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+# MSC1730: Mechanism for redirecting to an alternative server during login
+
+## Background/requirements
+
+This is a proposal for a mechanism for handling the following situation.
+
+A large, loosely-coupled organisation wants its members to be able to
+communicate with one another via Matrix. The organisation consists of several
+departments which are cooperative but prefer to host their own infrastructure.
+
+The organisation has an existing single-sign-on system which covers the entire
+organisation, and which they would like their members to use when
+authenticating to the Matrix system.
+
+## Proposal
+
+The response to `POST /_matrix/client/r0/login` currently includes the fields
+`user_id`, `access_token`, `device_id`, and the deprecated `home_server`.
+
+We will add to this the the field `well_known`, which has the same format as
+the [`/.well-known/matrix/client`
+object](https://matrix.org/docs/spec/client_server/r0.4.0.html#get-well-known-matrix-client).
+
+Servers MAY add this field to the login response if they wish to redirect
+clients to an alternative homeserver after login. Clients SHOULD use the
+provided `well_known` object to reconfigure themselves, optionally validating the
+URLs within.
+
+## Application
+
+Let's imagine for this description that our organisation is the University of
+Canadialand, which is divided into departments including Engineering, History,
+Physics, and so on.
+
+Central University IT currently host a SAML2-based single-sign-on system, which
+asks users to select their department, and then defers to the departmental
+authentication system to authenticate them. Note that the users do not have a
+globally-unique identifier.
+
+University IT now sets up a Matrix Homeserver instance, which they host at
+`https://matrix.ac.cdl`. They run a publicity campaign encouraging university
+members to use the service by configuring off-the-shelf Matrix clients to use
+the homeserver at `https://matrix.ac.cdl`. They may also release customised
+clients configured to use that URL by default.
+
+However, the departments actually want to host their own homeservers; these
+might be at `https://matrix.eng.ac.cdl`, `https://matrix.hist.ac.cdl`, etc. The
+central IT homeserver therefore redirects clients to the departmental
+homeserver after login.
+
+A complete login flow is as shown in the following sequence diagram:
+
+
+
+Note that this flow is complicated by the out-of-band SAML2 authentication. We
+envisage that a similar technique could also be used for a standard
+username/password authentication, however.
+
+## Rejected solutions
+
+Alternative solutions might include:
+
+### Have all users on one homeserver
+
+In many situtations, it might be more appropriate to have a single homeserver,
+so users' MXids would look like `@user:ac.cdl` instead of
+`@user:eng.ac.cdl`.
+
+However, there are circumstances where separate homeservers are required:
+
+* the departments may be only very loosely related
+* the departments may have privacy concerns
+* the dpeartments may be geographically distributed with slow or unreliable
+ links to the central system
+* load-balancing may be essential.
+
+### Tell users the C-S API for their home homeserver
+
+We could tell Engineering users to configure their clients with
+`https://matrix.eng.ac.cdl`, History users to use `https://matrix.hist.ac.cdl`,
+etc.
+
+The problems with this are:
+
+ * Each department must issue its own documentation and publicity advising how
+ to configure a Matrix client
+
+ * It becomes impractical to distribute preconfigured clients.
+
+### Proxy all C-S endpoints
+
+It would be possible for the the central homeserver to proxy all C-S
+interaction, as well as `/login`, directing requests to the right server for
+the user.
+
+This is unsatisfactory due to the additional latency imposed, the load on the
+central homeserver, and the fact that it makes the central server a single
+point of failure for the entire system.
+
+### Require clients to perform a .well-known lookup after login
+
+We could require clients to do a .well-known lookup on the domain of their MXID
+once they have discovered it from the `/login` response.
+
+This has the following problems:
+
+* In most cases this `.well-known` lookup will be entirely redundant. It adds
+ latency and overhead, and complicates client implementations.
+
+* It complicates deployment, since each department has to host a `.well-known`
+ file at their root domain.
+
+### Add an alternative redirection mechanism in the login flow
+
+We could specify that the `/login` response could contain a `redirect` field
+property instead of the existing `user_id`/`access_token`/`device_id`
+properties. The `redirect` property would give the C-S API of the target
+HS. The client would then repeat its `/login` request, and use the specified
+endpoint for all future C-S interaction.
+
+This approach would complicate client implementations.
