Networking Technology: IP Multicasting
Suppose I am a service provider, and providing internet service to a network of 1000 end users. Now, I want to provide another new service lets say IPTV to the users on the same network. How could I do that? Shall I use broadcast so that all the users get IPTV service? How do I provide IPTV service to the only users who are interested for it? Shall I use unicasting to send to the interested receivers?
Well, broadcasting would be a good idea if all the users are interested to use the service and pay for that. What if all the users are not interested and ready to pay for it? Hence I won’t use broadcasting.
What about the unicasting? Yes I can provide the IPTV service to interested receivers using unicasting.In this case what I have to do is i need to send the traffic on one by one basis. If there are 100 users out of 1000 then I have to send a same packet 100 times.oops! Network congestion, bandwidth bottleneck!! So let me not use unicasting for this. What next? Is there any way which can intelligently replicates the single packet sent towards only the branch where interested receivers are connected to? Of course, we have multicasting technology which makes the network intelligent enough to replicate the packets only in the branch receivers are connected to. In this article, I will briefly explain the concepts of multicast.
Multicast technology supports IP networks as well as the VPN networks.
Let’s categorize different aspects in multicast:
- Multicast group addressing
- Multicast group management
- IP Multicast routing
- Multicast Group addressing:
Class –D IP addresses (224.0.0.1-239.255.255.254) are used in the IP multicasting except for certain IP address which are reserved for the
Special purpose. That is IP multicast packets are sent to the group address which is essentially a class-D IP address. What happens when the packets at Layer-2?well, at layer-2 first bit in the first octet of MAC address is set to 1 and last 23 bits are mapped from the group address and destination MAC id is created and encapsulated for example, if group address is 224.0.0.10 then group mac address will be 01:00:5e:00:00:10.
- Multicast group management:
This is very important aspect in multicast. This refers to the management of receivers which are interested to receive the traffic destined for a particular group. We can achieve this using IGMP (Internet group management protocol) protocol. This facilitates end users to join or leave a particular group any time. If a user wants to receive the multicast traffic from a particular group, then user has to send a IGMP join message to the nearest router. This router keeps track of the receivers which have joined to a particular group and send the multicast traffic to receivers when the traffic destined for that group reach to the router. How can the router come to know if any receivers still are interested to receive the traffic? What if all the receivers connected to that router leaves the groups and don’t want to receive multicast traffic any more? Router does this sending IGMP query messages to local area network after certain periodic interval.
If router doesn’t get any IGMP response from the host in certain interval then it confirms that there are no more receivers in the local network and removes track of receivers versus multicast group. Again, if any receiver wants to join to a particular group then they can send IGMP join messages.
- IP Multicast routing:
The predominant multicast routing protocol in use on the Internet today is Protocol Independent Multicast, or PIM. PIM operates in two basic modes: sparse mode and dense mode. In addition, PIM can operate in sparse-dense mode, with some multicast groups configured as dense mode and others configured as sparse mode. Ideally PIM dense mode is used when the receivers in the network are densely populated; otherwise PIM sparse mode is used.
The type of PIM used on the Internet is PIM sparse mode. PIM sparse mode is so accepted that when the simple term “PIM” is used in an Internet context, some form of sparse mode operation is assumed.
PIM dense mode implements the same flood-and-prune mechanism that DVMRP and other dense mode routing protocols employ. The main difference between
DVMRP and PIM dense mode is that PIM dense mode introduces the concept of protocol independence. PIM dense mode can use the routing table populated by any underlying unicast routing protocol to perform reverse-path-forwarding (RPF) checks.
These are the major characteristics of PIM sparse mode. Routers with downstream receivers join a PIM sparse-mode tree through explicit join message.PIM sparse-mode RPs are the routers where receivers meet sources. Senders announce their existence to one or more RPs, and receivers query RPs to find multicast sessions. Once receivers get content from sources through the RP, the last-hop router (the router closest to the receiver) can optionally remove the RP from the shared distribution tree (*, G), if the new source-based tree (S,G) is shorter. Receivers then get content directly from the source. This transitional aspect of PIM sparse mode from shared to source-based tree is one of the major attractions of PIM. This feature prevents overloading the RP or surrounding core links. There are related issues regarding source, RPs, and receivers when sparse mode multicast is used:
1. Sources must be able to send to all RPs.
2. RPs must all know each other.
3. Receivers must send explicit joins to a known RP.
4. Receivers initially need to know only one RP (they will be informed of others).
5. Receivers can explicitly prune themselves from a tree.
6. Receivers that never transition to a source-based tree are effectively running
7. CBT.
PIM sparse mode has standard features for all of these issues.
Well, that’s all for Multicast basics. I will cover the advance multicasting in easier way in the next article. Hope this helps!
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