Quick answer
For the lowest delay use UDP multicast inside your own network (under a second) and SRT across the internet (about 0.5–3 seconds). RTMP is for pushing to YouTube or Facebook, and HLS reaches apps and browsers at scale but adds 10–30 seconds unless you use low-latency HLS.
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Why the output protocol matters
Every IPTV encoder has two jobs: compress, then deliver. Delivery depends on where the viewers are. A TV on the same hotel network, a studio in another country and a phone on 4G each need a different transport, and most encoders can send several at once.
Protocols compared
| Protocol | Best for | Works over internet? | Typical latency | Watch out for |
|---|---|---|---|---|
| UDP multicast | Many TVs on one private network | No | < 1 s | Needs IGMP snooping on switches |
| UDP / RTP unicast | One receiver, point to point | Poorly (no recovery) | < 1 s | Packet loss shows as glitches |
| SRT | Contribution over the internet | Yes | 0.5–3 s | Set latency ≈ 3–4× round-trip time |
| RTMP / RTMPS | Pushing to YouTube, Facebook, servers | Yes | 2–5 s | H.264 only on most platforms |
| RTSP | VMS, security systems, some players | Rarely | < 1 s | Firewall-unfriendly |
| HLS | Apps, browsers, phones at scale | Yes | 10–30 s (LL-HLS: 2–5 s) | Delay; needs a web server/CDN |
UDP multicast: the hotel and campus standard
With multicast the encoder sends one copy of each channel to a group address such as 239.10.10.1:1234. TVs and set-top boxes "join" the group they want, and switches with IGMP snooping forward the stream only to those ports. Ten or a thousand rooms, the channel crosses each link once.
- Use the 239.x.x.x (administratively scoped) range and document one address per channel.
- Set a TTL high enough to cross your routed VLANs (often 5–16), but keep it inside your network.
- Constant bitrate (CBR) keeps multicast traffic predictable.
SRT: reliable live video over the internet
SRT (Secure Reliable Transport) sends video over UDP but adds retransmission of lost packets, AES encryption and timing control. It is the modern choice for sending a feed from a venue to a studio, cloud server or IPTV headend across the public internet.
- Modes: caller (encoder dials out), listener (encoder waits) or rendezvous. Caller from the venue is easiest through firewalls.
- Latency setting: start at 3–4 times the round-trip time between the two ends (e.g. 120 ms RTT → 400–500 ms).
- Passphrase: always set one; it enables encryption.
RTMP: pushing to platforms
RTMP is old but still the common language of live platforms. Enter the platform's server URL and stream key in the encoder, and it pushes the stream. Use RTMPS (TLS) where supported. RTMP is for ingest — viewers do not watch RTMP directly.
HLS: reaching apps and browsers
HLS cuts the stream into short files listed in an .m3u8 playlist, served over normal HTTPS. It plays almost everywhere and scales through CDNs, which is why most OTT services use it. The trade-off is delay; low-latency HLS reduces it. If you have ever opened an .m3u8 link in VLC, you have used HLS — our M3U explainer covers the playlist side.
Quick decision guide
| You need to… | Use |
|---|---|
| Put channels on TVs in the same building | UDP multicast |
| Send a live feed from a venue to a studio or cloud | SRT |
| Go live on YouTube, Facebook or Twitch | RTMP / RTMPS |
| Let phones and browsers watch anywhere | HLS (via a server or CDN) |
| Feed a security or video-management system | RTSP |
Most real projects combine two: multicast inside the building plus SRT or RTMP to the outside world. When shopping, check the encoder can run those outputs at the same time — see the encoder buying checklist.
Frequently asked questions
What is an SRT encoder?
An encoder that can send its stream using SRT (Secure Reliable Transport), an open protocol that recovers lost packets and encrypts the stream, so live video survives the public internet with low delay.
Why can't I use UDP multicast over the internet?
Multicast is not routed across the public internet. It works inside networks you control (LAN, campus, operator network). For the internet use SRT, RTMP or HLS.
Is RTMP still worth using?
Yes, for pushing to platforms like YouTube, Facebook and Twitch, which still accept RTMP/RTMPS ingest. It is not a good choice for viewers to play directly.
Which protocol gives the lowest latency?
UDP multicast on a LAN (well under a second), then SRT (typically 0.5–3 s depending on the latency buffer you set). Standard HLS adds 10–30 s; low-latency HLS brings that down to a few seconds.