Monitor SRT Packet Loss and Latency with FFmpeg

Monitoring network performance is crucial for maintaining high-quality live streams. This article explains how to track real-time packet loss, latency, and other vital statistics of an active Secure Reliable Transport (SRT) connection using FFmpeg’s built-in logging and command-line options.

Enabling SRT Statistics in FFmpeg

FFmpeg leverages the official libsrt library to handle SRT streams. To monitor the connection’s health, you can pass the stats parameter directly inside the SRT target URL.

The stats parameter dictates how frequently FFmpeg outputs connection metrics to the console. The value is defined by the number of packets processed. For example, setting stats=100 will output performance statistics every 100 packets.

Example: Receiving an SRT Stream (Listener Mode)

To receive an SRT stream and output statistics to the console every 100 packets, use the following command:

ffmpeg -i "srt://0.0.0.0:9000?mode=listener&stats=100" -c copy output.mp4

Example: Sending an SRT Stream (Caller Mode)

To send a stream to a remote server and monitor the transmission statistics, use:

ffmpeg -re -i input.mp4 -c:v libx264 -f mpegts "srt://192.168.1.50:9000?mode=caller&stats=100"

Interpreting the Console Output

Once the connection is active, FFmpeg will continuously output statistics block data to the standard error (stderr) stream. Look for lines in the console output that look like this:

SRT stats: sid=0 bytesRecv=12450000 pkgRecv=9000 pkgRcvLoss=12 pkgRcvRetrans=8 rtt=15ms

To monitor packet loss and latency, focus on these key metrics:

• rtt (Round Trip Time): Measured in milliseconds (ms), this represents the current network latency. If the rtt value rises close to your configured SRT latency envelope, you may experience stream buffering.
• pkgRcvLoss / pkgSndLoss (Packet Loss): The total number of lost packets detected. pkgRcvLoss displays packets lost on the receiving end, while pkgSndLoss displays packets lost during transmission.
• pkgRcvRetrans / pkgSndRetrans (Retransmissions): The number of packets that were successfully recovered and retransmitted via SRT’s ARQ (Automatic Repeat reQuest) mechanism.

Best Practices for Monitoring

1. Adjust the Frequency: If the console output is too fast or too slow, adjust the stats value. For high-bitrate streams, stats=1000 is often sufficient, while lower-bitrate streams benefit from stats=100.

2. Redirect Logs for Analysis: If you want to analyze the data later or parse it using a script, redirect the standard error output to a log file:

   ffmpeg -i "srt://0.0.0.0:9000?mode=listener&stats=100" -c copy output.mp4 2> srt_stats.log