Setting SRT in Nimble Streamer

Secure Reliable Transport (SRT) is a transport technology that optimizes streaming performance across unpredictable networks. It's applied to contribution and distribution endpoints as part of a video stream workflow to deliver the best quality and lowest latency video at all times.

As audio/video packets are streamed from a source to a destination device, SRT detects and adapts to the real-time network conditions between the two endpoints. SRT helps compensate for jitter and bandwidth fluctuations due to congestion over noisy networks, such as the Internet. Its error recovery mechanism minimizes the packet loss typical of Internet connections. AES 128/256 bit encryption is also supported for end-to-end security, keeping your streams safe from prying eyes.

Our company, Softvelum LLC, became a member of SRT Alliance which is a group dedicated to managing and supporting the open source implementation of SRT. This alliance is accelerating interoperability of video streaming solutions and fostering collaboration with industry leaders to achieve lower latency Internet video transport. Softvelum is actively contributing to the community to improve the protocol and its ecosystem.

Nimble Streamer has full support for SRT:

• Both Push (Caller) and Listen delivery modes are supported.
• Both Listen and Pull receive modes can be used to get SRT streams for further processing.
• Rendezvous mode for sending and receiving is supported.
• Custom "latency" and "maxbw" parameters are supported for fine tuning.
• Forward error correction filter support.
• Multiple streams can be sent via single SRT channel.
• Multiple Pull-mode clients can be supported when working in "Listen" mode, see section "Set up sending via SRT" section for details.
• Deliver and receive SRT bonded streams with Nimble Streamer with libsrt 1.5.
• AES 128 bit encryption provides stream security.
• IPv4 and IPv6 support.
• SRT playback stats and protection.

"streamid" parameters support: 

• Nimble supports streamid in Push and Pull modes.
• In addition, Nimble supports streamid in receiver Listen mode as part of SRT PASSet feature set.
• Besides, RTMP-style SRT streamid in Listen mode for publisher is supported for easier output generation of live streams.

Using Nimble Streamer transmuxing engine, you may create any supported live protocol output from SRT, as well as receive any of them  to create SRT.

Let's see how SRT can be enabled and used in Nimble Streamer.

Besides reading this full guide you can take a look at these video tutorials:

• SRT + Nimble Streamer on Amazon EC2: Nimble installation and SRT setup
• NDI to SRT to AWS Elemental MediaConnect via Nimble Streamer
• SRT playlist has all other SRTT-related video which you may find useful.

Install SRT package

SRT is available via separate package for Nimble Streamer. You need to install it in order to use this protocol.

1. Install Nimble Streamer or upgrade it to latest version.
2. Follow this instruction to install SRT package. For macOS, use this Docker-based approach.

Now you may proceed with the setup. You may define both transmission and reception settings so Nimble Streamer could be used on both ends of delivery chain.

A. Set up sending via SRT 

Having any input via RTMP, RTSP, MPEG-TS or HLS, you may create outgoing SRT streams.

Go to Nimble Streamer -> Live streams settings menu. Click on UDP streaming tab.

Click on Add SRT setting button to see the following dialog for creating a new SRT channel.

IP and Port fields define the destination of your channel.

Mode field may be set to either Push, Listen or Rendezvous - those are the modes supported by SRT.

• Push will initiate the active sending of the selected channel to the destination IP/Port regardless of the recipient state. This is also referred to as "Caller" mode.
• Listen will make Nimble Streamer connect to destination IP/Port and will wait for the Pull command from remote server.
• Rendezvous mode is described below.

Listen mode allows processing streamid using "RTMP style", e.g. obtain existing live streams available in Nimble Streamer using "app/stream" notation. Read this article for more details. 

Mux rate is typically used for precise testing of channel throughput. Read this article for all details regarding mux rate setup.

Source applications names, streams names and their respective PMT PIDs, Video PIDs and Audio PIDs describe where the content is taken from for further transmission. You may define multiple sources - in this case SRT channel will have multiple streams in it.

MPEGTS In Source option can be used If you'd like to stream some existing MPEGTS input intact without any transmuxing, like pass though PIDs, multiple tracks, subtitles or any metadata that is not supported by Nimble. Read this article for more details.

In addition to mandatory parameters you may specify custom parameters and their values.

Custom parameters

latency parameter can be defined in milliseconds. We recommend setting it not less than "120". If you don't specify, Nimble will use 120 by default as set in SRT library. This specified delay causes an extra "wasted time" if the packet reached the destination quickly (and therefore it's way ahead of "time to play"), but when it was lost, this gives it an extra time to recover the packet before its "time to play" comes.

maxbw parameter (measured in bytes per second) defines maximum bandwidth which is allowed to be used by a single outgoing stream. If the network connection is not stable enough then a lot of SRT packets will be re-transmitted within a period specified by latency. The more streams from same server are used in unstable network, the more packets are lost and need to be re-transmitted. As a result, entire network becomes utilized. If maxbw parameter is set then even if network conditions are bad, each stream will not consume more than it's allowed to.
So maxbw needs to include original stream bandwidth and possible re-transmission traffic. We highly recommend setting maxbw parameter to avoid excessive bandwidth usage. Please read this article for more details.

If you don't know the exact values of your expected bandwidth, please set maxbw to 0 to let the SRT library auto-select bandwidth efficiently and handle re-transmission of lost packets.

