Multicast Traffic: What It Is and Why It Matters

What is multicast traffic?

IP Multicast traffic, or also known as IP multicast, is a method of transmitting information to a group of receivers (clients) that are configured for this purpose. Computers that are not specifically configured will not receive this network traffic and will be able to send and receive other types of traffic. In IPv4 networks there are a total of four different types of communications that can be carried out, these are the following:

  • Unicast: It is the most common type of communication, the address is unicast, that is, from a specific origin to a destination. We have a single sender and receiver, and it can be used to both send and receive data. This type of communication is widely used, for example, for web browsing, file transfer via Samba or FTP, or almost any other type of communication. If we want to send the same information to several users, we will have to send the data once to each of the recipients.
  • Broadcast: this type of communication allows data to be sent to all users on the same local network. We can send a message to the broadcast IP address (which is the last IP address of a subnet) and automatically the rest of the connected users will receive this communication. We have a special IP address that is that represents a broadcast to the entire local network, this IP address is widely used when we send a DHCP Discovery message, to try to discover where the DHCP server is located on the network.
  • Anycast: this type of communication is one to many, however, the data is not transmitted to all receivers, it will only be sent to the closest ones. This method is used by DNS servers to balance the data traffic between the different servers that are spread all over the world. Thanks to Anycast IP, the same DNS server (, for example) can have this address both in Spain and in the US, so that the dynamic routing protocols will be in charge of sending the request to the closest DNS server.

Finally, we have the IP multicast traffic which is what we are going to talk about in detail now. Multicast traffic is specifically associated with a group of “clients” interested in receiving that network traffic, if it is not in the multicast group, they will not receive the information, this is ideal for not collapsing the networks, or having to send copies of all packages to all customers. Communication is done once from the source Unicast IP address to the chosen multicast IP address, regardless of how many clients there are at this multicast address, everyone in the group will receive the datagrams.

Unicast and multicast traffic is clearly different, for this reason, there are different protocols designed specifically for multicast traffic that can only be used with this type of traffic, and cannot be used with Unicast. Most of the existing application protocols that use Multicast make use of the UDP transport layer protocol. The reason for using UDP is because it is a non-connection-oriented protocol, that is, a prior “handshake” is not necessary to start sending the information, but datagrams can be sent directly from one source to various destinations, in addition, the headend is really small so network traffic is greatly optimized by not having an additional overhead due to headers.

Some very popular protocols that are used with multicast traffic is RTP (Real-time Transport Protocol), it is an application-level protocol that is responsible for transmitting information in real time, such as, for example, audio and video on a TV platform or in a video conference, this protocol is used in both multicast and unicast. It is also often used in conjunction with RTSP (Real-time Streaming Protocol) and RTCP (RTP Control Protocol) is also used.

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IP addressing for multicast

The IP addressing of multicast traffic has a specific range, this range goes from IP address to are intended to be explicit multicast addresses, this range is usually called class D. These IP addresses are not assigned to traditional unicast addresses, it is a reserved range, in addition, within the specified range there are other subranges that should not be used by all applications, these ranges are the following:

  • – (224.0.0 / 24) Local Network Control Block: This IP addressing range is commonly used by interior gateway routing protocols that make use of multicast communication, such as RIP or OSPF. This addressing is for local multicast only, so it should not be forwarded by routers.

The rest of the range has been assigned to different applications over the years, or has simply been reserved by the IANA. The range is reserved for use with the SSM protocol. The range is used for management use, this is where network operators provide IPTV services for television over the Internet. You can visit the RFC3171 document where you will find all the IP Multicast addressing ranges that exist and those that are reserved.

What is multicast traffic for?

Multicast traffic is widely used in pay TV services of the different fiber operators in Spain, for example, Movistar. Thanks to IP multicasting, the operator can offer high-quality video and audio streaming to all clients who hire it, the decoder will be listening on an IP address specifically from Multicast to receive all the information, and the router will receive the channels by consulting it. through the RIPv2 protocol. Thanks to Movistar’s TV platform, all clients can receive the TV signal in their homes without any delay, pixelation or stoppages when thousands of clients are online watching a football match.

Other uses that can be given to multicast traffic is for videoconferences, although it is not very common. Multicast traffic can also be used in CCTV systems, where video sequences captured by the same camera can be viewed and recorded efficiently, this allows us to save a great bandwidth by not having to send it in duplicate or triplicate to the different recipients, only users who need to see it will see it, without saturating the network.

As you have seen, multicast traffic is used above all in operators’ IPTV services, so that everything works correctly it is essential that both the router and the switches (if they exist) correctly manage this multicast traffic. The most important protocol for the correct management of multicast traffic is the so-called IGMP Snooping for IPv4 networks, and the MLD protocol for IPv6 networks, these protocols are responsible for only sending the traffic to the computers that are actually “listening” to this traffic, and not to all computers on the network, to avoid its collapse or slowness.

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