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Introduction

Real-Time Messaging Protocol (RTMP) is a TCP-based protocol originally developed by Macromedia (later Adobe) for low-latency streaming of audio, video, and data. Though Flash is now deprecated, RTMP remains widely used in modern streaming pipelines — particularly as an ingest protocol for media servers and CDNs.

RTMP transmits multiplexed audio, video, control, and metadata over a single persistent TCP connection, making it suitable for live broadcasting and real-time applications.

Key Characteristics

TCP-based: Uses port 1935 by default; ensures reliable delivery.
Multiplexed streams: Combines audio, video, and metadata in one connection.
Low latency: Typically under 1 second in optimal conditions.
Chunk-based transmission: Splits messages into manageable chunks.
Persistent connection: Requires a continuous TCP session.
Metadata embedding: Supports AMF-encoded key/value data (e.g., onMetaData, onTextData).

Benefits and Limitations

Benefits

Low latency: Sub-second delay, ideal for live interaction.
Simple ingest: Widely used for sending live streams to CDNs and media servers.
Tooling support: Compatible with FFmpeg, GStreamer, OBS, and major platforms.
In-band metadata: Supports time-aligned metadata via AMF tags (e.g., onMetaData).
Reliable delivery: TCP ensures ordered, complete transmission of media data.

Limitations

Playback deprecated: No longer supported in modern browsers (Flash-dependent).
No built-in security: Use RTMPS (over TLS) for encryption.
TCP overhead: Less efficient in lossy or high-latency networks.
Codec limitations: Typically limited to H.264 and AAC; newer codecs (e.g., AV1, Opus) have limited support.

RidgeRun compatible products

RidgeRun products that support metadata streaming over RTMP include:

SEI: Metadata is embedded as SEI NAL units and carried within the video bitstream. Fully compatible with RTMP.
RTMP: To enable flexible metadata workflows over RTMP, RidgeRun extended the standard flvmux and flvdemux elements to support metadata injection and extraction

Usage Implications (RTMP+ Metadata)

Method	Description	Implications
RTMP + SEI	SEI is embedded in the H.264/H.265 bitstream and, when sent via RTMP inside FLV without transcoding, it’s typically preserved end-to-end and can be extracted with `seiextract`.	Benefits Simple integration: Works with `seiinject` and `seiextract` in GStreamer. TCP transport: NAT/firewall friendly. Limitations Limited player support: SEI often ignored unless explicitly parsed (e.g., via `seiextract`). Unencrypted: Use RTMPS for secure transmission.
RTMP + FLV (RTMP)	RidgeRun supports injecting session-level metadata into FLV streams over RTMP using fields like `meta-string`, `meta-binary`, and `GstFlvMeta`. Metadata is inserted at `flvmux` and extracted at the demuxer	Benefits In-band metadata: No side channel—metadata travels inside the FLV container. Simple integration: Works with `flvmux → rtmpsink` and `rtmpsrc → flvdemux`. Limitations Not frame-accurate: FLV metadata is session-level and not tightly coupled to individual frames. Use SEI or OBU for per-frame metadata. Unencrypted: Use RTMPS for secure transmission.

Examples

This section presents reference pipelines that demonstrate various combinations of metadata transmission and reception using RidgeRun’s products over the RTMP protocol. Each example typically includes a sender, a receiver, and the corresponding output, showcasing the complete end-to-end flow. The pipelines highlight different metadata injection and extraction methods, such as SEI and FLV (RTMP).

RTMP SERVER RTMP SERVER

Launch a local RTMP server using the command below. This allows you to push streams via rtmpsink and receive them via rtmpsrc, making it a common setup for local end-to-end testing of RTMP pipelines with metadata injection and extraction.

docker run --rm -it -p 1935:1935 -p 8080:80 --name rtmp alfg/nginx-rtmp

SEI

Sender

gst-launch-1.0 -v \
  videotestsrc is-live=true ! video/x-raw,framerate=30/1 ! \
  x264enc tune=zerolatency key-int-max=60 byte-stream=true ! \
  h264parse config-interval=-1 ! \
  seiinject metadata="HELLO WORLD" ! \
  h264parse config-interval=-1 ! video/x-h264,stream-format=avc,alignment=au ! \
  flvmux streamable=true ! \
  rtmpsink location="rtmp://127.0.0.1/live/test"

Receiver

GST_DEBUG=*seiextract*:MEMDUMP gst-launch-1.0 -v \
  rtmpsrc location="rtmp://localhost/live/mystream" do-timestamp=true ! \
  flvdemux ! h264parse ! seiextract ! fakesink silent=false

OutPut

0:00:03.001499949 83893 0x5ada291f8240 MEMDUMP           seiextract gstseiextract.c:299:gst_sei_extract_extract_h264_data:<seiextract0> ---------------------------------------------------------------------------
0:00:03.001505054 83893 0x5ada291f8240 MEMDUMP           seiextract gstseiextract.c:299:gst_sei_extract_extract_h264_data:<seiextract0> The extracted data is: 
0:00:03.001510345 83893 0x5ada291f8240 MEMDUMP           seiextract gstseiextract.c:299:gst_sei_extract_extract_h264_data:<seiextract0> 00000000: 48 45 4c 4c 4f 20 57 4f 52 4c 44 00              HELLO WORLD.    
0:00:03.001514089 83893 0x5ada291f8240 MEMDUMP           seiextract gstseiextract.c:299:gst_sei_extract_extract_h264_data:<seiextract0> ---------------------------------------------------------------------------

RTMP

Sender

gst-launch-1.0 -e \
  videotestsrc is-live=true pattern=smpte ! video/x-raw,framerate=30/1 ! \
  x264enc tune=zerolatency speed-preset=ultrafast key-int-max=60 byte-stream=false ! \
  video/x-h264,stream-format=avc,alignment=au ! \
  flvmux name=mux streamable=true meta-string="Hello World" ! \
  rtmpsink location="rtmp://127.0.0.1/live/test"

Receiver

GST_DEBUG=flv*:6,flvdemux:6,gstflv*:6 \
gst-launch-1.0 -m \
  rtmpsrc location="rtmp://127.0.0.1/live/test" ! \
  flvdemux attach-flvmeta=true flv-meta-signal=true name=d \
    d.video ! identity silent=false ! fakesink sync=false \
    d.audio ! fakesink sync=false

OutPut

0:00:13.553384696 731996 0x5598d3195920 DEBUG               flvdemux gstflvdemux.c:692:gst_flv_demux_clear_tags:<d> clearing taglist
0:00:13.553393214 731996 0x5598d3195920 DEBUG               flvdemux gstflvdemux.c:406:gst_flv_demux_parse_metadata_item:<d> tag name Server, tag type 2
0:00:13.553400465 731996 0x5598d3195920 DEBUG               flvdemux gstflvdemux.c:475:gst_flv_demux_parse_metadata_item:<d> Server => (string) Hello World

Documentation

Documentation

For more technical details and advanced usage, see the official RidgeRun wiki: GstRTMPMetadata Technical Wiki

References

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RidgeRun Metadata/Streaming Protocols/RTMP

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