In general, we highly encourage using both "latency" and "maxbw" parameters. Please read this article for all details.

passphrase and pbkeylen parameters are available to enable encryption for processed streams. The passphrase must be at least 10 symbols long - this is your password. The pbkeylen may be set to 16, 24 or 32 with 16 used by default. If these parameters are set during the transmission, you need to re-start the stream in order for encryption parameters to apply. Same set of parameters and values must be defined for both sender and receiver parts.

local_ip and local_port parameters define which interface will be used for sending the stream in Push mode.

stream_id can also be specified in case your receiving side supports it.

Once you save the settings, they will be added to the list and will be synced with the server.

One-to-many multipoint listener

Multiple Pull-mode clients may request stream when Nimble Streamer provides SRT in "Listen" mode, like TechEx or mwEdge software. So in order to provide multiple streams in that case, you need to enable multipoint listener feature by adding this parameter into Nimble config:

srt_multipoint_listener_enabled = true

Read configuration parameters reference for more details about managing Nimble Streamer parameters.


Now you may receive content via SRT on your destination. As we already mentioned, Nimble can be used to receive the SRT streams so let's see how you can do that.

B. Set up receiving of SRT

Having SRT input, you may transmux it into  RTMP, RTSP, MPEG-TS, HLS, MPEG-DASH or even Icecast outgoing streams.

Add incoming stream

Go to Nimble Streamer -> Live streams settings menu. Click on MPEGTS In tab.

Click on Add SRT stream button to see the following dialog for creating a new incoming SRT channel.

Receive mode specifies the mode used for obtaining the stream.

• Listen sets Nimble to wait for incoming data and process it as soon as it arrives. Your source needs to be set "Push" mode - see previous section.
• Pull sets Nimble to initiate the source server to start sending the data. Your source needs to be set to Listen mode.
• Rendezvous mode is described below.

IP Address and Port fields specify which interface will be used for connection.

Alias is the name that will be used in incoming streams list.

You should also use latency parameter, you also need to specify it in custom parameters. Read this article for get more details about this parameter usage.

Once you save the setting, it will be started for sync-up with your server.

You've specified incoming stream, now you need to add it to outgoing streams, like it's done for any other MPEG-TS streams. This needs to be done because each SRT channel may carry several streams. You may check Add outgoing stream checkbox to enter app and stream names to make it automatically.

Go to MPEGTS Out tab to see the list of outgoing streams. Click on Add outgoing stream.

Here you will define the names of applications and streams which will be used for playback and other use cases. You will select video and audio sources and if they have multiple streams in a single channel, then you'll select the required ones. Read this article for more details about adding outgoing streams.

To play the SRT stream, go to Nimble Streamer -> Live Streams menu, select the outgoing streams to see what your server now has to offer for playback.

C. Setting up Rendezvous mode

Rendezvous mode is an addition to the modes described above. It allows streaming over SRT in case if one of both sides of send-receive pair use firewalls.
Each side needs local and remote interfaces to be set up: Local IP with Local Port and Remote IP with Remote Port.

These interfaces are used for initiating the transmission and exchanging the data.

• Sender side will send data to Receiver Local IP/port.
• Receiver side will send data to Sender Local IP/port during initial negotiation.

You can refer to SRT Deployment Guide (the link will start download) to get more details about it.

The description below shows how you can set up the Rendezvous mode.

Sending in Rendezvous mode

To set up SRT sending you need to go to Nimble Streamer -> Live Streams Settings menu. You will see settings page, click on UDP streaming tab there. You will see a page similar to what you saw in Set up sending via SRT section above. Click on Add SRT setting button to see the dialog. there you need to choose Rendezvous from Mode dropdown list.

Here, a pair of 192.168.0.2 and its port is a Local interface of our server which is a source of a stream to send. The Remote pair is the destination.

Receiving in Rendezvous mode

Go to Nimble Streamer -> Live streams settings menu. Click on MPEGTS In tab to see a page similar to what you saw in Set up receiving of SRT section above. Click on Add SRT stream to see the dialog below. Select Rendezvous from the Mode drop-down.

Here, a pair of Remote IP and Port will point to a source of the stream, a pair of Local IP and Port will point to current receiver server. The rest of the setup is done the same way as in Set up receiving of SRT section.

That's it - your Nimble Streamer instance may now get live streams via SRT.

We keep improving this feature set, so let us know if you have any questions or concerns about it.

SRT Publisher Assistance Security Set

Nimble Streamer has flexible security and management feature set for SRT receiver Listen mode:

• Accept streamid parameter with "application/stream" format.
• Make per-application and per-stream authentication with user and password.
• Apply any SRT parameters to each individual stream and even individual publisher.
• Apply allow and deny lists for IP addresses on server and stream level.
• Manage published streams via publish control framework.

Read more details in feature overview.

Related documentation

• Reliable Low Latency Delivery with SRT+SLDP article in Haivision blog describes a combination of SRT and SLDP protocols for building reliable delivery networks.
• Glass-to-glass SRT delivery setup article describes Nimble Streamer, Larix Broadcaster and SLDP Player being set up for delivery of live stream via SRT.
• We highly recommend reading the article about latency and maxbw parameters usage.
• Set up OBS Studio SRT streaming with Larix Broadcaster and Nimble Streamer.
• NDI to SRT to AWS Elemental MediaConnect via Nimble Streamer video tutorial.
• Deliver SRT to transform into multicast UDP MPEG-TS video tutorial.
• Zabbix monitoring of Nimble Streamer and SRT streams

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