docs(reassembly): a fragment after its datagram completes starts a new buffer (#1507)

docs/source/pcapkit/foundation/reassembly/ip/index.rst

@@ -89,3 +89,7 @@ Algorithm
       flush all reassembly with this BUFID;
       DONE.
    }
+
+Because completion frees the buffer (:rfc:`791#section-3.2`), a fragment that
+arrives after its datagram has completed opens a new buffer, which is later
+reported as a separate incomplete datagram; this is intended (:issue:`1507`).

pcapkit/foundation/reassembly/ip.py

@@ -87,6 +87,16 @@ def reassembly(self, info: 'Packet[_AT]') -> 'None':
         Arguments:
             info: info dict of packets to be reassembled
 
+        Note:
+            A completed datagram's buffer is freed, as in the
+            :rfc:`791#section-3.2` reassembly procedure, so a fragment of it
+            that arrives afterwards -- a late duplicate, say -- opens a new
+            buffer. The caller sees that buffer as a separate incomplete
+            datagram, ``PARTIAL`` at the end of the capture or ``TIMEOUT`` if
+            it expires first. This is intended, is permitted by
+            :rfc:`8200#section-4.5`, and matches Linux, Zeek and Wireshark
+            (:issue:`1507`).
+
         """
         # clear cache
         self._flag_n = False