# srt-live-transmit

The srt-live-transmit tool is a universal data transport tool with a purpose to transport data between SRT and other medium. At the same time it is just a sample application to show some of the powerful features of SRT. We encourage you to use SRT library itself integrated into your products.

## Introduction

The srt-live-transmit can be both used as a universal SRT-to-something-else flipper, as well as a testing tool for SRT.

The general usage is the following:

``shell srt-live-transmit <input-uri> <output-uri> [options] ``

The following medium types are handled by srt-live-transmit:

- SRT - use SRT for reading or writing, in listener, caller or rendezvous mode, with possibly additional parameters

- UDP - read or write the given UDP address (also multicast)

- RTP - read RTP from the given address (also multicast)

- Local file - read or store the stream into the file

- Process’s pipeline - use the process’s stdin and stdout standard streams

Any medium can be used with any direction, although some of them may have special direction-dependent cases.

Mind that the URI has a standard syntax:

``yaml scheme://HOST:PORT/PATH?PARAM1=VALUE1&PARAM2=VALUE2&... ``

The first parameter is introduced with a ? and all following can be appended with an & character.

If you specify only the path (no :// specified), then the scheme defaults to file. The path can be also specified as relative this way. Note also that empty host (scheme://:PORT) defaults to 0.0.0.0, and an empty port (when there’s no :PORT part) defaults to port number 0.

Special options for particular medium may be specified in PARAM items. All options are medium-specific, although there may happen some options common for multiple media types.

Note also that the HOST part is always tried to be resolved as a name, if its form is not directly the IPv4 address.

### Example for Smoke Testing

First we need to start up the srt-live-transmit app, listening for unicast UDP TS input on port 1234 and making SRT available on port 4201. Note, these are randomly chosen ports. We also open the app in verbose mode for debugging:

``shell srt-live-transmit udp://:1234 srt://:4201 -v ``

Now we need to generate a UDP stream. ffmpeg can be used to generate bars and tone as follows, doing a simple unicast push to our listening srt-live-transmit application:

``shell ffmpeg -f lavfi -re -i smptebars=duration=300:size=1280x720:rate=30 -f lavfi -re -i sine=frequency=1000:duration=60:sample_rate=44100 -pix_fmt yuv420p -c:v libx264 -b:v 1000k -g 30 -keyint_min 120 -profile:v baseline -preset veryfast -f mpegts "udp://127.0.0.1:1234?pkt_size=1316" ``

You should see the stream connect in srt-live-transmit.

Now you can test in VLC (make sure you’re using the latest version!) - just go to file -> open network stream and enter srt://127.0.0.1:4201 and you should see bars and tone right away.

Or you can test using ffplay or ffprobe to inspect the stream:

``shell ffplay srt://127.0.0.1:4201 `` -or- ``shell ffprobe srt://127.0.0.1:4201 ``

If you’re having trouble, make sure this works, then add complexity one step at a time (multicast, push vs listen, etc.).

## URI Syntax

Transmission mediums are specified as the standard URI format:

``yaml SCHEME://HOST:PORT?PARAM1=VALUE1&PARAM2=VALUE2&... ``

The applications supports the following schemes:

- file - for file or standard input and output

- udp - UDP output (unicast and multicast)

- rtp - RTP input (unicast and multicast)

- srt - SRT connection

Note that this application doesn’t support file as a medium, but this can be handled by other applications from this project.

### Medium: FILE (including standard process pipes)

NB! File mode, except file://con, is not supported in the srt-file-transmit tool!

The general syntax is: file:///global/path/to/the/file. No parameters in the URL are extracted. There’s one (non-standard!) special case, though:

``yaml file://con ``

That is, con is used as a HOST part of the URI. If you use this URI for <input-uri>, then the data will be read from the standard input. If <output-uri>, the data will be send to the standard output. Be careful with options being specified together with having standard output as output URI - some of them are not allowed as the extra output controlled by options might interfere with the data output.

## Medium: UDP

UDP can only be used in listening mode for input, and in calling mode for output. Multicast Streaming is also possible, without any special declaration. Just use an IP address from the multicast range. The specification and meaning of the fields in the URI depend on the mode.

The PORT part is always mandatory and it designates either the port number for the target host or the port number to be bound to read from.

