Dynamic Host Configuration Protocol (DHCP) is a protocol based on the UDP and the network protocol used only within the LAN, the LAN environment mainly for large or there are more mobile office LAN environment devices, its main purpose is for the internal LAN equipment or network provider automatically assigns IP address and other parameters.
In simple terms, DHCP protocol is to allow hosts on the LAN to automatically obtain network parameters.
In the e-mail system, called mail user agent (Mail User Agent, MUA) for users to send and receive mail server. In addition, since e-mail system allows users to receive data still can be done in the case of off-line, definitely have to have a "mailbox" for saving the user's mail server, the name of the server as a mail delivery agent (Mail Delivery Agent, MDA ), whose job it is to save from the message transfer agent (Mail transfer agent, MTA) mail to the local inbox. Among them, the responsibilities of the MTA is dealing with different e-mail forwarding service mail between suppliers, forwarding messages from the MUA MTA to the appropriate server. For example, we send an e-mail mailbox to mailbox from Sina, the transmission process is shown in this email
Overall, the general network service program in the transmission of information like the call, both parties need to be online at the same time, and in the e-mail system, when users send e-mail without having to wait for delivery of the work can be completed offline. If the other party mail server (MTA) is down or the other side temporarily offline, Mail Server (MTA) will send the contents to be sent automatically temporarily saved to a local, and other testing to each other after the mail server will resume immediately delivered again during the general operation and maintenance personnel do not need maintenance processing, followed by the recipient (MUA) will be able to find this message in your mailbox.
Figure 15-2 during transmission of e-mail
