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4.8. Source-Specific Multicast

Source-Specific Multicast (SSM) is a simplified PIM-SM mode where receivers specify both group and source.

SSM Characteristics:

  • No RP Required: SSM doesn't use RPs or shared trees
  • Direct SPT: Receivers join source-specific trees directly
  • Address Range: IPv4 232.0.0.0/8, IPv6 FF3x::/32
  • IGMPv3/MLDv2: Requires IGMPv3 (IPv4) or MLDv2 (IPv6) for source specification

SSM Operation:

  1. Receiver uses IGMPv3 to specify (S,G) interest
  2. DR sends (S,G) Join directly towards source
  3. No (*,G) state created
  4. No Register messages needed
  5. Traffic flows on source-specific tree only

Benefits:

  • Simpler deployment (no RP configuration)
  • Better security (receiver specifies source)
  • Efficient forwarding (direct source tree)
note

For complete SSM specification and IGMPv3 integration, see Section 4.8 of RFC 7761 and RFC 4607.

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