In IPv4 networks there are different types of communications between the different hosts on the same network, usually Unicast traffic (traffic from one computer to another computer) is used for communication and data transfer, however, there is also traffic IP Multicast, or also known as multicast, whose objective is to only send information to those computers (clients) that are specifically configured to receive this network traffic. Today in this article we are going to explain in detail what multicast traffic is and why it is so important today in Internet TV.
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 source to a destination. We have a single sender and receiver, and it can be used for both sending and receiving 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 the rest of the connected users will automatically receive this communication. We have a special IP address that is 255.255.255.255 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 distributed throughout the world. Thanks to Anycast IPs, the same DNS server (8.8.8.8, 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 the 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 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.
IP addressing for multicast
The IP addressing of multicast traffic has a specific range, this range goes from IP address 224.0.0.0 to 239.255.255.255 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:
- 224.0.0.0 – 224.0.0.255 (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 224.0.0.0/8 range has been assigned to different applications over the years, or has just been reserved by the IANA. The range 223.0.0.0/8 is reserved for use with the SSM protocol. The 239.0.0.0/8 range is used for management use, this is where network operators provide IPTV services for television over the Internet. You can visit the document RFC3171 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 customers 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 the Movistar TV platform, all customers can receive the TV signal in their homes without any delay, pixelation or stoppages when there are thousands of connected customers 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 large bandwidth by not having to send it in duplicate or triplicate to the different recipients, only the users who need to see it will see it, without the need to saturate the network.
As you have seen, multicast traffic is used above all in IPTV services of the operators, so that everything works correctly it is essential that both the router and also 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.