Internet-Draft MIMI problem outline July 2022
Mahy Expires 12 January 2023 [Page]
Intended Status:
R. Mahy

More Instant Messaging Interoperability (MIMI) problem outline


Instant Messaging interoperability requirements have changed dramatically since the IETF last activity in the space. This document presents an outline of problems that need to be addressed to make possible interoperability of modern Instant Messaging systems. The goal of this problem outline is to point at more detailed drafts which spawn discussion and eventually spur the IETF standards process.

Status of This Memo

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This Internet-Draft will expire on 12 January 2023.

Table of Contents

1. Overview

The IETF has been working on Instant Messaging Interoperability since the late 1990s. At the time, different groups within IETF proposed separate protocol suites (SIMPLE, APEX, and XMPP) because the community could not come to consensus on a single protocol (arguably due to a lack of consensus on the additional requirements which made these proposals unique). In the interests of interoperability, the IMPP Working Group developed a general framework for interoperability [RFC3860], and the Common Presence and Instant Messaging (CPIM) Message Format [RFC3862], an interoperability format that could pass through gateways among these protocols, even when end-to-end encrypted.

The CPIM model assumed standalone encryption of each message using a protocol such as S/MIME [RFC8551] or PGP [RFC3156]. This model was not widely adopted, but many Instant Messaging systems around this time frame began to add optional end-to-end encryption with OTR (Off The Record) [OTR], and eventually incorporated variants of the Double Ratchet protocol [DoubleRatchet], originally popularized by Signal.

Today, group chats are the norm, most modern instant messaging systems are end-to-end encrypted (many by default or always) using a variant of DoubleRatchet, and the typical feature set includes a plethora of features not included in CPIM: plain text and rich text messaging, delivery notifications, read receipts, replies, reactions, editing or deleting previously sent messages, and expiring messages. Almost all systems provide a way to share files/audio/videos, and many support calling and/or conferencing features (often using WebRTC). Some IM vendors are implementing MLS [I-D.ietf-mls-protocol], a group key establishment protocol motivated by the desire for group chat with efficient end-to-end encryption.

Unfortunately, federation of these IM systems is still rare and interoperability of the major IM systems in almost non-existent. It would be incredibly beneficial to provide interoperable best practices and solutions which IM vendors can incorporate into modern IM systems. Indeed, large customers and governments are already putting pressure on these IM vendors. The European Union's Digital Markets Act [DMA] is a recent dramatic example.

Instant Messaging interoperability requirements have changed dramatically since the IETF last activity in the space. This document presents an outline of problems that need to be addressed to make possible interoperability of modern Instant Messaging systems. The goal of this problem outline is to point at more detailed drafts which spur discussion and eventually spur the IETF standards process.

The larger goals of MIMI (More Instant Messaging Interoperability) are to start discussion; gather requirements common to many IM systems, focusing on the most immediate needs first; develop requirements and frameworks; and eventually to identify and evaluate existing solutions to these specific problems; and to assemble standards and technology which already largely exist into profiles and best current practices. Where special expertise in another Working Group or standards body is required, that work would be delegated to the specialty group.

2. Interoperability Problem Areas

2.1. Naming schemes

IM systems have a number of identifiers with different characteristics which are relevant for interoperability.

  • Domain identitifer
  • Handle identifier
  • User or Account identifier
  • Client or Device identifier
  • Group Chat or Channel identifier
  • Group, Conversation, or Session identifiers
  • Team or Workspace identifier

These identifiers are discussed in detail in [I-D.mahy-mimi-identity] as well as how they can be represented using URIs.

2.2. End-to-end IM Identity

The largest and most widely deployed Instant Messaging (IM) systems support end-to-end message encryption using a variant of the Double Ratchet protocol [DoubleRatchet] popularized by Signal and the companion X3DH [X3DH] key agreement protocol. Many vendors are also keen to support the Message Layer Security (MLS) protocol [I-D.ietf-mls-protocol] and architecture [I-D.ietf-mls-architecture]. These protocols provide confidentiality of sessions (with Double Ratchet) and groups (with MLS) once the participants in a conversation have been identified. However, the current state of most systems require the end user to manually verify key fingerprints or blindly trust their instant messaging service not to add and remove participants from their conversations. This problem is exacerbated when these systems federate or try to interoperate.

This problem space is explored in [I-D.mahy-mimi-identity].

2.3. Discovery

2.3.1. Domain-level service discovery

The discovery of IM services using DNS SRV records is described in [RFC3861].

2.3.2. User discovery and discovery of device keying material


One vendor mentioned a strawperson outline for user discovery:

  • well-known URL with query format on each domain

    • search string

      • could be handle, internal user ID, internal device ID; search by anonymous credential?
      • what type of key are you searching for?

        • keypackages
        • user keys
        • domain keys
    • searcher identity and proof of identity (optional)
    • rate limiting

2.4. Profiles of security protocols to facilitate interoperable end-to-end encryption

Enabling strong user privacy has been a core concern of the IETF for decades, and was the main motivation for the CPIM message format. S/MIME and PGP where proposed for use with instant messaging systems, but never widely adopted. The first broad adoption of end-to-end encryption in messaging was with Off The Record (OTR) [OTR] introduced in 2004, which also included perfect forward secrecy (which protects past communications from future compromises). As OTR was available in XMPP clients, it was possible to use across domains.

