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Multicast Forwarding
Routers must know packet origination, rather than destination (opposite of unicast)
    ... origination IP address denotes known source
    ... destination IP address denotes unknown group of receivers
 Multicast routing utilizes Reverse Path Forwarding (RPF)
    ... Broadcast: floods packets out all interfaces except incoming from source; initially assuming every host on network is part of multicast group
    ... Prune: eliminates tree branches without multicast group members; cuts off transmission to LANs without interested receivers
    ... Selective Forwarding: requires its own integrated unicast routing protocol
Reverse Path Forwarding
Routers forward multicast datagrams received from incoming interface on distribution tree leading to source
Routers check the source IP address against their multicast routing tables (RPF check); ensure that the multicast datagram was received on the specified incoming interface
IANA Reserved Addresses
   IANA is the responsible Authority for the assignment of reserved class D addresses.  Other interesting reserved addresses are:
 224.0.0.2   - PIMv1   (ALL-ROUTERS - due to transport in IGMPv1)
 224.0.0.5   - OSPF ALL ROUTERS (RFC1583)
 224.0.0.6   - OSPF DESIGNATED ROUTERS (RFC1583)
 224.0.0.9   - RIP2   Routers
 224.0.0.13 - PIMv2
 224.0.1.39 - CISCO-RP-ANNOUNCE (Auto-RP)
 224.0.1.40 - CISCO-RP-DISCOVERY (Auto-RP)

“ftp://ftp.isi.edu/in-notes/iana/assignments/multicast-addresses” is the authoritative source for reserved multicast addresses.
 
Additional Information
   "Administratively Scoped IP Multicast", June 1997, has a good discussion on scoped addresses.  This document is available at:

       draft-ietf-mboned-admin-ip-space-03.txt
IANA Reserved Addresses
   IANA is the responsible Authority for the assignment of reserved class D addresses.  Other interesting reserved addresses are:
 224.0.0.2   - PIMv1   (ALL-ROUTERS - due to transport in IGMPv1)
 224.0.0.5   - OSPF ALL ROUTERS (RFC1583)
 224.0.0.6   - OSPF DESIGNATED ROUTERS (RFC1583)
 224.0.0.9   - RIP2   Routers
 224.0.0.13 - PIMv2
 224.0.1.39 - CISCO-RP-ANNOUNCE (Auto-RP)
 224.0.1.40 - CISCO-RP-DISCOVERY (Auto-RP)

“ftp://ftp.isi.edu/in-notes/iana/assignments/multicast-addresses” is the authoritative source for reserved multicast addresses.
 
Additional Information
   "Administratively Scoped IP Multicast", June 1997, has a good discussion on scoped addresses.  This document is available at:

       draft-ietf-mboned-admin-ip-space-03.txt
IANA Reserved Addresses
   IANA is the responsible Authority for the assignment of reserved class D addresses.  Other interesting reserved addresses are:
 224.0.0.2   - PIMv1   (ALL-ROUTERS - due to transport in IGMPv1)
 224.0.0.5   - OSPF ALL ROUTERS (RFC1583)
 224.0.0.6   - OSPF DESIGNATED ROUTERS (RFC1583)
 224.0.0.9   - RIP2   Routers
 224.0.0.13 - PIMv2
 224.0.1.39 - CISCO-RP-ANNOUNCE (Auto-RP)
 224.0.1.40 - CISCO-RP-DISCOVERY (Auto-RP)

“ftp://ftp.isi.edu/in-notes/iana/assignments/multicast-addresses” is the authoritative source for reserved multicast addresses.
 
Additional Information
   "Administratively Scoped IP Multicast", June 1997, has a good discussion on scoped addresses.  This document is available at:

       draft-ietf-mboned-admin-ip-space-03.txt
IGMP
   The primary purpose of IGMP is to permit hosts to commincate their desire to receive multicast traffic to the IP Multicast router(s) on the local network.  This, in turn, permits the IP Multicast router(s) to “Join” the specified multicast group and to begin forwarding the multicast traffic onto the network segment.

The initial specification for IGMP (v1) was documented in RFC 1112, Host Extensions for IP Multicasting.  Since that time, many problems and limitations with IGMPv1 have been discovered.  This has lead to the development of the IGMPv2 specification which was ratified in November, 1997 as RFC 2236.

Even before IGMPv2 had been ratified, work on the next generation of the IGMP protocol, IGMPv3, had already begun.  However, the IGMPv3 specification is still in the working stage and has not been implemented by any vendors.
IGMPv3 Example (future)
In this example, host “H1” has joined group 224.1.1.1 but only wishes to receive traffic from Source 1.1.1.1.  Using an as yet unspecified IGMPv3 mechanism, the host can inform the designated router, “R3”, that it is only interested in multicast traffic from Source 1.1.1.1 for Group 224.1.1.1.  Router “R3” could then potentially “prune” this specific (S,G) traffic source.