MSC3886: Simple client rendezvous capability

proposals/3886-simple-rendezvous-capability.md

@@ -0,0 +1,236 @@
+# MSC3886: Simple client rendezvous capability
+
+In MSCyyyy (to be published) a proposal is made to allow a user to login on a new device using an existing device by means of scanning a
+QR code.
+
+In order to facilitate this the two devices need some bi-directional communication channel which they can use to exchange
+information such as:
+
+- the homeserver being used
+- the user ID
+- facilitation of issuing a new access token
+- device ID for end-to-end encryption
+- device keys for end-to-end encryption
+
+To enable MSCyyyy and support any future proposals this MSC proposes a simple HTTP based protocol that can be used to
+establish a direct communication channel between two IP connected devices.
+
+It will work with devices behind NAT. It doesn't require homeserver administrators to deploy a separate server.
+
+## Proposal
+
+It is proposed that a general purpose HTTP based protocol be used to establish ephemeral bi-directional communication
+channels over which arbitrary data can be exchanged.
+
+A typical flow might look like this where device A is initiating the rendezvous with device B:
+
+```mermaid
+
+sequenceDiagram
+  participant A as Device A
+  participant R as Rendezvous Server
+  participant B as Device B
+  Note over A: Device A determines which rendezvous server to use
+
+  A->R: POST / Hello from A
+  R->A: 201 Created Location: /abc-def-123-456
+
+  A-->B: Rendezvous URI between clients, perhaps as QR code: e.g. https://rendzvous-server/abc-def-123-456
+
+  Note over A: Device A starts polling for contact at the rendezvous
+
+  B->R: GET <rendezvous URI>
+  R->B: 200 OK Hello from A
+
+  loop Device A polls for rendezvous updates
+    A->R: GET <rendezvous URI> If-None-Match: <ETag>
+    R->A: 304 Not Modified
+  end
+
+  B->R: PUT <rendezvous URI> Hello from B
+  R->B: 202 Accepted 
+
+  Note over A,B: Rendezvous now established
+```
+
+Please note that it is intentional that this protocol does nothing to ensure the integrity of the data exchanged at a rendezvous.
+
+### Protocol
+
+#### Create a new rendezvous point: `POST /`
+
+HTTP request headers:
+
+- `Content-Length` - required
+- `Content-Type` - optional, server should assume `application/octet-stream` if not specified
+
+HTTP request body:
+
+- any data up to maximum size allowed by the server
+
+HTTP response codes:
+
+- `201 Created` - rendezvous created
+- `403 Forbidden` - forbidden by server policy
+- `413 Payload Too Large` - the supplied payload is too large
+- `429 Too Many Requests` - the request has been rate limited
+
+HTTP response headers for `201 Created`:
+
+- `Location` - required, the allocated rendezvous URI which can be on a different server
+- `X-Max-Bytes` - required, the maximum allowed bytes for the payload
+- `ETag` - required, ETag for the current payload at the rendezvous point as per [RFC7232](https://httpwg.org/specs/rfc7232.html#header.etag)
+- `Expires` - required, the expiry time of the rendezvous as per [RFC7233](https://httpwg.org/specs/rfc7234.html#header.expires)
+- `Last-Modified` - required, the last modified date of the payload as per [RFC7232](https://httpwg.org/specs/rfc7232.html#header.last-modified)
+
+Example response headers:
+
+```http
+Location: /abcdEFG12345
+X-Max-Bytes: 10240
+ETag: VmbxF13QDusTgOCt8aoa0d2PQcnBOXeIxEqhw5aQ03o=
+Expires: Wed, 07 Sep 2022 14:28:51 GMT
+Last-Modified: Wed, 07 Sep 2022 14:27:51 GMT
+```
+
+#### Update payload at rendezvous point: `PUT <rendezvous URI>`
+
+HTTP request headers:
+
+- `Content-Length` - required
+- `Content-Type` - optional, server should assume `application/octet-stream` if not specified
+- `If-Match` - optional, as per [RFC7232](https://httpwg.org/specs/rfc7232.html#header.if-match) server will assume `*`
+if not specified
+
+HTTP request body:
+
+- any data up to maximum size allowed by the server
+
+HTTP response codes:
+
+- `202 Accepted` - payload updated
+- `404 Not Found` - rendezvous URI is not valid (it could have expired)
+- `412 Precondition Failed` - when `If-Match` is supplied and the ETag does not match
+- `413 Payload Too Large` - the supplied payload is too large
+- `429 Too Many Requests` - the request has been rate limited
+
+HTTP response headers for `202 Accepted` and `412 Precondition Failed`:
+
+- `ETag` - required, ETag for the current payload at the rendezvous point as per [RFC7232](https://httpwg.org/specs/rfc7232.html#header.etag)
+- `Expires` - required, the expiry time of the rendezvous as per [RFC7233](https://httpwg.org/specs/rfc7234.html#header.expires)
