note that TLS connections can be closed.

   of TLS records MUST NOT leak information about the type or contents

   of the cells.

   OR-to-OR connections are never deliberately closed.  When an OR

   starts or receives a new directory, it tries to open new

   connections to any OR it is not already connected to.

[not true, unused OR conns close after 5 mins too -RD]

   OR-to-OP connections are not permanent. An OP should close a

   TLS connections are not permanent. An OP or an OR may close a

   connection to an OR if there are no circuits running over the

   connection, and an amount of time (KeepalivePeriod, defaults to 5

   minutes) has passed.

   (As an exception, directory servers may try to stay connected to all of

   the ORs.)

3. Cell Packet format

   The basic unit of communication for onion routers and onion