| Internet-Draft | MIMI Content | March 2024 |
| Mahy | Expires 5 September 2024 | [Page] |
This document describes content semantics common in Instant Messaging (IM) systems and describes a profile suitable for instant messaging interoperability of messages end-to-end encrypted inside the MLS (Message Layer Security) Protocol.¶
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.¶
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.¶
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This Internet-Draft will expire on 5 September 2024.¶
Copyright (c) 2024 IETF Trust and the persons identified as the document authors. All rights reserved.¶
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
The terms MLS client, MLS group, and KeyPackage have the same meanings as in the MLS protocol [RFC9420]. Other relevant terminology may be found in [I-D.barnes-mimi-arch] and [I-D.ralston-mimi-terminology].¶
RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH. The source for this draft is maintained in GitHub. Suggested changes should be submitted as pull requests at https://github.com/ietf-wg-mimi/draft-ietf-mimi-content. Editorial changes can be managed in GitHub, but any substantive change should be discussed on the MIMI mailing list (mimi@ietf.org).¶
MLS [RFC9420] is a group key establishment protocol motivated by the desire for group chat with efficient end-to-end encryption. While one of the motivations of MLS is interoperable standards-based secure messaging, the MLS protocol does not define or prescribe any format for the encrypted "application messages" encoded by MLS. The development of MLS was strongly motivated by the needs of a number of Instant Messaging (IM) systems, which encrypt messages end-to-end using variations of the Double Ratchet protocol [DoubleRatchet].¶
End-to-end encrypted instant messaging was also a motivator for the Common Protocol for Instant Messaging (CPIM) [RFC3862], however the model used at the time assumed standalone encryption of each message using a protocol such as S/MIME [RFC8551] or PGP [RFC3156] to interoperate between IM protocols such as SIP [RFC3261] and XMPP [RFC6120]. For a variety of practical reasons, interoperable end-to-end encryption between IM systems was never deployed commercially.¶
There are now several instant messaging vendors implementing MLS, and the MIMI (More Instant Messaging Interoperability) Working Group is chartered to standardize an extensible interoperable messaging format for common features to be conveyed "inside" MLS application messages.¶
This document assumes that MLS clients advertise media types they support and can determine what media types are required to join a specific MLS group using the content advertisement extensions in Section 2.3 of [I-D.ietf-mls-extensions]. It allows implementations to define MLS groups with different media type requirements and allows MLS clients to send extended or proprietary messages that would be interpreted by some members of the group while assuring that an interoperable end-to-end encrypted baseline is available to all members, even when the group spans multiple systems or vendors.¶
Below is a list of some features commonly found in IM group chat systems:¶
IM systems have a number of types of identifiers. These are described in detail in [I-D.mahy-mimi-identity]. A few of these used in this document are:¶
This proposal relies on URIs for naming and identifiers. All the example use
the im: URI scheme (defined in [RFC3862]), but any instant messaging scheme
could be used.¶
Every MIMI content message has a message ID which is calculated from the
hash of the ciphertext of the message. When the content is end-to-encrypted
with MLS for a specific MLS group, the cipher suite for the group specifies a hash algorithm. The message ID is the first 32 octets of the hash of the MLSMessage struct using that hash algorithm.¶
As described in the the MIMI architecture {{?I-D.barnes-mimi-arch}}, one provider, called the hub, is responsible for ordering messages. The hub is also responsible for recording the time that any application message is accepted, and conveying it to any "follower" providers which receive messages from the group. It is represented as the whole number of milliseconds since the start of the UNIX epoch (01-Jan-1970 00:00:00 UTC). To the extent that the accepted timestamp is available to a MIMI client, the client can use it for fine grain sorting of messages into a consistent order.¶
This document also describes the semantics of a status report of other messages. Because some messaging systems deliver messages in batches and allow a user to mark several messages read at a time, the report format allows a single report to convey the read/delivered status of multiple messages (by message ID) within the same MLS group at a time.¶
Each MIMI Content message is a container format with two categories of information:¶
To focus on the semantics of a MIMI Content message, we use C/C++ struct notation to describe its data fields. These fields are numbered in curly braces for reference in the text. We do not propose any specific syntax for the format, but two reasonable constraints are:¶
typedef Octets[32] MessageId;
typedef uint64 Timestamp; // milliseconds since 01-Jan-1970
struct MimiContent {
MessageId replaces; // {1}
Octets topicId; // {2}
uint32 expires; // 0 = does not expire {3}
ReplyToInfo inReplyTo; // {4}
std::vector<MessageId> lastSeen; // {5}
NestablePart body; // {6}
};
¶
The replaces {1} data field indicates that the current message
is a replacement or update to a previous message whose message ID
is in the replaces data field. It is used to edit previously-sent
messages, delete previously-sent messages, and adjust reactions to
messages to which the client previously reacted.
