The More Instant Messaging Interoperability (MIMI) working group will specify the minimal set of mechanisms required to make modern Internet messaging services interoperable. Over time, messaging services have achieved widespread use, their feature sets have broadened, and their adoption of end-to-end encryption (E2EE) has grown, but the lack of interoperability between these services continues to create a suboptimal user experience. The standards produced by the MIMI working group will allow for E2EE messaging services for both consumer and enterprise to interoperate without undermining the security guarantees that they provide. The working group will aim to achieve the strongest usable security and privacy properties for each targeted functional requirement.

Recognizing the need for a standardized security protocol to support group key establishment, authentication, and confidentiality services for messaging, the IETF has specified the Messaging Layer Security (MLS) protocol [I-D.ietf-mls-protocol] and architecture [I-D.ietf-mls-architecture]. MLS is agnostic to the identity system used within any given messaging service; it provides confidentiality of sessions once the participants in a conversation have been identified. To achieve interoperable messaging, the MIMI working group will specify how one or more identity building block technologies (for example, X.509 certificates or Verifiable Credentials) can be used to establish end-to-end cryptographic identity across messaging services, assuming the use of MLS for key establishment.

Interoperable messaging in federated environments requires consensus on a common delivery service and a transport protocol between federated domains. Specifically, the MLS protocol requires ordering of handshake messages within groups to ensure clients can synchronize despite asynchronous message delivery. This working group will specify a flexible solution for transport and delivery that takes into account typical requirements and best practices from the industry.

Achieving interoperable messaging among MIMI-compliant services requires a solution for the introduction problem, i.e., the ability for a user in one application to take an identity of a target user along with the associated application, be granted permission to initiate communications, and be able to establish communications with the target. The working group will specify a solution to the introduction problem, together with best practice recommendations for functionality, configuration options, and other aspects. The working group may also choose to specify a solution to discover the set of preferred messaging services associated with a given identity. Express and implied user preferences about discoverability and reachability must be respected.

Modern messaging services commonly support numerous features including plain and rich text, delivery notifications, read receipts, replies, reactions, presence, and many more. The working group will identify an extensible baseline set of messaging features and specify a content format to allow this feature set to be implemented interoperably. This format must be usable in the presence of E2EE. In defining the format, the working group will seek to reuse existing primitives (especially existing semantics) including previously defined message headers, MIME types, and URIs where practical.

In its initial phase, the working group will focus on solutions for messaging. The working group will aim for general-purpose designs fit for both 1:1 and multiparty messaging. The working group will not standardize new audio/video signaling or media protocols but may recommend the use of existing protocols and suites such as SIP and WebRTC.

The following are out of scope for the working group:

* Metadata processing to manage spam and abuse

* Interoperable mechanisms for group administration or moderation across systems

* Extensions to the MLS protocol. If needed, requirements will be referred to the MLS working group or other relevant working groups in the security area.

* Definition of completely new identity formats or protocols.

* Extensions to SIP, SDP, MSRP, or WebRTC.

* Development of anti-spam or anti-abuse algorithms.

* Oracle or look-up services that reveal the list of messaging services associated with a given user identity without the user's permission.

Numerous prior attempts have been made to address messaging interoperability, including the IETF's extensive prior work on XMPP, SIP/SIMPLE, and their related messaging formats. The MIMI working group will draw lessons from these prior attempts, seek to avoid re-hashing old debates, and will focus on the minimal standards suite necessary to facilitate interoperability given the feature set of modern messaging applications.

MIMI will communicate with related working groups as needed, including MLS, STIR, OAUTH, GNAP, and the W3C Verifiable Credentials WG.