SRT Streaming

Overview

SRT (Secure Reliable Transport) is an open-source protocol that delivers low-latency video streaming over unreliable networks. It uses techniques like packet loss recovery and encryption to ensure high-quality streaming even in challenging network conditions.

Key Benefits

Low latency: Typically 1-4 seconds with packet recovery
Reliable: Automatic retransmission and forward error correction
Secure: AES encryption built-in
Firewall-friendly: Works with NAT and firewalls using caller/listener modes
Network resilience: Handles packet loss, jitter, and bandwidth fluctuations
Codec support: Agnostic - supports H.264, H.265, VP8, VP9, AAC, Opus

Publishing Streams

SRT Publisher Modes

SRT supports three connection modes:

Caller: Initiates connection to listener
Listener: Waits for incoming connections
Rendezvous: Both peers connect to each other

# Publish to Ant Media Server (caller mode)
ffmpeg -re -i input.mp4 \
  -c:v libx264 -preset veryfast -b:v 2500k -maxrate 2500k -bufsize 5000k \
  -g 60 -sc_threshold 0 \
  -c:a aac -b:a 128k -ar 44100 \
  -f mpegts \
  "srt://your-server.com:4200?streamid=publish/stream123&passphrase=yourpassword"

# With latency and other SRT parameters
ffmpeg -re -i input.mp4 \
  -c:v libx264 -preset ultrafast -tune zerolatency \
  -b:v 3000k -maxrate 3000k -bufsize 6000k \
  -c:a aac -b:a 128k \
  -f mpegts \
  "srt://your-server.com:4200?streamid=publish/stream123\
&latency=200\
&passphrase=secret\
&pbkeylen=16\
&lossmaxttl=40"

Hardware Encoders with SRT

Streaming → Encoder Settings:
  Protocol: SRT
  Mode: Caller
  Remote Host: your-server.com
  Remote Port: 4200
  Stream ID: publish/stream123
  Latency: 200ms
  Passphrase: yourpassword
  Encryption: AES-128 or AES-256

Playing Streams

SRT Playback

# Play SRT stream
ffplay -fflags nobuffer \
  "srt://your-server.com:4200?streamid=play/stream123&passphrase=yourpassword"

# Play with custom latency
ffplay -fflags nobuffer \
  "srt://your-server.com:4200?streamid=play/stream123&latency=1000&passphrase=secret"

Configuration Options

Server Configuration

# SRT settings in application.properties (Enterprise Edition)

# Enable SRT
settings.srtEnabled=true

# SRT port
settings.srtPort=4200

# Default latency (milliseconds)
settings.srtLatency=200

# Max bandwidth (bytes per second, 0 = unlimited)
settings.srtMaxBandwidth=0

# Passphrase for encryption (optional)
settings.srtPassphrase=

# Encryption key length (16, 24, or 32 for AES-128, 192, 256)
settings.srtPbkeylen=16

SRT URL Parameters

Common SRT parameters in URL query string:

srt://server:port?parameter=value

# Connection parameters
mode=caller|listener|rendezvous
streamid=publish/stream123  # Stream identifier
passphrase=secret           # Encryption passphrase
pbkeylen=16|24|32          # Key length (AES-128/192/256)

# Performance parameters  
latency=200                 # Latency in milliseconds (120-8000)
maxbw=0                     # Max bandwidth (0=unlimited)
lossmaxttl=40              # Max packet retransmit attempts
nakreport=true             # Enable periodic NAK reports
conntimeo=3000             # Connection timeout (ms)

# Advanced parameters
sndbuf=12058624            # Send buffer size (bytes)
rcvbuf=12058624            # Receive buffer size (bytes) 
mss=1500                   # Maximum segment size
fc=32                      # Flow control window size
iptos=0xB8                 # Type of service
ipttl=64                   # Time to live
ffs=25600                  # Flight flag size
ohead=25                   # Sender-side buffer overhead

Latency Configuration

Choose latency based on your use case:

Ultra-Low Latency (Local network):
  latency: 120-200 ms
  Use case: Local production, campus streaming

Low Latency (Good network):
  latency: 200-500 ms  
  Use case: Regional contribution, live events

Standard (Internet):
  latency: 500-2000 ms
  Use case: Remote contribution over internet

High Latency (Challenging network):
  latency: 2000-8000 ms
  Use case: Satellite, cellular, high packet loss

Security & Encryption

Enable AES Encryption

srt://server:4200?streamid=publish/stream123&passphrase=mysecret&pbkeylen=16

Stream ID Authentication

Use stream IDs to control access:

# Format: action/streamId/extra-params
publish/stream123
play/stream123  
publish/stream123/user=john&token=abc123

Troubleshooting

Connection Issues

Cannot connect to SRT server:

# Test SRT port
telnet your-server.com 4200

# Check firewall
sudo ufw allow 4200/udp
sudo firewall-cmd --add-port=4200/udp --permanent

# Verify SRT is enabled
curl http://your-server.com:5080/WebRTCAppEE/rest/v2/settings

Passphrase mismatch:

Error: "SRT Connection Error: Invalid passphrase"

Solution:
- Ensure client and server use same passphrase
- Check pbkeylen matches (16, 24, or 32)
- Verify no special characters causing encoding issues

Performance Issues

High packet loss:

# Increase latency to allow more retransmissions
latency=2000

# Increase max packet retransmit attempts  
lossmaxttl=100

# Enable stats monitoring
ffmpeg -v verbose -stats \
  -i "srt://server:4200?streamid=play/stream&latency=2000" \
  -f null -

Buffering/freezing:

