IP Multicast Service Models – Dense and Sparse Modes

IP Multicast Service Models Having Three Multicast Any-Source Multicast (ASM) Source-Filtered Multicast (SFM) and Source-Specific Multicast (SSM) which work as below.

Any-Source Multicast (ASM)

  • Destination ‘multicast’ address only defines ‘Group’ membership
  • Will accept from ‘any source’ (unicast)
  • First and oldest model, defined in RFC 1112

Source-Filtered Multicast (SFM)

  • Destination ‘multicast’ address only defines ‘Group’ membership
  • Adds capability to filter sources
  • Excluded List (mode), implies include everything else
  • Include List (mode), implies exclude everything else

Source-Specific Multicast (SSM)

  • Adds new concept of ‘Channel’ rather than Group
  • Source (unicast) and destination multicast Group address together define a single ‘Channel’ for membership
  • Like SFM Include List (mode) but with single source which is now significant for membership
  • (S,G) Channel is not the same as (S,G) Group
  • ASM and SFM receivers joining a Group receives from ALL sources unless filtered
  • (S,G) Group represents filtered source (include or exclude)
  • Multicast network may include all three models
  • ASM and SFM good for many-to-many interactions
  • SSM good for one-to-many interactions
  • Application may include two one-to-many, primary & Backup

Dense and Sparse Modes

  • Protocol Independent Multicasting Dense Mode (PIM-DM) uses a fairly simple approach to handle IP multicast routing.
  • The basic assumption behind PIM-DM is that the multicast packet stream has receivers at most locations.
  • An example of this might be a company presentation by the CEO or President of a company.
  • You opt out if you don’t want it
  • PIM Sparse Mode (PIM-SM) assumes relatively fewer receivers.
  • An example would be the initial orientation video for new employees.
  • You opt in if you want it Multicast Forwarding
  • Multicast Routing is backwards from Unicast Routing
  • Unicast Routing is concerned about where the packet is going or will need to go.
  • Multicast Routing is concerned about where the packet came from or will be coming from.
  • Multicast Routing uses “Reverse Path Forwarding” (RPF) and RPF Check
  • Which interface to expect multicast packet from source?
  • The interface would send unicast packet back to source or shared root!
  • Protects against multicast loops
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