RTP for broadcasting-over-IP use-cases in GStreamer: PTP, RFC7273 for Ravenna, AES67, SMPTE 2022 & SMPTE 2110

It’s that time of year again where the broadcast industry gathers at NAB show, which seems like a good time to me to revisit some frequently asked questions about GStreamer‘s support for various broadcasting related standards

Even more so as at this year’s NAB there seems to be a lot of hype about the new SMPTE 2110 standard, which defines how to transport and synchronize live media streams over IP networks, and which fortunately (unlike many other attempts) is based on previously existing open standards like RTP.

While SMPTE 2110 is the new kid on the block here, there are various other standards based on similar technologies. There’s for example AES67 by the Audio Engineering Society for audio-only, Ravenna which is very similar, the slightly older SMPTE 2022 and VSF TR3/4 by the Video Services Forum.

Other standards, like MXF for storage of media (which is supported by GStreamer since years), are also important in the broadcasting world, but let’s ignore these other use-cases here for now and focus on streaming live media.

Media Transport

All of these standards depend on RTP in one way or another, use PTP or similar services for synchronization and are either fully (as in the case of AES67/Ravenna) supported by GStreamer already or at least to a big part, with the missing parts being just some extensions to existing code.

There’s not really much to say here about the actual media transport as GStreamer has had solid support for RTP for a very long time and has a very flexible and feature-rich RTP stack that includes support for many optional extensions to RTP and is successfully used for broadcasting scenarios, real-time communication (e.g. WebRTC and SIP) and live-video streaming as required by security cameras for example.

Over the last months and years, many new features have been added to GStreamer’s RTP stack for various use-cases and the code was further optimized, and thanks to all that the amount of work needed for new standards based on RTP, like the beforementioned ones, is rather limited. For AES67 no additional work was needed to support it, for example.

The biggest open issue for the broadcasting-related standards currently is the need of further optimizations for high-resolution, high-framerate streaming of video. In these cases we currently run into performance problems due to the high amount of packets per second, and some serious optimizations would be needed. However there are already various ideas how to improve this situation that are just waiting to be implemented.

Synchronization

I previously already wrote about PTP in GStreamer, which is supported in GStreamer for synchronizing media and that support is now merged and has been included since the 1.8 release. In addition to that NTP is also supported now since 1.8.

In theory other clocks could also be used in some scenarios, like clocks based on a GPS receiver, but that’s less common and not yet supported by GStreamer. Nonetheless all the infrastructure for supporting arbitrary clocks exists, so adding these when needed is not going to be much work.

Clock Signalling

One major new feature that was added in the last year, for the 1.10 release of GStreamer, was support for RFC7273. While support for PTP in theory allows you to synchronize media properly if both sender and receiver are using the same clock, what was missing before is a way to signal what this specific clock exactly is and what offsets have to be applied. This is where RFC7273 becomes useful, and why it is used as part of many of the standards mentioned before. It defines a common interface for specifying this information in the SDP, which commonly is used to describe how to set up an RTP session.

The support for that feature was merged for the 1.10 release and is readily available.

Help needed? Found bugs or missing features?

While the basics are all implemented in GStreamer, there are still various missing features for optional extensions of the before mentioned standards or even, in some cases, required parts of the standard. In addition to that some optimizations may still be required, depending on your use-case.

If you run into any problems with the existing code, or need further features for the various standards implemented, just drop me a mail.

GStreamer is already used in the broadcasting world in various areas, but let’s together make sure that GStreamer can easily be used as a batteries-included solution for broadcasting use-cases too.

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