If the replaces field is empty (i.e. both the message ID
localPart and the domain are zero length), the receiver
assumes that the current message has not identified any special
relationship with another previous message.¶
The topicId {2} data field indicates that the current message is
part of a logical grouping of messages which all share the same
value in the topicId data field. If the topicId is zero length,
there is no such grouping.¶
The expires {3} data field is a hint from the sender to the receiver
that the message should be locally deleted and disregarded at a specific
timestamp in the future. Indicate a message with no specific expiration
time with the value zero. The data field is an unsigned integer number of
seconds after the start of the UNIX epoch. Using an 32-bit unsigned
integer allows expiration dates until the year 2106. Note that
specifying an expiration time provides no assurance that the client
actually honors or can honor the expiration time, nor that the end user
didn't otherwise save the expiring message (ex: via a screenshot).¶
The inReplyTo {4} data field indicates that the current message is
a related continuation of another message sent in the same MLS group.
It contains the message ID of the referenced message and the SHA-256
hash [RFC6234] of its MimiContent structure. If the message field is
empty (i.e. both the message ID localPart and the domain are
zero length), the receiver assumes that the current message has not
identified any special relationship with another previous message;
in that case the hash-alg is none and the replyToHash is zero
length.¶
The inReplyTo hash is a message digest used to make sure that a MIMI
message cannot refer to a sequence of referred messages which refers
back to itself. When replying a client checks if the referenced message
is itself a Reply. It compares the hash¶
When receiving a message, the client verifies that the hash is correct. Next it checks if the referenced message is itself a Reply. If so, it continues following the referenced messages, checking that neither the messageId nor the hash of any of referenced messages indicates a Reply which "loops" back to a message later in the inReplyTo chain.¶
enum HashAlgorithm {
none = 0,
sha256 = 1
};
struct ReplyToInfo {
MessageId message;
HashAlgorithm hashAlg;
Octets replyToHash; // empty or hash of body.content
};
¶
Note that a inReplyTo
always references a specific message ID. Even if the original message
was edited several times, a reply always refers to a specific version
of that message, and SHOULD refer to the most current version at the
time the reply is sent.¶
The lastSeen {5} data field indicates the latest message the sender
was aware of in the group. It is a list of message ids.¶
If the sender recently joined the group and has not yet seen any messages, the list is empty.¶
If the sender identifies a single message as unambiguously the latest
message in the group, the lastSeen list contains a single message id
from that message.¶
Imagine however that two users (Bob and Cathy) see a message from Alice
offering free Hawaiian pizza, and reply at the same time. Bob and Cathy both send
messages with their lastSeen including a single message id (Alice's)
message about pizza. Their messages don't need to be replies or reactions.
Bob might just send a message saying he doesn't like pineapple on pizza.
