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Meditate on why 104-short-descriptors cant work as written, and what needs to get solved before it can get implemented. svn:r9714
102 lines
4.2 KiB
Plaintext
102 lines
4.2 KiB
Plaintext
Filename: 104-short-descriptors.txt
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Title: Long and Short Router Descriptors
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Version: $Revision$
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Last-Modified: $Date$
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Author: Nick Mathewson
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Created:
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Status: Open
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Overview:
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This document proposes moving unused-by-clients information from regular
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router descriptors into a special "long form" router descriptor.
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It presents options; it is not yet a complete proposal.
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Proposal:
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Some of the costliest fields in the current directory protocol are ones
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that no client actually uses. In particular, the "read-history" and
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"write-history" fields are used only by the authorities for monitoring the
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status of the network. If we took them out, the size of a compressed list
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of all the routers would fall by about 60%. (No other disposable field
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would save more than 2%.)
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One possible solution here is that routers should generate and upload a
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short-form and long-form descriptor. Only the short-form descriptor should
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ever be used by anybody for routing. The long-form descriptor should be
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used only for analytics and other tools. (If we allowed people to route
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with long descriptors, we'd have to ensure that they stayed in sync with
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the short ones somehow. So let's not do that.) We can ensure that the
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short descriptors are used by only recommending those in the network
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statuses.
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Another possible solution would be to drop these fields from descriptors,
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and have them uploaded as a part of a separate "bandwidth report" to the
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authorities. This could help prevent the mistake of using long descriptors
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in the place of short ones.
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Other disposable fields:
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Clients don't need these fields, but removing them doesn't help bandwidth
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enough to be worthwhile.
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contact (save about 1%)
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fingerprint (save about 3%)
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We could represent these fields more succinctly, but removing them would
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only save 1%. (!)
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reject
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accept
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(Apparently, exit polices are highly compressible.)
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Issues:
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Indexing long descriptor or bandwidth reports presents an issue: right now
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the way to make sure you have the same copy of a descriptor as everyone
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else is to request the descriptor by its digest, and to make sure to that
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the digest you request is the one that the authorities like.
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Authorities should presumably list the digests of short descriptors, since
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that's what most everybody will be using. Including a second digest for
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long descriptors/bandwidth reports in the networkstatus would only bloat it
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with information nobody wants.
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Possible solutions are:
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- Drop the property that you can be sure of having the same long
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descriptor as others. This seems unoptimal.
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- Have a separate extra-information-status that also gets generated by the
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authorities; use it to tell which long descriptors others have. Also a
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pain.
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- Have short descriptors include a hash of the corresponding long
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descriptor/extra-info. This would keep the same order of magnitude
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performance increase (~59.2% savings as opposed to 61% savings.)
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This would require longdesc/extra-info downloaders to fetch
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router data before they could know which longdescs/extra info to fetch.
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- Have each authority make a signed concatenated "extra info" document,
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and hope we never need to reconcile them.
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- ????
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Migration:
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For long/short descriptor approach:
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* First:
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* Authorities should accept both, now, and silently drop short
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descriptors.
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* Routers should upload both once authorities accept them.
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* There should be a "long descriptor" url and the current "normal" URL.
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Authorities should serve long descriptors from both URLs.
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* Once tools that want long descriptors support fetching them from the
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"long descriptor" URL:
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* Have authorities remember short descriptors, and serve them from the
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'normal' URL.
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For bandwidth info approach:
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* First:
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* Rename it; it won't be just bandwidth forever.
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* Authorities should accept bandwidth info
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* Routers should upload bandwidth info once authorities accept it.
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* There should be a way to download bandwidth info
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* Once tools that want bandwidth info support fetching it:
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* Have routers stop including bandwidth info in their router
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descriptors.
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