SRT

Video: SRT Protocol Overview

Posted on by Russell Trafford-Jones

SRT’s ability to make lossy networks seem like perfect video circuits is increasingly well known, testified to by the SRT Alliance having just surpassed 400 member companies. But this isn’t your average ‘overview’, it dispenses with the technology introductions and goes straight into the detail so is ideal for people who already know the basics and want some deeper knowledge plus a look at the new features to come.

For those wanting an introduction, this article What is SRT? is a good starter which also links to two other intro videos. But today we’re going to join Haivision’s Maxim Sharabayko to look below the surface of SRT.

Maxim starts by introducing the open-source Git repository and the open-source integrations available before heading into the feature matrix. This shows what is and isn’t in SRT. We see that on top of ARQ, it has FEC, encryption, stream multiplexing and, soon, connection bonding. Addressing the major feature areas one by one, we start with connectivity.

SRT has two modes to establish a connection which Maixm shows on handshake diagrams. We can see that establishment need only take 2x round trips so is quick to establish. This allows Maxim to show how firewall traversal is accomplished, though NAT traversal is not yet implemented.

Next on the list of topics is access control whereby we need to ensure that only authorised users can gain access. This is achieved using the Stream ID field within SRT control packets which can contain up to 512 characters meaning it can be used to transfer usernames, passwords (in the form of keys) and requests. Maxim then explains the AES PSK encryption function and discusses the potential implementation of TLS and DTLS.

Content delivery is next under the magnifying glass starting with the structure of SRT packets and the difference between the two types: Data and Control, the former being restricted to only containing payload or FEC data. Maxim covers the positive acknowledgement which is contained with SRT with the range of received packets being acknowledged every 10ms and, where 64 packets come in less than 10ms, a low-overhead acknowledgement being sent for each group of 64 data packets. But of course, it’s the NAK packets which are the most important part of the protocol. Maxim explains they are able to send back one sequence number or a range of lost packets and talks about when they are sent. We see how this then fits into the Timestamp Based Packet Delivery (TSBPD) mechanism which itself is a feature of SRT which delivers packets to the receiver with the same timing as they arrived at the sender. The last thing we look at in the section is a worked example of Too-Late Packet Drop which explains when and why packets are dropped.

ARQ isn’t the only recovery mechanism in SRT, it also provides FEC and, soon, channel bonding. FEC’s can be useful but do have downsides which should be understood. There is a permanent bandwidth overhead, even when the circuit is working well, and a further latency is needed in order to generate the necessary recovery packets. Bonding allows you to stream the same stream over more than one circuit and use data from circuit B to fill in any gaps in circuit A, this technique is used in SMPTE ST 2022-7. Connection bonding, though, can also be used with multiple connections at once and having dynamic balancing across them. Maxim sums up the pros and cons of the different techniques in the table below.

Pros and cons of different packet recovery techniques. Source: Haivision

The talk finishes with a look at stream multiplexing, congestion control and ways in which you can use the SRT statistics which are constantly updated to manage your connectivity.

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Speakers

Maxim Sharabayko Maxim Sharabayko
Senior Software Developer,
Havision

Video: Demystifying Video Delivery Protocols

Posted on by Russell Trafford-Jones

Let’s face it, there are a lot of streaming protocols out there both for contribution and distribution. Internet ingest in RTMP is being displaced by RIST and SRT, whilst low-latency players such as CMAF and LL-HLS are vying for position as they try to oust HLS and DASH in existing services streaming to the viewer.

This panel, hosted by Jason Thibeault from the Streaming Video Alliance, talks about all these protocols and attempts to put each in context, both in the broadcast chain and in terms of its features. Two of the main contribution technologies are RIST and SRT which are both UDP-based protocols which implement a method of recovering lost packets whereby packets which are lost are re-requested from the sender. This results in a very high resilience to packet loss – ideal for internet deployments.

First, we hear about SRT from Maxim Sharabayko. He lists some of the 350 members of the SRT Alliance, a group of companies who are delivering SRT in their products and collaborating to ensure interoperability. Maxim explains that, based on the UDT protocol, it’s able to do live streaming for contribution as well as optimised file transfer. He also explains that it’s free for commercial use and can be found on github. SRT has been featured a number of times on The Broadcast Knowledge. For a deeper dive into SRT, have a look at videos such as this one, or the ones under the SRT tag.