Now Doug receives all these messages and replies
as well. Doug's message contains a lastSeen including the message id
list of both Bob's and Cathy's replies, effectively "merging" the order
of messages.¶
The next message after Doug's message contains a lastSeen containing
only the message id of Doug's message.¶
Every MIMI content message has a body {6} which can have multiple, possibly nested parts. A body with zero parts is permitted when deleting or unliking {7}. When there is a single body, its IANA media type, subtype, and parameters are included in the contentType field {8}.¶
typedef std::monostate NullPart; // {7}
struct SinglePart {
String contentType; // An IANA media type {8}
Octets content; // The actual content
};
struct ExternalPart {
String contentType; // An IANA media type {8}
String url; // A URL where the content can be fetched
uint32 expires; // 0 = does not expire
uint64 size; // size of content in octets
uint16 encAlg; // An IANA AEAD Algorithm number, or zero
Octets key; // AEAD key
Octets nonce; // AEAD nonce
Octets aad; // AEAD additional authentiation data
String description; // an optional text description
};
typedef std::vector<NestablePart> MultiParts;
enum PartSemantics { // {9}
nullPart = 0,
singlePart = 1, // the bodyParts is a single part
chooseOne = 2, // receiver picks exactly one part to process
singleUnit = 3 // receiver processes all parts as single unit
processAll = 4 // receiver processes all parts individually
};
enum Disposition {
unspecified = 0,
render = 1,
reaction = 2,
profile = 3,
inline = 4,
icon = 5,
attachment = 6,
session = 7,
preview = 8
};
struct NestablePart {
Disposition disposition; // {10}
String language; // {11}
uint16 partIndex; // {12}
PartSemantics partSemantics;
std::variant<NullPart,SinglePart,ExternalPart,MultiParts> part;
};
¶
With some types of message content, there are multiple media types associated with the same message which need to be rendered together, for example a rich-text message with an inline image. With other messages, there are multiple choices available for the same content, for example a choice among multiple languages, or between two different image formats. The relationship semantics among the parts is specified as an enumeration {9}.¶
The nullPart part semantic is used when there is no body part--for
deleting and unliking. The singlePart part semantic is used when
there is a single body part.¶
The chooseOne part semantic is roughly analogous to the semantics of the
multipart/alternative media type, except that the ordering of the
nested body parts is merely a preference of the sender. The receiver
can choose the body part among those provided according to its own
policy.¶
The singleUnit part semantic is roughly analogous to the semantics
of the multipart/related media type, in that all the nested body
parts at this level are part of a single entity (for example, a
rich text message with an inline image). If the receiver does not
understand even one of the nested parts at this level, the receiver
should not process any of them.¶
The processAll part semantic is roughly analogous to the semantics
of the multipart/mixed media type. The receiver should process as
many of the nested parts at this level as possible. For example, a
rich text document with a link, and a preview image of the link target
could be expressed using this semantic. Processing the preview image
is not strictly necessary for the correct rendering of the rich text
part.¶
The disposition {10} and language {11} of each part can be specified
for any part, including for nested parts. The disposition represents
the intended semantics of the body part or a set of nested parts.
It is inspired by the values in the Content-Disposition MIME header
[RFC2183].
The render disposition means that the content should be rendered
according to local policy. The inline dispositions means that the
content should be rendered "inline" directly in the chat interface.
The attachment disposition means that the content is intended to
be downloaded by the receiver instead of being rendered immediately.
The reaction disposition means that the content is a single
reaction to another message, typically an emoji, but which could be
an image, sound, or video. The reaction disposition was originally published
in [RFC9078], but was incorrectly placed in the Content Disposition
Parameters IANA registry instead of in the Content Disposition Values
registry.
The session disposition means that the content is a description of
a multimedia session, or a URI used to join one.
The preview disposition means that the content is a sender-generated
preview of something, such as the contents of a link.
The value of the language data field is an empty string or a
comma-separated list of one or more Language-tags as defined
in [RFC5646].¶
Each part also has an part index {12}, which is a zero-indexed, depth-first integer. It is used to efficiently refer to a specific body part (for example, an inline image) within another part. See {Nested body examples} for an example of how the part index is calculated.¶
It is common in Instant Messaging systems to reference external content via URI that will be processed automatically, either to store bulky content (ex: videos, images, recorded sounds) outside the the messaging infrastructure, or to access a specific service URI, for example, a media forwarding service for conferencing.¶
An ExternalPart is a convenient way to reference this content. It
provides a similar function to the message/external-body media type.
