[tor-bugs] #11197 [Obfsproxy]: obfsproxy should provide congestion feedback

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#11197: obfsproxy should provide congestion feedback
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 Reporter:  yawning                             |          Owner:  asn
     Type:  defect                              |         Status:  new
 Priority:  normal                              |      Milestone:
Component:  Obfsproxy                           |        Version:
 Keywords:  obfsproxy, congestion, bufferbloat  |  Actual Points:
Parent ID:                                      |         Points:
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 I went over this in IRC tonight to a poor GSOC student who was thinking
 about doing a CBR plugin, so I'll file a bug while it's fresh on my mind.

 Currently there is nothing in place to prevent unbound buffer growth in
 obfsproxy.  This problem arises when the bottleneck link is extremely
 narrow.

 For example, examine the following network topology:

      Client <-> obfsproxy <-> 14.4 kbit modem <-> ISP <-> 100 Mbit <->
 obfsproxy <-> Server

 The Client opens a connection, and initiates a bulk download from the
 Server.  Since there is no mechanism to indicate congestion, the outgoing
 buffer in the Server side obfsproxy process will grow because feedback
 from the Client in the form of the shrinking TCP/IP receive window will
 not get propagated.

 The same thing will happen on the Client side with a bulk upload, because
 the loopback interface has a gigantic amount of bandwidth compared to the
 bottleneck link.

 Twisted connections have a producer/consumer interface (and can handle
 stopping reading once the send buffer reaches a certain threshold
 'self.bufferSize'), so refactoring the base transport to use this
 interface to glue the upstream/downstream together would be the "correct"
 approach to solving this problem.

 See https://twistedmatrix.com/documents/current/core/howto/producers.html
 for more details.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/11197>
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