NIP-27 Multicasting

27.md

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+NIP-27
+======
+
+Multicasting
+------------
+
+`draft` `optional` `author:cameri`
+
+A new tag `m` meaning `multicast group` allow for events to be addressed to a
+group of clients simultaneously. Multicast in this scenario can be one-to-many
+or many-to-many.
+
+A relay MUST forward events containing `m` tags strictly to clients subscribed
+with a tag filter `#m` containing at least one of the values of such tags and
+refrain from forwarding such events to clients without such filter. The value
+of an `m` tag is case-sensitive.
+
+A client SHOULD send events containing `m` tags only to relays that explicitly
+support NIP-27. A NIP-16 supporting client MAY test for the capability by
+subscribing twice to a relay with and without the tag filter `#m` respectively
+and sending an ephemeral event with a random `m` tag of equal value.
+
+## Events
+
+Clients targeting a multicast group MAY include an `m` tag with a value set to
+an arbitrary string representing the multicast group.
+
+Suppose that Alice wants to multicast an event to a very special multicast group
+that covers specific topics. The multicast group is identified by the following
+string: `00000000839a8e6886ab5951d76f411475428afc90947ee320161bbf18eb6048`
+
+Alice would construct the following message and sends it to her favorite relay
+which happens to support multicasting:
+
+```json
+[
+  "EVENT",
+  {
+    "id": "<id>",
+    "pubkey": "<Alice's pubkey>",
+    "created_at": 1231469640,
+    "content": "Let's get the conversation started",
+    "tags": [
+      ["m", "00000000839a8e6886ab5951d76f411475428afc90947ee320161bbf18eb6048"],
+    ],
+    "sig": "<sig>"
+  }
+]
+```
+
+## Subscriptions
+
+Clients subscribing to a multicast group MUST include an `#m` tag filter with
+an array of multicast groups as values.
+
+Suppose that Bob and Charlie both share Alice's interest and would like to stay
+up to date. Both would construct the following message to subscribe:
+
+```
+[
+  "REQ",
+  "<subscriptionId>",
+  {
+    "#m": ["00000000839a8e6886ab5951d76f411475428afc90947ee320161bbf18eb6048"]
+  }
+]
+```