It optionally includes the size of the data in octets (or zero if
the length is not provided). It also includes an optional timestamp
after which the external content is invalid, expressed as seconds
since the start of the UNIX epoch (01-Jan-1970), or zero if the
content does not expire.¶
Typically, external content is encrypted with an ephemeral symmetric key before it is uploaded, and whatever is necessary for decryption is shared over the message channel.¶
It is a matter of local policy where the content is uploaded. Often in federated messaging systems, the sender of the content stores the external content in their own domain, but in some systems the content is stored in the "owning" or "hub" domain of the MLS group.¶
Before being uploaded, external content is encrypted with an
IANA-registered Authenticated Encryption with Additional Data (AEAD)
algorithm as described in [RFC5116]. The key, nonce, and additional
authenticated data (aad) values are set to the values used during the
encryption. Unless modified by an extension, the default value of the
aad is empty.¶
If the external URL is a service, the encAlg is set to zero, and the
key, nonce, and aad fields are zero length.¶
Implementations of this specification MUST implement the AES-128-GCM algorithm.¶
In addition to fields which are contained in a MIMI content message, there are also two fields which the implementation can definitely derive (the MLS group ID {13}, and the leaf index of the sender {14}). Many implementations could also determine one or more of: the sender's client identifier URL {15}, the user identifier URL of the credential associated with the sender {16}, and the identifier URL for the MIMI room {17}.¶
struct MessageDerivedValues {
MessageId messageId;
Timestamp hubAcceptedTimestamp;
Octets mlsGroupId; // value always available {13}
uint32 senderLeafIndex; // value always available {14}
IdUrl senderClientUrl; // {15}
IdUrl senderUserUrl; // "From" {16}
IdUrl roomUrl; // "To" {17}
};
¶
In the following examples, we assume that an MLS group is already established and that either out-of-band or using the MLS protocol or MLS extensions that the following is known to every member of the group:¶
Messages sent to an MLS group are delivered to every member of the group active during the epoch in which the message was sent.¶
For the sake of readability, all message IDs will be shown using only the first six octets of 32, for example: "\xe701beee59f9...".¶
In this example, Alice Smith sends a rich-text (Markdown) [RFC7763] message to the Engineering Team MLS group. The following values are derived from the client:¶
In addition, the messageId and timestamp are:¶
messageId = "\x28fd19857ad7..."; timestamp = 1644387225019; // 2022-02-08T22:13:45-00:00¶
Below are the relevant data fields set by the sender:¶
expires = 0;
body.disposition = render;
body.partIndex = 0;
body.contentType = "text/markdown;charset=utf-8";
body.content = "Hi everyone, we just shipped release 2.0." +
" __Good work__!";
¶
A reply message looks similar, but contains the message ID of the
original message in the inReplyTo data field. The derived MLS
group ID, URL, and name do not change in this example. The derived
senderClientId and senderLeafIndex are not especially relevant so
all but the user handle URL will be omitted.¶
The data fields needed:¶
inReplyTo.message: "\x28fd19857ad7...";
inReplyTo.hash-alg: sha256;
inReplyTo.replyToHash: "\xd3c14744d1791d02548232c23d35efa9" +
"\x7668174ba385af066011e43bd7e51501";
expires = 0;
body.disposition = render;
body.partIndex = 0;
body.contentType = "text/markdown;charset=utf-8";
body.content = "Right on! _Congratulations_ 'all!";
¶
A reaction looks like a reply, but uses the Disposition token of reaction. It is modeled on the reaction Content-Disposition token defined in [RFC9078]. Both indicate that the intended disposition of the contents of the message is a reaction.¶
The content in the sample message is a single Unicode heart character (U+2665). Discovering the range of characters each implementation could render as a reaction can occur out-of-band and is not within the scope of this proposal. However, an implementation which receives a reaction character string it does not recognize could render the reaction as a reply, possibly prefixing with a localized string such as "Reaction: ". Note that a reaction could theoretically even be another media type (ex: image, audio, or video), although not currently implemented in major instant messaging systems. Note that many systems allow mutiple independent reactions per sender.¶
