[tor-dev] Pre-draft of Proposal XXX: Extended ORPort and TransportControlPort

Nick Mathewson nickm at alum.mit.edu
Wed Mar 7 20:10:53 UTC 2012

On Thu, Jan 26, 2012 at 8:38 PM, George Kadianakis <desnacked at riseup.net> wrote:
> After discussion in tickets #4773 and #3587 this is a pre-draft of a
> proposal that revamps the Extended ORport, introduced in proposal 180,
> and specifies the new TransportControlPort.
> Comments are marked with '#', and there is also:
> https://trac.torproject.org/projects/tor/ticket/4773#comment:5
> Note that this proposal is not ready for prime time. It needs more
> thought and lots of editing to become functional and implementable.
> Inlining:
> Filename: X
> Title: Extended ORPort and TransportControlPort
> Author: X
> Created: X
> Status: worse-than-draft-still-probably-more-thoughtful-than-sopa

I've got a commitment to revise this and do a (not-yet-merged)
implementation by the middle of this month, so I guess I should get
moving on that!

> 1. Overview
>  Proposal 180 defined Tor pluggable transports, a way to decouple
>  protocol-level obfuscation from the core Tor protocol in order to
>  better resist client-bridge censorship. This is achieved by
>  introducing pluggable transport proxies, programs that obfuscate Tor
>  traffic to resist DPI detection.
>  Proposal 180 defined a way for pluggable transport proxies to
>  communicate with local tor clients and bridges, so as to exchange
>  traffic. This document extends this communication protocol, so that
>  pluggable transport proxies can exchange arbitrary operational
>  information and metadata with tor clients and bridges.
> 2. Motivation
>   The communication protocol specified in Proposal 180 gives a way
>   for transport proxies to announce the IP address of their clients
>   to tor. Still, modern pluggable transports might have more (?)
>   needs than this. For example:
>   1. Tor might want to inform pluggable transport proxies on how to
>      rate-limit incoming or outgoing connections.
>   2. Server pluggable transport proxies might want to pass client
>      information to an anti-active-probing system controlled by tor.
>   3. Tor might want to temporarily stop a transport proxy from
>      obfuscating traffic.
>   To satisfy the above use cases, there must be real-time
>   communication between the tor process and the pluggable transport
>   proxy. To achieve this, this proposal refactors the extended ORPort
>   protocol specified in Proposal 180, and introduces a new port,
>   TransportControlPort, whose sole role is the exchange of control
>   information between transport proxies and tor.

So before we get too far into this, let's do a protocol overview! I'd
suggest inserting something like this into the text, assuming that it
is actually what you meant:

"Server-side Transports need to talk to the Tor server about
individual connections, and continue to do so as the connection is
ongoing.  To do so,
the transports deliver each connection to an "Extended ORPort", where
they provide metadata and agree on an identifier for each tunneled
connection.  Once this handshake occurs, the OR protocol proceeds

Additionally, each transport maintains a single connection to Tor's
"TransportControlPort", where it receives instructions from Tor about
rate-limiting on individual connections.

> 3. The new extended ORPort protocol
>  Server transport proxies may need to connect to the bridge and pass
>  additional information about client connections that the bridge
>  would ordinarily receive from the kernel's TCP stack.  To do this,
>  they connect to the "extended server port" as given in
>  EXTENDED_SERVER_PORT, send a short amount of information, wait for a
>  response, and then send the user traffic on that port.
>  The extended server port protocol is as follows:
>     COMMAND [2 bytes, big-endian]
>     BODYLEN [2 bytes, big-endian]
>     BODY [BODYLEN bytes]
>     Commands sent from the transport proxy to the bridge are:
>     [0x0000] DONE: There is no more information to give. (body ignored)

Let's add, "the next bytes sent by the transport will be those
tunneled over it."

>     [0x0001] USERADDR: an address:port string that represents the user's
>       address.
>     [0x0002] WANT_CONTROL: A body-less message which indicates that
>       the transport proxy wants to use the TransportControlPort of
>       the bridge.  It SHOULD be followed by a CONTROL command from
>       the bridge, otherwise the transport may close the connection.
> # will this work?

Hm.  I think it'd be better to have this command mean "We support the
transportcontrolport protocol," not "you must use the
transportcontrolport protocol!"  After all, if Tor _doesn't_ tell the
transport about rate-limiting, it's not like anything breaks

>     Replies sent from tor to the proxy are:
>     [0x1000] OKAY: Send the user's traffic. (body ignored)
>     [0x1001] DENY: Tor would prefer not to get more traffic from
>       this address for a while. (body ignored)
>     [0x1002] CONTROL: a NUL-terminated "identifier" string, followed
>       by a second NUL-terminated string of the <address>:<port> of
>       the TransportControlPort. The pluggable transport proxy must
>       use the "identifier" to access the TransportControlPort.
> # pass TransportControlPort <address>:<port> through env. vars?

Seems wise, sure.

> # what should parties do when they receive a command they don't
> # understand?  should we enforce forward-compatibility with protocol
> # versioning or with "ignore commands you don't understand", or what?

Let's say "ignore."  If you want, we can reserve the top bit of each
command to indicate "you must understand this; if you don't, close the

>  [We could also use an out-of-band signalling method to tell Tor
>  about client addresses, but that's a historically error-prone way
>  to go about annotating connections.]

