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Saturday, August 24, 2013

Explain multicast routing?

- Multicast routing is also known as the IP multicast. 
- For sending the IP (internet protocol) data-grams to a group of receivers who are interested in receiving the data-grams, multicast routing is used.
- The data-grams are sent to all the receivers in just one transmission. 
Multicast routing has got a special use in the applications that require media streaming on private networks as well as internet. 
- Multicast routing is IP specific version. 
- A more general version is the multicast networking.
- Here, the multicast address blocks are especially reserved in IPv6 and IPv4. 
Broadcast addressing has been replaced by multicast addressing in IPv6. 
Broadcast addressing was used in IPv4. 
- RFC 1112 describes the multicast routing and in 1986 it was standardized. 

This technique is used for the following types of real – time communication over the IP infrastructure of the network:
Ø  Many – to – many
Ø  One – to – many

- It scales up to receiving population that is large enough and it does not require either knowledge regarding the receivers and the identity of the receivers. 
- Network infrastructure is used efficiently by the multicast efficiently and requires source sending packet to a large number of receivers only once. 
- The responsibility of the replication of the packet is of the nodes which are nothing but the routers and the network switches.
- The packet has to be replicated till it reaches the multiple receivers. 
- Also, it is important that the message is sent only once over the link.   
- UDP or the user data gram protocol is the mostly used protocol of low level. 
Even though if this protocol does not guarantees reliability i.e., the packets might get delivered or get lost. 
- There are other multicast protocols available that are reliable such as the PGM or the pragmatic general multicast. 

It has been developed for adding the following two things a top the IP multicast:
Ø  Retransmission and
Ø  Loss detection
The following 3 things are key elements of an IP multicast:
  1. Receiver driven tree creation
  2. Multicast distribution tree
  3. IP multicast group address
- The receivers and the sources use the last for sending as well as receiving the multicast messages. 
- The group address serves as the destination address of the data packets for the sources whereas it is used for informing the network whether or not the receivers want those packets.
- Receivers need a protocol for joining a group. 
- One most commonly used protocol for this purpose is the IGMP i.e., the internet group management protocol. 
- The multicast distribution trees are set up using this protocol. 
- Once a group has been joined by the receiver, the PIM (protocol independent multicast) protocol is used for constructing a multicast distribution tree for this group. 
- The multicast distribution trees set up with the help of this protocol are used for sending the multicast packets to the members of the multicast group. 

PIM can be implemented in any of the following variations:
  1. SM or sparse mode
  2. DM or dense mode
  3. SSM or source specified mode
  4. SDM or sparse – dense mode or bidirectional mode (bidir)

- Since 2006, the sparse mode is the most commonly used mode. 
- The last two variations are more scalable and simpler variations of PIM and are also popular. 
- An active source is not required for carrying out an IP multicast operation and knowing about the group’s receivers. 
- The receiver drives the construction of the IP multicast tree. 
- The network nodes which lie closer to receiver are responsible for initiating this construction.
- This multicast then scales to a receiver population that is large enough. 
- It is important for a multicast router to know which all multicast trees can be reached in the network. 
- Rather, it only requires knowledge of its downstream receivers. 
- This is how the multicast – addressed services can be scaled up. 

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