Multicasting Basics

 

Multicasting Basics 

Multicasting is a communication method in which one sender transmits a packet to multiple receivers belonging to a multicast group. Instead of sending separate copies to every receiver, a multicast address identifies the group.

1. Multicast Addressing

  • A multicast address identifies a group of receivers, not a single host.
  • The source address is a normal unicast address identifying the sender.
  • The destination address is the multicast address identifying the group.
  • A host belonging to n multicast groups has one unicast address + n multicast addresses.


2. IPv4 Multicast Address

IPv4 multicast addresses belong to:

224.0.0.0 – 239.255.255.255

or

224.0.0.0/4

The first 4 bits identify the multicast block, while the remaining 28 bits identify the multicast group.

Some important blocks:

BlockPurpose
224.0.0.0/24    Local network control
224.0.1.0/24    Internetwork control
232.0.0.0/8    Source-Specific Multicast (SSM)
233.0.0.0/8    GLOP block
239.0.0.0/8    Administratively scoped multicast

3. Collecting Group Membership Information

A multicast router must know which multicast groups have members on each of its interfaces.

Unlike unicast routing, this information is not automatically known because:

  • Group membership is independent of network prefixes.
  • Hosts can join or leave groups dynamically.

The Internet Group Management Protocol (IGMP) is used to collect this information.

IGMPv3 has two main message types:

  • Query — router asks hosts about their multicast-group interests.
  • Report — hosts respond with information about the groups and sources they are interested in.


4. Multicast Forwarding

Multicast forwarding differs from unicast forwarding in two important ways:

Unicast:

  • Packet normally goes through one outgoing interface.
  • Decision depends mainly on the destination address.

Multicast:

  • Packet may need to be sent through multiple outgoing interfaces to reach all group members.
  • Forwarding depends on both source and destination addresses.




5. Multicast Routing Trees

Two basic approaches are used:

A. Source-Based Tree

  • A separate tree is created for each source–group combination.
  • If there are n sources and m groups, potentially m × n trees are required.

B. Group-Shared Tree

  • Each group has a core/rendezvous-point router.
  • Sources first send packets to the core using unicast.
  • The core then multicasts the packets to group members.
  • Only m trees are needed for m groups.

Simple Picture

                 Source
                   |
                   |
                Router
              /    |    \
             /     |     \
           Host   Host   Host
            \      |      /
             \     |     /
              Multicast
                Group

Summary

Multicasting = one-to-many communication using a multicast group address, with IGMP used to learn group membership and multicast routing used to construct forwarding trees and deliver packets efficiently.

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