What is SRT?

Secure, Reliable Transport, known simply as SRT, is a protocol that allows unreliable networks like the internet to be used for reliable, encrypted, live video contribution. Created by Haivision and now an open-source technology with an IETF draft spec, the alliance of SRT users continues to grow as the technology continues to develop and add features. 

The foundation of SRT is its ability to resend lost data. Packets are counted in at the receiver end and if one packet is missing, it’s re-requested from the sender. Importantly, it’s only missing data which is flagged. This is different to most file transfers on the internet where every single is acknowledged.  This bogs down real-time media in red tape which we explained in more detail in the article RIST, SRT, QUIC – All on the run from micromanagement.

The receiver needs to hold on to the data it’s received for a short time to allow chance for the message to leave the receiver and arrive at the sender when a replacement packet is needed. It will also have to wait for the repeated packet to travel to the receiver. This delay is called the round trip time (RTT) and is the same as the time given by ping when testing a connection. You get to choose the amount of time the receiver holds on to the data to allow for re-requests and or typical conditions, the suggestion is that it’s set to 4 times the RTT. 

This technique is remarkably effective at recovering packets lost by networks and works for any type of network whether it be a microwave link, Wi-Fi, private circuit or the internet at large. This technique gives us the R in SRT which stands for ‘reliable’.

The S in SRT is for secure and this is delivered thanks to the encryption. All SRT links can be encrypted with AES 128, 192 or AES 256 encryption meaning only people who have the password (usually called a pre-shared key) can decrypt the media and watch it. Otherwise, it’s just noise. 

SRT is more than just these two features. For a deeper dive, check out this snazzily titled video: SRT – How the hot new UDP video protocol actually works under the hood given by Alex Converse from Twitch. For another perspective, Karel Boek, CEO of Reskenlund, introduced SRT in this talk and shows how to get the code yourself and build a live demo

Haivision are members of RIST which is a pan-industry collaboration to create an open standard for the same use cases which SRT meets. For more information about RIST, read our article RIST – Reliable Internet Streaming Transport. Sounds good! Tell me more…