SRTLA - SRT Link Aggregation

SRTLA bonds multiple network connections together for live video streaming, providing increased bandwidth and redundancy.

This is a fork of the BELABOX SRTLA project, with contributions from IRLToolkit, IRLServer, OpenIRL, and CeraLive.

What It Does

┌─────────────┐     ┌─────────────┐                ┌─────────────┐     ┌─────────────┐
│   Encoder   │────▶│ srtla_send  │═══════════════▶│ srtla_rec   │────▶│  SRT Server │
│   (SRT)     │     │             │  Multiple IPs  │             │     │             │
└─────────────┘     └─────────────┘                └─────────────┘     └─────────────┘
                          │
                    ┌─────┴─────┐
                    ▼     ▼     ▼
                  LTE   LTE   WiFi
                   1     2

• Combine bandwidth: 3× 5Mbps connections → ~15Mbps total
• Redundancy: One link fails, others continue
• Adaptive: Better links automatically get more traffic

Features

• Support for link aggregation across multiple network connections
• Automatic management of connection groups and individual connections
• Robust error handling and timeouts for inactive connections
• Logging of connection details for easy diagnostics
• Improved load balancing through ACK throttling
• Connection recovery mechanism for temporary network issues

Quick Start

Receiver (Server Side)

srtla_rec --srtla_port 5000 --srt_hostname 127.0.0.1 --srt_port 5001

Sender (Encoder Side)

With CeraUI (recommended): The IP list is managed automatically. CeraUI detects network interfaces, writes the IP file, and signals srtla_send when interfaces change.

Standalone usage:

1. Create IP list file:

   echo "10.0.0.10" > /tmp/srtla_ips   # usb0 IP
   echo "10.0.1.10" >> /tmp/srtla_ips  # usb1 IP  
   echo "192.168.1.50" >> /tmp/srtla_ips  # wlan0 IP

2. Start sender:

   srtla_send 5000 relay.example.com 5001 /tmp/srtla_ips

3. Configure encoder to send SRT to localhost:5000

4. When interfaces change, update the file and signal reload:

   kill -HUP $(pidof srtla_send)

Critical: Network Setup Required!

SRTLA will NOT work correctly without source-based routing!

Without it, all traffic goes through one interface regardless of which source IP is used.

See Network Setup Guide for step-by-step instructions.

Building

mkdir build && cd build
cmake ..
make
sudo make install

Dependencies:

• CMake 3.16+
• C++17 compiler
• spdlog (fetched automatically via CMake)
• argparse (included in deps/)

Documentation

Document	Description
Network Setup	Start here! Routing config, IP list management
How It Works	Protocol details, architecture, congestion control
Troubleshooting	Common issues and solutions
Connection Info Comparison	Connection metrics and comparison
Keepalive Improvements	Extended keepalive fix documentation

Note: If using CeraUI, the IP list (/tmp/srtla_ips) is managed automatically. See Managing the IP List for details.

Command Reference

srtla_send

srtla_send <listen_port> <srtla_host> <srtla_port> <ips_file> [--verbose]

Argument	Description	Default
listen_port	Port for local SRT encoder	5000
srtla_host	Remote SRTLA receiver hostname	127.0.0.1
srtla_port	Remote SRTLA receiver port	5001
ips_file	File with source IPs (one per line)	/tmp/srtla_ips
--verbose	Enable debug logging	off

Signals:

• SIGHUP: Reload IP list without restart

srtla_rec

srtla_rec --srtla_port <port> --srt_hostname <host> --srt_port <port> [--verbose] [--debug]

Argument	Description	Default
--srtla_port	Listen port for SRTLA connections	5000
--srt_hostname	Downstream SRT server	127.0.0.1
--srt_port	Downstream SRT port	4001
--verbose	Enable verbose logging	off
--debug	Enable debug logging	off

Technical Details

How It Works

1. srtla_rec creates a UDP socket for incoming SRTLA connections.
2. Clients register with srtla_rec and create connection groups.
3. Multiple connections can be added to a group.
4. Data is received across all connections and forwarded to the SRT server.
5. ACK packets are sent across all connections for timely delivery.
6. Inactive connections and groups are automatically cleaned up.

