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Encyclopedia > Post Office Protocol
The five-layer TCP/IP model
5. Application layer

DHCPDNSFTPGopherHTTPIMAP4IRCNNTPXMPPPOP3SIPSMTPSNMPSSHTELNETRPC • RTP • RTCPRTSPTLS/SSLSDPSOAPBGP • PPTP • L2TPGTPSTUNNTP • ... The TCP/IP model or Internet reference model, sometimes called the DoD model (DoD, Department of Defense), ARPANET reference model, is a layered abstract description for communications and computer network protocol design. ... The application layer is the seventh level of the seven-layer OSI model. ... (DHCP) is a set of rules used by a communications device such as a computer, router or network adapter to allow the device to request and obtain an IP address from a server which has a list of addresses available for assignment. ... It has been suggested that this article be split into multiple articles. ... “FTP” redirects here. ... Gopher is a distributed document search and retrieval network protocol designed for the Internet. ... Hypertext Transfer Protocol (HTTP) is a communications protocol used to transfer or convey information on the World Wide Web. ... The Internet Message Access Protocol (commonly known as IMAP or IMAP4, and previously called Internet Mail Access Protocol, Interactive Mail Access Protocol (RFC 1064), and Interim Mail Access Protocol[1]) is an application layer Internet protocol operating on port 143 that allows a local client to access e-mail on... “IRC” redirects here. ... The Network News Transfer Protocol or NNTP is an Internet application protocol used primarily for reading and posting Usenet articles, as well as transferring news among news servers. ... Extensible Messaging and Presence Protocol, or XMPP, is an open, XML-based protocol for near real-time extensible messaging and presence events. ... The Session Initiation Protocol (SIP) is an application-layer control (signaling) protocol for creating, modifying, and terminating sessions with one or more participants. ... Simple Mail Transfer Protocol (SMTP) is the de facto standard for e-mail transmissions across the Internet. ... The simple network management protocol (SNMP) forms part of the internet protocol suite as defined by the Internet Engineering Task Force (IETF). ... Secure Shell or SSH is a network protocol that allows data to be exchanged over a secure channel between two computers. ... This article or section includes a list of works cited or a list of external links, but its sources remain unclear because it lacks in-text citations. ... Remote procedure call (RPC) is a protocol that allows a computer program running on one computer to cause a subroutine on another computer to be executed without the programmer explicitly coding the details for this interaction. ... The Real-time Transport Protocol (or RTP) defines a standardized packet format for delivering audio and video over the Internet. ... RTP Control Protocol (RTCP) is a sister protocol of the Real-time Transport Protocol (RTP). ... The Real Time Streaming Protocol (RTSP), developed by the IETF and created in 1998 as RFC 2326, is a protocol for use in streaming media systems which allows a client to remotely control a streaming media server, issuing VCR-like commands such as play and pause, and allowing time-based... Transport Layer Security (TLS) and its predecessor, Secure Sockets Layer (SSL), are cryptographic protocols that provide secure communications on the Internet for such things as web browsing, e-mail, Internet faxing, instant messaging and other data transfers. ... Session Description Protocol (SDP), is a format for describing streaming media initialization parameters. ... This article is about the computer protocol. ... The Border Gateway Protocol (BGP) is the core routing protocol of the Internet. ... The Point-to-Point Tunneling Protocol (PPTP) is a method for implementing virtual private networks. ... In computer networking, the Layer 2 Tunneling Protocol (L2TP) is a tunneling protocol used to support virtual private networks (VPNs). ... GPRS Tunneling Protocol (or GTP) is an IP based protocol used within GSM and UMTS networks. ... This article is about the Internet protocol. ... The Network Time Protocol (NTP) is a protocol for synchronizing the clocks of computer systems over packet-switched, variable-latency data networks. ...

4. Transport layer

TCPUDPDCCPSCTPRSVP • ... In computing and telecommunications, the transport layer is the second highest layer in the four and five layer TCP/IP reference models, where it responds to service requests from the application layer and issues service requests to the Internet layer. ... The Transmission Control Protocol (TCP) is one of the core protocols of the Internet protocol suite, often simply referred to as TCP/IP. Using TCP, applications on networked hosts can create connections to one another, over which they can exchange streams of data using Stream Sockets. ... User Datagram Protocol (UDP) is one of the core protocols of the Internet protocol suite. ... The Datagram Congestion Control Protocol (DCCP) is a message-oriented transport layer protocol that is currently under development in the IETF. Applications that might make use of DCCP include those with timingconstraints on the delivery of data such that reliable in-order delivery, when combined with congestion control, is likely... In the field of computer networking, the IETF Signaling Transport (SIGTRAN) working group defined the Stream Control Transmission Protocol (SCTP) as a transport layer protocol in 2000. ... The Resource ReSerVation Protocol (RSVP), described in RFC 2205, is a transport layer protocol designed to reserve resources across a network for an integrated services Internet. ...

3. Network/Internet Layer

IP (IPv4IPv6) • IGMPICMPOSPFISISIPsecARPRARPRIP • ... The network layer is third layer out of seven in OSI model and it is the third layer out of five in TCP/IP model. ... The Internet Protocol (IP) is a data-oriented protocol used for communicating data across a packet-switched internetwork. ... Internet Protocol version 4 is the fourth iteration of the Internet Protocol (IP) and it is the first version of the protocol to be widely deployed. ... It has been suggested that IPv6 internet be merged into this article or section. ... The Internet Group Management Protocol is a communications protocol used to manage the membership of Internet Protocol multicast groups. ... The (ICMP) is one of the core protocols of the Internet protocol suite. ... The Open Shortest Path First (OSPF) protocol is a hierarchical interior gateway protocol (IGP) for routing in Internet Protocol, using a link-state in the individual areas that make up the hierarchy. ... Intermediate system to intermediate system (IS-IS), is an IGP routing protocol originally designed for CLNS as part of the OSI protocol stack and described in ISO 10589 . ... IPsec (IP security) is a suite of protocols for securing Internet Protocol (IP) communications by authenticating and/or encrypting each IP packet in a data stream. ... In computer networking, the Address Resolution Protocol (ARP) is the standard method for finding a hosts hardware address when only its network layer address is known. ... Reverse Address Resolution Protocol (RARP) is a network layer protocol used to obtain an IP address for a given hardware address (such as an Ethernet address). ... This article is chiefly about the Routing Information Protocol for IPv4 and IPv6. ...

