Multicast General Info

How it works:

• Source app sends UDP multicast traffic to group destination address.
• Interested receivers join group address by signaling to routers on local area network.
• Routers communicate to build loop free path (tree) from sender to receivers.
• Portion of network without receivers will not receive that traffic for the group.
  • Saving resources – Advantage of multicast.

Use cases:

• IPTV
• Videoconferencing
• VoIP On hold music.
• Large scale data center replication.
• Real time applications like stock tickers.

Group Addressing:
– Layer 3 and Layer 2 addressing

Control Plane:
– IGMP
– PIM
– MSDP
– MBGP

Data Plane
– Reverse Path Forwarding (RPF)
– Multicast Routing Table (MRIB/MFIB)

Multicast Group:

• Address agreed upon by sender and receiver.
  • Source sends to destination address (group address)
  • Receivers are listening for traffic heading to that group.
• Traffic always sent to a group, not group to sender.
• Groups use layer 2 and layer 3 addressing.

IPv4 Addressing:

• 224.0.0.0/4
• Link Local
  • 224.0.0.0/24
• Source Specific Multicast
  • 232.0.0.0/8
• Administratively Scoped
  • 239.0.0.0/8

Layer 2 Addressing:

• IPv4 address maps to MAC address which is forwarded for LAN.
• Allows L2 switches forwarding in intelligent way.

Control Plane:

• Who is sending traffic and to where?
• Who is receiving traffic and for what groups?
• How traffic should be forwarded
  • Tree
• Built with combo of IGMP and PIM/MSDP

IGMP:

• Used for host to signal to routers over L2/LAN.
  • Tells router that host is part of specific group.
• Version 1
  • Message Type:
    • Host membership query
    • Host membership report
  • Report used to join group
  • Query used by router to see if members of the group still exist.
  • Replaced by version 2
• Version 2
  • Adds
    • Querier election
      • If multiple routers on LAN segment that are running Multicast
    • Timers
      • Adds timers that speed up timeouts.
    • Group specific queries
      • Query sent to group address instead of all multicast hosts on segment.
    • Explicit Leave
      • Speeds up convergence when no hosts are part of group any longer.
  • Backwards compatible.
• Version 3
  • Supports Source Specific Multicast
    • v1 and v2
      • (*,G)
    • v3
      • (S,G)
    • Receiver already knows the source sending multicast traffic.

Multicast Routing:

• PIM
  • Router to router communication.
  • Builds loop free tree.
  • Versions 1 and 2
  • Sparse mode and Dense mode
• Dense Mode
  • Implicit join
  • All traffic unless you say you don’t want it.
  • Uses Flood and Prune behavior.
• Sparse Mode
  • Considered explicit join.
  • No traffic unless ask for it.
  • Uses Rendezvous Point to process join requests.

Multicast Data Plane:

• Traffic begins to flow once tree is built.
• Data Plane check prior to traffic flow
  • Reverse Path Forwarding (RPF)
    • Was traffic received on correct interface?
  • Multicast Routing Table (MRIB/MFIB)
    • What interface shuld I forward the packets out?

RPF Check:

• PIM does not exchange topology info with routers.
  • Multicast traffic comes in, router looks at source IP address and incoming interface.
  • Normal Unicast CEF is checked for reverse path back to source.
  • Logic:
    • If incoming multicast interface == outgoing unicast interface, check has passed.
    • If incoming multicast interface DOES NOT == outgoing unicast interface, then packet is dropped because RPF failed.
  • Basically PIM uses the normal unicast topology/FIB/routing information to figure out there’s a loop free path/multicast tree.
    • Opposite of each other.