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clean up my microdescriptors proposals now that i've slept on it
svn:r18171
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@ -13,14 +13,16 @@ Status: Open
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circuit-building protocol to fetch a server descriptor inline at each
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circuit extend, we instead put all of the information that clients need
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either into the consensus itself, or into a new set of data about each
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relay called a microdescriptor.
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relay called a microdescriptor. The microdescriptor is a direct
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transform from the relay descriptor, so relays don't even need to know
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this is happening.
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The goal is that descriptor elements that are small and frequently
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changing should go in the consensus itself, descriptor elements that
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are small and relatively static should go in the microdescriptor,
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and if we ever end up with descriptor elements that aren't small yet
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clients need to know them, we'll need to resume considering some design
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like the one in proposal 141.
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Descriptor elements that are small and frequently changing should go
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in the consensus itself, and descriptor elements that are small and
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relatively static should go in the microdescriptor. If we ever end up
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with descriptor elements that aren't small yet clients need to know
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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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2. Motivation
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@ -37,12 +39,14 @@ Status: Open
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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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of microdescriptor for each relay. Second, directory mirrors will serve
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microdescriptors. Third, clients will ask for them and then cache them.
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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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We also 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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@ -65,10 +69,13 @@ Status: Open
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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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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 all elements listed by a majority of authorities, sorted. The
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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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@ -89,7 +96,7 @@ Status: Open
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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 being lying or buggy. We should log it to track down why.)
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one of them lying or being buggy. We should log it to track down why.)
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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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@ -110,13 +117,15 @@ Status: Open
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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 puts down its
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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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3.2. Directory mirrors serve microdescriptors
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Directory mirrors should then read the microdescriptor-elements line
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from the consensus, and learn how to answer requests.
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from the consensus, and learn how to answer requests. (Directory mirrors
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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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@ -128,23 +137,23 @@ Status: Open
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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 1"
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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 is, since you can hash the elements
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to learn this.
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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".)
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"microdescriptor-header".)
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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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Is this smart?
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Unless you prefer that?
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Note that I put a "1" up there in the header line. It isn't part
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of what's hashed, though. Is there a way to put in a version that's
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more useful?
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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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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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@ -165,7 +174,8 @@ Status: Open
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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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until it hasn't been mentioned in any consensus for a week.
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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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3.3.1. Information leaks from clients
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@ -188,11 +198,9 @@ Status: Open
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be relatively easy to do.
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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. It
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would be great if we can squeeze this in during 0.2.1.x also, so once
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clients start to fetch them there will be many mirrors to choose from.
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(Are there reasonable ways to build only part of phase two in 0.2.1.x?)
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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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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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