network protocols handbook.pdf

36Protocols GuideTCP/IP - Application Layer ProtocolsProtocol NameSMTP: Simple Mail TransferProtocolProtocol DescriptionSimple Mail Transfer Protocol (SMTP) is a protocol designed totransfer electronic mail reliably and efficiently. SMTP is a mailservice modeled on the FTP file transfer service. SMTP transfersmail messages between systems and provides notificationregarding incoming mail.SMTP is independent of the particular transmission subsystemand requires only a reliable ordered data stream channel. An importantfeature of SMTP is its capability to transport mail acrossnetworks, usually referred to as “SMTP mail relaying”. A networkconsists of the mutually-TCP-accessible hosts on the public Internet,the mutually-TCP-accessible hosts on a firewall-isolatedTCP/IP Intranet, or hosts in some other LAN or WAN environmentutilizing a non-TCP transport-level protocol. Using SMTP,a process can transfer mail to another process on the same networkor to some other network via a relay or gateway processaccessible to both networks.In this way, a mail message may pass through a number of intermediaterelay or gateway hosts on its path from sender toultimate recipient. The Mail eXchanger mechanisms of the domainname system are used to identify the appropriate next-hopdestination for a message being transported.Protocol StructureSMTP commands are ASCII messages sent between SMTPhosts. Possible commands are as follows:Related protocolsPOP3, IMAP4, TCP, POP, FTPSponsor SourceSMTP is defined by IETF (http://www.ietf.org) in RFC 2821.Referencehttp://www.javvin.com/protocol/rfc2821.pdfSimple Mail Transfer ProtocolCommandDATAEXPN HELO HELP MAIL FROM NOOPQUITRCPT TO RSETSAML FROM SEND FROM SOML FROM TURNVRFY DescriptionBegins message composition.Returns names on the specified mail list.Returns identity of mail server.Returns information on the specified command.Initiates a mail session from host.Causes no action, except acknowledgementfrom server.Terminates the mail session.Designates who receives mail.Resets mail connection.Sends mail to user terminal and mailbox.Sends mail to user terminal.Sends mail to user terminal or mailbox.Switches role of receiver and sender.Verifies the identity of a user.