network protocols handbook.pdf

31Protocols GuideTCP/IP - Application Layer ProtocolsProtocol NamePOP and POP3: Post OfficeProtocol (version 3)Optional POP3 Commands:APOP name digest valid in the AUTHORIZATION stateTOP msg n valid in the TRANSACTION stateUIDL [msg]Protocol DescriptionThe Post Office Protocol is designed to allow a workstation todynamically access a mail drop on a server host. POP3 is theversion 3 (the latest version) of the Post Office Protocol. POP3allows a workstation to retrieve mail that the server is holding forit. POP3 transmissions appear as data messages between stations.The messages are either command or reply messages.There are several different technologies and approaches tobuilding a distributed electronic mail infrastructure: POP (PostOffice Protocol), DMSP (Distributed Mail System Protocol),and IMAP (Internet Message Access Protocol) among them. Ofthe three, POP is the oldest and consequently the best known.DMSP is largely limited to a single application, PCMAIL, and isknown primarily for its excellent support of “disconnected” operation.IMAP offers a superset of POP and DMSP capabilities,and provides good support for all three modes of remote mailboxaccess: offline, online, and disconnected.POP was designed to support “offline” mail processing, in whichmail is delivered to a server, and a personal computer user periodicallyinvokes a mail “client” program that connects to theserver and downloads all of the pending mail to the user’s ownmachine. The offline access mode is a kind of store-and-forwardservice, intended to move mail (on demand) from the mailserver (drop point) to a single destination machine, usually a PCor Mac. Once delivered to the PC or Mac, the messages arethen deleted from the mail server.POP3 is not designed to provide extensive manipulation operationsof mail on the server; which are done by a more advanced(and complex) protocol IMAP4. POP3 uses TCP as the transportprotocol.Protocol StructurePOP3 messages are ASCII messages sent between client andservers. POP3 Command Summary:POP3 Replies:+OK-ERRRelated protocolsSMTP, IMAP4, TCP, POPSponsor SourcePOP3 is defined by IETF (http://www.ietf.org) in RFC 1939.Referencehttp://www.javvin.com/protocol/rfc1939.pdfPost Office Protocol - Version 3CommandsUSERPASSSTATRETRDELELISTTOP QUITDescriptionName of userUser’s passwordInformation on messages in the serverNumber of message to getNumber of message to deleteNumber of message to showPrint X lines of the message starting from thebeginning (header included)Exit to POP3’s server