The following options are available through URI parameters:

- iptos: sets the IP_TOS socket option

- ttl: sets the IP_TTL or IP_MULTICAST_TTL option, depending on mode

- mcloop: sets the IP_MULTICAST_LOOP option (multicast mode only)

- rcvbuf: sets the SO_RCVBUF socket option

- sndbuf: sets the SO_SNDBUF socket option

- adapter: sets the local binding address

- source: uses IP_ADD_SOURCE_MEMBERSHIP, see below for details

For sending to unicast:

``yaml udp://TARGET:PORT?parameters... ``

- The HOST part (here: TARGET) is mandatory and designates the target host

- The iptos parameter designates the Type-Of-Service (TOS) field for

outgoing packets via IP_TOS socket option.

- The ttl parameter will set time-to-live value for outgoing packets via

IP_TTL socket options.

For receiving from unicast:

``yaml udp://LOCALADDR:PORT?parameters... ``

- The HOST part (here: LOCALADDR) designates the local interface to bind.

It’s optional (can be empty) and defaults to 0.0.0.0 (INADDR_ANY).

For multicast the scheme is:

``yaml udp://GROUPADDR:PORT?parameters... ``

- The HOST part (here: GROUPADDR) is mandatory always and designates the

target multicast group. The @ character is handled in this case, but it’s not necessary, as the IGMP addresses are recognized by their mask.

For sending to a multicast group:

- The iptos parameter designates the Type-Of-Service (TOS) field for

outgoing packets via IP_TOS socket option.

- The ttl parameter will set time-to-live value for outgoing packets via

IP_MULTICAST_TTL socket options.

- The adapter parameter can be used to specify the adapter to be set

through IP_MULTICAST_IF option to override the default device used for sending

For receiving from a multicast group:

- The adapter parameter can be used to specify the adapter through which

the given multicast group can be reached (it’s used to bind the socket)

- The source parameter enforces the use of IP_ADD_SOURCE_MEMBERSHIP

instead of IP_ADD_MEMBERSHIP and the value is set to imr_sourceaddr field.

Explanations for the symbols and terms used above can be found in POSIX manual pages, like ip(7) and on Microsoft docs pages under IPPROTO_IP.

### Medium: RTP

RTP is supported for input only.

All URI parameters described in the [Medium: UDP](#medium-udp) section above also apply to RTP. A further RTP-specific option is available as an URI parameter:

- rtpheadersize: sets the number of bytes to drop from the beginning of

each received packet. Defaults to 12 if not provided. Minimum value is 12.

A length of rtpheadersize bytes will always be dropped. If you wish to pass the entire packet, including RTP header, to the output medium, you should instead specify UDP as the input medium.

> NOTE: No effort is made in the initial implementation to attempt to parse the RTP headers in any way eg for validation, reordering, extracting timing, length detection of checking.

### Medium: SRT

Most important about SRT is that it can be either input or output and in both these cases it can work in listener, caller and rendezvous mode. SRT also handles several parameters special way, in addition to standard SRT options that can be set through the parameters.

SRT can be connected using one of three connection modes:

- caller: the “agent” (this application) sends the connection request to

the peer, which must be listener, and this way it initiates the connection.

- listener: the “agent” waits to be contacted by any peer caller.

Note that a listener can accept multiple callers, but srt-live-transmit does not use this ability; after the first connection, it no longer accepts new connections.

- rendezvous: A one-to-one only connection where both parties are

equivalent and both attempt to initiate a connection simultaneously. Whichever party happens to start first (or succeeds in punching through the firewall first) is considered to have initiated the connection.

This mode can be specified explicitly using the mode parameter. When it’s not specified, then it is derived based on the host part in the URI and the presence of the adapter parameter:

- Listener mode: if you leave the host part empty (adapter may be specified):

    - srt://:1234

- Caller mode: if you specify host part, but not adapter parameter:

    - srt://remote.host.com:1234

- Rendezvous mode: if you specify host AND adapter parameter:

    - srt://remote.host.com:1234&adapter=my.remote.addr

Sometimes the required parameter specification results in a different mode than desired; in this case you should specify the mode explicitly.

The interpretation of the host and port parts is the following:

- In LISTENER mode:

    - host part: the local IP address to bind (default: 0.0.0.0 - “all devices”)

    - port part: the local port to bind (mandatory)

    - adapter parameter: alternative for host part, e.g.:

``yaml srt://10.10.10.100:5001?mode=listener ``

or

``yaml srt://:5001?adapter=10.10.10.100 ``

- In CALLER mode:

    - host part: remote IP address to connect to (mandatory)

    - port part: remote port to connect to (mandatory)

    - port parameter: the local port to bind (default: 0 - “system autoselection”)

    - adapter parameter: the local IP address to bind (default: 0.0.0.0 - “system selected device”)

    - bind parameter: a shortcut to set adapter or port by specifying ADAPTER:PORT

``yaml srt://remote.host.com:5001 ``

``yaml srt://remote.host.com:5001?adapter=local1&port=4001&mode=caller ``

- In RENDEZVOUS mode: same as CALLER except that the local

port, if not specified by the port parameter, defaults to the value of the remote port (specified in the port part in the URI).