+- `Last-Modified` - required, the last modified date of the payload as per [RFC7232](https://httpwg.org/specs/rfc7232.html#header.last-modified)
+
+#### Get payload from rendezvous point: `GET <rendezvous URI>`
+
+HTTP request headers:
+
+- `If-None-Match` - optional, as per [RFC7232](https://httpwg.org/specs/rfc7232.html#header.if-none-match) server will
+only return data if given ETag does not match
+
+HTTP response codes:
+
+- `200 OK` - payload returned
+- `304 Not Modified` - when `If-None-Match` is supplied and the ETag does not match
+- `404 Not Found` - rendezvous URI is not valid (it could have expired)
+- `429 Too Many Requests` - the request has been rate limited
+
+HTTP response headers for `200 OK` and `304 Not Modified`:
+
+- `ETag` - required, ETag for the current payload at the rendezvous point as per [RFC7232](https://httpwg.org/specs/rfc7232.html#header.etag)
+- `Expires` - required, the expiry time of the rendezvous as per [RFC7233](https://httpwg.org/specs/rfc7234.html#header.expires)
+- `Last-Modified` - required, the last modified date of the payload as per [RFC7232](https://httpwg.org/specs/rfc7232.html#header.last-modified)
+
+- `Content-Type` - required for `200 OK`
+
+#### Cancel a rendezvous: `DELETE <rendezvous URI>`
+
+HTTP response codes:
+
+- `204 No Content` - rendezvous cancelled
+- `404 Not Found` - rendezvous URI is not valid (it could have expired)
+- `429 Too Many Requests` - the request has been rate limited
+
+### Maximum payload size
+
+The server should enforce a maximum payload size for the payload size. It is recommended that this be no less than 10KB.
+
+### Maximum duration of a rendezvous
+
+The rendezvous only needs to persist for the duration of the handshake. So a timeout such as 30 seconds is adequate.
+
+Clients should handle the case of the rendezvous being cancelled or timed out by the server.
+
+### ETags
+
+The ETag generated should be unique to the rendezvous point and the last modified time so that two clients can
+distinguish between identical payloads sent by either client.
+
+### CORS
+
+To support usage from web browsers, the server should allow CORS requests from any origin and expose the `ETag`,
+`Location` and `X-Max-Bytes` headers as:
+
+```http
+Access-Control-Allow-Headers: Content-Type,If-Match,If-None-Match
+Access-Control-Allow-Methods: GET,PUT,POST,DELETE
+Access-Control-Allow-Origin: *
+Access-Control-Expose-Headers: ETag,Location,X-Max-Bytes
+```
+
+### Choice of server
+
+Ultimately it will be up to the Matrix client implementation to decide which rendezvous server to use.
+
+However, it is suggested that the following logic is used by the device/client to choose the rendezvous server in order
+of preference:
+
+1. If the client is already logged in: try and use current homeserver.
+1. If the client is not logged in and it is known which homeserver the user wants to connect to: try and use that homeserver.
+1. Otherwise use a default server.
+
+## Potential issues
+
+Because this is an entirely new set of functionality it should not cause issue with any existing Matrix functions or capabilities.
+
+The proposed protocol requires the devices to have IP connectivity to the server which might not be the case in P2P scenarios.
+
+## Alternatives
+
+Try and do something with STUN or TURN or [COAP](http://coap.technology/).
+
+Rather than requiring the devices to poll for updated "long-polling" could be used instead similar to `/sync`.
+
+## Security considerations
+
+### Confidentiality of data
+
+Whilst the data transmitted can be encrypted in transit via HTTP/TLS the rendezvous server does have visibility over the
+data and can also perform man in the middle attacks.
+
+As such, for the purposes of authentication and end-to-end encryption the channel should be treated as untrusted and some
+form of secure layer should be used on top of the channel such as a Diffie-Hellman key exchange.
+
+### Denial of Service attack surface
+
+Because the protocol allows for the creation of arbitrary channels and storage of arbitrary data, it is possible to use
+it as a denial of service attack surface.
+
+As such, the following standard mitigations such as the following may be deemed appropriate by homeserver implementations
+and administrators:
+
+- rate limiting of requests
+- imposing a low maximum payload size (e.g. kilobytes not megabytes)
+- limiting the number of concurrent channels
+
+## Unstable prefix
+
+None.
+
+## Dependencies
+
+None.
+
+## Credits
+
+This proposal was influenced by https://wiki.mozilla.org/Services/KeyExchange which also has some helpful discussion
+around DoS mitigation.