Multicast Refresher

Multicast is a network communication method where one sender transmits data to multiple receivers simultaneously, but only to those that explicitly join the group. Multicast enables point-to-multipoint connections over Layer 3 networks utilizing IP addressing. 

Multicast sends traffic only to registered end-points that comprise a specific multicast group. This avoids flooding the entire network while still reaching numerous subscribers. 

It sits between Unicast (one to one) and Broadcast (one to everyone). 

Think of it as 1 → many (but only interested listeners).

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Multicast is used to optimize bandwidth and scalability:

• Avoids sending duplicate streams to each receiver
• Reduces load on servers and network links
• Ideal for real-time distribution

Common use cases:

• IPTV / video streaming
• Financial market data
• Software updates
• Live events

Multicast uses special IP ranges:

• IPv4: 224.0.0.0 – 239.255.255.255
• IPv6: ff00::/8
• See IANA links below for a deeper detailed view. 
• • e.g. SSM ("see Group Membership below") has a reserved part of the IP ranges there too (232.0.0.0/8 for IPv4)

Key idea:

• A multicast group address identifies a stream
• Receivers join or leave groups dynamically

Multicast Address Resources:

1. IANA: IPv4 Multicast Address Space
2. IANA: IPv6 Multicast Address Space

Hosts signal interest using:

• IGMP (Internet Group Management Protocol) → IPv4
• MLD (Multicast Listener Discovery) → IPv6

What happens:

1. Host sends join message
2. Router tracks group membership
3. Traffic is forwarded only where needed

Version Features:

IGMPv1/v2 and MLD: Hosts can only request to join a multicast group address and receive traffic from any source sending to that group. These do not have the capability to specify a particular sender or source; they only support Any-Source Multicast (ASM)

IGMPv3 and MLDv2: Allows hosts to specify "INCLUDE" (only receive from X sender(s)) or "EXCLUDE" (reject X sender(s)) source lists. These support Source-Specific Multicast (SSM), filtering specific sources and only receive data from desired senders.

Routers build distribution trees using multicast routing protocols like:

• PIM (Protocol Independent Multicast)

Two main modes:

• PIM Sparse Mode (PIM-SM) → most common, efficient for large networks
• PIM Dense Mode (PIM-DM) → floods then prunes (less common)

Goal:

Deliver traffic only to networks with active receivers.

Multicast uses:

• UDP (not TCP)

Implications:

• No built-in retransmissions
• No guaranteed delivery or ordering
• Applications and Networks must handle reliability if needed

Engineers should watch for:

• Network support (not all networks enable multicast)
• State scalability in routers
• Security (unauthorized joins, traffic leaks)
• Debugging complexity (harder than unicast)