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URI scheme for Matrix

This is a proposal of a URI scheme to identify Matrix resources in a wide range of applications (web, desktop, or mobile) both throughout Matrix software and (especially) outside it. It supersedes MSC455 in order to continue the discussion in the modern GFM style.

While Matrix has its own resource naming system that allows it to identify resources without resolving them, there is a common need to provide URIs to Matrix resources (e.g., rooms, users, PDUs) that could be transferred outside of Matrix and then resolved in a uniform way - matching URLs in World Wide Web.

Specific use cases include:

  1. Representation: as a Matrix user I want to refer to Matrix entities in the same way as for web pages, so that others could unambiguously identify the resource, regardless of the context or used medium to identify it to them (within or outside Matrix, e.g., in a web page or an email message).
  2. Inbound integration: as an author of Matrix software, I want to have a way to invoke my software from the operating environment to resolve a Matrix URI passed from another program. This is a case of, e.g., opening a Matrix client by clicking on a link from an email message.
  3. Outbound integration: as an author of Matrix software, I want to have a way to export identifiers of Matrix resources to non-Matrix environment so that they could be resolved in another time-place in a uniform way. An example of this case is the "Share via…" action in a mobile Matrix client.

Matrix identifiers as defined by the current specification have a form distinct enough from other identifiers to mostly fulfil the representation use case. Since they are not URIs, they can not cover the two integration use cases. https://matrix.to somehow compensates for this; however:

  • it requires a web browser to run JavaScript code that resolves identifiers (basically limiting first-class support to browser-based clients), and
  • it relies on matrix.to as an intermediary that provides that JavaScript code.

To cover the use cases above, the following scheme is proposed for Matrix URIs ([] enclose optional parts, {} enclose variables):

matrix:[//{authority}/]{type}/{id without sigil}[/{type}/{id without sigil}...][?{query}][#{fragment}]

with {type} defining the resource type (such as r, u or roomid - see the "Path" section in the proposal) and {query} containing additional hints or request details on the Matrix entity (see "Query" in the proposal). {authority} and {fragment} parts are reserved for future use; this proposal does not define them and implementations SHOULD ignore them for now.

This MSC does not introduce new Matrix entities, nor API endpoints - it merely defines a mapping between URIs with the scheme name matrix: and Matrix identifiers, as well as operations on them. The MSC should be sufficient to produce an implementation that would convert Matrix URIs to a series of CS API calls, entirely on the client side. It is recognised, however, that most of the URI processing logic can and should (eventually) be on the server side in order to facilitate adoption of Matrix URIs; further MSCs are needed to define details for that, as well as to extend the mapping to more resources (including those without equivalent Matrix identifiers, such as room state or user profile data).

The Matrix identifier (or identifiers) can be reconstructed from {id without sigil} by prepending a sigil character corresponding to {type}. To support a hierarchy of Matrix resources, more /{type}/{id without sigil} pairs can be appended, identifying resources within other resources. As of now, there's only one such case, with exactly one additional pair - pointing to an event in a room.

Examples:

  • Room #someroom:example.org: matrix:r/someroom:example.org
  • User @me:example.org: matrix:u/me:example.org
  • Event in a room: matrix:r/someroom:example.org/e/Arbitrary_Event_Id
  • A commit like this could make use of a Matrix URI in the form of <a href="{Matrix URI}">{Matrix identifier}</a>.

Proposal

Definitions

Further text uses the following terms:

  • Matrix identifier - one of identifiers defined by the current Matrix Specification,
  • Matrix URI - a uniform resource identifier proposed hereby, following the RFC-compliant URI format.
  • MUST/SHOULD/MAY etc. follow the conventions of RFC 2119.

Requirements

The following considerations drive the requirements for Matrix URIs:

  1. Follow existing standards and practices.
  2. Endorse the principle of the least surprise.
  3. Humans first, machines second.
  4. Cover as many entities as practical.
  5. URIs are expected to be extremely portable and stable; you cannot rewrite them once they are released to the world.
  6. Ease of implementation, allowing reuse of existing codes.

