Multicast is efficient but it doesn’t work “out of the box”. They typically use multicast for IPTV to deliver video to their own customers. The only place where you might see multicast on the Internet is your local ISP. These large video companies use LOTS of unicast traffic to deliver videos to their customers. What about the Internet? Since multicast is so much more efficient than unicast, large companies like Netflix and YouTube must use this to stream videos, right? Unfortunately, multicast on the Internet has never really been implemented. When using unicast, each additional host will increase the load and traffic rate. This reduces the load of the video server and network traffic in general. The switches and routers will only forward traffic to the hosts that want to receive it. The video server will only send the packets once. This time, we only have two hosts that are interested in receiving the video stream. Another issue with broadcast traffic is that routers do not forward broadcast traffic. The problem, however is that everyone in the broadcast domain will receive it…whether they like it or not. If our video server would broadcast its traffic, then the load on the video server will be reduced. Also, multicast only supports UDP traffic, so we can’t use the advantages of TCP, like windowing and acknowledgments. You will see that multicast requires some additional protocols to make it work. Are there any advantages? It’s simple since unicast works “out of the box”. The main problem with unicast traffic is that it is not scalable. What if there are 150 users that want to watch the movie? That’s when we start running into issues. What about the LAN on the left side? If these are Gigabit links, then a couple of hosts watching a movie will be no problem. When four more hosts would join on the right side, our WAN link would be completely saturated. Right now, we require 2x 6Mbps of bandwidth for H3 and H4. With four hosts, the video server will be streaming 4x 6Mbps = 24Mbps of traffic.Įach additional host that wants to receive this video stream will put more burden on the video server and require more bandwidth from the WAN link. When we are using unicast, the video server will send the packets to each individual host. The other two hosts are on another site that is connected through a 30 Mbit WAN link.Ī single HD video stream requires 6 Mbps of bandwidth. Let’s start with unicast:Ībove we have a small network with a video server that is streaming a movie and four hosts who want to watch the movie. You will see the advantages and disadvantages of each traffic type. Let’s imagine that we want to stream a high-definition video on the network using unicast, broadcast, or multicast. Why do you want to use multicast instead of unicast or broadcast? That’s best explained with an example. What if we want to send a message from one source to a group of receivers? That’s when we use multicast. If you want to send a message from one source to everyone, we use broadcast. We use unicast if you want to send a message from one source to one destination. There are three types of traffic that we can choose from for our networks: You will learn what multicast is, how it works, and the different protocols that are required.įirst of all, let’s talk about what multicast is… In this lesson, we’ll take a look at the basics of multicast.
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