+
+### Modify the single-sign-on flow
+
+It would be possible to modify the single-sign-on flow to allow an alternative
+homeserver to be specified for the final `m.login.token`-based call to
+`/login` (and subsequent C-S API calls).
+
+This is discussed in more detail in
+[MSC1731](https://github.com/matrix-org/matrix-doc/blob/rav/proposals/homeserver_in_sso_login/proposals/1731-redirect-in-sso-login.md).
+
+It has the disadvantage of limiting the solution to SSO logins. The solution
+presented in this proposal also extends to password-based logins.
+
+### Use a 3pid login flow
+
+It has been suggested that we could use a login flow based on third-party
+identifiers.
+
+In the current ecosystem, to do a 3pid login, clients must still be configured
+to send their `/login` request to a particular homeserver, which will then take
+them through an authentication process. We are therefore still left with the
+problem that we need to switch homeservers between login and initial sync.
+
+An alternative would be for clients to somehow know that they should go through
+the single-sign-on process *before* choosing a homeserver, and for the
+output of the single-sign-on process to indicate the homeserver to use. This
+would require either substantially customised Matrix clients, or substantial
+modifications to the login flow in Matrix, possibly involving authenticating
+against an identity server. The latter is something which could be considered,
+but the scope of the changes required make it impractical in the short/medium
+term.
diff --git a/proposals/images/1730-seq-diagram.1.svg b/proposals/images/1730-seq-diagram.1.svg
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+
\ No newline at end of file
diff --git a/proposals/images/1730-seq-diagram.txt b/proposals/images/1730-seq-diagram.txt
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+# https://sequencediagram.org/
+
+participantspacing equal
+participant Client
+participant matrix.ac.cdl
+participant SAML2 SSO system
+participant matrix.eng.ac.cdl
+
+activate Client
+
+Client->matrix.ac.cdl:""GET /_matrix/client/r0/login""
+activate matrix.ac.cdl
+Client<--matrix.ac.cdl:"""type": "m.login.sso"""
+deactivate matrix.ac.cdl
+# Start SSO flow: displayed in browser for web clients, or via fallback auth in embeddedbrowser for others
+Client->matrix.ac.cdl:""GET /_matrix/client/r0/login/sso/redirect\n--?redirectUrl=--""
+activate matrix.ac.cdl
+matrix.ac.cdl->matrix.ac.cdl:Generate SAML request
+Client<--matrix.ac.cdl:302 to SSO system
+deactivate matrix.ac.cdl
+Client->SAML2 SSO system:""GET /single-sign-on\n--?SAMLRequest=&RelayState=--""
+activate SAML2 SSO system
+Client<-->SAML2 SSO system: auth credentials
+SAML2 SSO system-->Client:auto-submitting HTML form including SAML Response
+deactivate SAML2 SSO system
+Client->matrix.ac.cdl:""POST /SAML2\n--SAMLResponse=\nRelayState=
+activate matrix.ac.cdl
+matrix.ac.cdl->matrix.ac.cdl:map user to HS
+Client<--matrix.ac.cdl:auto-submitting HTML form for target HS
+deactivate matrix.ac.cdl
+Client->matrix.eng.ac.cdl:""POST /SAML2\n--SAMLResponse=\nRelayState=
+activate matrix.eng.ac.cdl
+Client<--matrix.eng.ac.cdl:302 to clienturl with\n""--?loginToken=
+deactivate matrix.eng.ac.cdl
+Client->matrix.ac.cdl:""POST /_matrix/client/r0/login\n--{"type": "m.login.token","token": ""}
+activate matrix.ac.cdl
+matrix.ac.cdl->matrix.eng.ac.cdl:""POST /_matrix/client/r0/login\n--{"type": "m.login.token", "token": ""}
+activate matrix.eng.ac.cdl
+matrix.ac.cdl<--matrix.eng.ac.cdl:""--{"user_id": "@user:eng.ac.cdl", "access_token": "abc123",\n "well_known": {"m.homeserver": "..."}}
+deactivate matrix.eng.ac.cdl
+Client<--matrix.ac.cdl:""--{"user_id": "@user:eng.ac.cdl",\n "access_token": "abc123",\n "well-known": {"m.homeserver": "..."}}
+deactivate matrix.ac.cdl
+Client<-#0000FF>matrix.eng.ac.cdl: Matrix