SRT streaming from mobile device

SRT Streaming from Mobile Device

SRT (Secure Reliable Transport) — protocol for professional streaming over unreliable networks. Unlike RTMP it runs over UDP with error correction mechanism (ARQ/FEC), withstands up to 30% packet loss, and automatically recovers stream without restart. For mobile streaming from unstable LTE or public Wi-Fi — this is a principle advantage.

Where SRT Better than RTMP

RTMP at 5% packet loss starts buffering and falls apart. SRT with latency=500ms buffer holds at 20–25% loss. For on-location reporting, video production field shoots, remote broadcasts — SRT became standard in professional segment (OBS, vMix, Blackmagic accept SRT natively).

Latency: configurable via latency parameter. 200 ms — minimum for good network. 500–2000 ms — for mobile networks with jitter. Bigger buffer — more stability, more latency.

iOS: HaishinKit with SRT

HaishinKit supports SRT from version 2.x via SRTConnection:

import HaishinKit

let srtConnection = SRTConnection()
let srtStream = SRTStream(connection: srtConnection)

srtStream.videoSettings = VideoCodecSettings(
    videoSize: CGSize(width: 1920, height: 1080),
    bitRate: 5_000_000,
    profileLevel: kVTProfileLevel_H264_High_AutoLevel as String
)
srtStream.audioSettings = AudioCodecSettings(bitRate: 192_000)

try srtStream.attachCamera(AVCaptureDevice.default(.builtInWideAngleCamera, for: .video, position: .back))
try srtStream.attachAudio(AVCaptureDevice.default(for: .audio))

// Caller mode: we initiate connection
srtConnection.connect("srt://media-server.example.com:9998?latency=500&streamid=live/stream1")
srtStream.publish()

streamid — stream identifier on server (MediaMTX, SRT Live Server use it for routing). latency in milliseconds.

Reconnect on break: SRT can auto-reconnect within connectionTimeout. HaishinKit exposes SRTConnection.Event.connect with reason — handle in delegate.

Android: rtmp-rtsp-stream-client-java with SRT

Library rtmp-rtsp-stream-client-java supports SRT via SrtCamera2:

val srtCamera = SrtCamera2(binding.surfaceView, connectChecker)

srtCamera.prepareVideo(
    width = 1920, height = 1080, fps = 30,
    bitrate = 5_000_000,
    rotation = 0,
    profile = CodecUtil.H264_BASELINE
)
srtCamera.prepareAudio(bitrate = 192_000, sampleRate = 44100, isStereo = true)

srtCamera.startStream("srt://media-server.example.com:9998?streamid=live/stream1&latency=500")

Under the hood — native libsrt (C++), compiled for Android ABIs (arm64-v8a, armeabi-v7a, x86_64).

Alternative: FFmpegKit with libsrt

FFmpegKit compiled with libsrt — can use directly:

-f avfoundation -i 0:0 -c:v h264_videotoolbox -b:v 5M -f mpegts "srt://server:9998?latency=500&streamid=live/test"

Advantage: H.265 support (hevc_videotoolbox), more parameter control. Downside: FFmpegKit process doesn't manage camera directly — need separate capture via AVCaptureVideoDataOutput + pipe.

Listener vs Caller Mode

SRT supports two modes:

• Caller — mobile device initiates connection to server. Most common scenario.
• Listener — device listens for incoming connection. Useful for p2p without server or when server behind NAT.
• Rendezvous — both ends simultaneously initiate connection. For p2p through NAT without relay.

For mobile streaming Caller is used. Listener on mobile — rare scenario (IoT device on mobile network without public IP).

Encryption

SRT supports AES-128/256 encryption out of the box: passphrase and pbkeylen parameters in URL. This distinguishes SRT from RTMP — encryption built into protocol, not TLS wrapper.

srt://server:9998?passphrase=MySuperSecret&pbkeylen=32

Connection Quality Monitoring

SRT API provides statistics via srt_bistats(): pktSndLoss, pktRcvLoss, pktRetrans, mbpsSendRate, msRTT. HaishinKit exposes some stats in SRTStream.info. Display in UI: bitrate, RTT, packet loss — professional users appreciate this.

On high pktSndLoss (>5%) — automatically reduce videoBitrate and increase latency parameter (requires reconnection).

Server Compatibility

Timeline: integrate SRT streaming on one platform with connection quality monitoring — 2–4 work days.