How Multicast works

Multicast functionality is based on a group network model. This means that hosts that wish to receive data join a “multicast group” specific. Each multicast group has its own IP address, which is a special IP address in the range 224.0.0.0 to 239.255.255.255, known as a Class D IP address.

Once a host joins a multicast group, it has permission to receive any data sent to the group's IP address. When a host (or server) wants to send data, it sends the data to the group IP address, rather than sending it to a single IP address.

The routers and switches within the network are aware of these groups and are responsible for replicating the data and sending it to all the hosts that have joined that particular group.

Example

To illustrate this, let's say you have three hosts: A, B, and C. All three join multicast group 1, which has the IP address 224.0.0.1. Now, if you have a server that wants to send data to these three hosts, the server sends the data to 224.0.0.1.

The routers and switches on the network replicate this data and send it to A, B, and C. If a host D joins the group, it will automatically start receiving data sent to that IP address as well.

Multicast protocols

Multicast routing protocols are used to coordinate multicast traffic and ensure that data reaches all members of a group. The most common protocols include:

1. IGMP (Internet Group Management Protocol)

IGMP is a communications protocol used in IP networks to establish multicast group membership on a single network segment (a local network or LAN). It works between the host and the local router, allowing the host to inform the router that it wants to receive data sent to a specific multicast group address.

There are several versions of IGMP.

• • La version 1 only allows hosts to join groups.
  • La version 2 adds the ability for hosts to leave groups and also introduces a query mechanism for the router to determine which hosts still belong to a multicast group.
  • La version 3 adds the ability for hosts to indicate which specific source they would like to receive multicast traffic, known as “source-based multicast.”

2. PIM (Protocol Independent Multicast)

PIM is a routing protocol used to manage how packets are replicated across a network and sent to members of a multicast group.

Unlike other routing protocols, PIM is not used to choose a path through a network, but rather to construct distribution trees that define how packets are replicated and distributed to members of a multicast group.

There are two main types of PIM: PIM-SM (Sparse Mode) and PIM-DM (Dense Mode).

• • PIM-SM It is used in networks where multicast groups are dispersed and there are not many receivers.
  • PIM-DM It is used in networks where multicast receivers are densely packed.

3. DVMRP (Distance Vector Multicast Routing Protocol)

It is one of the oldest multicast routing protocols. It is based on the RIP (Routing Information Protocol) algorithm, which is a distance vector routing protocol.

DVMRP uses a technique known as traffic flooding, where multicast traffic is sent to all points on the network, and then routers drop traffic from networks that do not have members in the multicast group.

DVMRP constructs what is known as a spanning tree, which is a structure that defines how packets will be distributed across the network. Although DVMRP was one of the first multicast routing protocols and has been very influential, it is not as commonly used today due to the adoption of more modern and efficient protocols such as PIM.

Each of these protocols plays a unique role in facilitating multicast traffic, and will often work together to provide multicast services on a network.

For example, a host can use IGMP to inform a local router that it wants to join a multicast group, and then that router can use PIM to manage the distribution of packets to that host.

Multicast application in local and global networks

Multicast is commonly used in local networks (LANs). This is because most modern switches and routers on a LAN support multicast natively, and network administrators have complete control over the network, making multicast easier to implement and manage.

In a local network, multicast can be used for a variety of applications. These can include video or audio streaming, software distribution, systems updating, and many other applications that require simultaneous delivery of data to multiple hosts.

The use of multicast in a global network like the Internet is more complicated. Although the IP protocol supports multicast, not all routers on the Internet are configured to support it. This means that while you can send data to a group IP address, there is no guarantee that that data will reach all potential group members.

To overcome this limitation, techniques such as “IP Multicast over Unicast”, where multicast data is encapsulated in unicast packets for transport over the Internet. Although this can enable multicast data delivery over the Internet, it does not offer the same bandwidth efficiencies as native multicast.