Streaming target won't connect

An RTMP / SRT / NDI / Decklink target either fails to publish or drops out repeatedly.

First, check the target's last log entry. The error identifier (3500–3591 range — see Log identifiers) names the failure mode.

RTMP / RTMPS specifically:

• Wrong URL or stream key. Test the URL+key pair from the Composer host with ffmpeg:

  ffmpeg -re -f lavfi -i testsrc -t 10 -c:v libx264 -f flv rtmp://server/app/streamkey
  

  If ffmpeg can publish but Composer can't, the issue is Composer-side; otherwise it's the destination or the path between.
• Outbound TCP/1935 (or 443 for RTMPS) blocked. Try nc -zv server 1935 from the Composer host. Open the firewall hole if needed.
• TLS mismatch on RTMPS. Some RTMPS endpoints require a specific cert chain or SNI. Check the target server's accepted clients.
• Encoder timeout. Composer waits a fixed window for the destination to acknowledge each chunk. A slow or geographically distant destination can saturate that window. The fix on Composer's side is to lower the target bitrate; the fix on the destination side is a closer ingest.
• Provider-specific issues. NanoCosmos and similar providers each have their own RTMP-ingest configuration; consult their docs for the URL form and any auth headers, then configure Composer's RTMP target to match.

SRT specifically:

• Caller / Listener mismatch. Both endpoints must agree on who's the caller and who's the listener. Check the SRT URL's mode= parameter (or the equivalent target-property setting).
• Latency value too low. SRT targets need a latency window large enough to absorb the network's round-trip-time variance. Start at 200 ms and tune from there.
• Loopback failure on the same host. Same-host SRT loopback occasionally fails after a long uptime due to a Windows network-stack quirk. A reboot usually clears it before any deeper diagnosis is warranted.

NDI specifically:

• mDNS not resolving across subnets. NDI's discovery uses mDNS, which is link-local by default. Across subnets, configure NDI Access Manager with the destination IPs explicitly, or use NDI Discovery Server.
• Firewall blocking UDP. NDI uses a range of UDP ports for the actual stream. Open them on both endpoints.

Decklink specifically:

• Wrong device subindex. When multiple Decklink cards are installed, the target's device index addresses a specific port. Use Blackmagic Desktop Video Setup to confirm the index for the port you intend to use.
• Driver / firmware mismatch. Update Blackmagic Desktop Video to the latest version and reboot; firmware updates ship inside the driver installer.