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+Filename: 154-automatic-updates.txt
+Title: Automatic Software Update Protocol
+Version: $Revision$
+Last-Modified: $Date$
+Author: Matt Edman
+Created: 30-July-2008
+Status: Open
+Target: 0.2.1.x
+
+
+Scope
+
+  This proposal specifies the method by which an automatic update client can
+  determine the most recent recommended Tor installation package for the
+  user's platform, download the package, and then verify that the package was
+  downloaded successfully. While this proposal focuses on only the Tor
+  software, the protocol defined is sufficiently extensible such that other
+  components of the Tor bundles, like Vidalia, Polipo, and Torbutton, can be
+  managed and updated by the automatic update client as well.
+
+  The initial target platform for the automatic update framework is Windows,
+  given that's the platform used by a majority of our users and that it lacks
+  a sane package management system that many Linux distributions already have.
+  Our second target platform will be Mac OS X, and so the protocol will be
+  designed with this near-future direction in mind.
+
+  Other client-side aspects of the automatic update process, such as user
+  interaction, the interface presented, and actual package installation
+  procedure, are outside the scope of this proposal.
+
+
+Motivation
+
+  Tor releases new versions frequently, often with important security,
+  anonymity, and stability fixes. Thus, it is important for users to be able
+  to promptly recognize when new versions are available and to easily
+  download, authenticate, and install updated Tor and Tor-related software
+  packages.
+
+  Tor's control protocol [2] provides a method by which controllers can
+  identify when the user's Tor software is obsolete or otherwise no longer
+  recommended. Currently, however, no mechanism exists for clients to
+  automatically download and install updated Tor and Tor-related software for
+  the user.
+
+
+Design Overview
+
+  The core of the automatic update framework is a well-defined file called a
+  "recommended-packages" file. The recommended-packages file is accessible via
+  HTTP[S] at one or more well-defined URLs. An example recommended-packages
+  URL may be:
+
+    https://updates.torproject.org/recommended-packages
+
+  The recommended-packages document is formatted according to Section 1.2
+  below and specifies the most recent recommended installation package
+  versions for Tor or Tor-related software, as well as URLs at which the
+  packages and their signatures can be downloaded.
+
+  An automatic update client process runs on the Tor user's computer and
+  periodically retrieves the recommended-packages file according to the method
+  described in Section 2.0. As described further in Section 1.2, the
+  recommended-packages file is signed and can be verified by the automatic
+  update client with one or more public keys included in the client software.
+  Since it is signed, the recommended-packages file can be mirrored by
+  multiple hosts (e.g., Tor directory authorities), whose URLs are included in
+  the automatic update client's configuration.
+
+  After retrieving and verifying the recommended-packages file, the automatic
+  update client compares the versions of the recommended software packages
+  listed in the file with those currently installed on the end-user's
+  computer. If one or more of the installed packages is determined to be out
+  of date, an updated package and its signature will be downloaded from one of
+  the package URLs listed in the recommended-packages file as described in
+  Section 2.2.
+
+  The automatic update system uses a multilevel signing key scheme for package
+  signatures. There are a small number of entities we call "packaging
+  authorities" that each have their own signing key. A packaging authority is
+  responsible for signing and publishing the recommended-packages file.
+  Additionally, each individual packager responsible for producing an
+  installation package for one or more platforms has their own signing key.
+  Every packager's signing key must be signed by at least one of the packaging
+  authority keys.
+
+
+Specification
+
+  1. recommended-packages Specification
+
+  In this section we formally specify the format of the published
+  recommended-packages file.
+
+  1.1. Document Meta-format
+
+  The recommended-packages document follows the lightweight extensible
+  information format defined in Tor's directory protocol specification [1]. In
+  the interest of self-containment, we have reproduced the relevant portions
+  of that format's specification in this Section. (Credits to Nick Mathewson
+  for much of the original format definition language.)
+
+  The highest level object is a Document, which consists of one or more
+  Items.  Every Item begins with a KeywordLine, followed by zero or more
+  Objects. A KeywordLine begins with a Keyword, optionally followed by
+  whitespace and more non-newline characters, and ends with a newline.  A
+  Keyword is a sequence of one or more characters in the set [A-Za-z0-9-].
+  An Object is a block of encoded data in pseudo-Open-PGP-style
+  armor. (cf. RFC 2440)
+
+  More formally:
+
+    Document     ::= (Item | NL)+
+    Item         ::= KeywordLine Object*
+    KeywordLine  ::= Keyword NL | Keyword WS ArgumentChar+ NL
+    Keyword      ::= KeywordChar+
+    KeywordChar  ::= 'A' ... 'Z' | 'a' ... 'z' | '0' ... '9' | '-'
+    ArgumentChar ::= any printing ASCII character except NL.