# Increase receive buffer
rcvbuf=12058624

# Adjust latency based on RTT
# Rule of thumb: latency = 4 * RTT
latency=400  # for 100ms RTT

# Check network stats
srt-live-transmit -statsout -1 \
  "srt://server:4200?streamid=play/stream123" \
  "udp://127.0.0.1:5000"

Debug Commands

# Monitor SRT statistics with srt-stats-live-transmit
srt-live-transmit \
  -statsout 1000 \
  "srt://server:4200?streamid=play/stream123" \
  "file://output.ts"

# Analyze stream with FFprobe
ffprobe -v quiet -print_format json -show_streams \
  "srt://server:4200?streamid=play/stream123&passphrase=secret"

# Check SRT connection stats
netstat -su | grep -i udp

# Monitor bandwidth usage
iftop -f "udp port 4200"

# Packet capture
sudo tcpdump -i any udp port 4200 -w srt_capture.pcap

Best Practices

Production Recommendations

Set appropriate latency: Start with 200ms, increase if packet loss occurs
Use encryption: Always use passphrase in production
Monitor stats: Enable SRT statistics for health monitoring
Configure firewalls: Allow UDP traffic on SRT port
Use caller mode: More firewall-friendly than listener mode
Implement reconnection: Handle connection drops gracefully

Quality Settings by Use Case

ffmpeg -re -i input.mp4 \
  -c:v libx264 -preset ultrafast -tune zerolatency \
  -b:v 5000k -maxrate 5000k -bufsize 10000k \
  -g 30 -sc_threshold 0 \
  -c:a aac -b:a 192k \
  -f mpegts \
  "srt://server:4200?streamid=publish/sports&latency=120&passphrase=secret"

Bandwidth Optimization

# Limit bandwidth for specific use cases

# 5 Mbps limit
maxbw=625000  # bytes per second

# Calculate bandwidth:
# video bitrate + audio bitrate + 15% overhead
# Example: 3000k video + 128k audio = 3128k = 391KB/s
# With overhead: 391 * 1.15 = 450 KB/s
maxbw=450000

Integration Examples

Pull SRT Stream to Ant Media Server

# Create stream source pointing to external SRT stream
curl -X POST "https://your-server.com:5443/WebRTCAppEE/rest/v2/broadcasts/create" \
  -H "Content-Type: application/json" \
  -d '{
    "name": "SRT Stream",
    "streamId": "srt_stream_123",
    "type": "streamSource",
    "streamUrl": "srt://external-server.com:9000?streamid=input&latency=1000"
  }'

# Start pulling
curl -X POST "https://your-server.com:5443/WebRTCAppEE/rest/v2/broadcasts/srt_stream_123/start"

# Stop pulling  
curl -X POST "https://your-server.com:5443/WebRTCAppEE/rest/v2/broadcasts/srt_stream_123/stop"

Convert SRT to Other Formats

Ant Media Server automatically converts SRT input to:

WebRTC for browser playback
HLS for mobile/smart TV
DASH for adaptive streaming
RTMP for restreaming

Enable in configuration:

settings.hlsMuxingEnabled=true
settings.dashMuxingEnabled=true  
settings.webRTCEnabled=true

SRT vs Other Protocols

Feature	SRT	RTMP	WebRTC	RTSP
Latency	1-4s	3-10s	Less than 1s	1-3s
Reliability	High	Medium	Medium	Low
Encryption	Built-in	RTMPS	Built-in	RTSPS
Firewall	Good	Good	Excellent	Poor
Packet Loss	Handles well	Drops stream	Adapts	Stutters
Use Case	Contribution	Ingestion	Interactive	CCTV

Choose SRT for:

Remote contribution feeds
Bonded cellular streaming
Satellite links
Long-distance streaming
High-quality contribution with reliability

Get Started

Core Concepts

Deployment

Configuration

Streaming

Features

Security

Storage

SDKs

Operations

Overview

Key Benefits

Publishing Streams

SRT Publisher Modes

Hardware Encoders with SRT

Playing Streams

SRT Playback

Configuration Options

Server Configuration

SRT URL Parameters

Latency Configuration

Security & Encryption

Enable AES Encryption

Stream ID Authentication

Troubleshooting

Connection Issues

Performance Issues

Debug Commands

Best Practices

Production Recommendations

Quality Settings by Use Case

Bandwidth Optimization

Integration Examples

Pull SRT Stream to Ant Media Server

Convert SRT to Other Formats

SRT vs Other Protocols

Build docs developers (and LLMs) love

Get Started

Core Concepts

Deployment

Configuration

Streaming

Features

Security

Storage

SDKs

Operations

​Overview

​Key Benefits

​Publishing Streams

​SRT Publisher Modes

​Hardware Encoders with SRT

​Playing Streams

​SRT Playback

​Configuration Options

​Server Configuration

​SRT URL Parameters

​Latency Configuration

​Security & Encryption

​Enable AES Encryption

​Stream ID Authentication

​Troubleshooting

​Connection Issues

​Performance Issues

​Debug Commands

​Best Practices

​Production Recommendations

​Quality Settings by Use Case

​Bandwidth Optimization

​Integration Examples

​Pull SRT Stream to Ant Media Server

​Convert SRT to Other Formats

​SRT vs Other Protocols

Build docs developers (and LLMs) love

Overview

Key Benefits

Publishing Streams

SRT Publisher Modes

Hardware Encoders with SRT

Playing Streams

SRT Playback

Configuration Options

Server Configuration

SRT URL Parameters

Latency Configuration

Security & Encryption

Enable AES Encryption

Stream ID Authentication

Troubleshooting

Connection Issues

Performance Issues

Debug Commands

Best Practices

Production Recommendations

Quality Settings by Use Case

Bandwidth Optimization

Integration Examples

Pull SRT Stream to Ant Media Server

Convert SRT to Other Formats

SRT vs Other Protocols