The following requirements resulted from these drivers:

  1. Matrix URI MUST comply with RFC 3986 and RFC 7595.
  2. By definition, Matrix URI MUST unambiguously identify a resource in a Matrix network, across servers and types of resources. This means, in particular, that two Matrix identifiers distinct by Matrix Specification MUST NOT have Matrix URIs that are equal in RFC 3986 sense (but two distinct Matrix URIs MAY map to the same Matrix identifier).
  3. References to the following entities MUST be supported:
    1. User IDs (@user:example.org)
    2. Room IDs (!roomid:example.org)
    3. Room aliases (#roomalias:example.org)
    4. Event IDs ($arbitrary_eventid_with_or_without_serverpart)
  4. The mapping MUST take into account that some identifiers (e.g. aliases) can have non-ASCII characters - reusing RFC 3987 is RECOMMENDED, but an alternative encoding can be used if there are reasons for that.
  5. The mapping between Matrix identifiers and Matrix URIs MUST be extensible (without invalidating previous URIs) to:
    1. new classes of identifiers (there MUST be a meta-rule to produce a new mapping for IDs following the &somethingnew:example.org pattern assumed for Matrix identifiers);
    2. new ways to navigate to and interact with objects in Matrix (e.g., we might eventually want to have a mapping for room-specific user profiles).
  6. The mapping MUST support decentralised as well as centralised IDs. This basically means that the URI scheme MUST have provisions for mapping of identifiers with :<serverpart> but it MUST NOT require :<serverpart> to be there.
  7. Matrix URI SHOULD allow encoding of action requests such as joining a room.
  8. Matrix URI SHOULD have a human-readable, if not necessarily human-friendly, representation - to allow visual sanity-checks. In particular, characters escaping/encoding should be reduced to bare minimum in that representation. As food for thought, see Wikipedia: Clean URL, aka SEF URL and a use case from RFC 3986.
  9. It SHOULD be easy to parse Matrix URI in popular programming languages: e.g., one should be able to use parseUri() to dissect a Matrix URI into components in JavaScript.
  10. The mapping SHOULD be consistent across different classes of Matrix identifiers.
  11. The mapping SHOULD support linking to unfederated servers/networks (see also matrix-doc#2309 that calls for such linking).

The syntax and mapping discussed below meet all these requirements except the last one that will be addressed separately. Further extensions MUST NOT reduce the supported set of requirements.

Syntax and high-level processing

The proposed generic Matrix URI syntax is a subset of the generic URI syntax defined by RFC 3986:

MatrixURI = "matrix:" hier-part [ "?" query ] [ "#" fragment ]
hier-part = [ "//" authority "/" ] path

As mentioned above, this MSC assumes client-side URI processing (i.e. mapping to Matrix identifiers and CS API requests). However, even when URI processing is shifted to the server side the client will still have to parse the URI at least to remove the authority and fragment parts (if either exists) before sending the request to the server (more on that below).

Scheme name

The proposed scheme name is matrix. RFC 7595 states:

if there’s one-to-one correspondence between a service name and
a scheme name then the scheme name should be the same as
the service name.

Other considered options were mx and web+matrix; comments to MSC455 mention two scheme names proposed and one more has been mentioned in #matrix-core:matrix.org.

The scheme name is a definitive indication of a Matrix URI and MUST NOT be omitted. As can be seen below, Matrix URI rely heavily on relative references and omitting the scheme name makes them indistinguishable from a local path that might have nothing to do with Matrix. Clients MUST NOT try to parse pieces like r/MyRoom:example.org as Matrix URIs; instead, users should be encouraged to use Matrix identifiers for in-text references (#MyRoom:example.org) and client applications SHOULD turn them into hyperlinks to Matrix URIs.

Authority

Basing on the definition in RFC 3986, this MSC restricts the authority part to never have a userinfo component, partially to prevent confusion concerned with the @ character that has its own meaning in Matrix, but also because this component has historically been a popular target of abuse.

authority = host [ ":" port ]

Further definition of syntax or semantics for the authority part is left for future MSCs. Clients MUST parse the authority part as per RFC 3986 (i.e. the presence of an authority part MUST NOT break URI parsing) but SHOULD NOT use data from the authority part other than for experiments or research.

