matrix-spec-proposals/proposals/2312-matrix-uri.md

# 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 of it. It supersedes 
[MSC455](https://github.com/matrix-org/matrix-doc/issues/455) 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 in UI: as a Matrix user I want to refer to Matrix entities
   in the same way as for web pages, so that others could “follow the link”
   I sent them (not necessarily through Matrix, it can be, e.g., a web page or
   email) in order to access the referred resource.
1. 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 in an email program.
1. Outbound integration: as an author of Matrix software, I want to have a way
   to export identifiers of Matrix resources as URIs to non-Matrix environment
   so that they could be resolved in another time-place in a uniform way.
   This is a case of "Share via…" action in a mobile Matrix client.

https://matrix.to somehow compensates for the lack of dedicated URIs; however:
* it addresses use case (1) in a somewhat clumsy way (resolving a link needs
  two interactions with the user instead of one), and 
* it can only deal with (2) within a web browser (basically limiting
  first-class support to browser-based clients).

To cover the use cases above, the following scheme is proposed for Matrix URIs
(`[]` enclose optional parts, `{}` enclose variables):
```text
matrix:[//{authority}/]{type}/{id without sigil}[/{more type/id pairs}][?{query}]
```
with `type` defining the resource type (such as `user` 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).
The Matrix identifier (or identifiers) can be reconstructed from the URI by
taking the sigil that corresponds to `type` and appending `id without sigil`
to it. To support a hierarchy of Matrix resources, `more type/id pairs` series
is used to reconstruct inner identifiers (as of now, there can be only one
inner identifier, pointing to an event in a room).

This proposal defines initial mapping of URIs to Matrix identifiers and actions
on corresponding resources; the scheme and mapping are subject
to further extension.

Examples:
* Room `#someroom:example.org`:
  `matrix:room/someroom:example.org`
* Unfederated room `#internalroom:internal.example.org`:
  `matrix:room/internalroom:internal.example.org`. This can only be resolved by a client connected to the appropriate server, likely internal.example.org in this case, so it can be hinted at: `matrix:room/internalroom:internal.example.org&via=internal.example.org`. See the [Discussion points and tradeoffs](#discussion-points-and-tradeoffs) section.
* Event in a room:
  `matrix:room/someroom:example.org/event/Arbitrary_Event_Id`
* [A commit like this](https://github.com/her001/steamlug.org/commit/2bd69441e1cf21f626e699f0957193f45a1d560f)
  could make use of a Matrix URI in the form of
  `<a href="{Matrix URI}">{Matrix identifier}</a>`.
  
This MSC does not introduce new Matrix entities, nor API endpoints -
it merely defines a mapping from a URI with the scheme name `matrix:`
to Matrix identifiers and actions on them. It is deemed 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 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.

## Proposal

Further text uses “Matrix identifier” with a meaning of identifiers
as described by [Matrix Specification](https://matrix.org/docs/spec/),
and “Matrix URI” with a meaning of an identifier following
the RFC-compliant URI format proposed hereby.

### Requirements

The following considerations drive the requirements for Matrix URIs:
1. Follow existing standards and practices.
1. Endorse the principle of least surprise.
1. Humans first, machines second.
1. Cover as many entities as practical.
1. URIs are expected to be extremely portable and stable;
   you cannot rewrite them once they are released to the world.
1. Ease of implementation, allowing reuse of existing codes.

