Filename: 105-handshake-revision.txt
Title: Version negotiation for the Tor protocol.
Author: Nick Mathewson, Roger Dingledine
Created: Jan 2007
Status: Closed
Implemented-In: 0.2.0.x
Overview:
This document was extracted from a modified version of tor-spec.txt that we
had written before the proposal system went into place. It adds two new
cells types to the Tor link connection setup handshake: one used for
version negotiation, and another to prevent MITM attacks.
This proposal is partially implemented, and partially proceded by
proposal 130.
Motivation: Tor versions
Our *current* approach to versioning the Tor protocol(s) has been as
follows:
- All changes must be backward compatible.
- It's okay to add new cell types, if they would be ignored by previous
versions of Tor.
- It's okay to add new data elements to cells, if they would be
ignored by previous versions of Tor.
- For forward compatibility, Tor must ignore cell types it doesn't
recognize, and ignore data in those cells it doesn't expect.
- Clients can inspect the version of Tor declared in the platform line
of a router's descriptor, and use that to learn whether a server
supports a given feature. Servers, however, aren't assumed to all
know about each other, and so don't know the version of who they're
talking to.
This system has these problems:
- It's very hard to change fundamental aspects of the protocol, like the
cell format, the link protocol, any of the various encryption schemes,
and so on.
- The router-to-router link protocol has remained more-or-less frozen
for a long time, since we can't easily have an OR use new features
unless it knows the other OR will understand them.
We need to resolve these problems because:
- Our cipher suite is showing its age: SHA1/AES128/RSA1024/DH1024 will
not seem like the best idea for all time.
- 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 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
harder.
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 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 that's an extra step and it might be nicer to
learn the information in the course of the regular protocol.
Proposal:
1.0. Version numbers
The node-to-node TLS-based "OR connection" protocol and the multi-hop
"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. The connection protocol affects:
- Initial connection setup, link encryption, transport guarantees,
etc.
- 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. 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 protocols,
- 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 can be considered to be version 0 of each, and are not
supported.
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.)
VersionsLen [2 byte]
Versions [VersionsLen bytes]
"Versions" is a sequence of VersionsLen bytes. Each value between 1 and
127 inclusive represents a single version; current implementations MUST
ignore other bytes. Parties should list all of the versions which they
are able and willing to support. Parties can only communicate if they
have some connection protocol version in common.
Version 0.2.0.x-alpha and earlier don't understand VERSIONS cells,
and therefore don't support version negotiation. Thus, waiting until
the other side has sent a VERSIONS cell won't work for these servers:
if the other side sends no cells back, it is impossible to tell
whether they
have sent a VERSIONS cell that has been stalled, or whether they have
dropped our own VERSIONS cell as unrecognized. Therefore, we'll
change the TLS negotiation parameters so that old parties can still
negotiate, but new parties can recognize each other. Immediately
after a TLS connection has been established, the parties check
whether the other side negotiated the connection in an "old" way or a
"new" way. If either party negotiated in the "old" way, we assume a
v1 connection. Otherwise, both parties send VERSIONS 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
has a recognized command, and is _not_ a VERSIONS cell, we assume a
v1 protocol.
(For more detail on the TLS protocol change, see forthcoming draft
proposals from Steven Murdoch.)
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, 509 bytes of payload).
[NOTE: The VERSIONS cell is assigned the command number 7.]
2.2. MITM-prevention and time checking
If we negotiate a v2 connection or higher, the second cell we send SHOULD
be a NETINFO cell. Implementations SHOULD NOT send NETINFO cells at other
times.
A NETINFO cell contains:
Timestamp [4 bytes]
Other OR's address [variable]
Number of addresses [1 byte]
This OR's addresses [variable]
Timestamp is the OR's current Unix time, in seconds since the epoch. If
an implementation receives time values from many ORs that
indicate that its clock is skewed, it SHOULD try to warn the
administrator. (We leave the definition of 'many' intentionally vague
for now.)
Before believing the timestamp in a NETINFO cell, implementations
SHOULD compare the time at which they received the cell to the time
when they sent their VERSIONS cell. If the difference is very large,
it is likely that the cell was delayed long enough that its
contents are out of date.
Each address contains Type/Length/Value as used in Section 6.4 of
tor-spec.txt. The first address is the one that the party sending
the NETINFO cell believes the other has -- it can be used to learn
what your IP address is if you have no other hints.
The rest of the addresses are the advertised addresses of the party
sending the NETINFO cell -- we include them
to block a man-in-the-middle attack on TLS that lets an attacker bounce
traffic through his own computers to enable timing and packet-counting
attacks.
A Tor instance should use the other Tor's reported address
information as part of logic to decide whether to treat a given
connection as suitable for extending circuits to a given address/ID
combination. When we get an extend request, we use an
existing OR connection if the ID matches, and ANY of the following
conditions hold:
- The IP matches the requested IP.
- We know that the IP we're using is canonical because it was
listed in the NETINFO cell.
- We know that the IP we're using is canonical because it was
listed in the server descriptor.
[NOTE: The NETINFO cell is assigned the command number 8.]
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
feature 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 allows version negotiation
to
succeed.
[Resolution: Reserve the high bit and the v0 value for later use. If
we ever have more live versions than we can fit in a cell, we've made a
bad design decision somewhere along the line.]
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 may leak our
experience with the server in question.
Discussion: Advertising versions in routerdescs and networkstatuses.
In network-statuses:
The networkstatus "v" line now has the format:
"v" IMPLEMENTATION IMPL-VERSION "Link" LINK-VERSION-LIST
"Circuit" CIRCUIT-VERSION-LIST NL
LINK-VERSION-LIST and CIRCUIT-VERSION-LIST are comma-separated lists of
supported version numbers. IMPLEMENTATION is the name of the
implementation of the Tor protocol (e.g., "Tor"), and IMPL-VERSION is the
version of the implementation.
Examples:
v Tor 0.2.5.1-alpha Link 1,2,3 Circuit 2,5
v OtherOR 2000+ Link 3 Circuit 5
Implementations that release independently of the Tor codebase SHOULD NOT
use "Tor" as the value of their IMPLEMENTATION.
Additional fields on the "v" line MUST be ignored.
In router descriptors:
The router descriptor should contain a line of the form,
"protocols" "Link" LINK-VERSION-LIST "Circuit" CIRCUIT_VERSION_LIST
Additional fields on the "protocols" line MUST be ignored.
[Versions of Tor before 0.1.2.5-alpha rejected router descriptors with
unrecognized items; the protocols line should be preceded with an "opt"
until these Tors are obsolete.]
Security issues:
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. (First introduce the new version to servers,
and have clients admit to using it only when configured to do so for
testing. Then, once many servers are running the new protocol
version, enable its use by default.)
- 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.