Next Kieran Kunhya explains that RIST was a response to an industry request to have a vendor-neutral protocol for reliable delivery over the internet or other dedicated links. Not only does vendor-neutrality help remove reticence for users or vendors to adopt the technology, but interoperability is also a key benefit. Kieran calls out hitless switching across multiple ISPs and cellular. bonding as important features of RIST. For a summary of all of RIST’s features, read this article. For videos with a deeper dive, have a look at the RIST tag here on The Broadcast Knowledge.

Demystifying Video Delivery Protocols from Streaming Video Alliance on Vimeo.

Barry Owen represents WebRTC in this webinar, though Wowza deal with many protocols in their products. WebRTC’s big advantage is sub-second delivery which is not possible with either CMAF or LL-HLS. Whilst it’s heavily used for video conferencing, for which it was invented, there are a number of companies in the streaming space using this for delivery to the user because of it’s almost instantaneous delivery speed. Whilst a perfect rendition of the video isn’t guaranteed, unlike CMAF and LL-HLS, for auctions, gambling and interactive services, latency is always king. For contribution, Barry explains, the flexibility of being able to contribute from a browser can be enough to make this a compelling technology although it does bring with it quality/profile/codec restrictions.

Josh Pressnell and Ali C Begen talk about the protocols which are for delivery to the user. Josh explains how smoothstreaming has excited to leave the ground to DASH, CMAF and HLS. They discuss the lack of a true CENC – Common Encryption – mechanism leading to duplication of assets. Similarly, the discussion moves to the fact that many streaming services have to have duplicate assets due to target device support.

Looking ahead, the panel is buoyed by the promise of QUIC. There is concern that QUIC, the Google-invented protocol for HTTP delivery over UDP, is both under standardisation proceedings in the IETF and is also being modified by Google separately and at the same time. But the prospect of a UDP-style mode and the higher efficiency seems to instil hope across all the participants of the panel.

Watch now to hear all the details!
Speakers

Ali C. Begen Ali C. Begen
Technical Consultant, Comcast
Kieran Kunhya Kieran Kunhya
Founder & CEO, Open Broadcast Systems
Director, RIST Forum
Barry Owen Barry Owen
VP, Solutions Engineering
Wowza Media Systems
Joshua Pressnell Josh Pressnell
CTO,
Penthera Technologies
Maxim Sharabayko Maxim Sharabayko
Senior Software Developer,
Haivision
Jason Thibeault Moderator: Jason Thibeault
Executive Director,
Streaming Video Alliance

Video: Latency Still Sucks (and What You Can Do About It)

Posted on by Russell Trafford-Jones

The streaming industry is on an ever-evolving quest to reduce latency to bring it in line with, or beat linear broadcasts and to allow business models such as gaming to flourish. When streaming started, latency of a minute or more was not uncommon and whilst there are some simple ways to improve that, getting down to the latency of digital TV, approximately 5 seconds, is not without challenges. Whilst the target of 5 seconds works for many use cases, it’s still not enough for auctions, gambling or ‘gamification’ which need sub-second latency.

In this panel, Jason Thielbaut explores how to reduce latency with Casey Charvet from Gigcasters, Rob Roskin from CenturyLink and Haivision VP Engineering, Marc Cymontkowski. This wide-ranging discussion covers CDN caching, QUIC and HTTP/3, encoder settings, segmented Vs. non-segmented streaming, ingest and distribution protocols.

Key to the discussion is differentiating the ingest protocol from the distribution protocol. Often, just getting the content into the cloud quickly is enough to bring the latency into the customer’s budget. Marc from Haivision explains how SRT achieves low-latency contribution. SRT allows unreliable networks like the Internet to be used for reliable, encrypted video contribution. Created by Haivision and now an Open Source technology with an IETF draft spec, the alliance of SRT users continues to grow as the technology continues to develop and add features. SRT is a ‘re-request’ technology meaning it achieves its reliability by re-requesting from the encoder any data it missed. This is in contrast to TCP/IP which acknowledges every single packet of data and is sent missing data when acknowledgements aren’t received. Doing it the SRT, way really makes the protocol much more efficient and able to cope with real-time media. SRT can also encrypt all traffic which, when sending over the internet, is extremely important even if you’re not sending live-sports. In this video, Marc makes the point that SRT also recovers the timing of the stream so that the data comes out the SRT pipe in the same ‘shape’ as it went in. Particularly with VBR encoding, your decoder needs to receive the same peaks and troughs as the encoder created to save it having to buffer the input as much. All this included, SRT manages to deliver a transport latency of around 2.5 times the round trip time.

