What is Multicast?
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Multicast is a network delivery method that sends a single copy of data to many receivers at once. Instead of the source generating one stream per viewer, it sends one stream to a group address, and the network duplicates packets only where the paths to interested receivers diverge. For live video, that means bandwidth stays roughly flat whether 50 people are watching or 50,000.
In contrast, unicast is a one-to-one protocol that requires an individual copy of a data file be sent to each user. Unicast uses n times the amount of bandwidth as multicast, where n equals the number of users requesting the file.
In a multicast architecture, senders and receivers belonging to a group use a single destination IP address to communicate. The network, with multicast capabilities enabled on the routers and switches, is responsible for delivering packets from any sender in the group simultaneously to all the receivers.
Multicast has been part of IP networking since the 1980s — RFC 1112, the standard that defined how hosts support IP multicasting, was published in August 1989. It’s a proven technology and many enterprises continue to rely on it to efficiently distribute large amounts of data.
As enterprises move more internal communication to video, multicast eCDNs have become the standard way to carry live streams across corporate networks without rebuilding them.
How Multicast Works
When it comes to streaming video, the purpose of a multicast eCDN is to make video playable in a browser using the protocol. This process uses two software components to distribute video instead of using the HTTP transport layer.
A sender sits on the network to retrieve live HLS or DASH video streams from the video source, encapsulate them in a multicast transport mechanism, and send them out over the multicast-enabled network.
Viewing devices on the network, such as personal computers, host a receiver client that is capable of tapping into the multicast broadcast. When the video reaches the viewing device, the receiver unencapsulates it and makes it available to the video player in the browser.
Multicast vs. Unicast vs. Broadcast
The three IP delivery methods differ in one respect: who receives a copy.
| Unicast | Multicast | Broadcast | |
|---|---|---|---|
| Who receives it | One specific device | Only devices that joined the group | Every device on the subnet |
| Copies sent by source | One per viewer | One, total | One, total |
| Bandwidth as audience grows | Rises linearly | Effectively flat | Flat, but wasted on uninterested devices |
| Crosses routers | Yes | Yes, where configured | No |
| Typical use | Web pages, VOD | Live enterprise video, IPTV, market data | ARP, DHCP discovery |
Unicast is what your network does by default, and it is why a 10,000-person all-hands can saturate a corporate WAN: the source sends 10,000 separate streams. Broadcast avoids that duplication but is indiscriminate — it reaches every device on the segment whether or not anyone is watching, and it stops at the router. Multicast is the middle path: one stream from the source, duplicated by the network only where the paths to interested receivers diverge.
Multicast Addresses and Groups
Every multicast conversation happens on a group address rather than a device address. In IPv4, RFC 1112 reserved the range 224.0.0.0 to 239.255.255.255 for host groups — the addresses formerly known as class D. A few multicast addresses are permanently assigned: 224.0.0.1 always refers to the multicast group of all IP hosts on a subnet, and 224.0.0.0 is never assigned to any group.
A receiver does not connect to a sender. It tells its local router which multicast group it wants to join, using the Internet Group Management Protocol (IGMP). RFC 1112 describes IGMP as the protocol IP hosts use “to report their host group memberships to any immediately-neighboring multicast routers.” Those routers then build a delivery path back toward the source. When the last receiver on a segment leaves the multicast group, the router stops forwarding that multicast traffic to the segment entirely.
This is why multicast traffic behaves so differently from unicast at scale. The sender never learns how many receivers exist, and it never has to. Everything that makes IP multicast efficient happens in the network, between the routers, not at the source.
What Makes Multicast so Efficient
A multicast eCDN only sends traffic to subnets with active receivers (clients requesting the content). Therefore, in a very large network, the traffic does not traverse all network segments, but only the segments needed to ensure all group members receive the traffic.
In addition, multicast sends only one copy of the data on each network segment, and every multicast-enabled router and switch sends the traffic only to segments that have active multicast group members. Even if multiple receivers are on a segment, only one copy of the data is sent. All members receive and read that same copy.