The authority part may eventually be used to indicate access to a Matrix resource (such as a room or a user) specifically through a given entity. See "Ideas for further evolution".

Path

This MSC restricts the very wide definition of path in RFC 3986, to a simple pattern that allows to easily reconstruct a Matrix identifier or a chain of identifiers and also to locate a certain sub-resource in the scope of a given Matrix entity:

path = entity-descriptor ["/" entity-descriptor]
entity-descriptor = nonid-segment / type-qualifier id-without-sigil
nonid-segment = segment-nz ; as defined in RFC 3986, see also below
type-qualifier = segment-nz "/" ; as defined in RFC 3986, see also below
id-without-sigil = string ; as defined in Matrix identifier spec, see below

The path component consists of 1 or more descriptors separated by a slash (/) character. This is a generic pattern intended for reusing in future extensions.

This MSC only proposes mappings along type-qualifier id-without-sigil syntax; nonid-segment is unused and reserved for future use. For the sake of integrity future nonid-segment extensions must follow the ABNF for segment-nz as defined in RFC 3986.

This MSC defines the following type specifiers: u (user id, sigil @), r (room alias, sigil #), roomid (room id, sigil !), and e (event id, sigil $). This MSC does not define a type specifier for sigil + (groups aka communities or, in the more recent incarnation, spaces); a separate MSC can introduce the specifier, along with the parsing/construction logic and relevant CS API invocations, following the framework of this proposal.

The following type specifiers proposed in earlier editions of this MSC and already in use in several implementations, are deprecated: user, room, and event. Client applications MAY parse these specifiers as if they were u, r, and e respectively; they MUST NOT emit URIs with the deprecated specifiers. The rationale behind the switch is laid out in "Alternatives".

As of this MSC, u, r, and roomid can only be at the top level. The type e (event) can only be used on the 2nd level and only under r or roomid; this is driven by the current shape of Client-Server API that does not provide a non-deprecated way to retrieve an event without knowing the room (see MSC2695 and MSC2779 that may change this).

Further MSCs may introduce navigation to more top-level as well as non-top-level objects; see "Ideas for further evolution" to get inspired. These new proposals SHOULD follow the generic grammar laid out above, adding new type and nonid-segment specifiers and/or allowing them in other levels, rather than introduce a new grammar. It is recommended to only use abbreviated single-letter specifiers if they are expected to be user visible and convenient for type-in; if a URI for a given resource type is usually generated (e.g. because the corresponding identifier is not human-friendly), it's RECOMMENDED to use full (though short) words to avoid ambiguity and confusion.

id-without-sigil is defined as the string part of Matrix Common identifier format with percent-encoded characters that are NEITHER unreserved, sub-delimiters, : nor @, as per RFC 3986 rule for pchar. This notably exempts : from percent-encoding but includes /.

See the rationale behind dropping sigils and the respective up/downsides in "Discussion points and tradeoffs" as well as "Alternatives" below.

Query

Matrix URI can optionally have the query part. This MSC defines the general form for the query and two "standard" query items; further MSCs may add to this as long as RFC 3986 is followed.

query = query-element *( "&" query-item )
query-item = action / routing / custom-query-item
action = "action=" ( "join" / "chat" )
routing = "via=” authority
custom-query-item = custom-item-name "=" custom-item-value
custom-item-name = 1*unreserved ; reverse-DNS name; see below
custom-item-value = ; see below

The action query item is used in contexts where, on top of identifying the Matrix entity, a certain action is requested on it. This proposal describes two possible actions:

  • action=join is only valid in a URI resolving to a Matrix room; applications MUST ignore it if found in other contexts and MUST NOT generate it for other Matrix resources. This action means that a client application SHOULD attempt to join the room specified by the URI path using the standard CS API means.
  • action=chat is only valid in a URI resolving to a Matrix user; applications MUST ignore it if found in other contexts and MUST NOT generate it for other Matrix resources. This action means that a client application SHOULD open a direct chat window with the user specified by the URI path; clients supporting canonical direct chats SHOULD open the canonical direct chat.

