Receiving via SRT

The Secure Reliable Transport (SRT) protocol is an open-source video streaming protocol designed to provide low-latency, high-quality video streaming over unreliable networks. SRT uses end-to-end encryption and provides a range of features designed to enhance reliability and security, including error correction, congestion control, and retransmission of lost packets. SRT is often used for live video streaming applications, such as sports and news broadcasts, where maintaining a reliable, high-quality connection is critical.

Address format

SRT can be received in two modes:

• Caller mode - Astra sends a request to the SRT server and receives content in response. This is the most popular variant
• Listener mode - Astra waits for the SRT server to establish a connection and receives content in the request. This is a point-to-point mode

The address format depends on the selected mode.

Caller mode

srt://address:port[#options]

• address - remote server IPv4 address or hostname
• port - remote port

Example:

• srt://example.com:3001 - send request to example.com

Listener mode

In listener mode, the address format is similar to the UDP address, with the addition of the @ symbol to indicate the local interface name.

srt://[interface]@:port[#options]

• interface - local interface name where to listen for connection. By default, Astra waits for a connection on all interfaces
• port - local port to accept incoming connections
• options - additional options for SRT protocol

Examples:

• srt://@:3001 - wait for connection on any interface
• srt://eth0@:3001 - wait for connection on interface eth0

Caution

Avoid using ports in the dynamic port range when configuring Listener mode. In Linux, the dynamic port range is typically 32768-60999 (you can check the range on your system with cat /proc/sys/net/ipv4/ip_local_port_range). Using ports in this range may cause conflicts with ports that Linux allocates dynamically for outbound connections, such as SRT Caller mode connections.

Options

• timeout=N - restarts the receiver if no data is received within a defined interval, in seconds. Default: 5 seconds
• latency=N - maximum accepted transmission latency, in milliseconds. Default: 120 milliseconds
• packetfilter=S - injects extra processing instructions at the beginning and/or end of a transmission to implement Forward Error Correction (FEC). Read more{target=“_blank”} in the official documentation
• passphrase=S – password for encrypted transmission. Length: 10-79 characters
• pbkeylen=N – crypto key length in bytes. Values: 16, 24, 32. Default: 0
• streamid=ID – stream identifier provided to the SRT server in caller mode
• no_tsbpdmode – turns off timestamp-based packet delivery mode
• oheadbw - limits bandwidth overhead, in percent. Range: 5-100. Default: 25

Web Interface

To configure SRT reception, go to New Stream or the settings of an existing stream and set Input Type to SRT.

• Input type: Select SRT to enable SRT-specific configuration options.
• SRT mode: Determines whether Astra acts as a Caller or Listener. Select Listener to wait for incoming connections.
• Local interface: The network interface for receiving the stream. Default is Any interface.
• Port: The network port for receiving the stream.
• Timeout: Time in milliseconds to wait for incoming data before timing out. Default: 5000 ms (5 seconds).
• Latency: Delay in milliseconds to improve stability. Default: 120ms. Higher latency increases stability but adds delay. See Latency for SRT Receiving for more details.
• Passphrase: Password for secure communication. Must match the sender’s passphrase.
• Crypto key length: Length of the cryptographic key (16, 24, or 32 bytes). Default: 16 bytes (128 bits).
• Timestamp-based packet delivery mode: When enabled, uses timestamps to ensure correct packet order. Default: Disabled.