- 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- Broadcast addressing has been replaced by multicast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:
- Receiver
driven tree creation
- Multicast
distribution tree
- 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:
- SM
or sparse mode
- DM
or dense mode
- SSM
or source specified mode
- 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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