Spec: clarify port definition

doc/of-spec/openflow-spec-v0.9.0.tex

@@ -49,6 +49,8 @@ Version 1.0 of this document will be the first for which official vendor support
 An OpenFlow Switch consists of a \emph{flow table}, which performs packet lookup and forwarding, and a \emph{secure channel} to an external controller (Figure \ref{fig:flow table and controller}).  The controller manages the switch over the secure channel using the OpenFlow protocol.
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 The flow table contains a set of flow entries (header values to match packets against), activity counters, and a set of zero or more actions to apply to matching packets.  All packets processed by the switch are compared against the flow table.  If a matching entry is found, any actions for that entry are performed on the packet (e.g., the action might be to forward a packet out a specified port).  If no match is found, the packet is forwarded to the controller over the secure channel.  The controller is responsible for determining how to handle packets without valid flow entries, and it manages the switch flow table by adding and removing flow entries.
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+Flow entries may forward packets to one or more OpenFlow ports.   In general, these are physical ports, but the protocol does not preclude abstractions like port aggregations or VLAN traffic on a port appearing as an OpenFlow port.  OpenFlow ports have limited state such as ``up'', ``down'' and whether spanning tree flood packets should be forwarded out the port.  Additional configuration of ports may handled by the OpenFlow configuration protocol.  There are several OpenFlow virtual ports used to indicate, for example, flooding or the ingress port (see \ref{ft:actions}).
 
 \section{Flow Table}
 This section describes the components of flow table entries and the process by which incoming packets are matched against flow table entries.