MIMI Transport

Introduction This document describes an HTTP-based transport layer protocol for use with all MIMI sub-protocols. The MIMI transport protocol provides an envelope for for MIMI transaction types and takes care of message authentication. This document also describes the endpoints that are served by the individual MIMI functionalities.

Authentication The MIMI transport protocol uses the mutually authenticated mode of TLS to provide authentication for server-to-server communication. TODO: More information specific to how TLS should be used, i.e. mandate best practices that make sense in a mutually authenticated scenario that involves two WebPKI based certificates.

Framing The MIMI transport protocol uses a simple framing structure that includes the transaction payload, as well as a small header that signals the protocol version, as well as the transaction type contained in the envelope. ; } TransactionEnvelope; ]]>

Endpoint Discovery A messaging provider that wants to query the endpoint of another messaging provider first has to discover the fully qualified domain name under which Delivery Service of that provider can be reached. It does so by performing a GET request to provider.com/.well-known/mimi/ds-domain. provider.com could for example answer by providing the domain ds.provider.com (assuming that this is where it responds to the REST endpoints defined below).

REST Endpoints The following REST endpoints can then be used to access the different functionalities of the Delivery Service. As the Delivery Service relies on TLS encoded structs, all requests to endpoints described below should be marked as Content-type: application/octet-stream. All structs and concepts referred to below are defined in draft-robert-mimi-delivery-service, where their underlying functionality is defined in more detail.

Process Group Message This REST endpoint provides access to all operations associated with an existing MLS group on the Delivery Service such as delivering application messages, adding group members, removing group members, updating key material, etc. The payloads for this endpoint are generally provided (and signed) by a member of the corresponding group rather than the service provider of that member. The exact operation, as well as the target group ID is determined by the payload itself rather than an HTTP header, the path or any other query parameter.

Welcome Information Through this endpoint, a provider can obtain information required to join the group for clients that have already received a Welcome message. The DS responds with the group’s RatchetTree, as well as authentication information of existing group members.

External Commit Information Guest providers can use this endpoint to obtain information that allows a client to join a group without a Welcome message from an existing group member.

Verification Key This allows guest providers to obtain the verification key of this provider. This allows other providers to authenticate queries originating from this provider.

Deliver Connection Request This endpoint lets other providers deliver connection establishment request to clients of this provider.

Deliver Message An owning provider can deliver messages from one of its owned groups to this endpoint, if one of the group’s clients is associated with this provider.

Connection KeyPackage Retrieval Allows another provider to retrieve KeyPackages for use during the connection establishment process between two users.

Group KeyPackage Retrieval Allows another provider to retrieve KeyPackages that can be used to add another user or one of its clients to an existing group.

Rate-limiting The MIMI transport protocol itself doesn’t include any rate-limiting measures. However, traditional rate-limiting (e.g. based on sender IP) can be applied, as well as rate-limiting based on information in the message body such as Group ID (e.g. in the case of the /welcome_information endpoint) or group member (in the case of the /group_operation endpoint). More fine-grained rate-limiting can be applied through the use of the emerging Privacy Pass protocol (draft-ietf-privacypass-auth-scheme).

IANA Considerations IANA has created the following registries: * Transaction Types

Transaction Types An transaction type denotes the nature of a given payload in the context of the MIMI protocol. The transaction type is a string that is composed of substrings separated by dots. The first substring is "mimi", followed by the document that defines the corresponding transaction payload, which in turn is followed by the name of the transaction. The MIMI transaction specified in the MIMI DS document that signals the addition of a client would for example be denoted by the string ~~~ "mimi.delivery-service.add" ~~~