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+Firefox System Add-on Update Protocol
+=====================================
+This document describes the protocol that Firefox uses when retrieving updates
+for System Add-ons from the automatic update service (AUS, currently `Balrog`_),
+and the expected behavior of Firefox based on the updater service's response.
+
+.. _Balrog: https://wiki.mozilla.org/Balrog
+
+System Add-ons
+--------------
+System add-ons:
+
+* Are add-ons that ship with Firefox, are hidden from the UI, and cannot be
+  disabled
+* Can be updated by Firefox depending on the AUS response to Firefox's update
+  request
+* Are stored in two locations:
+
+  * The **default** set ships with Firefox and is stored in the application
+    directory.
+  * The **update** set is stored in the user’s profile directory. If an add-on
+    is both in the update and default set, the update version gets precedence.
+
+Update Request
+--------------
+To determine what updates to install, Firefox makes an HTTP **GET** request to
+AUS once a day via a URL of the form::
+
+  https://aus5.mozilla.org/update/3/SystemAddons/%VERSION%/%BUILD_ID%/%BUILD_TARGET%/%LOCALE%/%CHANNEL%/%OS_VERSION%/%DISTRIBUTION%/%DISTRIBUTION_VERSION%/update.xml
+
+The path segments surrounded by ``%`` symbols are variable fields that Firefox
+fills in with information about itself and the environment it's running in:
+
+``VERSION``
+  Firefox version number
+``BUILD_ID``
+  Build ID
+``BUILD_TARGET``
+  Build target
+``LOCALE``
+  Build locale
+``CHANNEL``
+  Update channel
+``OS_VERSION``
+  OS Version
+``DISTRIBUTION``
+  Firefox Distribution
+``DISTRIBUTION_VERSION``
+  Firefox Distribution version
+
+Update Response
+---------------
+AUS should respond with an XML document that looks something like this:
+
+.. code-block:: xml
+
+  <?xml version="1.0"?>
+  <updates>
+    <addons>
+      <addon id="flyweb@mozilla.org" URL="https://ftp.mozilla.org/pub/system-addons/flyweb/flyweb@mozilla.org-1.0.xpi" hashFunction="sha512" hashValue="abcdef123" size="1234" version="1.0"/>
+      <addon id="pocket@mozilla.org" URL="https://ftp.mozilla.org/pub/system-addons/pocket/pocket@mozilla.org-1.0.xpi" hashFunction="sha512" hashValue="abcdef123" size="1234" version="1.0"/>
+    </addons>
+  </updates>
+
+* The root element is ``<updates>``, used for all updater responses.
+* The only child of ``<updates>`` is ``<addons>``, which represents a list of
+  system add-ons to update.
+* Within ``<addons>`` are several ``<addon>`` tags, each one corresponding to a
+  system add-on to update.
+
+``<addon>`` tags **must** have the following attributes:
+
+``id``
+  The extension ID
+``URL``
+  URL to a signed XPI of the specified add-on version to download
+``hashFunction``
+  Identifier of the hash function used to generate the hashValue attribute.
+``hashValue``
+  Hash of the XPI file linked from the URL attribute, calculated using the function specified in the hashValue attribute.
+``size``
+  Size (in bytes) of the XPI file linked from the URL attribute.
+``version``
+  Version number of the add-on
+
+Update Behavior
+---------------
+After receiving the update response, Firefox modifies the **update** add-ons
+according to the following algorithm:
+
+1. If the ``<addons>`` tag is empty (``<addons></addons>``) in the response,
+   **remove all system add-on updates**.
+2. If no add-ons were specified in the response (i.e. the ``<addons>`` tag
+   is not present), do nothing and finish.
+3. If the **update** add-on set is equal to the set of add-ons specified in the
+   update response, do nothing and finish.
+4. If the set of **default** add-ons is equal to the set of add-ons specified in
+   the update response, remove all the  **update** add-ons and finish.
+5. Download each add-on specified in the update response and store them in the
+   "downloaded add-on set". A failed download **must** abort the entire system
+   add-on update.
+6. Validate the downloaded add-ons. The following **must** be true for all
+   downloaded add-ons, or the update process is aborted:
+
+   a. The ID and version of the downloaded add-on must match the specified ID or
+      version in the update response.
+   b. The hash provided in the update response must match the downloaded add-on
+      file.
+   c. The downloaded add-on file size must match the size given in the update
+      response.
+   d. The add-on must be compatible with Firefox (i.e. it must not be for a
+      different application, such as Thunderbird).
+   e. The add-on must be packed (i.e. be an XPI file).
+   f. The add-on must be restartless.
+   g. The add-on must be signed by the system add-on root certificate.
+
+6. Once all downloaded add-ons are validated, install them into the profile
+   directory as part of the **update** set.