For both actions, where applicable, client applications SHOULD ask for user confirmation or at least notify the user before joining or creating a new room. Conversely, no additional confirmation/notification is necessary when the action leads to opening a room the user is already a member of.

It is worth reiterating on the (blurry) distinction between URIs with action and those without:

  • a URI with no action simply identifies the resource; if the context implies an operation, it is usually focused on the retrieval of the resource, in line with RFC 3986 (see also the next paragraph);
  • a URI with action in the query means that a client application should (but is not obliged to) perform that action, with precautions as described above.

In some cases a client application may have no meaningful way to immediately perform the default operation suggested by this MSC (see below); e.g., the client may be unable to display a room before joining it, while the URI doesn't have action=join. In these cases client applications are free to do what's best for user experience (e.g., suggest joining the room), even if that means performing an action on a URI with no action in the query.

The routing query (via=) indicates servers that are likely involved in the room (see also the feature of matrix.to). In the meantime, it is proposed that this routing query be used not only with room ids in a public federation but also when a URI refers to a resource in a non-public Matrix network (see the question about closed federations in "Discussion points and tradeoffs"). Note that authority in the definition above is only a part of the query parameter grammar; it is not proposed here to generate or interpret the authority part of the URI.

Clients MAY introduce and recognise custom query items, according to the following rules:

  • the name of a custom item MUST follow the reverse-DNS (aka "Java package") naming convention, as per MSC2758 - e.g., a custom action item for Element clients would be named io.element.action, for Quaternion - com.github.quaternion.action, etc.
  • the value of the item can be any content but its representation in the URI MUST follow the general RFC requirements for the query part; on top of that, if the raw value contains & it MUST be percent-encoded.
  • clients SHOULD respect standard query items over their own ones; e.g., if a URI contains both action and the custom client action, the standard action should be respected as much as possible. Client authors SHOULD strive for consistent experience across their and 3rd party clients, anticipating that the same user may happen to have both their client and a 3rd party one.

Client authors are strongly encouraged to standardise custom query elements that gain adoption by submitting an MSC defining them in a way compatible across the client ecosystem.

Recommended implementation

URI parsing algorithm

The reference algorithm of parsing a Matrix URI follows. Note that, although clients are encouraged to use lower-case strings in their URIs, all string comparisons are case-INsensitive.

  1. Parse the URI into main components (scheme name, authority, path, query, and fragment), decoding special or international characters as directed by RFC 3986 and (for IRIs) RFC 3987. Authors are strongly RECOMMENDED that they find an existing implementation of that step for their language and SDK, rather than implement it from scratch based on RFCs.

  2. Check that scheme name is exactly matrix, case-insensitive. If the scheme name doesn't match, exit parsing: this is not a Matrix URI.

  3. Split the path into segments separated by / character; several subsequent / characters delimit empty segments, as advised by RFC 3986.

  4. Check that the URI contains either 2 or 4 segments; if it's not the case, fail parsing; the Matrix URI is invalid.

  5. To construct the top-level (primary) Matrix identifier:

    a. Pick the leftmost segment of path until / (path segment) and match it against the following list to produce sigil-1:

    • u (or, optionally, user - see "Path") -> @
    • r (or, optionally, room) -> #
    • roomid -> !
    • any other string, including an empty one -> fail parsing: the Matrix URI is invalid.

    b. Pick the next (2nd) leftmost path segment:

    • if the segment is empty, fail parsing;
    • otherwise, percent-decode the segment (unless the initial URI parse has already done that) and make mxid-1 by prepending sigil-1.