+    WS           ::= (SP | TAB)+
+    Object       ::= BeginLine Base-64-encoded-data EndLine
+    BeginLine    ::= "-----BEGIN " Keyword "-----" NL
+    EndLine      ::= "-----END " Keyword "-----" NL
+
+    The BeginLine and EndLine of an Object must use the same keyword.
+
+  In our Document description below, we also tag Items with a multiplicity in
+  brackets. Possible tags are:
+
+    "At start, exactly once": These items MUST occur in every instance of the
+    document type, and MUST appear exactly once, and MUST be the first item in
+    their documents.
+
+    "Exactly once": These items MUST occur exactly one time in every
+    instance of the document type.
+
+    "Once or more": These items MUST occur at least once in any instance
+    of the document type, and MAY occur more than once.
+
+    "At end, exactly once": These items MUST occur in every instance of
+    the document type, and MUST appear exactly once, and MUST be the
+    last item in their documents.
+
+  1.2. recommended-packages Document Format
+
+  When interpreting a recommended-packages Document, software MUST ignore
+  any KeywordLine that starts with a keyword it doesn't recognize; future
+  implementations MUST NOT require current automatic update clients to
+  understand any KeywordLine not currently described.
+
+  In lines that take multiple arguments, extra arguments SHOULD be
+  accepted and ignored.
+
+  The currently defined Items contained in a recommended-packages document
+  are:
+
+    "recommended-packages-format" SP number NL
+
+      [Exactly once]
+
+      This Item specifies the version of the recommended-packages format that
+      is contained in the subsequent document. The version defined in this
+      proposal is version "1". Subsequent iterations of this protocol MUST
+      increment this value if they introduce incompatible changes to the
+      document format and MAY increment this value if they only introduce
+      additional Keywords.
+
+    "published" SP YYYY-MM-DD SP HH:MM:SS NL
+
+      [Exactly once]
+
+      The time, in GMT, when this recommended-packages document was generated.
+      Automatic update clients SHOULD ignore Documents over 60 days old.
+
+    "tor-stable-win32-version" SP TorVersion NL
+
+      [Exactly once]
+
+      This keyword specifies the latest recommended release of Tor's "stable"
+      branch for the Windows platform that has an installation package
+      available. Note that this version does not necessarily correspond to the
+      most recently tagged stable Tor version, since that version may not yet
+      have an installer package available, or may have known issues on
+      Windows.
+
+      The TorVersion field is formatted according to Section 2 of Tor's
+      version specification [3].
+
+    "tor-stable-win32-package" SP Url NL
+
+      [Once or more]
+
+      This Item specifies the location from which the most recent
+      recommended Windows installation package for Tor's stable branch can be
+      downloaded.
+
+      When this Item appears multiple times within the Document, automatic
+      update clients SHOULD select randomly from the available package
+      mirrors.
+
+    "tor-dev-win32-version" SP TorVersion NL
+
+      [Exactly once]
+
+      This Item specifies the latest recommended release of Tor's
+      "development" branch for the Windows platform that has an installation
+      package available. The same caveats from the description of
+      "tor-stable-win32-version" also apply to this keyword.
+
+      The TorVersion field is formatted according to Section 2 of Tor's
+      version specification [3].
+
+    "tor-dev-win32-package" SP Url NL
+
+      [Once or more]
+
+      This Item specifies the location from which the most recent recommended
+      Windows installation package and its signature for Tor's development
+      branch can be downloaded.
+
+      When this Keyword appears multiple times within the Document, automatic
+      update clients SHOULD select randomly from the available package
+      mirrors.
+
+    "signature" NL SIGNATURE NL
+
+      [At end, exactly once]
+
+      The "SIGNATURE" Object contains a PGP signature (using a packaging
+      authority signing key) of the entire document, taken from the beginning
+      of the "recommended-packages-format" keyword, through the newline after
+      the "signature" Keyword.
+
+
+  2. Automatic Update Client Behavior
+
+  The client-side component of the automatic update framework is an
+  application that runs on the end-user's machine. It is responsible for
+  fetching and verifying a recommended-packages document, as well as
+  downloading, verifying, and subsequently installing any necessary updated
+  software packages.
+
+  2.1. Download and verify a recommended-packages document
+
+  The first step in the automatic update process is for the client to download
+  a copy of the recommended-packages file. The automatic update client
+  contains a (hardcoded and/or user-configurable) list of URLs from which it
+  will attempt to retrieve a recommended-packages file.
+
+  Connections to each of the recommended-packages URLs SHOULD be attempted in
+  the following order:
+
+    1) HTTPS over Tor
+    2) HTTP over Tor
+    3) Direct HTTPS
+    4) Direct HTTP
+
+  If the client fails to retrieve a recommended-packages document via any of
+  the above connection methods from any of the configured URLs, the client
+  SHOULD retry its download attempts following an exponential back-off
+  algorithm. After the first failed attempt, the client SHOULD delay one hour
+  before attempting again, up to a maximum of 24 hours delay between retry
+  attempts.