The following requirements resulted from these drivers
(see [RFC 2119](https://www.ietf.org/rfc/rfc2119.txt) for conventions
around MUST/SHOULD/MAY):
1. Matrix URI MUST comply with
   [RFC 3986](https://tools.ietf.org/html/rfc3986) and
   [RFC 7595](https://tools.ietf.org/html/rfc7595).
1. 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](https://matrix.org/docs/spec/appendices.html#identifier-grammar)
   MUST NOT have Matrix URIs that are equal in
   [RFC 3986](https://tools.ietf.org/html/rfc3986) sense
   (but two distinct Matrix URIs MAY map to the same Matrix identifier).
1. References to the following entities MUST be supported:
   1. User IDs (`@user:example.org`)
   1. Room IDs (`!roomid:example.org`)
   1. Room aliases (`#roomalias:example.org`)
   1. Event IDs (`$arbitrary_eventid_with_or_without_serverpart`)
1. The mapping MUST take into account that some identifiers
   (e.g. aliases) can have non-ASCII characters - reusing
   [RFC 3987](https://tools.ietf.org/html/rfc3987) is RECOMMENDED
   but an alternative encoding can be used if there are reasons
   for that.
1. 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);
   1. 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).
1. The mapping MUST support decentralised as well as centralised IDs.
   This basically means that the URI scheme MUST have provisions
   for mapping of `:<serverpart>` but it MUST NOT require
   `:<serverpart>` to be there.
1. Matrix URI SHOULD allow encoding of action requests such as joining
   a room.
1. 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 a food for thought, see
   [Wikipedia: Clean URL, aka SEF URL](https://en.wikipedia.org/wiki/Clean_URL) and
   [a very relevant use case from RFC 3986](https://tools.ietf.org/html/rfc3986#section-1.2.1).
1. 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 in JavaScript.
1. The mapping SHOULD be consistent across different classes of
   Matrix identifiers.
1. The mapping SHOULD support linking to unfederated servers/networks
   (see also
   [matrix-doc#2309](https://github.com/matrix-org/matrix-doc/issues/2309)
   that calls for such linking).

The syntax and mapping discussed below meet all these requirements.
Further extensions MUST comply to them as well.