  6. If sigil-1 is ! or # and the URI path has exactly 4 segments, it may be possible to construct the 2nd-level Matrix identifier to point to an event inside the room identified by mxid-1:

    a. Pick the next (3rd) path segment:

    • if the segment is exactly e (or, optionally, event), proceed;
    • otherwise, including the case of an empty segment (trailing /, e.g.), fail parsing.

    b. Pick the next (4th) leftmost path segment:

    • if the segment is empty, fail parsing;
    • otherwise, percent-decode the segment (unless the initial URI parse has already done that) and make mxid-2 by prepending $.

  7. Split the query into items separated by & character; several subsequent & characters delimit empty items, ignored by this algorithm.

    a. If query contains one or more items starting with via=: for each item, treat the rest of the item as a percent-encoded homeserver name to be used in routing.

    b. If query contains one or more items starting with action=: treat the last such item as an instruction, as this proposal defines in query.

Clients MUST implement proper percent-decoding of the identifiers; there's no liberty similar to that of matrix.to.

Operations on Matrix URIs

The main purpose of a Matrix URI is accessing the resource specified by the identifier. This MSC defines the "default" operation (in the sense of RFC 7595) that a client application SHOULD perform when the user activates (e.g. clicks on) a URI; further MSCs may introduce additional operations enabled either by passing an action value in the query part, or by other means.

The classes of URIs and corresponding default operations (along with relevant CS API calls) are collected below. The table assumes that the operations are performed on behalf (using the access token) of the user @me:example.org:

URI class/example Interactive operation Non-interactive operation / Involved CS API
User Id (no action in URI):
matrix:u/her:example.org		 Outside the room context: show user profile
Inside the room context: mention the user in the current room (client-local operation)		 No default non-interactive operation
GET /profile/@her:example.org/display_name
GET /profile/@her:example.org/avatar_url
User Id (action=chat):
matrix:u/her:example.org?action=chat		 1. Confirm with the local user if needed (see "Query")
2. Open the room as defined in the next column		 If canonical direct chats are supported: GET /_matrix/client/r0/user/@me:example.org/dm?involves=@her:example.org
Without canonical direct chats:
1. GET /user/@me:example.org/account_data/m.direct
2. Find the room id for @her:example.org in the event content
3. if found, return this room id; if not, POST /createRoom with "is_direct": true and return id of the created room
Room (no action in URI):
matrix:roomid/rid:example.org
matrix:r/us:example.org		 Attempt to "open" (usually: display the timeline at the latest or last remembered position) the room No default non-interactive operation
API: Find the respective room in the local /sync cache or
GET /rooms/!rid:example.org/...
Room (action=join):
matrix:roomid/rid:example.org?action=join&via=example2.org
matrix:r/us:example.org?action=join		 1. Confirm with the local user if needed (see "Query")
2. Attempt to join the room		 POST /join/!rid:example.org?server_name=example2.org
POST /join/#us:example.org
Event:
matrix:r/us:example.org/e/lol823y4bcp3qo4
matrix:roomid/rid:example.org/event/lol823y4bcp3qo4?via=example2.org		 1. For room aliases, resolve an alias to a room id (see the next column)
2. Attempt to retrieve (see the next column) and display the event;
3. If the event could not be retrieved due to access denial and the current user is not a member of the room, the client MAY offer the user to join the room and try to open the event again		 Non-interactive operation: return event or event content, depending on context
API: find the event in the local /sync cache or
GET /directory/room/%23us:example.org (to resolve alias to id)
GET /rooms/!rid:example.org/event/lol823y4bcp3qo4?server_name=example2.org

URI construction algorithm

The following algorithm assumes a Matrix identifier that follows the high-level grammar described in the specification. Clients MUST ensure compliance of identifiers passed to this algorithm.

For room and user identifiers (including room aliases):

  1. Remove the sigil character from the identifier and match it against the following list to produce prefix-1:
    • @ -> u/
    • # -> r/
    • ! -> roomid/
  2. Build the Matrix URI as a concatenation of:
    • literal matrix:;
    • prefix-1;
    • the remainder of identifier (id without sigil), percent-encoded as per RFC 3986.

