A multicast is similar to a broadcast in the sense that its target is a number of machines on a network, but not all. Where a broadcast is directed to all hosts on the network, a multicast is directed to a group of hosts. The hosts can choose whether they wish to participate in the multicast group often done with the Internet Group Management Protocol , whereas in a broadcast, all hosts are part of the broadcast group whether they like it or not!
As you are aware, each host on an Ethernet network has a unique MAC address, so here's the million dollar question: How do you talk to a group of hosts our multicast group , where each host has a different MAC address, and at the same time ensure that the other hosts, which are not part of the multicast group, don't process the information? You will soon know exactly how all this works. In order to explain Multicasting the best I can and to make it easier for you understand, I decided to break it down into 3 sections:.
The brief diagram below shows you the relationship between the 3 and how they complete the multicasting model:. When a computer joins a multicast group, it needs to be able to distinguish between normal unicasts which are packets directed to one computer or one MAC address and multicasts.
With hardware multicasting, the network card is configured, via its drivers, to watch out for particular MAC addresses in this case, multicast MAC addresses apart from its own.
When the network card picks up a packet which has a destination MAC that matches any of the multicast MAC addresses, it will pass it to the upper layers for further processing. Ethernet uses the low-order bit of the high-order octet to distinguish conventional unicast addresses from multicast addresses. To understand this, we need to analyse the destination MAC address of a unicast and multicast packet, so you can see what we are talking about:. The following picture is an example of my workstation When my gateway receives the packet, it knows it's a unicast packet as explained in the above picture.
Let's now have a look at the MAC address of a multicast packet. Keep in mind, a multicast packet is not directed to one host but a number of hosts, so the destination MAC address will not match the unique MAC address of any computer, but the computers which are part of the multicast group will recognise the destination MAC address and accept it for processing. The following multicast packet was sent from my NetWare server.
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Table 4 describes the significant fields in an IGMPv3 report message. Block of fields containing information regarding the sender's membership with a single multicast group on the interface from which the report was sent. IGMPv3 supports applications that explicitly signal sources from which they want to receive traffic. With IGMPv3, receivers signal membership to a multicast host group in the following two modes:. The default behavior for a Layer 2 switch is to forward all multicast traffic to every port that belongs to the destination LAN on the switch.
This behavior reduces the efficiency of the switch, whose purpose is to limit traffic to the ports that need to receive the data. RGMP is used on routed segments that contain only routers, such as in a collapsed backbone. The result is that, with CGMP, IP multicast traffic is delivered only to those Catalyst switch ports that are attached to interested receivers.
All other ports that have not explicitly requested the traffic will not receive it unless these ports are connected to a multicast router. Multicast router ports must receive every IP multicast data packet. When a host joins a multicast group part A in the figure , it multicasts an unsolicited IGMP membership report message to the target group The switch receives this CGMP join message and then adds the port to its content-addressable memory CAM table for that multicast group.
All subsequent traffic directed to this multicast group will be forwarded out the port for that host. The Layer 2 switches were designed so that several destination MAC addresses could be assigned to a single physical port. This allows switches to be connected in a hierarchy and also allows many multicast destination addresses to be forwarded out a single port. The router port also is added to the entry for the multicast group. Multicast routers must listen to all multicast traffic for every group because the IGMP control messages also are sent as multicast traffic.
When the switch hears the IGMP host report from a host for a particular multicast group, the switch adds the port number of the host to the associated multicast table entry. When the switch hears the IGMP leave group message from a host, the switch removes the table entry of the host.
Because IGMP control messages are sent as multicast packets, they are indistinguishable from multicast data at Layer 2. CGMP is a better option for low-end switches without special hardware. They both depend on IGMP control messages that are sent between the hosts and the routers to determine which switch ports are connected to interested receivers. Switched Ethernet backbone network segments typically consist of several routers connected to a switch without any hosts on that segment.
RGMP must be enabled on the routers and on the Layer 2 switches. A multicast router indicates that it is interested in receiving a data flow by sending an RGMP join message for a particular group part A in Figure The switch then adds the appropriate port to its forwarding table for that multicast group—similar to the way it handles a CGMP join message. IP multicast data flows will be forwarded only to the interested router ports part B in Figure When the router no longer is interested in that data flow, it sends an RGMP leave message and the switch removes the forwarding entry.
