feat: SRTLA support

[thejoeejoee]

Adds an SRTLA (SRT Link Aggregation) receiver server that accepts bonded connections from BELABOX-compatible encoders and proxies aggregated traffic to the local SRT server.

What

• Full SRTLA protocol implementation: REG1/REG2/REG3 handshake, keepalive, multi-connection aggregation with per-connection ACKs
• SRTLA↔SRT correlation: when SRT accepts a connection from the SRTLA backend, the group is labeled with the stream path
• Path-labeled Prometheus metrics: srtla_groups, srtla_groups_conns_active, srtla_groups_bytes_received, srtla_groups_bytes_forwarded
• Enriched lifecycle logging with group short ID in all events
• IPv6/dual-stack support with family-aware loopback resolution
• Configuration via srtla / srtlaAddress in mediamtx.yml

Config

srtla: yes
srtlaAddress: :5000

Closes #3029

[thejoeejoee]

• is SRT-SRTLA linker worth it? adds a lot of correlation code, but adds debugging/metrics benefits
• receiver only, no sender — follow-up if needed
• group timeout 30s hardcoded — should probably be configurable, but fine for now
• address-based correlation is fragile behind NAT — if SRT conn comes from different IP than SRTLA reg, it won't match. in practice this works because both go through the same UDP socket on localhost, but worth knowing
• no max groups/conns limit — could be DoS vector in theory

[thejoeejoee]

2026/05/29 14:29:32 INF MediaMTX dev, darwin, arm64
2026/05/29 14:29:32 INF [RTSP] listener opened on :8554 (TCP/RTSP), :8000 (UDP/RTP), :8001 (UDP/RTCP)
2026/05/29 14:29:32 INF [RTMP] listener opened on :1935 (TCP/RTMP)
2026/05/29 14:29:32 INF [HLS] listener opened on :8888 (TCP/HTTP)
2026/05/29 14:29:32 INF [WebRTC] listener opened on :8889 (TCP/HTTP), :8189 (UDP/ICE)
2026/05/29 14:29:32 INF [SRT] listener opened on :8890 (UDP)
2026/05/29 14:29:32 INF [SRTLA] listener opened on :8891 (UDP)
^[2026/05/29 14:33:18 INF [SRT] [conn [::1]:52107] opened
2026/05/29 14:33:18 INF [path mystream] stream is available and online, 2 tracks (H264, MPEG-4 Audio)
2026/05/29 14:33:18 INF [SRT] [conn [::1]:52107] is publishing to path 'mystream'
2026/05/29 14:33:46 INF [RTMP] [conn [::1]:49652] opened
2026/05/29 14:33:46 INF [RTMP] [conn [::1]:49652] is reading from path 'mystream', 2 tracks (H264, MPEG-4 Audio)

$ srtla_send 5000 127.0.0.1 8891 ./ips

ffmpeg -re -f lavfi -i testsrc=size=1280x720:rate=30 -f lavfi -i sine=frequency=1000 -c:v libx264 -preset ultrafast -tune zerolatency -c:a aac -f mpegts "srt://127.0.0.1:5000?streamid=publish:mystream&pkt_size=1316"
Input #0, lavfi, from 'testsrc=size=1280x720:rate=30':
  Duration: N/A, start: 0.000000, bitrate: N/A
  Stream #0:0: Video: wrapped_avframe, rgb24, 1280x720 [SAR 1:1 DAR 16:9], 30 fps, 30 tbr, 30 tbn
Input #1, lavfi, from 'sine=frequency=1000':
  Duration: N/A, start: 0.000000, bitrate: 705 kb/s
  Stream #1:0: Audio: pcm_s16le, 44100 Hz, mono, s16, 705 kb/s
Stream mapping:
  Stream #0:0 -> #0:0 (wrapped_avframe (native) -> h264 (libx264))
  Stream #1:0 -> #0:1 (pcm_s16le (native) -> aac (native))
Press [q] to stop, [?] for help
[libx264 @ 0x155e07df0] using SAR=1/1
[libx264 @ 0x155e07df0] using cpu capabilities: ARMv8 NEON DotProd I8MM
[libx264 @ 0x155e07df0] profile High 4:4:4 Predictive, level 3.1, 4:4:4, 8-bit
Output #0, mpegts, to 'srt://127.0.0.1:5000?streamid=publish:mystream&pkt_size=1316':
  Metadata:
    encoder         : Lavf62.12.101
  Stream #0:0: Video: h264, yuv444p(tv, progressive), 1280x720 [SAR 1:1 DAR 16:9], q=2-31, 30 fps, 90k tbn
    Metadata:
      encoder         : Lavc62.28.101 libx264
    Side data:
      CPB properties: bitrate max/min/avg: 0/0/0 buffer size: 0 vbv_delay: N/A
  Stream #0:1: Audio: aac (LC), 44100 Hz, mono, fltp, 69 kb/s
    Metadata:
      encoder         : Lavc62.28.101 aac
frame= 4411 fps= 30 q=15.0 size=    7522KiB time=00:02:27.03 bitrate= 419.1kbits/s speed=   1x elapsed=0:02:26.51

❯ ffprobe "rtmp://localhost:1935/mystream"
Input #0, flv, from 'rtmp://localhost:1935/mystream':
  Duration: N/A, start: 171.061000, bitrate: N/A
  Stream #0:0: Video: h264 (High 4:4:4 Predictive), yuv444p(progressive), 1280x720 [SAR 1:1 DAR 16:9], 30.30 fps, 30 tbr, 1k tbn, start 175.000000
  Stream #0:1: Audio: aac (LC), 44100 Hz, mono, fltp, start 171.061000

# SRTLA groups
srtla_groups{id="21...",path="mystream"} 1
srtla_groups_conns_active{id="21...",path="mystream"} 1
srtla_groups_bytes_received{id="21...",path="mystream"} 843368
srtla_groups_bytes_forwarded{id="21...",path="mystream"} 843368