For event identifiers (assuming they need the room context, see MSC2695 and MSC2779 that may change this):

  1. Take the event's room id or canonical alias and build a Matrix URI for them as described above.
  2. Append to the result of previous step:
    • literal e/;
    • the event id after removing the sigil ($) and percent-encoding.

Clients MUST implement proper percent-encoding of the identifiers; there's no liberty similar to that of matrix.to.

Discussion and non-normative statements

Ideas for further evolution

This MSC is obviously just the first step, keeping the door open for extensions. Here are a few ideas:

  • Add new actions; e.g. leaving a room (action=leave).

  • Add specifying a segment of the room timeline (from=$evtid1&to=$evtid2).

  • Unlock bare event ids (matrix:e/$event_id) - subject to change in other areas of the specification.

  • Bring tangible semantics to the authority part. The main purpose of the authority part, as per RFC 3986, is to identify the entity governing the namespace for the rest of the URI. The current MSC rules out the userinfo component but leaves it to a separate MSC to define semantics of the remaininghost[:port] piece.

    Importantly, future MSCs are advised against using the authority part for routing over federation (the case for via= query items), as it would be against the spirit of RFC 3986. The authority part can be used in cases when a given Matrix entity is only available from certain servers (the case of closed federations or non-federating servers).

    While being a part of the original proposal in an attempt to address the respective case, the definition of the authority semantics has been dropped as a result of the subsequent discussion. A further MSC may approach the same case (and/or others) and define the meaning of the authority part (either on the client- or even on the server-side - provided that using Matrix URIs on the server-side brings some other value along the way). This might not necessarily be actual DNS hostnames even - one (quite far-fetched for now) idea to entertain might be introducing some decentralised system of "network names" in order to equalise "public" and "non-public" federations.

    Along the same lines, if providing any part of user credentials via the authority part is found to be of considerable value in some case, a separate MSC could both reinstate it in the grammar and define how to construct, parse, and use it - provided that the same MSC addresses the security concerns associated with such URIs.

  • One could conceive a URI mapping of avatars in the form of matrix:u/uid:matrix.org/avatar/room:matrix.org (a user’s avatar for a given room).

  • As described in "Alternatives", a synonymous system can be introduced that uses Matrix identifiers with sigils by adding another path prefix (e.g., matrix:id/%23matrix:matrix.org). However, such MSC would have to address the concerns of possible confusion arising from having two similar but distinct notations.

  • Interoperability of Matrix URIs with Linked Data.

Past discussion points and tradeoffs

The below documents the discussion and outcomes in various prior forums; further discussion should happen in GitHub comments.

  1. Why no double-slashes in a typical URI? Because // is used to mark the beginning of an authority part. RFC 3986 explicitly forbids to start the path component with // if the URI doesn't have an authority component. In other words, // implies a centre of authority, and the (public) Matrix federation is not supposed to have one; hence no // in most URIs.
  2. Why do type specifiers use singular rather than plural as is common in RESTful APIs? This is no more relevant with single-letter type specifiers. The answer below is provided for history only. Unlike in actual RESTful APIs, this MSC does not see rooms/ or users/ as collections to browse. The type specifier completes the id specification in the URI, defining a very specific and easy to parse syntax for that. Future MSCs may certainly add collection URIs, but it is recommended to use more distinct naming for such collections. In particular, rooms/ is ambiguous, as different sets of rooms are available to any user at any time (e.g., all rooms known to the user; or all routable rooms; or public rooms known to the user's homeserver).
  3. Should we advise using the query part for collections then? Not in this MSC but that can be considered in the future.
  4. Why can't event URIs use the fragment part for the event ID? Because fragment is a part processed exclusively by the client in order to navigate within a larger document, and room cannot be considered a "document". Each event can be retrieved from the server individually, so each event can be viewed as a self-contained document. When/if URI processing is shifted to the server-side, servers are not even going to receive fragments (as per RFC 3986), which is why usage of fragments to remove the need for percent-encoding in other identifiers would lead to URIs that cannot be resolved on servers. Effectively, all clients would have to implement full URI processing with no chance to offload that to the server. For that reason fragments, if/when ever employed in Matrix, only should be used to pinpoint a position within events and for similar strictly client-side operations.
  5. How does this MSC work with closed federations? If you need to communicate a URI to the bigger world where you cannot expect the consumer to know in advance which federation they should use - supply any server of the closed federation in the authority part. Users inside the closed federation can omit the authority part if they know the URI is not going to be used outside this federation. Clients can facilitate that by having an option to always add or omit the authority part in generated URIs for a given user account. As of now, use via= in order to point to a homeserver in the closed federation. The authority part may eventually be used for that (or for some other case - see the previous section).