+
+  After successfully downloading a recommended-packages file, the automatic
+  update client will verify the signature using one of the public keys
+  distributed with the client software. If more than one recommended-packages
+  file is downloaded and verified, the file with the most recent "published"
+  date that is verified will be retained and the rest discarded.
+
+  2.2. Download and verify the updated packages
+
+  The automatic update client next compares the latest recommended package
+  version from the recommended-packages document with the currently installed
+  Tor version. If the user currently has installed a Tor version from Tor's
+  "development" branch, then the version specified in "tor-dev-*-version" Item
+  is used for comparison. Similarly, if the user currently has installed a Tor
+  version from Tor's "stable" branch, then the version specified in the
+  "tor-stable-*version" Item is used for comparison. Version comparisons are
+  done according to Tor's version specification [3].
+
+  If the automatic update client determines an installation package newer than
+  the user's currently installed version is available, it will attempt to
+  download a package appropriate for the user's platform and Tor branch from a
+  URL specified by a "tor-[branch]-[platform]-package" Item. If more than one
+  mirror for the selected package is available, a mirror will be chosen at
+  random from all those available.
+
+  The automatic update client must also download a ".asc" signature file for
+  the retrieved package. The URL for the package signature is the same as that
+  for the package itself, except with the extension ".asc" appended to the
+  package URL.
+
+  Connections to download the updated package and its signature SHOULD be
+  attempted in the same order described in Section 2.1.
+
+  After completing the steps described in Sections 2.1 and 2.2, the automatic
+  update client will have downloaded and verified a copy of the latest Tor
+  installation package. It can then take whatever subsequent platform-specific
+  steps are necessary to install the downloaded software updates.
+
+  2.3. Periodic checking for updates
+
+  The automatic update client SHOULD maintain a local state file in which it
+  records (at a minimum) the timestamp at which it last retrieved a
+  recommended-packages file and the timestamp at which the client last
+  successfully downloaded and installed a software update.
+
+  Automatic update clients SHOULD check for an updated recommended-packages
+  document at most once per day but at least once every 30 days.
+
+
+  3. Future Extensions
+
+  There are several possible areas for future extensions of this framework.
+  The extensions below are merely suggestions and should be the subject of
+  their own proposal before being implemented.
+
+  3.1. Additional Software Updates
+
+  There are several software packages often included in Tor bundles besides
+  Tor, such as Vidalia, Privoxy or Polipo, and Torbutton. The versions and
+  download locations of updated installation packages for these bundle
+  components can be easily added to the recommended-packages document
+  specification above.
+
+  3.2. Including ChangeLog Information
+
+  It may be useful for automatic update clients to be able to display for
+  users a summary of the changes made in the latest Tor or Tor-related
+  software release, before the user chooses to install the update. In the
+  future, we can add keywords to the specification in Section 1.2 that specify
+  the location of a ChangeLog file for the latest recommended package
+  versions. It may also be desirable to allow localized ChangeLog information,
+  so that the automatic update client can fetch release notes in the
+  end-user's preferred language.
+
+  3.3. Weighted Package Mirror Selection
+
+  We defined in Section 1.2 a method by which automatic update clients can
+  select from multiple available package mirrors. We may want to add a Weight
+  argument to the "*-package" Items that allows the recommended-packages file
+  to suggest to clients the probability with which a package mirror should be
+  chosen. This will allow clients to more appropriately distribute package
+  downloads across available mirrors proportional to their approximate
+  bandwidth.
+
+
+Implementation
+
+  Implementation of this proposal will consist of two separate components.
+
+  The first component is a small "au-publish" tool that takes as input a
+  configuration file specifying the information described in Section 1.2 and a
+  private key. The tool is run by a "packaging authority" (someone responsible
+  for publishing updated installation packages), who will be prompted to enter
+  the passphrase for the private key used to sign the recommended-packages
+  document. The output of the tool is a document formatted according to
+  Section 1.2, with a signature appended at the end. The resulting document
+  can then be published to any of the update mirrors.
+
+  The second component is an "au-client" tool that is run on the end-user's
+  machine. It periodically checks for updated installation packages according
+  to Section 2 and fetches the packages if necessary. The public keys used
+  to sign the recommended-packages file and any of the published packages are
+  included in the "au-client" tool.
+
+
+References
+
+  [1] Tor directory protocol (version 3),
+  https://tor-svn.freehaven.net/svn/tor/trunk/doc/spec/dir-spec.txt
+
+  [2] Tor control protocol (version 2),
+  https://tor-svn.freehaven.net/svn/tor/trunk/doc/spec/control-spec.txt
+
+  [3] Tor version specification,
+  https://tor-svn.freehaven.net/svn/tor/trunk/doc/spec/version-spec.txt
+