### Syntax and high-level processing

The proposed generic Matrix URI syntax is a subset of the generic
URI syntax
[defined by RFC 3986](https://tools.ietf.org/html/rfc3986#section-3):
```text
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 find
the authority part (or lack of it) and remove the fragment part
before sending the request to the server (more on that below).

#### Scheme name
The proposed scheme name is `matrix`.
[RFC 7595](https://tools.ietf.org/html/rfc7595) 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](https://github.com/matrix-org/matrix-doc/issues/455)
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](https://tools.ietf.org/html/rfc3986#section-4.2) 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 `room/MyRoom:example.org` as Matrix URIs; instead,
users should be encouraged to use Matrix IDs for in-text references
(`#MyRoom:example.org`) and client applications should do
the heavy-lifting of turning them into hyperlinks to Matrix URIs.

#### Authority

The authority part is used for the specific case of getting access
to a Matrix resource (such as a room or a user) through a given server.
```text
authority = host [ “:” port ]
```

Here's an example of a Matrix URI with an authority part
(the authority part is `example.org:682` here):
`matrix://example.org:682/roomid/Internal_Room_Id:example2.org`.

The authority part, as defined above, is reserved for future MSCs.
Clients SHOULD NOT use data from the authority part other than for
experimental or further research purposes.

#### Path
Unlike the very wide definition of path in RFC 3986, this MSC
restricts the path component of a Matrix URI to a simple
pattern that allows to easily reconstruct a Matrix identifier or
a chain of identifiers:
```text
path = type “/” id-without-sigil [“/” path]
type = “user” / “roomid” / “room” / “event” / “group”
id-without-sigil = string ; see below
```
The path component consists of 1 or more type-id pairs separated
by slash character both inside the pair and between pairs. While most
of the URIs considered in this MSC do not need any sort of hierarchy,
one case is standing out: as of now, events require rooms to be
resolved so an event URI for `$eventid` in the room
`!roomid:example2.org` would be
`matrix:roomid/roomid:example2.org/event/eventid`.

This MSC defines the following type specifiers:
`user` (user id, sigil `@`), `roomid` (room id, sigil `!`),
`room` (room alias, sigil `#`), and `event` (event id, sigil `$`).
The type `group` (group/community id, sigil `+`) is reserved for future use.

`id-without-sigil` is defined as the `string` part of Matrix
[Common identifier format](https://matrix.org/docs/spec/appendices#common-identifier-format)
with percent-encoded characters that are NEITHER unreserved, sub-delimiters, `:` nor `@`,
[as per RFC 3986 rule for pchar](https://tools.ietf.org/html/rfc3986#appendix-A).
This notably exempts `:` from percent-encoding but includes `/`.

See the rationale behind dropping sigils and the respective up/downsides in
"Discussion points and tradeoffs" below.
  
Further MSCs may introduce navigation to more top-level as well as
non-top-level objects; see "Further evolution" for some ideas.

#### Query

Matrix URI can optionally have
[the query part](https://tools.ietf.org/html/rfc3986#section-3.4).
This MSC defines only two specific forms for the query; further MSCs
may add to this as long as RFC 3986 is followed.
```text
query = query-element *( “&” query-element )
query-element = action / routing
action = “action=" ( "join" / "chat" )
routing = “via=” authority
```

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 client applications
  SHOULD attempt to join it 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. A URI with this action that a client application
  SHOULD open a direct chat window with the user; clients supporting
  [canonical direct chats](https://github.com/matrix-org/matrix-doc/pull/2199)
  SHOULD open the canonical direct chat.
  
For both actions, where applicable, client applications SHOULD ask for user
confirmation or at least make the user aware if the action leads
to joining or creating a new room rather than switching to a prior one.

The routing query (`via=`) indicates servers that are likely involved in
the room (see also
[the feature of matrix.to](https://matrix.org/docs/spec/appendices#routing)).
It is proposed to use the routing query to be used not only for resolving
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 grammar as defined in the respective section;
it is not proposed here to generate or read the authority part of the URI.


### URI semantics and parsing algorithm

The main purpose of a Matrix URI is accessing the resource specified by the
identifier, and the primary action is loading the contents of a document
corresponding to a given resource. This MSC defines the "default" action
that a client application SHOULD perform when the user activates
(e.g. clicks on) a URI; further MSCs may introduce additional actions enabled
either by passing an `action` value in the query part, or by other means.

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](https://tools.ietf.org/html/rfc3986) and
   (for IRIs) [RFC 3987](https://tools.ietf.org/html/rfc3987). Authors are
   strongly RECOMMENDED to find an existing implementation of that step for
   their language and SDK, rather than implement it from scratch based on RFCs.

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

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

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

1. 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`:
      - `user` -> `@`
      - `roomid` -> `!`
      - `room` -> `#`
      - `group` -> `+`
      - 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 and make `mxid-1` by
        concatenating `sigil-1` and the result of percent-decoding.

1. 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 `event`, proceed;
      - otherwise, including the case of an empty segment, fail parsing.
    
   b. Pick the next (4th) leftmost path segment:
      - if the segment is empty, fail parsing;
      - otherwise, percent-decode the segment and make `mxid-2` by
        prepending `$` to the result of percent-decoding.
      
1. 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 string as a percent-encoded homeserver name to be used in
      [routing](https://matrix.org/docs/spec/appendices#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](#query).

The classes of URIs and corresponding default actions (along with relevant
CS API calls) are collected as follows. This is non-normative and just provides
a reference and examples of handling various kinds of URIs.

* User ID:
  - URI example: `matrix:user/me:example.org` or
    (decentralised user id, future) `matrix:user/me_in_matrix`
  - Possible default actions:
    - Show user profile
    (`GET /profile/@me:example.org/...`, `GET /profile/@me_in_matrix/...`);
    - Mention the user in the current room (client-local operation)
* Room ID:
  - URI example: `matrix:roomid/rid:example.org` or
    (decentralised id, future) `matrix:roomid/lol823y4bcp3qo4`
  - Default action: attempt to "open" the room (usually means the client
    at least displays the room timeline at the latest or
