[tor-relays] SENDME CELL

David Goulet dgoulet at torproject.org
Tue Apr 30 20:11:51 UTC 2019

Sorry for the very bad Subject line. Thank you Mutt :).


On 30 Apr (15:59:23), David Goulet wrote:
> On 30 Apr (12:06:31), marziyeh latifi wrote:
> > Hello,
> > I have two questions about SENDME cell in TOR:
> > Is SENDME a Control cell or a Relay cell?
> A SENDME cell is a control cell. See:
> https://gitweb.torproject.org/torspec.git/tree/tor-spec.txt#n1531
> However, SENDME exists at the stream level which affects only a specific
> stream within the circuit and thus their StreamID will be non zero which
> explains the "sometimes" in the spec.
> > Which node does  interpret  SENDME cells?The node that receives them or the
> > edge node(client/exit)?
> SENDMEs are always end-to-end so only the end points interpret them.
> For example, when a client starts a download of a large file from an Exit
> node, the Exit will send some data (up to what we call the circuit window
> size) and wait for a SENDME cell _from_ the client to send more. In order
> word, the client sends the SENDME towards the Exit to tell it "please send me
> more data".
> Now, if the client uploads data to the Exit, this is reverse where the Exit
> will send a SENDME when it is ready to receive more data.
> All this is managed by the package and delivery window maintained at each end
> points. See Section 7 of torspec.txt for more details.
> Cheers!
> David
> -- 
> 71kN/ro+ccyP6zH5RukUX1TNXn7KjZ+E8ffp3xaYOzg=

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