inReplyTo.message: "\x28fd19857ad7...";
inReplyTo.hash-alg: sha256;
inReplyTo.replyToHash: "\xd3c14744d1791d02548232c23d35efa9" +
"\x7668174ba385af066011e43bd7e51501";
expires = 0;
body.disposition = reaction;
body.partIndex = 0;
body.contentType = "text/plain;charset=utf-8";
body.content = "\u2665"; \\ ♥
¶
In instant messaging systems and social media, a mention allows special formatting and behavior when a name, handle, or tag associated with a known group is encountered, often when prefixed with a commercial-at "@" character for mentions of users or a hash "#" character for groups or tags. A message which contains a mention may trigger distinct notifications on the IM client.¶
We can convey a mention by linking the user handle URI, or group URI in Markdown or HTML rich content. For example, a mention using Markdown is indicated below.¶
expires = 0;
body.disposition = render;
body.partIndex = 0;
body.contentType = "text/markdown;charset=utf-8";
body.content = "Kudos to [@Alice Smith](im:alice-smith@example.com)"
+ "for making the release happen!";
¶
The same mention using HTML [W3C.CR-html52-20170808] is indicated below.¶
body.contentType = "text/html;charset=utf-8";
body.content = "<p>Kudos to <a href='im:alice-smith@example.com'>" +
"@Alice Smith</a> for making the release happen!</p>"
¶
Unlike with email messages, it is common in IM systems to allow the sender of a message to edit or delete the message after the fact. Typically the message is replaced in the user interface of the receivers (even after the original message is read) but shows a visual indication that it has been edited.¶
The replaces data field includes the message ID of the message to
edit/replace. The message included in the body is a replacement for the message
with the replaced message ID.¶
Here Bob Jones corrects a typo in his original message:¶
replaces: "\xe701beee59f9..."; expires = 0; body.disposition = render; body.partIndex = 0; body.contentType = "text/markdown;charset=utf-8"; body.content = "Right on! _Congratulations_ y'all!";¶
Note that replies and reactions always refer to a specific message id, and therefore a specific "version" of a message, which could have been edited before and/or after the message id referenced in the reply or reaction. It is a matter of local policy how to render (if at all) a reaction to a subsequently edited message.¶
In IM systems, a delete means that the author of a specific message has retracted the message, regardless if other users have read the message or not. Typically a placeholder remains in the user interface showing that a message was deleted. Replies which reference a deleted message typically hide the quoted portion and reflect that the original message was deleted.¶
If Bob deleted his message instead of modifying it, we would represent it
using the replaces data field, and using an empty body (NullPart),
as shown below.¶
replaces: "\xe701beee59f9..."; expires = 0; body.disposition = render; body.partSemantics = nullPart; body.part = NullPart;¶
In most IM systems, not only is it possible to react to a message ("Like"), but it is possible to remove a previous reaction ("Unlike"). This can be accomplished by deleting the message which creates the original reaction¶
If Cathy removes her reaction, we would represent the removal using a
replaces data field with an empty body, referring to the message which
created the reaction, as shown below.¶
replaces: "\x1a771ca1d84f..."; expires = 0; body.disposition = reaction; body.partIndex = 0; body.partSemantics = nullPart; body.part = NullPart;¶
Expiring messages are designed to be deleted automatically by the receiving client at a certain time whether they have been read or not. As with manually deleted messages, there is no guarantee that an uncooperative client or a determined user will not save the content of the message, however most clients respect the convention.¶
The expires data field contains the timestamp when the message can be deleted.
