[tor-bugs] #4485 [Analysis]: 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:     
     Type:  task                      |         Status:  new
 Priority:  normal                    |      Milestone:     
Component:  Analysis                  |        Version:     
 Keywords:  performance, flowcontrol  |         Parent:     
   Points:                            |   Actualpoints:     
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 Ian and Mashael opined a year or so ago that our stream-level sendmes are
 not helpful, and perhaps harmful.

 I'm inclined to agree. All they do is introduce yet another reason for us
 to sometimes not send a cell that we would otherwise send.

 They were originally designed for fairness between streams in a given
 circuit, but once you have more than two streams in the circuit, I think
 the stream-level sendmes start to get in the way rather than help.

 Step one is that we should analyze whether this is really true. Are there
 any cases where stream-level sendmes are still necessary?  Do they help
 enough in the two-stream case to merit keeping them around? Do they harm
 enough in the three-stream case to merit taking them out?

 Step two is to write a patch that rips them out assuming everybody in the
 network is running that patch, and then runs some private networks to make
 sure nothing crashes (easy to notice) or wedges (harder to notice).

 Step three is to simulate this patched Tor in Shadow and/or ExperimenTor
 to see if there are any surprises on the performance front.

 Step four is to design a backward-compatible way to phase them out, and
 get that started.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/4485>
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