``yaml srt://remote.host.com:5001?mode=rendezvous `` (uses remote.host.com port 5001 for a remote host and the default network device for routing to this host; the connection from the peer is expected on that device and port 5001)

``yaml srt://remote.host.com:5001?port=4001&adapter=local1 `` (uses remote.host.com port 5001 for a remote host and the peer is expected to connect to local1 address and port 4001)

IMPORTANT information about IPv6.

This application can also use an address specified as IPv6 with the following restrictions:

1. The IPv6 address in the URI is specified in square brackets: e.g. srt://[::1]:5000.

2. In listener mode, if you leave the host empty, the socket is bound to INADDR_ANY for IPv4 only. If you want to make it listen on IPv6, you need to specify the host as ::. NOTE: Don’t use square brackets syntax in the adapter parameter specification, as in this case only the host is expected.

3. If you bind to an IPv6 wildcard address (with listener mode, or when using the bind option), setting the ipv6only option to 0 or 1 is obligatory, as it is a part of the binding definition. If you set it to 1, the binding will apply only to IPv6 local addresses, and if you set it to 0, it will apply to both IPv4 and IPv6 local addresses. See the [SRTO_IPV6ONLY](../API/API-socket-options.md#SRTO_IPV6ONLY) option description for details.

4. In rendezvous mode you may only interconnect both parties using IPv4, or both using IPv6. Unlike listener mode, if you want to leave the socket default-bound (you don’t specify adapter), the socket will be bound with the same IP version as the target address. If you do specify adapter, then both this address and the target address must be of the same family.

Examples:

- srt://:5000 defines listener mode with IPv4.

- srt://[::]:5000 defines caller mode (!) with IPv6.

- srt://[::]:5000?mode=listener&ipv6only=1 defines listener mode with IPv6.

    Only connections from IPv6 callers will be accepted.

- srt://192.168.0.5:5000?mode=rendezvous will make a rendezvous connection

    with local address INADDR_ANY (IPv4) and port 5000 to a destination with port 5000.

- srt://[::1]:5000?mode=rendezvous&port=4000 will make a rendezvous

    connection with local address inaddr6_any (IPv6) and port 4000 to a destination with port 5000.

- srt://[::1]:5000?adapter=127.0.0.1 - this URI is invalid

    (different IP versions for binding and target address in rendezvous mode)

Some parameters handled for SRT medium are specific, all others are socket options. The following parameters are handled in a special way by srt-live-transmit:

- mode: enforce caller, listener or rendezvous mode

- port: enforce the outgoing port (the port number that will be set in the UDP packet as a source port when sent from this host). Not used in listener mode.

- blocking: sets the SRTO_RCVSYN for input medium or SRTO_SNDSYN for output medium

- timeout: sets SRTO_RCVTIMEO for input medium or SRTO_SNDTIMEO for output medium

- adapter: sets the local IP address to bind

All other parameters are SRT socket options. The Values column uses the following type specification:

- bool. Possible values: yes/no, on/off, true/false, 1/0.

- bytes positive integer [1; INT32_MAX].

- ms - positive integer value of milliseconds.