Common Uses for Multicast
Multicast is used wherever the same data has to reach many endpoints at the same moment:
- Live enterprise video — all-hands meetings, executive broadcasts, town halls and training sessions delivered to an entire workforce over the corporate network.
- IPTV and digital signage — television distribution across campuses, hospitals, hotels and trading floors.
- Financial market data — exchange price feeds, where every subscriber must receive the same tick at the same instant.
- Software and OS deployment — imaging hundreds of machines from a single source.
- Service discovery — protocols such as mDNS and SSDP, which is how a laptop finds a printer on the local network.
The common thread is simultaneity. If viewers can arrive at different times and watch from different points in a file, a video caching or peer-to-peer approach may suit better. If everyone has to see the same frame at the same second, multicast is the most efficient way to do it.
Strengths and Weaknesses
Multicast is a robust delivery option for live streaming. As a network protocol it is highly deterministic, and its bandwidth cost stays effectively flat as audience size grows — which is what makes it well suited to large enterprises.
Despite being a fundamental part of IP networks for decades, multicast may require some configuration on your network routers and switches, as well as Wi-Fi network. But given its ubiquity, this configuration is well understood and not complex. Today, you must install client software on the receiving devices. Finally, users may experience slightly slower start times for video than they will with either video caching or peer-to-peer because it is based on a push architecture.
When to Use a Multicast eCDN
You might be asking yourself, “How do I know which eCDN is right for my enterprise?” Below are the prime use cases for a multicast eCDN.
- You have a multicast network: Your network needs to be multicast-enabled to use this eCDN. Not every network has multicast turned on, and that’s the real prerequisite — not a preference. If only part of your network is multicast-enabled, video caching or peer-to-peer can cover the rest.
- Live streaming events are a priority: Multicast is the most efficient way to stream live video to large audiences: one stream reaches every viewer, so bandwidth stays flat as your audience grows. Because it’s push-based, it’s built for scheduled, live events — not video on demand.
- Security is paramount: A multicast eCDN runs 100% behind your firewall, so you’re not opening your network to a cloud-based delivery path to stream internally. For security teams evaluating a streaming solution, that’s the difference between adding a new attack surface and not.
- You want network stability and predictability: A given stream is sent only once per network segment, no matter how many viewers are watching it. A 4 Mbps stream never uses more than 4 Mbps on any segment — in SD, HD or 4K, you can predict the load an event will put on your network before it happens, not after.
- You rely on Wi-Fi: Most commercial-grade access points support multicast, so you can configure your wireless network for it and predict bandwidth the same way you would on a wired segment. Peer-to-peer delivery, by contrast, can double bandwidth consumption over Wi-Fi.
Next-generation Multicast
Vbrick Multicast is a next-generation eCDN that overlays your existing network infrastructure to deliver high-quality, stable video to all your viewers. You can easily install our lightweight, software solution on existing infrastructure. So, you don’t need to upgrade your network or buy proprietary hardware.
Vbrick Multicast is the only vendor-neutral solution for HLS and DASH video. Once deployed, you can use it to distribute video from any and all streaming platforms in use in your enterprise. And because it is deployed 100% behind the firewall, you have total control over your eCDN environment. In addition, we’ve built-in security controls to prevent unauthorized access to your videos and technologies like our patented forward error correction to create an optimal viewer experience.
- Predictable, deterministic video delivery network
- Supports any HTML5 browser-based video player
- Only direct replacement for legacy solutions
- Secure: Deploys 100% behind the firewall with end-to-end video encryption
- Event simulations to silently test your network before live events
- Best eCDN for live video over Wi-Fi
- Patented forward error correction and bandwidth smoothing technologies
- High-availability with heartbeat monitoring
- Centralized management, monitoring and insightful event analytics
- DVR capability allows users to pause and time slip video playback
You can learn more about Vbrick Multicast by reading our solution brief (keep its existing link), or see how multicast compares with the other eCDN delivery methods in our guide to enterprise content delivery networks.