Multicast is communication between a single sender and multiple receivers on a network. or It is a technique to send a single message to multiple user using a single address (known as multicast address).Simple examples for Multicasting are sending an e-mail message to a mailing list,Teleconferencing ,videoconferencing or watching a video on YouTube.
What Is Multicast?
Multicast is a technology used to address multiple hosts as a group. A host multicasts to a group of hosts by sending an IP packet to a special IP address that identifies the group. An IP address is used to designate a group of hosts. The IP address that defines a multicast group is in the range of 224.0.0.0 to 239.255.255.255, defined as Class D addresses. Each group has its specific IP address in that range, so many groups can be defined at the same time by using a different IP address.The membership in a group is dynamic because hosts can join and leave the group as they want.
Multicast differs from broadcast because broadcasting involves sending a packet to all hosts, without exception, on the specified network, whereas multicast sends a packet to the group of hosts. Those hosts that are not part of the group will not process the multicast packet since it is not addressed to them.
Joining the Group :
To join a multicast group either by a user who wants to listen to a multimedia channel or by a specific application, it is actually very simple: the kernel has to be configured to process the IP multicast address of the group or channel. Then, all packets with an IP destination address as the group IP multicast address will be processed by the host and sent to the upper layer applications.
IGMP ??
But, if the multicast channel comes from far away (meaning that it comes from another network), it is possible that this channel is not currently multicasted on the local network of the host. In this case, the host will have to tell its neighbor router that it wants to listen to this multicast channel. This is done using the Internet Group Management Protocol (IGMP). Then this router will try to get that channel from its source and send the multicast packets of that channel to the local network. This entire process involves routing multicast over the larger network.
When a host joins a multicast group, it sends a report to the all-hosts group (224.0.0.1) on its local network. Routers can then learn who is joining which group. When a host is leaving a multicast group, it sends a report to the all-routers group (224.0.0.2) on its local network. Routers also send queries periodically to an all-hosts group address to request reports of group membership to all hosts on each multicast network to which it connects. In this way, a multicast router knows the membership of all groups for all multicast hosts.
Multicast Routing Protocols ??
Since multicast needs special processing for routers, the IETF defines multicast routing protocols to help routers control the routing of those multicast channels over the networks. The first one was Distance Vector Multicast Routing Protocol (DVMRP), which has been used a lot; however, the RFC (RFC1075) now has experimental status, which means that its implementation is not recommended. This multicast routing protocol was based on the Routing Information Protocol (RIP), and inherits its simplicity. Other protocols have been developed for multicast routing, like the Protocol Independent Multicast (PIM) protocol and Multicast extensions to OSPF (MOSPF).
Why Multicast ?
Why Multicast ?
The first and most well-known use of multicast is to save bandwidth when casting a video conference to a number of users. But multicast can do more.
Efficiency of Bandwidth Usage and Scaling
Multicast has been used since the beginning to save bandwidth, especially for any content that is for many users, for example, audio and video. Other examples are net news sent to many servers and software upgrades for a whole network sent to all hosts. All these examples save bandwidth by sending only one copy of the content whatever the number of clients, instead of one copy per client in the standard unicast way.
Discovering
Many discussions on IP are related to making it easier and autoconfigurable, and enabling devices to discover “automatically” servers, services, and the like. Examples of this are wireless and small devices that do not have permanent memory, and have to discover where they are, who they are, and which services are available. Multicast is very interesting, because hosts that need to discover things can multicast their request on specific channels in order to get more information from listening servers. This is actually being implemented in the Service Location Protocol (SLP). IPv6 uses multicast to get this autoconfiguration working.
Efficient Channel
Without multicast, the only way to send a packet to computers that have the same characteristics (like running a specific protocol) or are routers, was either to know the addresses of them, which is a very difficult task when the number is important, or to use broadcast. If broadcasts were used for all those purposes, then all computers would have to process those packets, even if the packet is not for them. Not only that, but broadcast is limited to the link-local, where multicast can be used over multiple networks. So multicast is an easy way to send information to unknown parties that share the same characteristics without disturbing others.
Many discussions on IP are related to making it easier and autoconfigurable, and enabling devices to discover “automatically” servers, services, and the like. Examples of this are wireless and small devices that do not have permanent memory, and have to discover where they are, who they are, and which services are available. Multicast is very interesting, because hosts that need to discover things can multicast their request on specific channels in order to get more information from listening servers. This is actually being implemented in the Service Location Protocol (SLP). IPv6 uses multicast to get this autoconfiguration working.
Efficient Channel
Without multicast, the only way to send a packet to computers that have the same characteristics (like running a specific protocol) or are routers, was either to know the addresses of them, which is a very difficult task when the number is important, or to use broadcast. If broadcasts were used for all those purposes, then all computers would have to process those packets, even if the packet is not for them. Not only that, but broadcast is limited to the link-local, where multicast can be used over multiple networks. So multicast is an easy way to send information to unknown parties that share the same characteristics without disturbing others.
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