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[or-cvs] r9713: Try to nail down versions and version negotiation more thoro (in tor/trunk: . doc/spec/proposals)
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- Subject: [or-cvs] r9713: Try to nail down versions and version negotiation more thoro (in tor/trunk: . doc/spec/proposals)
- From: nickm@xxxxxxxx
- Date: Fri, 2 Mar 2007 15:00:33 -0500 (EST)
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Date: 2007-03-02 15:00:33 -0500 (Fri, 02 Mar 2007)
New Revision: 9713
r12374@Kushana: nickm | 2007-03-02 13:12:09 -0500
Try to nail down versions and version negotiation more thoroughly. Document some issues and ideas. Try to make things more extensible.
Property changes on: tor/trunk
svk:merge ticket from /tor/trunk [r12374] on c95137ef-5f19-0410-b913-86e773d04f59
--- tor/trunk/doc/spec/proposals/105-handshake-revision.txt 2007-03-02 20:00:30 UTC (rev 9712)
+++ tor/trunk/doc/spec/proposals/105-handshake-revision.txt 2007-03-02 20:00:33 UTC (rev 9713)
@@ -15,7 +15,7 @@
This is an open proposal.
+Motivation: Tor versions
Our *current* approach to versioning the Tor protocol(s) has been as
@@ -46,34 +46,80 @@
- There are many ideas circulating for multiple cell sizes; while it's
not obvious whether these are safe, we can't do them at all without a
mechanism to permit them.
- - There are many ideas circulating for alternative cell relay rules:
- they don't work unless they can coexist in the current network.
+ - There are many ideas circulating for alternative circuit building and
+ cell relay rules: they don't work unless they can coexist in the
+ current network.
- If our protocol changes a lot, it's hard to describe any coherent
version of it: we need to say "the version that Tor versions W through
X use when talking to versions Y through Z". This makes analysis
+Motivation: Preventing MITM attacks
+ TLS prevents a man-in-the-middle attacker from reading or changing the
+ contents of a communication. It does not, however, prevent such an
+ attacker from observing timing information. Since timing attacks are some
+ of the most effective against low-latency anonymity nets like Tor, we
+ should take more care to make sure that once we're not only talking to who
+ we think we're talking to, but that we're using the network path we
+ believe we're using.
+Motivation: Signed clock information
+ It's very useful for Tor instances to know how skewed they are relative
+ to one another. The only way to find out currently has been to download
+ directory information, and check the Date header--but this is not
+ authenticated, and hence subject to modification on the wire. Using
+ BEGIN_DIR to create an authenticated directory stream through an existing
+ circuit is better, but only works when the other party serves directory
1.0. Version numbers
The node-to-node TLS-based "OR connection" protocol and the multi-hop
- "circuit" protocol are versioned quasi-independently. (Certain versions
+ "circuit" protocol are versioned quasi-independently.
+ Of course, some dependencies will continue to exist: Certain versions
of the circuit protocol may require a minimum version of the connection
- protocol to be used.)
+ protocol to be used. The connection protocol affects:
+ - Initial connection setup, link encryption, transport guarantees,
+ - The allowable set of cell commands
+ - Allowable formats for cells.
+ The circuit protocol determines:
+ - How circuits are established and maintained
+ - How cells are decrypted and relayed
+ - How streams are established and maintained.
Version numbers are incremented for backward-incompatible protocol changes
only. Backward-compatible changes are generally implemented by adding
additional fields to existing structures; implementations MUST ignore
- fields they do not expect.
+ fields they do not expect. Unused portions of cells MUST be set to zero.
+ Though versioning the protocol will make it easier to maintain backward
+ compatibility with older versions of Tor, we will nevertheless continue to
+ periodically drop support for older protocol,
+ - to keep the implementation from growing without bound,
+ - to limit the maintenance burden of patching bugs in obsolete Tors,
+ - to limit the testing burden of verifying that many old protocol
+ versions continue to be implemented properly, and
+ - to limit the exposure of the network to protocol versions that are
+ expensive to support.
+ The Tor protocol as implemented through the 0.1.2.x Tor series will be
+ called "version 1" in its link protocol and "version 1" in its relay
+ protocol. Versions of the Tor protocol so old as to be incompatible with
+ Tor 0.1.2.x
2.1. VERSIONS cells
When a Tor connection is established, both parties normally send a
VERSIONS cell before sending any other cells. (But see below.)