+
+Notes on the update process:
+
+* Add-ons are considered "equal" if they have the same ID and version number.
+
+Examples
+--------
+The follow section describes common situations that we have or expect to run
+into and how the protocol described above handles them.
+
+For simplicity, unless otherwise specified, all examples assume that there are
+two system add-ons in existence: **FlyWeb** and **Pocket**.
+
+Basic
+~~~~~
+A user has Firefox 45, which shipped with FlyWeb 1.0 and Pocket 1.0. We want to
+update users to FlyWeb 2.0. AUS sends out the following update response:
+
+.. code-block:: xml
+
+  <updates>
+    <addons>
+      <addon id="flyweb@mozilla.org" URL="https://ftp.mozilla.org/pub/system-addons/flyweb/flyweb@mozilla.org-2.0.xpi" hashFunction="sha512" hashValue="abcdef123" size="1234" version="2.0"/>
+      <addon id="pocket@mozilla.org" URL="https://ftp.mozilla.org/pub/system-addons/pocket/pocket@mozilla.org-1.0.xpi" hashFunction="sha512" hashValue="abcdef123" size="1234" version="1.0"/>
+    </addons>
+  </updates>
+
+Firefox will download FlyWeb 2.0 and Pocket 1.0 and store them in the profile directory.
+
+Missing Add-on
+~~~~~~~~~~~~~~
+A user has Firefox 45, which shipped with FlyWeb 1.0 and Pocket 1.0. We want to
+update users to FlyWeb 2.0, but accidentally forget to specify Pocket in the
+update response. AUS sends out the following:
+
+.. code-block:: xml
+
+  <updates>
+    <addons>
+      <addon id="flyweb@mozilla.org" URL="https://ftp.mozilla.org/pub/system-addons/flyweb/flyweb@mozilla.org-2.0.xpi" hashFunction="sha512" hashValue="abcdef123" size="1234" version="2.0"/>
+    </addons>
+  </updates>
+
+Firefox will download FlyWeb 2.0 and store it in the profile directory. Pocket
+1.0 from the **default** location will be used.
+
+Remove all system add-on updates
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+A response from AUS with an empty add-on set will *remove all system add-on
+updates*:
+
+.. code-block:: xml
+
+  <updates>
+    <addons></addons>
+  </updates>
+
+Rollout
+~~~~~~~
+A user has Firefox 45, which shipped with FlyWeb 1.0 and Pocket 1.0. We want to
+rollout FlyWeb 2.0 at a 10% sample rate. 10% of the time, AUS sends out:
+
+.. code-block:: xml
+
+  <updates>
+    <addons>
+      <addon id="flyweb@mozilla.org" URL="https://ftp.mozilla.org/pub/system-addons/flyweb/flyweb@mozilla.org-2.0.xpi" hashFunction="sha512" hashValue="abcdef123" size="1234" version="2.0"/>
+      <addon id="pocket@mozilla.org" URL="https://ftp.mozilla.org/pub/system-addons/pocket/pocket@mozilla.org-1.0.xpi" hashFunction="sha512" hashValue="abcdef123" size="1234" version="1.0"/>
+    </addons>
+  </updates>
+
+With this response, Firefox will download Pocket 1.0 and FlyWeb 2.0 and install
+them into the profile directory.
+
+The other 90% of the time, AUS sends out an empty response:
+
+.. code-block:: xml
+
+  <updates></updates>
+
+With the empty response, Firefox will not make any changes. This means users who
+haven’t seen the 10% update response will stay on FlyWeb 1.0, and users who have
+seen it will stay on FlyWeb 2.0.
+
+Once we’re happy with the rollout and want to switch to 100%, AUS will send the
+10% update response to 100% of users, upgrading everyone to FlyWeb 2.0.
+
+Rollback
+~~~~~~~~
+This example continues from the “Rollout” example. If, during the 10% rollout,
+we find a major issue with FlyWeb 2.0, we want to roll all users back to FlyWeb 1.0.
+AUS sends out the following:
+
+.. code-block:: xml
+
+  <updates>
+    <addons>
+      <addon id="flyweb@mozilla.org" URL="https://ftp.mozilla.org/pub/system-addons/flyweb/flyweb@mozilla.org-1.0.xpi" hashFunction="sha512" hashValue="abcdef123" size="1234" version="1.0"/>
+      <addon id="pocket@mozilla.org" URL="https://ftp.mozilla.org/pub/system-addons/pocket/pocket@mozilla.org-1.0.xpi" hashFunction="sha512" hashValue="abcdef123" size="1234" version="1.0"/>
+    </addons>
+  </updates>
+
+For users who have updated, Firefox will download FlyWeb 1.0 and Pocket 1.0 and
+install them into the profile directory. For users that haven’t yet updated,
+Firefox will see that the **default** add-on set matches the set in the update
+ping and clear the **update** add-on set.