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Major revision of proposal 158.
The big changes are to go from a "caches compute the micro-descriptor" format to an "authorities generate microdescriptors" format. See or-dev discussions of January 2009 for full rationales.
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@ -4,6 +4,15 @@ Author: Roger Dingledine
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Created: 17-Jan-2009
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Status: Open
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0. History
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15 May 2009: Substantially revised based on discussions on or-dev
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from late January. Removed the notion of voting on how to choose
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microdescriptors; made it just a function of the consesus method.
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(This lets us avoid the possibility of "desynchronization.")
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Added suggestion to use a new consensus flavor. Specified use of
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SHA256 for new hashes. -nickm
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1. Overview
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This proposal replaces section 3.2 of proposal 141, which was
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@ -22,6 +31,10 @@ Status: Open
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them, we'll need to resume considering some design like the one in
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proposal 141.
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Note also that any descriptor element which clients need to use to
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decide which servers to fetch info about, or which servers to fetch
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info from, needs to stay in the consensus.
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2. Motivation
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See
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@ -34,89 +47,69 @@ Status: Open
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3. Design
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There are three pieces to the proposal. First, authorities will list in
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their votes (and thus in the consensus) what relay descriptor elements
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are included in the microdescriptor, and also list the expected hash
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their votes (and thus in the consensus) the expected hash
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of microdescriptor for each relay. Second, directory mirrors will serve
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microdescriptors. Third, clients will ask for them and cache them.
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3.1. Consensus changes
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V3 votes should include a new line:
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microdescriptor-elements bar baz foo
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listing each descriptor element (sorted alphabetically) that authority
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included when it calculated its expected microdescriptor hashes.
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If the authorities choose a consensus method of a given version or
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later, a microdescriptor format is implicit in that version.
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A microdescriptor should in every case be a pure function of the
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router descriptor and the conensus method.
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We also need to include the hash of each expected microdescriptor in
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In votes, need to include the hash of each expected microdescriptor in
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the routerstatus section. I suggest a new "m" line for each stanza,
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with the base64 of the hash of the elements that the authority voted
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for above.
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with the base64 of the SHA256 hash of the router's microdescriptor.
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For every consensus method that an authority supports, it includes a
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separate "m" line in each router section of its vote, containing:
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"m" SP methods SP digest NL
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where methods is a comma-separated list of the consensus methods
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that the authority believes will produce "digest".
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(As with base64 encoding of SHA1 hashes in consensuses, let's
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omit the trailing =s)
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The consensus microdescriptor-elements and "m" lines are then computed
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as described in Section 3.1.2 below.
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I believe that means we need a new consensus-method "6" that knows
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how to compute the microdescriptor-elements and add "m" lines.
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(This means we need a new consensus-method that knows
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how to compute the microdescriptor-elements and add "m" lines.)
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3.1.1. Descriptor elements to include for now
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To start, the element list that authorities suggest should be
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family onion-key
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(Note that the or-dev posts above only mention onion-key, but if
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we don't also include family then clients will never learn it. It
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seemed like it should be relatively static, so putting it in the
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microdescriptor is smarter than trying to fit it into the consensus.)
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We could imagine a config option "family,onion-key" so authorities
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could change their voted preferences without needing to upgrade.
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In the first version, the microdescriptor should contain the
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onion-key element and the family element from the router descriptor.
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3.1.2. Computing consensus for microdescriptor-elements and "m" lines
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One approach is for the consensus microdescriptor-elements line to
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include every element listed by a majority of authorities, sorted. The
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problem here is that it will no longer be deterministic what the correct
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hash for the "m" line should be. We could imagine telling the authority
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to go look in its descriptor and produce the right hash itself, but
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we don't want consensus calculation to be based on external data like
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that. (Plus, the authority may not have the descriptor that everybody
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else voted to use.)
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When we generating a consensus, we use whichever m line
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unambiguously corresponds to the descriptor digest that will be
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included in the consensus. (If there are multiple m lines for that
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descriptor digest, we use whichever is most common. If they are
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equally common, we break ties in the favor of the lexically
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earliest. Either way, we should log a warning: That's likely a
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bug.)
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The better approach is to take the exact set that has the most votes
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(breaking ties by the set that has the most elements, and breaking
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ties after that by whichever is alphabetically first). That will
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increase the odds that we actually get a microdescriptor hash that
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is both a) for the descriptor we're putting in the consensus, and b)
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over the elements that we're declaring it should be for.
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The "m" lines in a consensus contain only the digest, not a list of
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consensus methods.
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Then the "m" line for a given relay is the one that gets the most votes
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from authorities that both a) voted for the microdescriptor-elements
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line we're using, and b) voted for the descriptor we're using.
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3.1.3. A new flavor of consensus
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(If there's a tie, use the smaller hash. But really, if there are
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multiple such votes and they differ about a microdescriptor, we caught
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one of them lying or being buggy. We should log it to track down why.)
