Proposal: Provide diffs between consensuses
peter at palfrader.org
Fri Jun 13 22:44:03 UTC 2008
- How old can a consensus be for us to try to fetch a diff.
We could of course always try to fetch a diff, but how smart
is it to let a dirserver know that we were last online 3.8 days
- Do we agree on using the ed patch format?
- What to do when the client already has the latest consensus
Title: Provide diffs between consensuses
Author: Peter Palfrader
Tor clients and servers need a list of which relays are on the
network. This list, the consensus, is created by authorities
hourly and clients fetch a copy of it, with some delay, hourly.
This proposal suggests that clients download diffs of consensuses
once they have a consensus instead of hourly downloading a full
After implementing proposal 138 which removes nodes that are not
running from the list a consensus document is about 92 kilobytes
in size after compression.
The diff between two consecutive consensus, in ed format, is on
average 13 kilobytes compressed.
If a client has a consensus that is recent enough it SHOULD
try to download a diff to get the latest consensus rather than
fetching a full one.
[XXX: what is recent enough?
time delta in hours / size of compressed diff
[ size of gzip compressed "diff -e" between the consensus on
2008-06-01-00:00:00 and the following consensuses that day.
Consensuses have been modified to exclude down routers per
proposal 138. ]
Data suggests that for the first few hours diffs are very useful,
saving about 60% for the first three hours, 30% for the first 10,
and almost nothing once we are past 16 hours.
Directory authorities and servers need to keep up to X [XXX: depends
on how long clients try to download diffs per above] old consensus
documents so they can build diffs. They should offer a diff to the
most recent consensus at the URL
where hash is the full digest of the consensus the client currently
has, and FPRLIST is a list of (abbreviated) fingerprints of
authorities the client trusts.
Servers will only return a consensus if more than half of the requested
authorities have signed the document, otherwise a 404 error will be sent
back. The fingerprints can be shortened to a length of any multiple of
two, using only the leftmost part of the encoded fingerprint. Tor uses
3 bytes (6 hex characters) of the fingerprint. (This is just like the
conditional consensus downloads that Tor supports starting with
If a server cannot offer a diff from the consensus identified by the
hash but has a current consensus it MUST return the full consensus.
[XXX: what should we do when the client already has the latest
consensus? I can think of the following options:
- send back 3xx not modified
- send back 200 ok and an empty diff
- send back 404 nothing newer here.
I currently lean towards the empty diff.]
4. Diff Format
Diffs start with the token "network-status-diff-version" followed by a
space and the version number, currently "1".
If a document does not start with network-status-diff it is assumed
to be a full consensus download and would therefore currently start
with "network-status-version 3".
Following the network-status-diff header line is a diff, or patch, in
limited ed format. We choose this format because it is easy to create
and process with standard tools (patch, diff -e, ed). This will help
us in developing and testing this proposal and it should make future
[ If at one point in the future we decide that the space benefits from
a custom diff format outweighs these benefits we can always
introduce a new diff format and offer it at for instance
We support the following ed commands, each on a line by itself:
- "<n1>d" Delete line n1
- "<n1>,<n2>d" Delete lines n1 through n2, including
- "<n1>c" Replace line n1 with the following block
- "<n1>,<n2>c" Replace lines n1 through n2, including, with the
- "<n1>a" Append the following block after line n1.
- "a" Append the following block after the current line.
- "s/.//" Remove the first character in the current line.
Note that line numbers always apply to the file after all previous
commands have already been applied.
The "current line" is either the first line of the file, if this is
the first command, the last line of a block we added in an append or
change command, or the line immediate following a set of lines we just
deleted (or the last line of the file if there are no lines after
The replace and append command take blocks. These blocks are simply
appended to the diff after the line with the command. A line with
just a period (".") ends the block (and is not part of the lines
to add). Note that it is impossible to insert a line with just
a single dot. Recommended procedure is to insert a line with
two dots, then remove the first character of that line using s/.//.
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