Yes; let's not do that.

> The new TransportControlPort protocol
>  The TransportControlPort protocol is as follows:
>     COMMAND [2 bytes, big-endian]
>     BODYLEN [2 bytes, big-endian]
>     BODY [BODYLEN bytes]
>     Association commands sent from the transport proxy to the bridge
>     are:
>     [0x0000] ASSOCIATE: a NUL-terminated "identifier" string. See
>      'Association' section below.

Hm.  I think that each command should have an associated identifier,
and that identifiers should be (say) 16-byte binary values.  All this
hex encoding/decoding seems pointless, since this isn't a
text-oriented protocol.  So how about
     ConnectionID [16 bytes]
     Command [2 bytes]
     Bodylen [2 bytes]
     Body [bodylen bytes]

>     Association commands sent from the bridge to the transport proxy
>     are:
>     [0x1000] ASSOCIATED: Sent upon receiving a legit ASSOCIATE
>       command from a transport proxy. (body ignored)
>     [0x1001] NOT_ASSOCIATED: Sent after the bridge receives a
>       non-legit ASSOCIATE command from a transport proxy. Also sent
>       when the bridge receives a non-ASSOCIATE command from a
>       non-associated transport proxy. Upon sending this command, the
>       bridge SHOULD close the connection. (body ignored)
>     Configuration commands sent from the transport proxy to the
>     bridge:
>     [0x0001] RATE_LIMITED: Message confirming that the rate limiting
>       request of the bridge was carried out successfully (body
>       ignored). See the 'Rate Limiting' section below.
>     [0x0001] NOT_RATE_LIMITED: Message notifying that the transport
>       proxy failed to carry out the rate limiting request of the
>       bridge (body ignored). See the 'Rate Limiting' section below.
>     Configuration commands sent from the bridge to the transport
>     proxy are:
>     [0x1002] RATE_LIMIT: Carries information on how the pluggable
>       transport proxy should rate-limit its traffic. See the 'Rate
>       Limiting' section below.
> # what should parties do when they receive a command they don't understand?

Send an "unrecognized command error", perhaps.  Or ignore it.  If the
latter, let's add a way to declare what version of this protocol you
will understand.

> 3.1. Association and identifier creation
> For Tor and a transport proxy to communicate using the
> TransportControlPort, an identifier must have already been negotiated
> using the 'CONTROL' command of Extended ORPort.
> The TransportControlPort identifier should not be predictable by a
> user who hasn't received a 'CONTROL' command from the Extended
> ORPort. For this reason, the TransportControlPort identifier should
> not be cryptographically-weak or deterministically created.
> Tor should create its identifiers by generating 16 bytes of random
> data and hashing them with the SHA256 cryptographic hash function.
> The identifier string transmitted with the 'CONTROL' command should be
> the hex representation of the SHA256 output.

The hashing step seems pointless; why not just generate 16 random
bytes and use those?

> 4. Configuration commands
> 4.1. Rate Limiting
> A tor relay should be able to inform a transport proxy in real-time
> about its rate-limiting needs.
> This can be achieved by using the TransportControlPort and sending a
> 'RATE_LIMIT' command to the transport proxy.
> The body of the 'RATE_LIMIT' command should carry two integers, in
> NUL-terminated ASCII string format, representing the bandwidth rate
> and bandwidth burst in 'bytes per second', that the transport proxy
> must set.
> # better transmit format? After reading langsec.org, I prefer to avoid
> # length fields. Not that this format is bug-proof...

4 bytes, big-endian, I'd say.

> When the transport proxy sets the appropriate rate limiting, it should
> send back a 'RATE_LIMITED' command. If it fails while setting up rate
> limiting, it should send back a 'NOT_RATE_LIMITED' command.
> After sending a 'RATE_LIMIT' command. the tor bridge might want to
> stop pushing data to the transport proxy, till it receives a
> 'RATE_LIMITED' command. If, instead, it receives a 'NOT_RATE_LIMITED'
> command it might want to shutdown its connections to the transport
> proxy.
> # is this realistic?

Hm.  There probably also wants to be an overall rate limit that
applies to all connections.  Also, there should be a way for the
transport to report to Tor how many bytes it's actually using, I
think, if the bytes on the wire are more vebose than the traffic they

> 5. Security Considerations
> Extended ORPort or TransportControlPort do _not_ provide link
> confidentiality, authentication or integrity. Sensitive data, like
> cryptographic material, should not be transferred through them.
> Note that an attacker with superuser access, is able to sniff network
> traffic, and capture TransportControlPort identifiers and any data
> passed through those ports.
> # Is it worth adding an SSL layer (passing pub. key fpr via
> # env. vars?)? :/

I say no; though it probably _is_ worthwhile to say "This should only
use localhost, and should shout very loudly if you try to bind or
connect somewhere else with it."

> # Talk about Incentives of tor or transport proxies to comply to the
> # wishes of each other. Ways to detect nonconformism. (threat
> # model. Should tor speak with 3v1l transport proxies in the first
> # place?)

I disagree with that; the transport proxies are run by the bridge
operators and need to do more or less what they're supposed to do.
Trying to get proxies to be sandboxable or something seems like a much
bigger and sorta unrelated task.


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