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Rather than inserting "m" lines in the current consensus format,
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they should be included in a new consensus flavor (see proposal
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162).
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If there are no such votes, then we leave out the "m" line for that
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relay. That means clients should avoid it for this time period. (As
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an extension it could instead mean that clients should fetch the
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descriptor and figure out its microdescriptor themselves. But let's
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not get ahead of ourselves.)
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This flavor can safely omit descriptor digests.
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It would be nice to have a more foolproof way to agree on what
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microdescriptor hash each authority should vote for, so we can avoid
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missing "m" lines. Just switching to a new consensus-method each time
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we change the set of microdescriptor-elements won't help though, since
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each authority will still have to decide what hash to vote for before
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knowing what consensus-method will be used.
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We still need to descide whether to move ports into microdescriptors
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or not. In either case, they can be removed from the current "ns"
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flavor of consensus, since no current clients use them, and they
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take up about 5% of the compressed consensus.
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Here's one way we could do it. Each vote / consensus includes
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the microdescriptor-elements that were used to compute the hashes,
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and also a preferred-microdescriptor-elements set. If an authority
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has a consensus from the previous period, then it should use the
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consensus preferred-microdescriptor-elements when computing its votes
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for microdescriptor-elements and the appropriate hashes in the upcoming
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period. (If it has no previous consensus, then it just writes its
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own preferences in both lines.)
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This new consensus flavor should be signed with the sha256 signature
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format as documented in proposal 162.
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3.2. Directory mirrors serve microdescriptors
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@ -125,8 +118,10 @@ Status: Open
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continue to serve normal relay descriptors too, a) to serve old clients
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and b) to be able to construct microdescriptors on the fly.)
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The microdescriptors with hashes <D1>,<D2>,<D3> should be available at:
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http://<hostname>/tor/micro/d/<D1>+<D2>+<D3>.z
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The microdescriptors with base64 hashes <D1>,<D2>,<D3> should be available at:
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http://<hostname>/tor/micro/d/<D1>-<D2>-<D3>.z
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(We use base64 for size and for consistency with the consensus
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format. We use -s instead of +s to separate these items, since
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All the microdescriptors from the current consensus should also be
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available at:
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@ -134,24 +129,9 @@ Status: Open
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so a client that's bootstrapping doesn't need to send a 70KB URL just
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to name every microdescriptor it's looking for.
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The format of a microdescriptor is the header line
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"microdescriptor-header"
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followed by each element (keyword and body), alphabetically. There's
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no need to mention what hash it's for, since it's self-identifying:
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you can hash the elements to learn this.
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(Do we need a footer line to show that it's over, or is the next
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microdescriptor line or EOF enough of a hint? A footer line wouldn't
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hurt much. Also, no fair voting for the microdescriptor-element
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"microdescriptor-header".)
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Microdescriptors have no header or footer.
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The hash of the microdescriptor is simply the hash of the concatenated
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elements -- not counting the header line or hypothetical footer line.
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Unless you prefer that?
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Is there a reasonable way to version these things? We could say that
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the microdescriptor-header line can contain arguments which clients
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must ignore if they don't understand them. Any better ways?
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elements.
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Directory mirrors should check to make sure that the microdescriptors
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they're about to serve match the right hashes (either the hashes from
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@ -168,10 +148,14 @@ Status: Open
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When a client gets a new consensus, it looks to see if there are any
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microdescriptors it needs to learn. If it needs to learn more than
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some threshold of the microdescriptors (half?), it requests 'all',
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else it requests only the missing ones.
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else it requests only the missing ones. Clients MAY try to
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determine whether the upload bandwidth for listing the
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microdescriptors they want is more or less than the download
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bandwidth for the microdescriptors they do not want.
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Clients maintain a cache of microdescriptors along with metadata like
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when it was last referenced by a consensus. They keep a microdescriptor
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when it was last referenced by a consensus, and which identity key
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it corresponds to. They keep a microdescriptor
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until it hasn't been mentioned in any consensus for a week. Future
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clients might cache them for longer or shorter times.
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@ -192,14 +176,11 @@ Status: Open
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Phase one, the directory authorities should start voting on
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microdescriptors and microdescriptor elements, and putting them in the
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consensus. This should happen during the 0.2.1.x series, and should
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be relatively easy to do.
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consensus.
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Phase two, directory mirrors should learn how to serve them, and learn
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how to read the consensus to find out what they should be serving. This
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phase could be done either in 0.2.1.x or early in 0.2.2.x, depending
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on how messy it turns out to be and how quickly we get around to it.
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how to read the consensus to find out what they should be serving.
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Phase three, clients should start fetching and caching them instead
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of normal descriptors. This should happen post 0.2.1.x.
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of normal descriptors.
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