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