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SSM / Unicast-Prefix-Based Multicast (RFC 3306)

The SSM tool builds and decodes unicast-prefix-based IPv6 multicast group addresses as defined by RFC 3306 and RFC 4607. These are the FF3x::/12 group addresses used by Source-Specific Multicast where the group identifier is partially derived from a unicast prefix owned by the operator. Embedding the unicast prefix in the multicast address removes the need for a separate group-ID coordination protocol — every operator can carve groups out of any unicast prefix they hold.

Address layout

|   8   | 4 | 4 |   8    |   8    |     64      |     32     |
+-------+---+---+--------+--------+-------------+------------+
| 0xFF  | 11RPT |  scope | reserv | plen | network prefix    | group ID |
+-------+-------+--------+--------+-------------+------------+
  • Byte 0 — 0xFF multicast prefix.
  • Byte 1 — flag nibble (0RPT) and 4-bit scope. SSM uses flags = 0x3 (P + T set) which yields the FF3x::/12 range. Embedded-RP groups (R + P + T = 0x7, FF7x::/12) are out of scope and routed to the separate embedded-RP tool.
  • Byte 2 — reserved, always 0x00.
  • Byte 3 — prefix length (0..64). Only the upper 64 bits of the unicast prefix fit in the multicast address, so RFC 3306 caps the prefix length at 64.
  • Bytes 4–11 — first 8 bytes of the unicast prefix (host bits zeroed).
  • Bytes 12–15 — 32-bit group ID, big-endian.

Worked example

The §6 example from RFC 3306 — global-scope SSM group derived from 2001:db8::/32 with group ID 0x12345678 — produces:

ff3e:20:2001:db8::1234:5678

The drawer round-trips this address: encode → decode → encode yields the same canonical lowercase form.

Tool drawer

Open the IPv6 tab and click SSM (RFC 3306), or follow the deep-link button on the multicast scope decoder when the input is recognised as a prefix-based group.

The drawer has two modes:

  • Encode — supply a unicast prefix in <ipv6>/<n> form (length 0..64), a scope (1..15; 14 / 0xE for global), and a 32-bit group ID (decimal or 0x-prefixed hex). The tool masks any host bits in the prefix automatically and emits the canonical group address.
  • Decode — supply an SSM group address. The tool reverses the encoding and reports the embedded unicast prefix, prefix length, scope, and 32-bit group ID. Any address outside FF3x::/12 (or with a flag nibble different from 0x3) is rejected.

Shareable URLs

All drawer inputs round-trip through ?ssm6_mode=…&ssm6_unicast_prefix=… &ssm6_scope=…&ssm6_group_id=…&ssm6_ipv6=… query parameters. The multicast scope decoder's "Open in SSM tool" deep-link uses this path.

REST API

POST /api/v1/ssm6 — see the OpenAPI spec for the full request / response schema. Two body shapes are accepted, distinguished by mode:

{ "mode": "encode", "unicast_prefix": "2001:db8::/32", "scope": 14, "group_id": 305419896 }

{ "mode": "decode", "ipv6": "FF3E:20:2001:db8::1234:5678" }

Both modes return the same response shape: address, scope, prefix_length, unicast_prefix, group_id. Decode mode additionally echoes the input string. Embedded-RP groups (FF7x::/12) are rejected with HTTP 400 — use the embedded-RP tool for those.

References

  • RFC 3306 — Unicast-Prefix-based IPv6 Multicast Addresses
  • RFC 4291 §2.7 — IPv6 multicast address layout and flags
  • RFC 4607 — Source-Specific Multicast for IP
  • RFC 7346 — IPv6 multicast scope value updates