URI param | Values | SRT Option | Description |
——————– | —————- | ————————- | ———– |
congestion | {live, file} | SRTO_CONGESTION | Type of congestion control. |
conntimeo | ms | SRTO_CONNTIMEO | Connection timeout. |
cryptomode | 0..2 | SRTO_CRYPTOMODE | Cryptographic mode. |
drifttracer | bool | SRTO_DRIFTTRACER | Enable drift tracer. |
enforcedencryption | bool | SRTO_ENFORCEDENCRYPTION | Reject connection if parties set different passphrase. |
fc | bytes | SRTO_FC | Flow control window size. |
groupconnect | {0, 1} | SRTO_GROUPCONNECT | Accept group connections. |
groupminstabletimeo`| 60.. `ms | `SRTO_GROUPMINSTABLETIMEO`| Group minimum stability timeout. |
inputbw | bytes | SRTO_INPUTBW | Input bandwidth. |
iptos | 0..255 | SRTO_IPTOS | IP socket type of service |
ipttl | 1..255 | SRTO_IPTTL | Defines IP socket “time to live” option. |
ipv6only | -1..1 | SRTO_IPV6ONLY | Allow only IPv6. |
kmpreannounce | 0.. | SRTO_KMPREANNOUNCE | Duration of Stream Encryption key switchover (in packets). |
kmrefreshrate | 0.. | SRTO_KMREFRESHRATE | Stream encryption key refresh rate (in packets). |
latency | 0.. | SRTO_LATENCY | Defines the maximum accepted transmission latency. |
linger | 0.. | SRTO_LINGER | Link linger value |
lossmaxttl | 0.. | SRTO_LOSSMAXTTL | Packet reorder tolerance. |
maxbw | 0.. | SRTO_MAXBW | Bandwidth limit in bytes |
mininputbw | 0.. | SRTO_MININPUTBW | Minimum allowed estimate of SRTO_INPUTBW |
messageapi | bool | SRTO_MESSAGEAPI | Enable SRT message mode. |
minversion | maj.min.rev | SRTO_MINVERSION | Minimum SRT library version of a peer. |
mss | 76.. | SRTO_MSS | MTU size |
nakreport | bool | SRTO_NAKREPORT | Enables/disables periodic NAK reports |
oheadbw | 5..100 | SRTO_OHEADBW | limits bandwidth overhead, percents |
packetfilter | string | SRTO_PACKETFILTER | Set up the packet filter. |
passphrase | string | SRTO_PASSPHRASE | Password for the encrypted transmission. (must be 10 to 79 characters) |
payloadsize | 0.. | SRTO_PAYLOADSIZE | Maximum payload size. |
pbkeylen | {16, 24, 32} | SRTO_PBKEYLEN | Crypto key length in bytes. |
peeridletimeo | ms | SRTO_PEERIDLETIMEO | Peer idle timeout. |
peerlatency | ms | SRTO_PEERLATENCY | Minimum receiver latency to be requested by sender. |
rcvbuf | bytes | SRTO_RCVBUF | Receiver buffer size |
rcvlatency | ms | SRTO_RCVLATENCY | Receiver-side latency. |
retransmitalgo | {0, 1} | SRTO_RETRANSMITALGO | Packet retransmission algorithm to use. |
sndbuf | bytes | SRTO_SNDBUF | Sender buffer size. |
snddropdelay | ms | SRTO_SNDDROPDELAY | Sender’s delay before dropping packets. |
streamid | string | SRTO_STREAMID | Stream ID (settable in caller mode only, visible on the listener peer). |
tlpktdrop | bool | SRTO_TLPKTDROP | Drop too late packets. |
transtype | {live, file} | SRTO_TRANSTYPE | Transmission type |
tsbpdmode | bool | SRTO_TSBPDMODE | Timestamp-based packet delivery mode. |

The list of socket options can also be found in SRT header file srt.h (SRT_SOCKOPT enum type). Please note that the set of available options may be version dependent. All options are available under the lowercase name of the option without the SRTO_ prefix. For example, SRTO_PASSPHRASE can be set using a passphrase parameter. The mapping table srt_options can be found in common/socketoptions.hpp file.

Important thing about the options (which holds true also for options for TCP and UDP, even though it’s not described anywhere explicitly) is that there are two categories of options:

- PRE options: these options must be set to the socket prior to connecting and they cannot be altered after the connection is made. A PRE option set to a listening socket will be also derived by the socket returned by srt_accept().

- POST options: these options can be set to a socket at any time. The option set to a listening socket will not be derived by an accepted socket.

You don’t have to worry about that actually - the application is aware of this and it sets these options at appropriate time.

Note also that blocking option has no practical use for users. Normally the non-blocking mode is used only when you have an event-driven application that needs a common signal bar for multiple event sources, or you prefer fibers to threads, when working with multiple SRT sockets in one application. The srt-live-transmit application isn’t defined this way. This makes that the practical result of non-blocking mode here is that it uses polling on exactly one socket with infinite timeout. Every reading and writing operation will then return always without blocking, but when they report the “again” situation the application will stall on srt_epoll_wait() call. This option then exists for the testing purposes, as well as educational, to serve as an example of how your application should use the non-blocking mode.

## Command-Line Options

The following options are available in the application. Note that some may affect specifically only selected type of medium.

Options usually have values and they are set using colon: for example, -t:60. Alternatively you can also separate them by a space, but this space must be part of the parameter and not extracted by a shell (using “ “ quotes or backslash).

