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Add specification for half-open connections, modified from last week's

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conversation enough to correspond to real TCP.


svn:r338
This commit is contained in:
Nick Mathewson
2003-06-19 23:23:35 +00:00
parent ace7cc92a9
commit bb17a8d76e
1 changed files with 27 additions and 11 deletions
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trunk/doc/tor-spec.txt
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@@ -487,24 +487,40 @@ which reveals the downstream node.
cells, echo their contents to the corresponding TCP stream.
[XXX Mention zlib encoding. -NM]
5.2. Closing connections
5.2. Closing streams
[Note -- TCP streams can only be half-closed for reading. Our
Bickford's conversation was incorrect. -NM]
Because TCP connections can be half-open, we follow an equivalent
to TCP's FIN/FIN-ACK/ACK protocol to close streams.
When one side of the TCP stream is closed, the corresponding edge
node sends a RELAY_END cell along the circuit; upon receiving a
RELAY_END cell, the edge node closes its side of the corresponding
TCP stream (by sending a FIN packet), but continues to accept and
package incoming data until both sides of the TCP stream are
closed. At that point, the edge node sends a second RELAY_END
cell, and drops its record of the stream.
A exit conneection can have a TCP stream in one of three states:
'OPEN', 'DONE_PACKAGING', and 'DONE_DELIVERING'. For the purposes
of modeling transitions, we treat 'CLOSED' as a fourth state,
although connections in this state are not, in fact, tracked by the
onion router.
For creation and handling of RELAY_EXTEND and RELAY_EXTENDED cells,
see section 4. For creating and handling of RELAY_SENDME cells,
see section 6.
A stream begins in the 'OPEN' state. Upon receiving a 'FIN' from
the corresponding TCP connection, the edge node sends a 'RELAY_END'
cell along the circuit and changes its state to 'DONE_PACKAGING'.
Upon receiving a 'RELAY_END' cell, an edge node sends a 'FIN' to
the corresponding TCP connection (e.g., by calling
shutdown(SHUT_WR)) and changing its state to 'DONE_DELIVERING'.
When a stream in already in 'DONE_DELIVERING' receives a 'FIN', it
also sends a 'RELAY_END' along the circuit, and changes its state
to 'CLOSED'. When a stream already in 'DONE_PACKAGING' receives a
'RELAY_END' cell, it sends a 'FIN' and changes its state to
'CLOSED'.
[Note: Please rename 'RELAY_END2'. :) -NM ]
If an edge node encounters an error on any stram, it sends a
'RELAY_END2' cell along the circuit (if possible) and closes the
TCP connection immediately. If an edge node receives a
'RELAY_END2' cell for any stream, it closes the TCP connection
completely, and sends nothing along the circuit.
6. Flow control