In SSL, sessions are used to describe an ongoing relationship between two entities. Each SSL connection involves one session at a time, but that session may be used on many connections between those entities, simultaneously or sequentially. The session used on a connection may also be replaced by a different session. Sessions are created, or rejoined, as part of the SSL handshaking protocol. Sessions may be invalidated due to policies affecting security or resource usage, or by an application explicitly calling <code>invalidate</code>. Connection management policies are typically used to tune performance.