Multicast-capable routers create distribution trees that control the path that IP multicast traffic takes through the network in order to deliver traffic to all receivers. The two basic types of multicast distribution trees are source trees and shared trees, which are described in the following sections. The simplest form of a multicast distribution tree is a source tree with its root at the source and branches forming a spanning tree through the network to the receivers.
Because this tree uses the shortest path through the network, it is also referred to as a shortest path tree SPT. Figure 11 shows an example of an SPT for group Using this notation, the SPT for the example shown in Figure 11 would be The S, G notation implies that a separate SPT exists for each individual source sending to each group—which is correct.
For example, if Host B is also sending traffic to group Unlike source trees that have their root at the source, shared trees use a single common root placed at some chosen point in the network. This shared root is called a rendezvous point RP. Figure 12 shows a shared tree for the group This shared tree is unidirectional. Source traffic is sent towards the RP on a source tree. The traffic is then forwarded down the shared tree from the RP to reach all of the receivers unless the receiver is located between the source and the RP, in which case it will be serviced directly.
In this example, multicast traffic from the sources, Hosts A and D, travels to the root Router D and then down the shared tree to the two receivers, Hosts B and C. Both source trees and shared trees are loop-free. Messages are replicated only where the tree branches.
Members of multicast groups can join or leave at any time; therefore the distribution trees must be dynamically updated. When all the active receivers on a particular branch stop requesting the traffic for a particular multicast group, the routers prune that branch from the distribution tree and stop forwarding traffic down that branch.
If one receiver on that branch becomes active and requests the multicast traffic, the router will dynamically modify the distribution tree and start forwarding traffic again. Source trees have the advantage of creating the optimal path between the source and the receivers. This advantage guarantees the minimum amount of network latency for forwarding multicast traffic.
However, this optimization comes at a cost: The routers must maintain path information for each source. In a network that has thousands of sources and thousands of groups, this overhead can quickly become a resource issue on the routers. Memory consumption from the size of the multicast routing table is a factor that network designers must take into consideration. Shared trees have the advantage of requiring the minimum amount of state in each router. This advantage lowers the overall memory requirements for a network that only allows shared trees.
The disadvantage of shared trees is that under certain circumstances the paths between the source and receivers might not be the optimal paths, which might introduce some latency in packet delivery.
Network designers must carefully consider the placement of the rendezvous point RP when implementing a shared tree-only environment. In unicast routing, traffic is routed through the network along a single path from the source to the destination host.
A unicast router does not consider the source address; it considers only the destination address and how to forward the traffic toward that destination. The router scans through its routing table for the destination address and then forwards a single copy of the unicast packet out the correct interface in the direction of the destination. In multicast forwarding, the source is sending traffic to an arbitrary group of hosts that are represented by a multicast group address.
The multicast router must determine which direction is the upstream direction toward the source and which one is the downstream direction or directions. If there are multiple downstream paths, the router replicates the packet and forwards it down the appropriate downstream paths best unicast route metric —which is not necessarily all paths. Forwarding multicast traffic away from the source, rather than to the receiver, is called Reverse Path Forwarding RPF. RPF is described in the following section.
PIM uses the unicast routing information to create a distribution tree along the reverse path from the receivers towards the source. The multicast routers then forward packets along the distribution tree from the source to the receivers.
RPF is a key concept in multicast forwarding. If you have any questions feel free to leave a comment in our forum. Awesome Rene!!!
You are the only one who can teach in a very clear way. God Bless you. Hi , nice explanation. But I have a doubt. Then why do data streaming sites like youtube , netflix etc donot use multicast instead of using a vast amount of unicast addresses.
Is the lesser amount of multicast group addresses in Class D the reason? Kindly explain this to me. First of all, services like Netflix and Youtube are not streaming services. They are video on demand services. This means that they implicitly require unicast functionality. The video that you watch on Youtube is being watched only by you at that specific time and no one else.
Someone else may click on it a few seconds before or after you, but this is a uniquely separate event. Streaming services that are candidates for using multicast include online radio and the broadcasting of live events over the Internet. However there are two reasons w. Ask a question or join the discussion by visiting our Community Forum. Skip to content Search for: Search.
You are here: Home » Multicast. Lesson Contents. What about the Internet? Since multicast is so much more efficient than unicast, large companies like Netflix and Youtube must be using this to stream videos right?
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