    the last remembered position - `GET /rooms/!rid:example.org/...`,
    `GET /rooms/!lol823y4bcp3qo4/...`)
* Joining by Room ID:
  - URI example: `matrix:roomid/rid:example.org?action=join&via=example2.org`
  - Default action: if needed (see the section about the query part) ask
    the user to confirm the intention; then join the room
    (`POST /join/!rid:example.org?server_name=example2.org`)
* Room alias:
  - URI example: `matrix:room/us:example.org`
  - Default action: resolve the alias to room id
    (`GET /directory/room/#us:example.org` if needed) and attempt to "open"
    the room (same as above)
* Joining by Room alias:
  - URI example: `matrix:room/us:example.org?action=join`
  - Default action: if needed (see the section about the query part) ask
    the user to confirm the intention; then join the room
    (`POST /join/#us:example.org`)
* Event ID (as used in
  [room version 3](https://matrix.org/docs/spec/rooms/v3) and later):
  - URI example (aka "event permalink"):
    `matrix:room/us:example.org/event/UnpaddedBase64` or
    (additional routing is needed for room id)
    `matrix:roomid/rid:example.org/event/UnpaddedBase64?via=example2.org`
  - Default action:
    1. For room aliases, resolve an alias to a room id (see above)
    1. If the event is in the room that the user has joined, retrieve
       the event contents (`GET /rooms/!rid:example.org/event/UnpaddedBase64` or
       `GET /rooms/!rid:example.org/context/UnpaddedBase64`) and display them
       to the user 
    1. Otherwise try to retrieve the event in the same way but in case of
       access failure the client MAY offer the user to join the room; if
       the user agrees and joining succeeds, retry the step above.
* Group ID:
  - URI example: `matrix:group/them:matrix.org`
  - Default action: reserved for future use


## 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.
1. _Why do type specifiers use singular rather than plural
   as is common in RESTful APIs?_
   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).
1. _Should we advise using the query part for collections then?_
   Not in this MSC but that can be considered in the future.
1. _Why not Reddit-style single-letter type specifiers? That's almost
   as compact as a sigil, still pretty clearly conveys the type,
   and nicely avoids the singular vs. plural confusion._
   Reddit-style prefixes would eventually produce bigger ambiguity as
   primary notation; but they can be handy as shortcuts. As discussed
   further below, the current proposal provides enough space to define
   synonyms; this may need some canonicalisation service from homeservers
   so that we don't have to enable synonyms at each client individually.
1. _Why can an event URI not 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).
1. _Interoperability with
   [Linked Data](https://en.wikipedia.org/wiki/Linked_data)_ is out of
   scope of this MSC but worth being considered separately.
1. _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.~~
   Use `via=` in order to point to a homeserver in the closed federation.
   The authority part may eventually be used for that but further discussion
   is needed on how clients should support without compromising privacy
   (see https://github.com/matrix-org/matrix-doc/pull/2312#discussion_r348960282
   for the original concern).


## Further evolution

This section is non-normative.

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:event/$event_id`) - subject to changes in
  other areas of the specification.
* One area of possible evolution is bringing tangible semantics to
  the authority part. The main purpose of the authority part,
  [as per RFC 3986](https://tools.ietf.org/html/rfc3986#section-3.2),
  is to identify the authority governing the namespace for the rest
  of the URI. This MSC reuses the RFC definitions for
  [`host`](https://tools.ietf.org/html/rfc3986#section-3.2.2) and
  [`port`](https://tools.ietf.org/html/rfc3986#section-3.2.3).
  RFC 3986 also includes provisions for user information -
  this MSC explicitly excludes them. If providing a user identity
  in the authority part is found to be of value in some case,
  this should be addressed in a separate MSC.

  Importantly, the authority part is _not_ intended for usage in routing
  over federation; rather, it is for cases when a given Matrix
  entity is not expected to be reachable through federation (such as
  unfederated rooms or non-public Matrix networks). Sending requests
  to the server resolved from the authority part means that the client
  should be, as the name implies, _authorised_ by the authority server
  to access the requested resource. That, in turn, implies that the resource
  is either available to guests on the authority server, or the end user
  must be authenticated (and their access rights checked)
  on that server in order to access the resource. While being a part
  of the original proposal, the semantics for the authority part have
  been dropped from the normative part as a result of MSC discussion.
* One could conceive a URI mapping of avatars in the form of
  `matrix:user/uid:matrix.org/avatar/room:matrix.org`
  (a user’s avatar for a given room).
* As described below in "Alternatives", one can introduce a synonymous
  system that uses Matrix identifiers with sigils by adding another path
  prefix (`matrix:id/%23matrix:matrix.org`).


## Alternatives

### URNs

The discussion in
[MSC455](https://github.com/matrix-org/matrix-doc/issues/455)
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](https://tools.ietf.org/html/rfc8141)) 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.


### "Full REST" 

Yet another alternative considered was to go "full REST" and build
a more traditionally looking URL structure 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 (`example.org` above is a server part of an alias,
not the name of a hypothetical homeserver that should be used to resolve
the URI).


### Minimal syntax

One early but still viable 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. Bare Matrix identifiers have
the same applicable range without deceptive looks.


### Minimal syntax based on path 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/$event_id` (NB: RFC3986 doesn't require `$`
to be encoded). This is considerably more concise and encoding is only
needed for `#` - quite unfortunately, this is one of the most used sigils
in Matrix. E.g., `matrix:/%23matrix:matrix.org` would be a URI for
Matrix HQ chat room.

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 to use `#` without encoding on many URI parsers. It is
still not fully RFC3986-compliant but the bigger problem is that putting
the identifying part to the fragment rules out using URIs in client-server
communication. Effectively all clients will have to implement full URI
processing with no chance to offload that to the server.

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 even 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.

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 so this MSC recommends to ask for a user consent -
recognising that not all clients are in position for that.


## 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:room/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.