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DNS server information with FTP and SMTP protocols

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FTP, SMTP and DNS 2: Application Layer 1 FTP: separate control, data connections TCP control connection port 21  FTP client contacts FTP server     at port 21, specifying TCP as transport protocol Client obtains authorization over control connection Client browses remote directory by sending commands over control connection. When server receives a command for a file transfer, the server opens a TCP data connection to client After transferring one file, server closes connection. FTP client TCP data connection port 20 FTP server  Server opens a second TCP data connection to transfer another file.  Control connection: “out of band”  FTP server maintains “state”: current directory, earlier authentication 2: Application Layer 2 Electronic Mail outgoing message queue user mailbox user agent Three major components:  user agents  mail servers mail server SMTP  simple mail transfer protocol: SMTP User Agent  a.k.a. “mail reader”  composing, editing, reading mail messages  e.g., Eudora, Outlook, elm, Netscape Messenger  outgoing, incoming messages stored on server SMTP mail server user agent SMTP user agent mail server user agent user agent user agent 2: Application Layer 3 Electronic Mail: mail servers user agent Mail Servers  mailbox contains incoming messages for user  message queue of outgoing (to be sent) mail messages  SMTP protocol between mail servers to send email messages ? ...
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