The semantics of the header are that the message is automatically deleted
by the receiving clients at the indicated time without user interaction or
network connectivity necessary.¶
expires = 1644390004; // ~10 minutes later
body.disposition = render;
body.partIndex = 0;
body.contentType = "text/markdown;charset=utf-8";
body.content = "__*VPN GOING DOWN*__\n" +
"I'm rebooting the VPN in ten minutes unless anyone objects."
¶
An ExternalPart is a convenient way to present both "attachments" and (possibly inline rendered) content which is too large to be included in an MLS application message. The disposition data field is set to inline if the sender recommends inline rendering, or attachment if the sender intends the content to be downloaded or rendered separately.¶
body.disposition = attachment; body.expires = 0; body.contentType = "video/mp4"; body.URL = "https://example.com/storage/bigfile.mp4"; body.size = 708234961; body.encAlg = 0x0001; // AES-128-GCM body.key = "\x21399320958a6f4c745dde670d95e0d8"; body.nonce = "\xc86cf2c33f21527d1dd76f5b"; body.aad = ""; body.description = "2 hours of key signing video";¶
Other dispositions of external content are also possible, for example an external GIF animation of a rocket ship could be used with a reaction disposition.¶
Joining a conference via an external URL is possible. The link could be
rendered to the user, requiring a click. Alternatively the URL could be
rendered the
disposition could be specified as session which could be processed
differently by the client (for example, alerting the user or presenting
a dialog box).
Further discussion of calling and conferencing functionality is out-of-scope
of this document.¶
body.expires = 0; body.url = "https://example.com/join/12345"; body.description = "Join the Foo 118 conference"; body.expires = 1699671600; // 10-Nov-2023 19:00 UTC body.contentType = ""; // contentType not relevant body.size = 0; // no defined size body.encAlg = 0; // no encryption body.key = ""; body.nonce = ""; body.aad = "";¶
As popularized by the messaging application Slack, some messaging
applications have a notion of a Topic or message Thread
(not to be confused with message threading as used in email).
Clients beginning a new "topic" populate the topicId with a unique
opaque octet string. This could be the message ID of the first message
sent related to the topic. Subsequent messages may include the same
topicId for those messages to be associated with the same topic. The sort order
for messages within a thread uses the timestamp field. If more than
one message has the same timestamp, the lexically lowest message ID
sorts earlier.¶
In instant messaging systems, read receipts typically generate a distinct indicator for each message. In some systems, the number of users in a group who have read the message is subtly displayed and the list of users who read the message is available on further inspection.¶
Of course, Internet mail has support for read receipts as well, but the existing message disposition notification mechanism defined for email in [RFC8098] is completely inappropriate in this context:¶
Instead we would like to be able to include status changes about multiple messages in each report, the ability to mark a message delivered, then read, then unread, then expired for example.¶
The proposed format below, application/mimi-message-status is sent by one member of an MLS group to the entire group and can refer to multiple messages in that group. The format contains its own timestamp, and a list of message ID / status pairs. As the status at the recipient changes, the status can be updated in a subsequent notification.¶
enum MessageStatus {
unread = 0,
delivered = 1,
read = 2,
expired = 3,
deleted = 4,
hidden = 5,
error = 6
};
struct PerMessageStatus {
MessageId messageId;
MessageStatus status;
};
struct MessageStatusReport {
unit64 timestamp;
// a vector of message statuses in the same MLS group
std::vector<PerMessageStatus> statuses;
};
¶
timestamp = 1644284703227; statuses[0].messageId = "\x4dcab7711a77..."; statuses[0].status = read; statuses[1].messageId = "\x285f75c46430..."; statuses[1].status = read; statuses[2].messageId = "\xc5e0cd6140e6..."; statuses[2].status = unread; statuses[3].messageId = "\x5c95a4dfddab..."; statuses[3].status = expired;¶
As the MIMI Content container is just a container, the plain text or rich text messages sent inside, and any image or other formats needs to be specified. Clients compliant with MIMI MUST be able to receive the following media types:¶
Note that it is acceptable to render the contents of a received markdown document as plain text.¶
