RTP/UDP/SRT URL Format

In caller mode, a source or destination device acts as the initiator of an SRT connection. The caller device must know the IP address and port number of the listener. The URL in caller mode should contain the specific address of the source or destination for the SRT packets, for example:

srt://10.186.3.41:9000

In this case, an application requesting or sending the stream will reach out to the specified address and request that an SRT connection is made to the application on UDP port 9000. Before the request and in case the request is terminated or the connection is lost, no traffic will flow from the source to the receiver (unlike with a static unicast configuration where the sender continuously streams packets regardless of any receiver state). This is useful when defining and starting a new receiving operation without reconfiguration of a sender, for example, a mobile device requesting the output of the Cinegy Air engine or Cinegy Multiviewer over Wi-Fi or LTE, for a short confidence check of that output.

A further example is that the Cinegy Playout engine can be a caller into Cinegy Multiviewer and, conversely, Cinegy Multiviewer can be a caller into the Cinegy Playout Engine. In both scenarios the video data will always flow from the Cinegy Playout engine to Cinegy Multiviewer.

In listener mode, a device waits for a request to open the SRT connection. The listener device waits for SRT requests on a certain port on either a specific or all network adapters. The URL for listener mode to use all local adapters will be:

srt://0.0.0.0:9000

Whereas, if the application needs to listen on a specific local address, simply replace the IP component with that of the desired network adapter.

In this case, the application listening will accept the first valid SRT connection sent to UDP port 9000. This allows a listener to wait for a calling device to choose to make a request, allowing the said application to wait in a configured state while the caller is set up or adjusted later. This is useful for defining the listener that can retain a static configuration at runtime while callers are altered, for example allowing Cinegy Multiviewer to listen for incoming streams from a mobile device or uplinked studio.

Cinegy products support encrypted SRT streams and will automatically decrypt them for you on input with an SRT passphrase specified in the dedicated field for output streams and directly in the input stream URL as follows:

srt://1.2.3.4:8888/?pass=AAAAABBBBB

or

srt://1.2.3.4:8888/?serviceId=2001&videoPid=2110&audioPids=2120&pass=AAAAABBBBB

SRT protocol supports the concept of a delay buffer that provides an operating window for lost packets to be recovered. The value could be zero, if using only forward-error-correction modes (zero retransmissions) or it could be up to a few seconds. The SRT communication protocol works by both parties of the exchange sharing the latency parameters required for the recovery window, where the highest value of the two is taken as the value that shall be used for the duration of the session.

When using SRT with Cinegy products, latency may be controlled on any stream receiver through adjustment of the 'latency' URL parameter. Any stream generator starts with a default latency value of 120 milliseconds, and this acts as the lowest value for SRT latency any session may use when a Cinegy product is the source.

The receiving device requests an explicit latency by expressing the desired milliseconds of delay in the SRT URL:

srt://1.2.3.4:8888/?latency=250

This allows receivers to have a different error correction buffer depending on circumstance – a LAN-connected device might leave latency at the default (unspecified), but a 4G-connected devices might request 300 ms from the same source.

Choosing what latency values are required is not simple, but should be at least 2x the RTT (round-trip-time) of the connection as a minimum. The RTT of the link can be estimated through use of tools like ping, or can be tracked through the SRT connection statistics exposed by the Cinegy Playout engine.