Two-phase Registration Process

• Sender (conn 0): SRTLA_REG1 (contains sender-generated random ID)
• Receiver: SRTLA_REG2 (contains full ID with receiver-generated values)
• Sender (conn 0): SRTLA_REG2 (with full ID)
• Receiver: SRTLA_REG3
• Additional connections follow a similar pattern

Error Handling

The receiver can send error responses:

• SRTLA_REG_ERR: Operation temporarily failed
• SRTLA_REG_NGP: Invalid ID, group must be re-registered

Enhanced Load Balancing and Recovery

This version includes improvements to address key issues in the original implementation:

Connection Recovery

In the original implementation, connections with temporary problems were completely disabled. Now:

• Connections showing signs of recovery enter a "recovery mode"
• These connections receive more frequent keepalive packets for a set period (5 seconds)
• After successful recovery, they are fully reactivated for data transmission
• Recovery attempts are abandoned after a certain time if unsuccessful

ACK Throttling for Load Balancing

The central innovation is ACK throttling for load distribution:

1. The SRT/SRTLA client (srtla_send) selects connections based on a score derived from window size and in-flight packets
2. The window size in the client is adjusted when ACKs are received
3. By selectively throttling ACK frequency, we indirectly control how quickly the window grows
4. This causes the client to prefer better connections without requiring client modifications

Connection Quality Assessment

Connection quality is assessed by measuring:

• Bandwidth Performance: Compares actual bandwidth to expected bandwidth
• Packet Loss: Higher loss rates lead to more error points
• Dynamic Bandwidth Evaluation: Connections evaluated against median or minimum thresholds
• Grace Period: New connections receive a 10-second grace period before penalties

Weight levels:

• 100% (WEIGHT_FULL): Optimal connection
• 85% (WEIGHT_EXCELLENT): Excellent connection
• 70% (WEIGHT_DEGRADED): Slightly impaired connection
• 55% (WEIGHT_FAIR): Fair connection
• 40% (WEIGHT_POOR): Severely impaired connection
• 10% (WEIGHT_CRITICAL): Critically impaired connection

Configuration Parameters

Adjustable parameters for optimization:

Parameter	Default	Description
KEEPALIVE_PERIOD	1s	Interval for keepalive packets during recovery
RECOVERY_CHANCE_PERIOD	5s	Period for connection recovery attempt
CONN_QUALITY_EVAL_PERIOD	5s	Interval for evaluating connection quality
ACK_THROTTLE_INTERVAL	100ms	Base interval for ACK throttling
MIN_ACK_RATE	20%	Minimum ACK rate to keep connections alive
MIN_ACCEPTABLE_TOTAL_BANDWIDTH_KBPS	1000	Minimum total bandwidth for acceptable streaming
GOOD_CONNECTION_THRESHOLD	50%	Threshold for considering a connection "good"
CONNECTION_GRACE_PERIOD	10s	Grace period before applying penalties

Socket Information

srtla_rec creates information files about active connections under /tmp/srtla-group-[PORT]. These files contain the client IP addresses connected to a specific socket.

Setup Checklist

• Routing tables added to /etc/iproute2/rt_tables
• DHCP hook installed for USB/Ethernet modems
• NetworkManager dispatcher installed for WiFi
• Public DNS configured
• UDP buffer sizes increased (sysctl)
• Firewall allows UDP traffic
• Source IPs file created
• Verified with ip route get ... from <source_ip>

Support the Project

If you find SRTLA useful, consider supporting CeraLive development:

• Ko-fi
• PayPal

License

GNU Affero General Public License v3.0 (AGPL-3.0)

Copyright (C) 2020-2021 BELABOX project
Copyright (C) 2024 IRLToolkit Inc.
Copyright (C) 2024 OpenIRL
Copyright (C) 2025 IRLServer.com
Copyright (C) 2025 CeraLive

You can use, modify, and distribute this code according to the terms of the AGPL-3.0.