Alternatives

Using full words for all types

During its draft state, this MSC was proposing type specifiers using full words (user, room, event etc.), arguing that abbreviations can be introduced separately as synonyms. Full words have several shortcomings pointed out in discussions across the whole period of preparation, namely:

  • The singular vs. plural choice (see also "Past discussion points")
  • Using English words raises a question about eventual support of localised URI variants (matrix:benutzer/..., matrix:usuario/... etc.) catering to international audience, that would add complication to the Matrix technology.
  • Abbreviated forms are popularised by Reddit and make URIs shorter which is crucial for the outbound integration case (see the introduction).

Meanwhile, using u/r/e for users, rooms and events has the following advantages:

  1. there's a strong Reddit legacy, with users across the world quite familiar with the abbreviated forms (and r/ coincidentally standing for sub-Reddits links to which have basically the same place in the Reddit ecosystem as Matrix room aliases have in the Matrix ecosystem);
  2. matrix.to links to users and room aliases are heavily used throughout Matrix, specifically in end-user-facing contexts (see also use cases in the introductory section of this MSC);
  3. the singular vs. plural (room or rooms?) confusion is avoided;
  4. it's shorter, which is crucial for typing the URI in an external medium.

The rationale behind not abbreviating roomid/ is a better distinction between room aliases and room ids; also, since room ids are almost never typed in manually, the advantages (3) and (4) above don't hold.

For these reasons, it was decided in the end to use the single-letter style for types most used in the outbound integration case. It's still possible to reinstate full words as synonyms some time down the road, with the caveat that a canonicalisation service from homeservers may be needed to avoid having to enable synonyms at each client individually.

URNs

The discussion in MSC455 mentions an option to standardise URNs rather than URLs/URIs, with the list of resolvers being user-specific. While a URN namespace such as urn:matrix:, along with a URN scheme, might be deemed useful once we shift to (even) more decentralised structure of the network, urn: URIs must be managed entities (see RFC 8141) which is not always the case in Matrix (consider room aliases, e.g.).

With that said, a URN-styled (matrix:room:example.org:roomalias) option was considered. However, Matrix already uses colon (:) as a delimiter of id parts and, as can be seen above, reversing the parts to meet the URN's hierarchical order would look confusing for Matrix users (as in example above - is room a part of the identifier or the type signifier?).

"Full REST"

Yet another alternative considered was to go "full REST" and structure URLs in a more traditional way with serverparts coming first, followed by type grouping (sic - not specifiers), and then by localparts, i.e. matrix://example.org/rooms/roomalias. This is even more difficult to comprehend for a Matrix user than the previous alternative and besides it conflates the notion of an authority server with that of a namespace discriminator: clients would not connect to example.org to resolve the alias above, they would still connect to their own homeserver.

Minimal syntax

One early proposal was to simply prepend matrix: to a Matrix identifier (without encoding it), assuming that it will only be processed on the client side. The massive downside of this option is that such strings are not actual URIs even though they look like ones: most URI parsers won't handle them correctly. As laid out in the beginning of this proposal, Matrix URIs are not striving to preempt Matrix identifiers; instead of trying to produce an equally readable string, one should just use identifiers where they work. Why Matrix identifiers look the way they look is way out of the MSC scope to discuss here.

Minimal syntax based on the path component and percent-encoding

A simple modification of the previous option is much more viable: proper percent-encoding of the Matrix identifier allows to use it as a URI path part. A single identifier packed in a URI could look like matrix:/encoded_id_with_sigil; an event-in-a-room URI would be something like matrix:/roomid_or_alias/$event_id (NB: RFC 3986 doesn't require $ to be encoded). This is considerably more concise and encoding is only needed for #.