2.4.1. Protocols based on Double Ratchet

Signal introduced what is now know as the Double Ratchet protocol in 2013. Today there are over a dozen implementations of variations of Double Ratchet. While the differences among these variations tend to be small, there is little emphasis on interoperability.

Tens of instant messaging applications implement some form of end-to-end encryption using a protocol based on the Double Ratchet protocol. Double Ratchet was originally referred to as Axolotl Ratchet when it was introduced in 2013 and popularized in the Signal application. Most applications using Double Ratchet also use [X3DH] for initial key agreement. However the initial setup of encryption sessions among these applications are often incompatible.

Most implementations of Double Ratchet use a fixed ciphersuite and have no content negotiation mechanism.

2.4.2. Instant Messaging using Messaging Layer Security

Messaging Layer Security (MLS) [I-D.ietf-mls-protocol] is a group key agreement protocol with application message encryption and authentication. As described in the MLS architecture [I-D.ietf-mls-architecture], the protocol does not define the specific behavior of the Distribution Service (DS) or the Authentication Service (AS).

Some specific issues involving the use of MLS is a multi-domain Instant Messaging context are discussed in the MLS federation draft [I-D.ietf-mls-federation].

More documentation on the use of MLS in the Instant Messaging Context: (ex: long-lived persistent groups) would be invaluable.

2.5. Content negotiation

Content negotiation is protocol specific. In MLS, the requirements for content negotiation are discussed in the MLS architecture document [I-D.ietf-mls-architecture] and a specific content negotiation mechanism that can be used within MLS is described in [I-D.mahy-mls-content-neg].

2.6. Content format interoperability

The expectation of basic or common features in IM systems has grown. Below is a list of some features commonly found in most IM group chat systems:

  • plain text and rich text messaging
  • delivery notifications
  • read receipts
  • replies
  • reactions
  • edit or delete previously sent messages
  • expiring messages
  • knock / ping
  • shared files/audio/videos
  • calling / conferencing

Once messages are encrypted end-to-end there is no further opportunity for content negotiation. Exploring requirements, semantics, and an example common format for messages, which would allow proprietary messages or extensions to be delivered in parallel to the same users is described in [I-D.mahy-mimi-content]. It discusses all of the features above except for calling and conferencing.

2.7. Administrative setup of federation

(ex: agreement on certificates, contact information, abuse policies). TBC.

2.8. Authorization features

Is it necessary to standardize IM application authorization features such as moderation roles?

3. IANA Considerations

This document requires no action of IANA.

4. Security Consideration


5. Normative References

Mahy, R., "More Instant Messaging Interoperability (MIMI) message content", Work in Progress, Internet-Draft, draft-mahy-mimi-content-00, , <>.
Mahy, R., "More Instant Messaging Interoperability (MIMI) Identity Concepts", Work in Progress, Internet-Draft, draft-mahy-mimi-identity-00, , <>.
Peterson, J., "Address Resolution for Instant Messaging and Presence", RFC 3861, DOI 10.17487/RFC3861, , <>.

6. Informative References

Perrin, T. and M. Marlinspike, "The Double Ratchet Algorithm", , <>.
Beurdouche, B., Rescorla, E., Omara, E., Inguva, S., Kwon, A., and A. Duric, "The Messaging Layer Security (MLS) Architecture", Work in Progress, Internet-Draft, draft-ietf-mls-architecture-08, , <>.
Omara, E. and R. Robert, "The Messaging Layer Security (MLS) Federation", Work in Progress, Internet-Draft, draft-ietf-mls-federation-01, , <>.
Barnes, R., Beurdouche, B., Robert, R., Millican, J., Omara, E., and K. Cohn-Gordon, "The Messaging Layer Security (MLS) Protocol", Work in Progress, Internet-Draft, draft-ietf-mls-protocol-16, , <>.
Mahy, R., "Content Negotiation for Message Layer Security (MLS)", Work in Progress, Internet-Draft, draft-mahy-mls-content-neg-00, , <>.
Borisov, N., Goldberg, I., and E. Brewer, "Off-the-Record Communication, or, Why Not To Use PGP", , <>.
Elkins, M., Del Torto, D., Levien, R., and T. Roessler, "MIME Security with OpenPGP", RFC 3156, DOI 10.17487/RFC3156, , <>.
Peterson, J., "Common Profile for Instant Messaging (CPIM)", RFC 3860, DOI 10.17487/RFC3860, , <>.
Klyne, G. and D. Atkins, "Common Presence and Instant Messaging (CPIM): Message Format", RFC 3862, DOI 10.17487/RFC3862, , <>.
Schaad, J., Ramsdell, B., and S. Turner, "Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 4.0 Message Specification", RFC 8551, DOI 10.17487/RFC8551, , <>.
Marlinspike, M. and T. Perrin, "The X3DH Key Agreement Protocol", , <>.

Author's Address

Rohan Mahy