The following MIME types are RECOMMENDED:¶
Clients compliant with this specification must be able to download
ExternalParts with http and https URLs, and decrypt downloaded content
encrypted with the AES-128-GCM AEAD algorithm.¶
As most messaging systems are proprietary, standalone systems, it is useful to allow clients to send and receive proprietary formats among themselves. Using the functionality in the MIMI Content container, clients can send a message using the basic functionality described in this document AND a proprietary format for same-vendor clients simultaneously over the same group with end-to-end encryption. An example is given in the Appendix.¶
This document proposes registration of a media subtype with IANA.¶
Type name: application Subtype name: mimi-content Required parameters: none Optional parameters: none Encoding considerations: This message type should be encoded as binary data Security considerations: See Section A of RFC XXXX Interoperability considerations: See Section Y.Z of RFC XXXX Published specification: RFC XXXX Applications that use this media type: Instant Messaging Applications Fragment identifier considerations: N/A Additional information: Deprecated alias names for this type: N/A Magic number(s): N/A File extension(s): N/A Macintosh file type code(s): N/A Person & email address to contact for further information: IETF MIMI Working Group mimi@ietf.org¶
This document proposes registration of a media subtype with IANA.¶
Type name: application Subtype name: mimi-message-status Required parameters: none Optional parameters: none Encoding considerations: This message type should be encoded as binary data Security considerations: See Section A of RFC XXXX Interoperability considerations: See Section Y.Z of RFC XXXX Published specification: RFC XXXX Applications that use this media type: Instant Messaging Applications Fragment identifier considerations: N/A Additional information: Deprecated alias names for this type: N/A Magic number(s): N/A File extension(s): N/A Macintosh file type code(s): N/A Person & email address to contact for further information: IETF MIMI Working Group mimi@ietf.org¶
The following cases are examples of nonsensical values that most likely represent malicious messages. These should be logged and discarded.¶
sender of the message¶
message IDs¶
timestamps¶
inReplyTo¶
topicId¶
expires¶
lastSeen¶
body¶
For the avoidance of doubt, the following cases may be examples of legitimate use cases, and should not be considered the result of a malicious sender.¶
The timestamp is the time a message is accepted by the hub provider. As such, the hub provider can manipulate the timestamp, and the sending provider can delay sending messages selectively to cause the timestamp on a hub to be later.¶
TODO: Discuss how to sanity check lastSeen, timestamp and the MLS epoch and generation, and the limitations of this approach.¶
This document includes a mechanism where the sender can offer alternate versions of content in a single message. For example, the sender could send:¶
A malicious client could use this mechanism to send content that will appear different to a subset of the members of a group and possibly elicit an incorrect or misleading response.¶
Message as seen by Alice (manager) Xavier: Do you want me to reserve a room for the review meeting? Message as seen by Bob (Alice's assistant) Xavier: @Bob I need to pickup Alice's Ferarri keys. She'll confirm Reply sent by Alice Alice: Yes please.¶
Both Markdown and HTML support links. Using the example of an https link,
if the rendered text and the link target match exactly or are canonically
equivalent, there is no need for confirmation if the end user selects the link.¶
[example.com/foobar](https://example.com/foobar) [https://example.com/foobar](https://example.com/foobar) [https://example.com:443/foobar](https://example.com/foobar)¶
However, if the link text is different, or the scheme is downgraded from https to http, the user should be presented with an alert warning that the text is not the same.¶
[https://example.com/foobar](https://spearphishers.example/foobar) [https://example.com/foobar](http://example.com/foobar)¶
An IM URI link to a user who has a member client in the MLS group in which the message was sent is considered a mention. Clients may support special rendering of mentions instead of treating them like any other type of link. In Markdown and HTML, the display text portion of a link is considered a rendering hint from the sender to the receiver of the message. The receiver should use local policy to decide if the hint is an acceptable local representation of the user represented by the link itself. If the hint is not an acceptable representation, the client should instead display its canonical representation for the user.¶
