Multicast Routing: Definition, Protocols, and Examples

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Multicast routing is a protocol that sends one copy of data to multiple users simultaneously on a closed network. The most popular uses of multicasting include videoconferencing and streaming media on organizational networks such as hotels or campuses.

Multicast routing has numerous other uses, though. Here’s a look at some of the most common ones, how it operates, the classifications of multicast routing, and the most well-known protocols and examples that solidify it as a timeless technique.

What is the purpose of multicasting?

The principal purpose Multicasting sends only one copy of a data packet to selective destinations so that only devices that request the information receive it. Before getting there, routing tables identify endpoints to ensure they have the correct address.

In contrast, unicasting sends one packet to one source, while broadcasting sends one packet to all sources simultaneously.

The purpose of this selective delivery is multifold. First, it optimizes loop-free topology, allowing the protocol to discover the shortest paths for data reception. Second, it only attempts to send data packets one time, hitting nodes again if they submit requests.

In general, multicast has low bandwidth requirements because it only sends one information stream. This makes it ideal for large quantities of data hitting several software and technologies, like media. 

There’s potential to increase or decrease bandwidth use, because not every recipient will make persistent requests if they miss the multicast. However, repeated attempts from a few users could increase traffic. 

Finally, multicasting is extremely scalable, as it makes data accessible to large numbers of people without the cumbersome nature of a broadcast. It should disperse traffic so it’s easy to oversee, preventing congestion and allowing room for support interference when needed.

How multicast routing works

Multicast routing begins by sending a select group of receivers the data, which they filter out to other necessary receivers like tree branches. That’s why these setups are often called multicast trees. 

It’s another way multicasting reduces latency and bandwidth because it doesn’t attempt to send more transmissions than it can handle. Diversifying the nodes responsible for dispersing the information helps receivers access data promptly.

Multicasting requires a server capable of handling multiple protocols. Otherwise, it won’t operate well on LANs or WANs. Protocol independent multicast (PIM) and internet group management protocol (IGMP) allow routers to fashion the tree branches and access data. IGMP informs multicast rendezvous points on how many recipients there are and where they are located.

Multicasting tree classification

Two multicast routing classifications demonstrate the protocol’s versatility: source-based tree (SBT) and group shared tree (GST). They also accentuate how multicast routing must work with additional protocols for purpose optimization.

Source-based tree (SBT)

In an SBT approach, the origin of the multicast routing creates a tree, and every group is directly traceable back to the starting source. At the same time, each node can continue to develop other branches because they each keep transmitting the source.

SBTs are annotated as source, group (S, G). They attempt to find the quickest route to as many receivers as possible, trimming the distance the data travels each time it’s transferred. Thus it’s also known as a shortest-path tree (SPT).

Group shared tree (GST)

GSTs send packets from multiple sources to converge at a rendezvous point (RP). The RP then forwards those packets down the shared tree.

GSTs are annotated as (*, G), using an asterisk (*) in place of the source, since the source may be any number of different origins beyond the RP.

Transmission can occur bidirectionally in this setup. Therefore, not every member has to be attached to the starting router or core.

How is multicast routing supported?

Multicast routing is not a default and may not be supported on every network. Where supported, users can toggle it on and off through network settings. You can configure your router to support any multicast routing protocol. The host is then assigned an IP address that receivers downstream can easily identify. 

It uses a multicast forwarding information base (MFIB) and sends information to the routing information base (MRIB). These keep communications open with other protocols and track data transmission speed and bytes.

Top 3 multicast routing protocols

Several routing protocols enhance multicasting capabilities, but the most widely used are dense mode, sparse mode, and reverse path forwarding.

Dense mode

In dense mode, the source will distribute the data to every subnet included in the multicast network. This is also called flooding because it sends packets with less discretion than other protocol variants. 

Not every router has to accept the package, though. Prune messages can notify the sending nodes to exclude them from receiving information, which helps trim unnecessary data emissions. 

PIM protocol can work in a dense mode multicast routing type because it creates as many short pathways to recipients as possible.

Sparse mode

Whereas dense mode mass-releases data to connected networks, sparse mode only takes the time to do so if there’s a request. 

Protocols like this reduce bandwidth usage and allow nodes joining the multicast group to receive data over time when they initiate a specific request. 

Like dense mode, there is also a PIM sparse model.

Reverse Path Forwarding

Reverse path forwarding implements a self-check so nodes can’t receive the same information they sent, because the sender won’t look at that interface. 

Though multicast routing optimizes for short paths, it can sometimes lead to loops when packets continually find other connected nodes. In such cases it can unintentionally waste resources by sending data back to the original sender. Reverse path forwarding allows for more intentional sending and receiving of data.

4 examples of multicasting

There are many uses of multicast routing, but some of the most common are media streaming on IPTV, videoconferencing, file distribution, and Internet of Things (IoT) devices.

It may be helpful to contextualize multicasting with these examples to understand its most relevant uses, as well as what sets it apart from other routing protocols. Even though unicasting or broadcasting could suffice to deliver media or mass amounts of data, it may not always be the optimal solution.

  • Streaming: This is one of the most widely used multicasting applications, allowing video or other media to be delivered to users across a corporate campus, school, or hotel property. Live streaming of events and concerts is also sometimes done via multicast.
  • Videoconferencing: From Zoom calls to mass video training sessions, multicasting allows many entrants to participate in the same event without overwhelming the server.
  • File distribution: Over-the-air patch management, operating system imaging, and even financial stock tickers can all be multicast over corporate networks.
  • IoT devices: The sensors embedded into IoT devices translate and send information to databases, AI, and countless other sources when it collects info. These systems may rely on multicasting to keep tabs on cybersecurity and ensure the correct recipients.

Multicasting could apply to large-scale professional training experiences or gaming events through virtual reality. The possibilities are continuing to expand as new technologies come into the fold, stretching the abilities of what and how multicasts can transmit experiences across networks.

It should be noted that content delivery networks (CDNs) such as Netflix, although sharing many characteristics with multicasting, are typically done via unicast. Although this may increase latency on the organization’s home network, it enables greater flexibility in dispersing media on demand over the internet.

Bottom line: Multicast routing protocol

Multicast routing is necessary for managing modern technological uses, like videoconferencing, and accepting data demand as it ramps up to new heights from remote workers to a more tech-dedicated younger generation. 

Routers must receive manual configuration to support it, but these measures reserve multicast routing to the most optimized situations, keeping digital resources flowing to necessary parties efficiently and safely. 

Keep your enterprise network running strong no matter what you need to do on it with one of the best enterprise Wi-Fi providers.

Devin Partida
Devin Partida
Devin Partida is a contributing writer for Enterprise Networking Planet who writes about business technology, cybersecurity, and innovation. Her work has been featured on Yahoo! Finance, Entrepreneur, Startups Magazine, and many other industry publications. She is also the Editor-in-Chief of ReHack.

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