At the top of the stack (the collection of layers) are the user programs which are generating and consuming the bit streams. For example, the UNIX mail program is such a user program. Some person wishing to send email to someone uses the program on a terminal. When the email is ready to be sent, the mail program encapsulates it in a packet format and uses SMTP to deliver it to the network for delivery. Only the computer doesn't directly send SMTP packets over the network. Instead it uses TCP to open up a reliable connection (see next section) with the destination computer.