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+Mach ports are [[capabilities]].
+
+A Mach port is a kernel queue.  Each port has associated with
+it a receive right and one or more send and send-once rights.
+A queue can hold a number of messages.  Once the queue is full,
+the send blocks until their is space to enqueue the message
+(this is interruptible via a timeout mechanism).
+
+A receive right designates a queue and authorizes the holder to
+dequeue messages from the queue, and to create send and send-once
+rights.
+
+Send and send-once rights designate a queue and authorize the
+hold to enqueue messages (in the case of a send-once right,
+a single message).  Enqueuing a message is equivalent to
+[[invoke|invoking]] a capability.
+
+Send and receive rights are named using local names.  Each
+task has associated with it a port address space.  A ports
+are addressed via this table.  Each task thus has its own
+private [[NamingContext]] for ports.
+
+Ports can be [[delegate]]d in an IPC message.  When the
+receiver dequeues the message, the right is made available
+to it.
+
+A thread can only block receiving on a single port.  To work
+around this, the concept of a port set was introduced.  A receive
+right can be added to (at most) one port set.  When a thread
+receives from a port set, it dequeues from any of the ports that
+has a message available.
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