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

   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]

   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.

   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.  

   Because TCP connections can be half-open, we follow an equivalent

   to TCP's FIN/FIN-ACK/ACK protocol to close streams.

   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.

   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