Monday, 10 June 2019

Introduction to Multicast



In this chapter we’ll see the basics of multicast. 

First let’s talk about what multicast is… well in simple words we can say answer multicast is sending a message from a single source to selected multiple destinations.


There are three types of traffic that we can choose from for our networks:

Unicast
Broadcast
Multicast


Unicast If we want to send a message from one source to one destination, we use unicast.

broadcast If we want to send a message from one source to everyone, we use broadcast.

multicast if  we want to send a message from one source to a group of receivers? Then we use multicast..

Why do we want to use multicast instead of unicast or broadcast? 

Multimedia using Unicast 









Application sends one copy of each packet to every client. Used when only few client need to access the application. If the message has to be sent to a large group, the same information has to be carried multiple time, even on the same link. Bandwidth usage is proportional to number of users. Router make individual routing decision based on each pair of source and destination.


Multimedia traffic using broadcast


Application sends only copy of each packet using a broadcast. All end hosts need to process even if they don’t want. Not recommended implantation for applications delivering data, voice, or video to multiple receivers.

Multimedia traffic using multicast


The most efficient solution – in between broadcast and unicast. Server sends one copy of each packet to a special address that represent multiple clients. Server send out a single data stream to multiple clients.

Advantage of multicast

It saves bandwidth and controls network traffic by forcing the network to replicate packets only when necessary. Reduce network bandwidth consumption and host processing. Control network traffic and reduce server and CPU loads. 


Multicast Components

first we use a designated range of IP address that is used for multicast traffic. We use the class D range for this: 224.0.0.0 to 239.255.255.255. These addresses are only used as destination addresses,but not as source addresses. The source IP address will be the device that is sending the multicast traffic.

we use IGMP ( internet group management protocol ) for hosts to tell the routers when and which multicast traffic they want to receive.

To help the switch figure out where to forward multicast traffic, we can use IGMP Snooping. 


we use a multicast routing protocol: why? we see in the next chapter.
  • DVMRP (Distance Vector Multicast Routing Protocol)
  • MOSPF (Multicast Open Shortest Path First)
  • PIM (Protocol Independent Multicast)
how multicast work ?

step 1- Multicast server application configured with layer 3 address (class- D)
step 2- Multicast application installed on all the hosts.
step 3- Indicate router that thy want to receive multicast traffic for group (IGMP)
step 4- Multicast routing protocol forward multicast server. (PIM)
step 5- Calculate layer 2 multicast MAC address (IGMP Snoping/ CGMP)

This is basic of multicast, 
Thanks for reading 









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