Create NIP-91

91.md

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+NIP-91
+======
+
+Protocol Negotiation
+--------------------
+
+`draft` `optional`
+
+This NIP defines a method for clients to negotiate extension protocols with relays.
+
+## Protocol Names
+
+Each protocol has a **protocol name** that can uniquely identify the protocol. Currently the following formats are recommended:
+- `nip-XX-<description>`: For protocols standardized in NIPs. Experimental protocols are recommended to have a unique description to not conflict.
+- `<namespace>/<protocol>`: For custom protocols. The namespace should uniquely identify the standardizing body for this protocol with a DNS name. (example: `example.com/example-protocol`)
+- `x/<protocol>`: For custom protocols without a namespace.
+
+Any protocol name outside this list MUST NOT be used.
+
+## Protocol Offers
+
+The relay may send a protocol offer as a `PN-OFFER` message:
+```jsonc
+[
+  "PN-OFFER",
+  {
+    "example.com/example-protocol": {...},
+    "nip-XX-example-protocol": {...}
+  }
+]
+```
+
+The first entry MUST be an object, with the keys being protocol names, and the values being objects. The format of the object is up to the protocol to define.
+
+A protocol offer can be sent multiple times in a connection, such as to update availability based off of their [NIP-42](./42.md) authentication status.
+
+Clients SHOULD be able to handle protocol offers being able to be withdrawn. The removal of a protocol from a `PN-OFFER` does not affect existing active protocols, a `PN-SHUTDOWN` must be sent if that is required. 
+
+## Protocol Requests
+
+A client may request a protocol offered by the relay be enabled with the `PN-REQUEST` message:
+```jsonc
+[
+  "PN-REQUEST",
+  "example.com/example-protocol",
+  {...}
+]
+```
+
+The first entry MUST be the protocol name, and the second entry MUST be a protocol-dependent object for the protocol enable request.
+
+The relay SHOULD respond with a `PN-OK` or `PN-ERROR` message:
+```jsonc
+[
+  "PN-OK",
+  "example.com/example-protocol",
+  {...}
+]
+```
+```jsonc
+[
+  "PN-ERROR",
+  "example.com/example-protocol",
+  "improper protocol request data"
+]
+```
+
+In both messages, the first entry MUST be the protocol name. The second entry depends on the type:
+- `PN-OK`: Response data for the protocol negotiation. After this message, the protocol can be used.
+- `PN-ERROR`: An error message.
+
+An error message SHOULD start with a single-word prefix, followed by a colon (:) and space character, and a human readable message. The following types are defined:
+- `error`: An internal error has occurred in the relay.
+- `not-offered`: The protocol was not offered. This may happen due to a `PN-REQUEST` being sent at the same time as a new `PN-OFFER`.
+- `bad-data`: The client has sent improper protocol negotiation data.
+
+## Protocol Shutdown
+
+The relay or client may shut down the usage of the protocol any time with a `PN-SHUTDOWN` message:
+```jsonc
+[
+  "PN-SHUTDOWN",
+  "example.com/example-protocol"
+]
+```
+
+After this message, the sending party will no longer accept and send new protocol messages, and the receiver should not continue attempting to use the protocol functionality.