Add extended-monitor spec (#466)

Author: emersion

extensions/extended-monitor.md

@@ -0,0 +1,73 @@
+---
+title: "`extended-monitor` Extension"
+layout: spec
+work-in-progress: true
+extends:
+  - monitor
+  - away-notify
+  - account-notify
+  - chghost
+  - setname
+copyrights:
+  -
+    name: "Simon Ser"
+    email: "[email protected]"
+    period: "2021"
+---
+
+
+## Notes for implementing work-in-progress version
+
+This is a work-in-progress specification.
+
+Software implementing this work-in-progress specification MUST NOT use the
+unprefixed `extended-monitor` capability name. Instead, implementations SHOULD
+use the `draft/extended-monitor` capability name to be interoperable with other
+software implementing a compatible work-in-progress version.
+
+The final version of the specification will use an unprefixed capability name.
+
+
+## Description
+
+With the help of extensions such as [`away-notify`](away-notify.html),
+[`account-notify`](account-notify.html), [`chghost`](chghost.html) and
+[`setname`](setname.html), clients are able to keep other users' metadata
+up-to-date with their local state when they share a channel. However, clients
+are not able to do so when they don't share a channel.
+
+The [`monitor`](monitor.html) extension allows clients to track when another
+user goes offline or comes online. This specification extends MONITOR to also
+include AWAY, ACCOUNT, CHGHOST and SETNAME notifications.
+
+The `extended-monitor` capability advertises that the server supports sending
+such extended notifcations for monitored nicks. When enabled by the client:
+
+- If `away-notify` is also enabled, the client will get AWAY notifications for
+  monitored nicks.
+- If `account-notify` is also enabled, the client will get ACCOUNT
+  notifications for monitored nicks.
+- If `chghost` is also enabled, the client will get CHGHOST notifications for
+  monitored nicks.
+- If `setname` is also enabled, the client will get SETNAME notifications for
+  monitored nicks.
+
+## Privacy considerations
+
+This extension allows users to monitor personal information of other users.
+Since the IRC protocol doesn't provide any way to atomically change this
+personal information (nick, host, realname all at the same time), it may be
+possible for an outside observer to track nick changes:
+
+- Observer monitors nick A and nick B.
+- User with nick A changes their nick to nick B, then changes their host and
+  realname.
+- Observer receives `RPL_MONOFFLINE` for nick A, `RPL_MONONLINE` for nick B,
+  and then receives notifications for host and realname changes.
+
+In this scenario, it's possible for the observer to figure out that nick A and
+nick B are owned by the same user by comparing the host and realname.
+
+For this reason, privacy conscious clients are advised to disconnect and
+re-connect to the IRC server as a way to atomically update personal
+information.