Haivision are members of RIST which is a similar technology to SRT. Marc explains that RIST is approaching the problem from a standards perspective; taking IETF RFCs and applying them to RTP. SRT took a more pragmatic way forward by creating a binary which implemented the features and by making this open source for interoperability.

The video finishes with a Q&A covering HTTP Header compression, recommended size of HLS chunks, peer-to-peer streaming and latency requirements for VoD.

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Speakers

Rob Roskin Rob Roskin
Principal Solutions Architect,
Level3 Communications
Marc Cymontkowski Marc Cymontkowski
VP Engineering – Cloud,
Haivision
Casey Charvet Casey Charvet
Managing Director,
Gigcasters
Jason Thibeault Jason Thibeault
Executive Director,
Streaming Video Alliance

Video: How to Build an SRT Streaming Flow from Encoder to Edge

Posted on by Russell Trafford-Jones

SRT is an enabler for contribution over the internet – whether point to point, or cloud egress/ingress. In recent weeks here on The Broadcast Knowledge we have seen different takes on how SRT, short for Secure Reliable Transport, and RIST can be used including from Open Broadcast Systems.

Here, Karel Boek, CEO of Raskenlund a Norwegian consultancy company for streaming, explains SRT and builds a workflow as a live demo showing how you can implement it quickly and easily. He starts by explaining where SRT sits and what it’s for. SRT makes contribution over the internet possible because it has a very light-touch away of recovering missing packets which are inevitable on internet links. Karel covers Haivision’s creation of SRT and the SRT Alliance that has grown out of that which now boast 350 members. The protocol being Open Source – and now an IETF Draft – means that a lot of companies have been happy to adopt the protocol. There are frequent plugfests, one has just concluded, where vendors test compatibility with the increasing set of features offered in SRT.

‘Secure’ is the ‘S’ in SRT’s name which is because the stream can be easily encrypted as part of the protocol. This is an important aspect in enabling sports and enterprise contribution in the cloud given the security that no-one can watch the feed before it gets to its destination.

‘Reliable’ is the key offer for SRT as that’s the number one problem with the internet and other networks where not all packets get delivered. TCP/IP is a great protocol on which most webpages are delivered. It’s fantastic for file delivery since every single packet gets acknowledged and there really isn’t any way that a file can get to the other end without being completely intact. Live streams can’t afford the overhead of counting in and counting out every packet so SRT’s ability to request only the missing packets is very important. It should be noted that this ability is also in Zixi, ARQ and RIST.

Karel compares SRT with other protocols including RTMP, MPEG-2 Transport Streams amongst others. He is careful to separate HLS, MPEG-DASH and WebRTC as ‘last-mile protocols’ in order to make a differentiation between those which content providers use to move video around as part of production and those which are used for distribution. RTMP’s use is still notable but diminishing particularly in Europe and the American markets. But the idea of MPEG-TS over UDP is still the best way to deliver within a building. Outside of the building, you would then want to protect it at least with FEC, with SMPTE 2022-7 or, better, with a protocol such as RIST or SRT. Karel mentions the details of the Simple Profile of RIST which was, by design, missing the features Karel notes are absent. We’ve heard here on The Broadcast Knowledge that these features have been delivered as planned in the Main Profile.

In the final part of this talk, Karel builds, live, an example workflow which combines both Wowza and SRTHub to create an end-to-end workflow. This is a great way of demonstrating how quickly you can create a workflow with SRT. There are plenty of SRT-enabled encoders and senders which is one of the ways we can judge the success of the SRT Alliance. Similarly whilst Haivision’s SRTHub is a useful product which brings things together in the cloud or on-prem, but Techex’s MWEdge and Videoflow’s DVG can do similar or more, each with their own advantages.

Overwell the takeaway from this talk from Raskenlund is that internet contribution is sorted, it’s now for your to choose how to do it and with whom. To that end, the talk ends with a Q&A from people wondering exactly that.

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Speaker

Karel Boek Karel Boek
CEO,
Raskenlund