[tor-bugs] #9349 [Flashproxy]: flashproxy facilitator: Allow clients to specify transports

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#9349: flashproxy facilitator: Allow clients to specify transports
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     Reporter:  asn         |      Owner:  dcf
         Type:  task        |     Status:  new
     Priority:  normal      |  Milestone:
    Component:  Flashproxy  |    Version:
   Resolution:              |   Keywords:
Actual Points:              |  Parent ID:  #7167
       Points:              |
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Comment (by infinity0):

 I think the idea of a transport chain needs work. I don't believe what has
 been said so far is precisely coherent:

 - the browser proxy communicates to client and server via TCP channels,
 *regardless of the contents*. Having "tcp" as a valid transport name like
 "obfs|tcp" doesn't make sense. "" (empty string) would be the appropriate
 name for the transport chain that carries raw user data.
 - I also don't understand this whole "outermost transport" thing, since
 the browser proxy just passes bytes and doesn't need to speak the
 outermost transport in order to do this.
 - However, in order to *match* client vs server, it needs to match the
 entire chain. a client that speaks "obfs|websocket" isn't going to be able
 to talk to a server that speaks "xxx|websocket"

 Here is my understanding:

 Certain types of PTs are what I'll call a "byte-transform" PT - i.e. at
 its heart, it transforms input bytes to some output bytes, and the
 underlying transport *mechanism* (TCP in this case) is undisturbed.
 obfsproxy is a "byte-transform" PT, but flashproxy isn't, since it does
 extra stuff to the underlying channel, so that the output of flashproxy
 cannot by fed into a "byte-transform" PT.

 A transport chain only makes sense if each component in the chain is a
 byte-transform PT. A browser proxy can pass the data stream transparently,
 or it can strip off layers in order to adapt between client/servers:

 - a client of a|b|c can talk to a server a|b|c and the proxy doesn't need
 to do anything, just pass bytes
 - a client of a|b|c can talk to a server a|d|e but the proxy needs to
 apply the transformation e(d(b-inv(c-inv(_)))) to the data stream from the
 client, and vice-versa from the server. (This is essentially what
 [comment:12 arlolra] did in his "raw tcp" commit.)

 (A more general framework generalises the idea of a "byte-transform" PT -
 each PT has an *input* interface, and an *output* interface, then you can
 chain PTs by matching input interfaces to output interfaces. But we can
 stick with byte-to-byte PTs for now.)

-- 
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/9349#comment:22>
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