[erlang-questions] What is Cookie in distribution message as in {2, Cookie, ToPid}?

[Leo Liu]

http://www.erlang.org/doc/apps/erts/erl_dist_protocol.html Mentions
`cookie' in the handshake and also in the messages between connected
nodes.

Since cookie is not sent in clear text it is odd to see it used in:

SEND

    {2, Cookie, ToPid}

REG_SEND

    {6, FromPid, Cookie, ToName}

Ideas? Also what are the differences between EXIT and EXIT2:

EXIT

    {3, FromPid, ToPid, Reason}

EXIT2

    {8, FromPid, ToPid, Reason}

How to instruct an Erlang node to log distribution messages it receives?

Thanks,
Leo