Quite unfortunately, # is one of the two sigils in Matrix most relevant to integration cases. The other one is @; it doesn't need encoding except in the authority part - which is why the form above uses a leading / that puts the identifier in the path part instead of what parsers treat as the authority part. # has to be encoded wherever it appears, making a URI for Matrix HQ, the first chat room many new users join, look like matrix:/%23matrix:matrix.org. Beyond first-time usage, this generally impacts the "napkin" case from RFC 3986 that the Requirements section of this MSC mentions. Until we have applications generally recognising Matrix identifiers in the same way e-mail addresses are recognised without prefixing mailto:, we should live with the fact that people will have to produce Matrix URIs by hand in various instances, from pen-and-paper to other instant messengers.

Putting the whole id to the URI fragment (matrix:#id_with_sigil or, following on the matrix.to tradition, matrix:#/id_with_sigil for readability) allows using # without encoding on many URI parsers. It is still not fully RFC-compliant and rules out using URIs by homeservers (see also "Past discussion points" on using fragments to address events).

Regardless of the placement (the fragment or the path), one more consideration is that the character space for sigils is extremely limited and Matrix identifiers are generally less expressive than full-blown URI paths. Not that Matrix showed a tendency to produce many classes of objects that would warrant a dedicated sigil but that cannot be ruled out. Rather than rely on the institute of sigils, this proposal gives an alternative more extensible syntax that can be used for more advanced cases - as a uniform way to represent arbitrary sub-objects (with or without Matrix identifier) such as user profiles, or a notifications feed for the room - and also, if ever needed, as an escape hatch to a bigger namespace if we hit shortage of sigils.

The current proposal is also flexible enough to incorporate the minimal syntax of this option as an alternative to its own notation - e.g., a further MSC could enable matrix:id/%23matrix:matrix.org as a synonym for matrix:room/matrix:matrix.org.

Potential issues

Despite the limited functionality of URIs as proposed in this MSC, Matrix authors are advised to use tools that would process URIs just like an HTTP(S) URI instead of making home-baked parsers/emitters. Even with that in mind, not all tools normalise and sanitise all cases in a fully RFC-compliant way. This MSC tries to keep the required transformations to the minimum and will likely not bring much grief even with naive implementations; however, as functionality of Matrix URI grows, the number of corner cases will increase.

Security/privacy considerations

This MSC mostly builds on RFC 3986 but tries to reduce the scope as much as possible. Notably, it avoids introducing complex traversable structures and further restricts the URI grammar to the necessary subset. In particular, dot path segments (. and ..), while potentially useful when URIs become richer, would come too much ahead of time for now. Care is taken to not make essential parts of the URI omittable to avoid even accidental misrepresentation of a local resource for a remote one in Matrix and vice versa.

As mentioned in the authority part section, the MSC intentionally doesn't support conveying any kind of user information in URIs.

The MSC strives to not be prescriptive in treating URIs except the action query parameter. Actions without user confirmation may lead to unintended leaks of certain metadata and/or changes in the account state with respect to Matrix. To reiterate, clients SHOULD ask for a user consent if/when they can unless applying the action doesn't lead to sending persistent (message or state) events on user's behalf.

Conclusion

A dedicated URI scheme is well overdue for Matrix. Many other networks already have got one for themselves, benefiting both in terms of branding (compare matrix:r/weruletheworld:example.org vs. #weruletheworld:example.org from the standpoint of someone who hasn't been to Matrix) and interoperability (matrix.to requires opening a browser while clicking a tg: link dumped to the terminal application will open the correct application for Telegram without user intervention or can even offer to install one, if needed). The proposed syntax makes conversion between Matrix URIs and Matrix identifiers as easy as a bunch of string comparisons or regular expressions; so even though client-side processing of URIs might not be optimal longer-term, it's a very simple and quick way that allows plenty of experimentation early on.