For example, in the first examples, the sender expresses no preference about how to render the mention. In the second example, the sender requests that the mention is rendered as the literal URI. In the third example, the sender requests the canonical handle for Alice. In the fourth example, the sender requests Alice's first name.¶
<im:alice-smith@example.com> [im:alice-smith@example.com](im:alice-smith@example.com) [@AliceSmith](im:alice-smith@example.com) [Alice](im:alice-smith@example.com)¶
Note that in some clients, presence of a mention for the local user may result in a different notification policy.¶
If the client does not support special rendering of mentions, the application, should render the text like any other link.¶
Delivery and Read Receipts can provide useful information inside a group, or they can reveal sensitive private information. In many IM systems there is are per-group policies for and/or delivery read receipts:¶
In the first case, everyone in the group would have to claim to support read receipts to be in the group and agree to the policy of sending them whenever a message was read. A user who did not wish to send read receipts could review the policy (automatically or manually) and choose not to join the group. Of course, requiring read receipts is a cooperative effort just like using self-deleting messages. A malicious client could obviously read a message and not send a read receipt, or send a read receipt for a message that was never rendered. However, cooperating clients have a way to agree that they will send read receipts when a message is read in a specific group.¶
In the second case, sending a read receipt would be at the discretion of each receiver of the message (via local preferences).¶
Example of body needed to send this profile and a proprietary messaging protocol simultaneously.¶
body = new NestablePart(); body.disposition = render; body.language = ""; body.partIndex = 0; body.partSemantics = chooseOne; s = new SinglePart(); s.contentType = "application/mimi-content"; s.content = "\xabcdef0123456789...."; standardPart = new NestablePart() standardPart.disposition = render; standardPart.language = ""; standardPart.partIndex = 1; standardPart.partSemantics = singlePart; standardPart.part = s; p = new SinglePart(); p.contentType = "application/vnd.examplevendor-fancy-im-message"; p.content = "\x0123456789abcdef...."; proprietaryPart = new NestablePart() proprietaryPart.disposition = render; proprietaryPart.language = ""; proprietaryPart.partIndex = 2; proprietaryPart.partSemantics = singlePart; proprietaryPart.part = p; body.part = new MultiParts(); body.part.push(standardPart); body.part.push(proprietaryPart);¶
This shows sending a reaction with multiple separate emojis.¶
TBC¶
This example shows separate English and French versions of HTML message with inline images. Each of the images is presented in alternate formats: an animated GIF, and a single PNG.¶
TBC¶
In a heterogenous group of IM clients, it is often desirable to send more than one
media type as alternatives, such that IM clients have a choice of which media
type to render. For example, imagine an IM group containing a set of clients
which support a common video format and a subset which only support animated GIFs.
The sender could use a MultiParts NestablePart with chooseOne semantics
containing both media types. Every client in the group chat could render something
resembling the media sent. This is analogous to the multipart/alternative [RFC2046]
media type.¶
Likewise it is often desirable to send more than one media type intended to be
rendered together as in (for example a rich text document with embedded images),
which can be represented using a MultiParts NestablePart with processAll
semantics. This is analogous to the multipart/mixed [RFC2046] media type.¶
Some implementors complain that the multipart types are unnatural to use inside a binary protocol which requires explicit lengths such as MLS [RFC9420]. Concretely, an implementation has to scan through the entire content to construct a boundary token which is not contained in the content.¶
Note that there is a minor semantic difference between multipart/alternative and
MultiParts with chooseOne semantics. In multipart/alternative, the parts are
presented in preference order by the sender. With MultiParts the receiver
chooses its "best" format to render according to its own preferences.¶
render and inline dispositions¶