[tcpm] Handling of FIN in the reassembly queue

The TCP reassembly queue is very good at handling and queuing out of
order data.  However correctly handling and queuing an out of order
FIN seems to be a bit tricky for a number of (broken) edge cases:

  1. FIN points into the middle of already queued data (or hole);
       Ignore the FIN and integrate the data?
       Ignore the whole segment with the bogus FIN?
       Integrate the data, move the FIN and flush excess data?

  2. FIN was already received and another one arrives pointing
     into the middle of already queue data (or hole);
       Ignore the FIN and integrate the data?
       Ignore the whole segment with the bogus FIN?
       Integrate the data, move the FIN and flush excess data?

  3. FIN was already received and another one arrives pointing
     beyond the previous one;
       Ignore the FIN and integrate the data (if within previous FIN)?
       Ignore the whole segment with the bogus FIN?
       Integrate the data and move the FIN?

  4. FIN without data pointing beyond already queued data;
       Queuing a standalone FIN w/o data and not directly attaching
       to the end of the queue complicates the implementation a lot.
       Is it acceptable to ignore the FIN for the moment and have it
       retransmitted (with or without data) later?

For reference the traditional reassembly queue implementation doesn't
care and queues any segment with FIN in any place.  Depending on the
hole situation this can lead to strange behavior and only ever partly
dequeued data.

Any input, insight and discussion welcome. :-)

-- 
Andre

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