- NumVersions [1 byte]
+ NumVersions [1 byte]
Versions [NumVersions bytes]
"Versions" is a sequence of NumVersions link connection protocol versions,
@@ -94,10 +140,14 @@
cells listing all their supported versions. Upon receiving the
other party's VERSIONS cell, the implementation begins using the
highest-valued version common to both cells. If the first cell from
- the other party is _not_ a VERSIONS cell, we assume a v1 protocol.
+ the other party has a recognized command, and is _not_ a VERSIONS cell, we
+ assume a v1 protocol.
- Implementations MUST discard cells that are not the first cells sent on a
+ Implementations MUST discard VERSIONS cells that are not the first
+ recognized cells sent on a connection.
+ The VERSIONS cell must be sent as a v1 cell (2 bytes of circuitID, 1
+ byte of command, 590 bytes of payload).
2.2. MITM-prevention and time checking
@@ -125,3 +175,81 @@
The second address is the one that the party sending the VERSIONS cell
believes the other has -- it can be used to learn what your IP address
is if you have no other hints.
+Discussion: Versions versus feature lists
+ Many protocols negotiate lists of available features instead of (or in
+ addition to) protocol versions. While it's possible that some amount of
+ version negotiation could be supported in a later Tor, we should prefer to
+ use protocol versions whenever possible, for reasons discussed in
+ the "Anonymity Loves Company" paper.
+Discussion: Bytes per version, versions per cell
+ This document provides for a one-byte count of how many versions a Tor
+ supports, and allows one byte per version. Thus, it can only support only
+ 254 more versions of the protocol beyond the unallocated v0 and the
+ current v1. If we ever need to split the protocol into 255 incompatible
+ versions, we've probably screwed up badly somewhere.
+ Nevertheless, here are two ways we could support more versions:
+ - Change the version count to a two-byte field that counts the number of
+ _bytes_ used, and use a UTF8-style encoding Versions 0 through 127
+ take one byte to encode; versions 128 through 2047 take two bytes to
+ encode, and so on. We wouldn't need to parse any version higher than
+ 127 right now, since all bytes used to encode higher versions would
+ have their high bit set.
+ We'd still have a limit of 380 simultaneously versions that could be
+ declared in any version. This is probably okay.
+ - Decide that if we need to support more versions, we can add a
+ MOREVERSIONS cell that gets sent before the VERSIONS cell. The spec
+ above requires Tors to ignore unrecognized cell types that they get
+ before the first VERSIONS cell, and still allow version negotiation to
+Discussion: Reducing round-trips
+ It might be appealing to see if we can cram more information in the
+ initial VERSIONS cell. For example, the contents of NETINFO will pretty
+ soon be sent by everybody before any more information is exchanged, but
+ decoupling them from the version exchange increases round-trips.
+ Instead, we could speculatively include handshaking information at
+ the end of a VERSIONS cell, wrapped in a marker to indicate, "if we wind
+ up speaking VERSION 2, here's the NETINFO I'll send. Otherwise, ignore
+ this." This could be extended to opportunistically reduce round trips
+ when possible for future versions when we guess the versions right.
+ Of course, we'd need to be careful about using a feature like this:
+ - We don't want to include things that are expensive to compute,
+ like PK signatures or proof-of-work.
+ - We don't want to speculate as a mobile client it will leak our
+ experience with the server in question.
+Discussion: Advertising versions in routerdescs and networkstatuses.
+ Client partitioning is the big danger when we introduce new versions; if a
+ client supports some very unusual set of protocol versions, it will stand
+ out from others no matter where it goes. If a server supports an unusual
+ version, it will get a disproportionate amount of traffic from clients who
+ prefer that version. We can mitigate this somewhat as follows:
+ - Do not have clients prefer any protocol version by default until that
+ version is widespread.
+ - Do not multiply protocol versions needlessly.
+ - Encourage protocol implementors to implement the same protocol version
+ sets as some popular version of Tor.
+ - Disrecommend very old/unpopular versions of Tor via the directory
+ authorities' RecommmendedVersions mechanism, even if it is still
+ technically possible to use them.