- -timeout, -t, -to - Sets the timeout for any activity from any medium (in seconds). Default is 0 for infinite (that is, turn this mechanism off). The mechanism is such that the SIGALRM is set up to be called after the given time and it’s reset after every reading succeeded. When the alarm expires due to no reading activity in defined time, it will break the application. Notes: - The alarm is set up after the reading loop has started, not when the application has started. That is, a caller will still wait the standard timeout to connect, and a listener may wait infinitely until some peer connects; only after the connection is established is the alarm counting started. - The timeout mechanism doesn’t work on Windows at all. It behaves as if the timeout was set to -1 and it’s not modifiable.

- -timeout-mode, -tm - Timeout mode used. Default is 0 - timeout will happen after the specified time. Mode 1 cancels the timeout if the connection was established.

- -st, -srctime, -sourcetime - Enable source time passthrough. Default: disabled. It is recommended to build SRT with monotonic (-DENABLE_MONOTONIC_CLOCK=ON) or C++ 11 steady (-DENABLE_STDCXX_SYNC=ON) clock to use this feature.

- -buffering - Enable source buffering up to the specified number of packets. Default: 10. Minimum: 1 (no buffering).

- -chunk, -c - use given size of the buffer. The default size is 1456 bytes, which is the maximum payload size for a single SRT packet.

- -verbose, -v - Display additional information on the standard output. Note that it’s not allowed to be combined with output specified as file://con.

- -statsout - SRT statistics output: filename. Without this option specified, the statistics will be printed to the standard output.

- -pf, -statspf - SRT statistics print format. Values: json, csv, default. After a comma, options can be specified (e.g. “json,pretty”).

- -s, -stats, -stats-report-frequency - The frequency of SRT statistics collection, based on the number of packets.

- -loglevel - lowest logging level for SRT, one of: fatal, error, warn, note, debug (default: warn)

- -logfa, -lfa - selected FAs in SRT to be logged (default: all are enabled). See the list of FAs running -help:logging.

- -logfile:logs.txt - Output of logs is written to file logs.txt instead of being printed to stderr.

- -help, -h - Show help.

- -version - Show version info.

## Testing Considerations

Before starting any test with srt-live-transmit please make sure your video source works properly. For example: if you use VLC as a test player, send a UDP stream directly to it before routing it through srt-live-transmit.

For any MPEG-TS UDP based source make sure it has packet sizes of 1316 bytes. When using ffmpeg like in the “Example for Smoke Testing” section above set the pkt_size=1316 parameter in case your input is a continuous data stream like from a file, camera or data-generator.

When leaving the LAN for testing, please keep an eye on statistics and make sure your round-trip-time (RTT) is not drifting. It’s recommended to set the latency 3 to 4 times higher than RTT. Especially on wireless links such as WLAN, Line-of-Sight Radio (LOS) and mobile links such as LTE/4G or 5G the RTT can vary a lot.

If you perform tests on the public Internet, consider checking your firewall rules. The SRT listener must be reachable on the chosen UDP port. Same applies to routers using NAT. Please set a port forwarding rule with protocol UDP to the local IP address of the SRT listener.

The initiation of an SRT connection (handshake) is decoupled from the stream direction. The sender of a stream can be an SRT listener or an SRT caller, as long as the receiving end uses the opposite connection mode. Typically you use the SRT listener on the receiving end, since it is easier to configure in terms of firewall/router setup. It also makes sense to leave the Sender in listener mode when trying to connect from various end points with possibly unknown IP addresses.

### UDP Performance

Performance issues concerning reading from UDP medium were reported in [#933](https://github.com/Haivision/srt/issues/933) and [#762](https://github.com/Haivision/srt/issues/762).

The dedicated research showed that at high and bursty data rates (~60 Mbps) the epoll_wait(udp_socket) is not fast enough to signal about the possibility of reading from a socket. It results in losing data when the input bitrate is very high (above 20 Mbps).

PR [#1152](https://github.com/Haivision/srt/pull/1152) (SRT v1.4.2 and above) adds the possibility of setting the buffer size of the UDP socket in srt-live-transmit. Having a bigger buffer of UDP socket to store incoming data, srt-live-transmit handles higher bitrates.

The following steps have to be performed to use the bigger UDP buffer size.

### Increase the system-default max rcv buffer size

``bash $ cat /proc/sys/net/core/rmem_max 212992 $ sudo sysctl -w net.core.rmem_max=26214400 net.core.rmem_max = 26214400 $ cat /proc/sys/net/core/rmem_max 26214400 ``

### Specify the size of the UDP socket buffer via the URI

Example URI:

``bash "udp://:4200?rcvbuf=67108864" ``

Example full URI:

``bash ./srt-live-transmit "udp://:4200?rcvbuf=67108864" srt://192.168.0.10:4200 -v ``