[tor-bugs] #4485 [Tor]: Research: can we get rid of the stream-level sendme cells

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#4485: Research: can we get rid of the stream-level sendme cells
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 Reporter:  arma                                      |          Owner:  arma            
     Type:  project                                   |         Status:  new             
 Priority:  normal                                    |      Milestone:  Tor: unspecified
Component:  Tor                                       |        Version:                  
 Keywords:  performance flowcontrol SponsorF20121101  |         Parent:                  
   Points:                                            |   Actualpoints:                  
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Changes (by arma):

 * cc: iang, kevin, malsabah (added)


Comment:

 Replying to [ticket:4485 arma]:
 > Step one is that we should analyze whether this is really true. Are
 there any cases where stream-level sendmes are still necessary?

 Ah ha! Based on discussion on tor-dev, I believe the answer here is yes,
 there is such a case: circuit-level flow control regulates the number of
 cells that can be in flight, but stream-level flow control is needed to
 push back on streams that shouldn't send any more cells yet because the
 other end hasn't reading enough yet. We don't want to block the circuit
 just because one of its streams is blocked, so we need some separate
 control mechanism.

 In fact, if I am understanding things correctly, I believe this is a flaw
 with the n23 design as well -- it lacks any stream-level pushback for
 streams that are blocking on writes.

 (It's interesting to note that none of the experiments in the
 Defenestrator paper, and also none of the standard torperf experiments,
 expose the "how do we push back if the application stops reading" issue.)

-- 
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/4485#comment:12>
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