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some notes on how exactly to do this microdescriptor thing.

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Roger Dingledine 2009-01-18 09:51:09 +00:00
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Filename: xxx-microdescriptors.txt
Title: Clients download consensus + microdescriptors
Version: $Revision$
Last-Modified: $Date$
Author: Roger Dingledine
Created: 17-Jan-2009
Status: Open
1. Overview
This proposal replaces section 3.2 of proposal 141, called "Fetching
descriptors on demand". Rather than modifying the circuit-building
protocol to fetch a server descriptor inline at each circuit extend,
we instead put all of the information that clients need either into
the consensus itself, or into a new set of data about each relay
called a microdescriptor. The goal is that descriptor elements that
are small and frequently changing should go in the consensus itself,
descriptor elements that are small and relatively static should go in
the microdescriptor, and if we ever end up with descriptor elements
that aren't small yet clients need to know them, we'll need to resume
considering some design like the one in proposal 141.
2. Motivation
See
http://archives.seul.org/or/dev/Nov-2008/msg00000.html and
http://archives.seul.org/or/dev/Nov-2008/msg00001.html and especially
http://archives.seul.org/or/dev/Nov-2008/msg00007.html
for a discussion of the options and why this is currently the best
approach.
3. Design
There are three pieces to the proposal. First, authorities will list
in their votes (and thus in the consensus) what relay elements are
included in the microdescriptor, and also list the expected hash of
microdescriptor for each relay. Second, directory mirrors will serve
microdescriptors. Third, clients will ask for them and then cache them.
3.1. Consensus changes
V3 votes should include a new line:
microdescriptor-elements bar baz foo
We also need to include the hash of each expected microdescriptor in
the routerstatus section. I suggest a new "m" line for each stanza,
with the base64 of the hash of the elements that the authority voted
for above.
The consensus microdescriptor-elements and "m" lines are then computed
as described in Section 3.1.2 below.
I believe that means we need a new consensus-method "6" that knows
how to compute the microdescriptor-elements and add "m" lines.
3.1.1. Descriptor elements to include for now
To start, the element list that authorities suggest should be
family onion-key
(Note that the or-dev posts above only mention onion-key, but if
we don't also include family then clients will never learn it. It
seemed like it should be relatively static, so putting it in the
microdescriptor is smarter than trying to fit it into the consensus.)
3.1.2. Computing consensus for microdescriptor-elements and "m" lines
One approach is for the consensus microdescriptor-elements line to
include all elements listed by a majority of authorities, sorted. The
problem here is that it will no longer be deterministic what the correct
hash for the "m" line should be. We could imagine telling the authority
to go look in its descriptor and produce the right hash itself, but
we don't want consensus calculation to be based on external data like
that. (Plus, the authority may not have the descriptor that everybody
else voted to use.)
The better approach is to take the exact set that has the most votes
(breaking ties by the set that has the most elements, and breaking
ties after that by whichever is alphabetically first). That will
increase the odds that we actually get a microdescriptor hash that
is both a) for the descriptor we're putting in the consensus, and b)
over the elements that we're declaring it should be for.
Then the "m" line for a given relay is the one that gets the most votes
from authorities that a) voted for the microdescriptor-elements line
we're using, and b) voted for the descriptor we're using.
(If there's a tie, use the smaller hash. But really, if there are
multiple such votes and they differ about a microdescriptor, we caught
one of them being lying or buggy. We should log it to track down why.)
If there are no such votes, then we leave out the "m" line for that
relay. That means clients should avoid it for this time period. (As
an extension it could instead mean that clients should fetch the
descriptor and figure out its microdescriptor themselves. But let's
not get ahead of ourselves.)
It would be nice to have a more foolproof way to agree on what
microdescriptor hash each authority should vote for, so we can avoid
missing "m" lines. Just switching to a new consensus-method each time
we change the set of microdescriptor-elements won't help though, since
each authority will still have to decide what hash to vote for before
knowing what consensus-method will be used.
Here's one way we could do that. Each vote / consensus includes both
the microdescriptor-elements that were used to compute the hashes,
and also a preferred-microdescriptor-elements set. If an authority
has a consensus from the previous period, then it should use the
consensus preferred-microdescriptor-elements when computing its votes
for microdescriptor-elements and the appropriate hashes in the upcoming
period. (If it has no previous consensus, then it just puts down its
own preferences in both lines.)
3.2. Directory mirrors serve microdescriptors
Directory mirrors should then read the microdescriptor-elements line
from the consensus, and learn how to answer requests.
The microdescriptors with hashes <D1>,<D2>,<D3> should be available at:
http://<hostname>/tor/micro/d/<D1>+<D2>+<D3>.z
All the microdescriptors from the current consensus should also be
available at:
http://<hostname>/tor/micro/all.z
so a client that's bootstrapping doesn't need to send a 70KB URL just
to name every microdescriptor it's looking for.
The format of a microdescriptor is the header line
"microdescriptor 1"
followed by each element (keyword and body), alphabetically. There's
no need to mention what hash it is, since you can hash the elements
to learn this.
(Do we need a footer line to show that it's over, or is the next
microdescriptor line or EOF enough of a hint? A footer line wouldn't
hurt much. Also, no fair voting for the microdescriptor-element
"microdescriptor".)
The hash of the microdescriptor is simply the hash of the concatenated
elements -- not counting the header line or hypothetical footer line.
Is this smart?
Note that I put a "1" up there in the header line. It isn't part
of what's hashed, though. Is there a way to put in a version that's
more useful?
Directory mirrors should check to make sure that the microdescriptors
they're about to serve match the right hashes (either the hashes from
the fetch URL or the hashes from the consensus, respectively).
We will probably want to consider some sort of smart data structure to
be able to quickly convert microdescriptor hashes into the appropriate
microdescriptor. Clients will want this anyway when they load their
microdescriptor cache and want to match it up with the consensus to
see what's missing.
3.3. Clients fetch them and cache them
When a client gets a new consensus, it looks to see if there are any
microdescriptors it needs to learn. If it needs to learn more than
some threshold of the microdescriptors (half?), it requests 'all',
else it requests only the missing ones.
Clients maintain a cache of microdescriptors along with metadata like
when it was last referenced by a consensus. They keep a microdescriptor
until it hasn't been mentioned in any consensus for a week.
3.3.1. Information leaks from clients
If a client asks you for a set of microdescs, then you know she didn't
have them cached before. How much does that leak? What about when
we're all using our entry guards as directory guards, and we've seen
that user make a bunch of circuits already?
Fetching "all" when you need at least half is a good first order fix,
but might not be all there is to it.
4. Transition and deployment
Phase one, the directory authorities should start voting on
microdescriptors and microdescriptor elements, and putting them in the
consensus. This should happen during the 0.2.1.x series, and should
be relatively easy to do.
Phase two, directory mirrors should learn how to serve them, and learn
how to read the consensus to find out what they should be serving. It
would be great if we can squeeze this in during 0.2.1.x also, so once
clients start to fetch them there will be many mirrors to choose from.
(Are there reasonable ways to build only part of phase two in 0.2.1.x?)
Phase three, clients should start fetching and caching them instead
of normal descriptors. This should happen post 0.2.1.x.