffer that briefly holds data to maintain steady playback, similar to how you adjust your pace to walk in sync with someone moving slower.
• Encryption: It wraps your video in a protective layer so no one can peek at it during transit, similar to sending your content in a locked briefcase.
• Low Latency: It’s built for speed, cutting delays to milliseconds and making it ideal for real-time broadcasts like sports or remote production.

SRT vs Other Protocols

Use Cases: Real-World Scenarios

• Live News Broadcasting
  News reporters in remote locations send SRT streams to the studio over 4G or LTE, eliminating the need for a satellite truck.
• Remote Sports Production
  Live camera feeds from multiple stadiums are delivered via SRT to a centralized control room for coordination.
• Hybrid Cloud Playout
  Cloud production platforms securely retrieve streams from on-premise sources using SRT.

Conclusion

SRT isn’t just another streaming protocol. It represents a turning point in how we think about delivering live video. Born from a need to overcome the pitfalls of public networks, it combines security, reliability, and low latency in a way that traditional protocols simply can’t match.

In a world where live content travels across continents in milliseconds, SRT makes it possible to stream smoothly and reliably, whether you’re broadcasting breaking news, streaming a concert, or powering an OTT platform. It transforms the unreliable internet into a trusted broadcast highway.