2. Data link layer

802.11ATMDTMToken RingEthernetFDDIFrame RelayGPRS • EVDO • HSPA • HDLC • PPP • ... This article does not cite any references or sources. ... IEEE 802. ... The introduction to this article provides insufficient context for those unfamiliar with the subject matter. ... Dynamic synchronous Transfer Mode , or DTM for short, is a network protocol. ... Token-Ring local area network (LAN) technology was developed and promoted by IBM in the early 1980s and standardised as IEEE 802. ... Ethernet is a large, diverse family of frame-based computer networking technologies that operates at many speeds for local area networks (LANs). ... In computer networking, fiber-distributed data interface (FDDI) is a standard for data transmission in a local area network that can extend in range up to 200 km (124 miles). ... In the context of computer networking, frame relay consists of an efficient data transmission technique used to send digital information quickly and cheaply in a relay of frames to one or many destinations from one or many end-points. ... General Packet Radio Service (GPRS) is a Mobile Data Service available to users of Global System for Mobile Communications (GSM) and IS-136 mobile phones. ... Evolution-Data Optimized or Evolution-Data only, abbreviated as EV-DO or EVDO and often EV, is one telecommunications standard for the wireless transmission of data through radio signals, typically for broadband Internet access. ... High-Speed Packet Access (HSPA) is a collection of mobile telephony protocols that extend and improve the performance of existing UMTS protocols. ... High-Level Data Link Control (HDLC) is a bit-oriented synchronous data link layer protocol developed by the International Organization for Standardization (ISO). ... In computing, the Point-to-Point Protocol, or PPP, is commonly used to establish a direct connection between two nodes. ...

1. Physical layer

Ethernet physical layerISDNModemsPLCSONET/SDHG.709Optical FiberWiFiWiMAXCoaxial CableTwisted Pair • ... This article does not cite any references or sources. ... IEEE photograph of a diagram with the original terms for describing Ethernet drawn by Robert M. Metcalfe around 1976. ... // Integrated Services Digital Network (ISDN) is a circuit-switched telephone network system, designed to allow digital transmission of voice and data over ordinary telephone copper wires, resulting in better quality and higher speeds than that is available with the PSTN system. ... A modem (from modulate and demodulate) is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. ... For other uses, see Power band. ... It has been suggested that this article be split into articles entitled Synchronous optical networking, SONET and Synchronous digital hierarchy. ... There are very few or no other articles that link to this one. ... Optical fibers An optical fiber (or fibre) is a glass or plastic fiber designed to guide light along its length by confining as much light as possible in a propagating form. ... Wi-Fi (or Wi-fi, WiFi, Wifi, wifi), short for Wireless Fidelity, is a set of standards for wireless local area networks (WLAN) currently based on the IEEE 802. ... WiMAX, the Worldwide Interoperability for Microwave Access, is a telecommunications technology aimed at providing wireless data over long distances in a variety of ways, from point-to-point links to full mobile cellular type access. ... Coaxial Cable For the weapon, see coaxial weapon. ... 25 Pair Color Code Chart 10BASE-T UTP Cable Twisted pair cabling is a common form of wiring in which two conductors are wound around each other for the purposes of cancelling out electromagnetic interference known as crosstalk. ...

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In computing, local e-mail clients use the Post Office Protocol version 3 (POP3), an application-layer Internet standard protocol, to retrieve e-mail from a remote server over a TCP/IP connection. Many subscribers to individual Internet service provider e-mail accounts access their e-mail with client software that uses POP3. RAM (Random Access Memory) Look up computing in Wiktionary, the free dictionary. ... This article does not cite any references or sources. ... The application layer is the seventh level of the seven-layer OSI model. ... An Internet standard is a specification for an innovative internetworking technology or methodology, which the Internet Engineering Task Force (IETF) ratified as an open standard after the innovation underwent peer review. ... For other senses of this word, see protocol. ... Wikipedia does not yet have an article with this exact name. ... A mail transfer agent or MTA (also called a mail server, or a mail exchange server in the context of the Domain Name System) is a computer program or software agent which transfers electronic mail messages from one computer to another. ... The Internet protocol suite is the set of communications protocols that implement the protocol stack on which the Internet and most commercial networks run. ... “ISP” redirects here. ...

Contents

Overview

POP3 has made earlier versions of the protocol obsolete, POP (informally called POP1 and POP2). In contemporary usage, the less precise term POP almost always means POP3 in the context of e-mail protocols.


The design of POP3 and its procedures supports end-users with intermittent connections (such as dial-up connections), allowing these users to retrieve e-mail when connected and then to view and manipulate the retrieved messages without needing to stay connected. Although most clients have an option to leave mail on server, e-mail clients using POP3 generally connect, retrieve all messages, store them on the user's PC as new messages, delete them from the server, and then disconnect. In contrast, the newer, more capable Internet Message Access Protocol (IMAP) supports both connected and disconnected modes of operation. E-mail clients using IMAP generally leave messages on the server until the user explicitly deletes them. This and other facets of IMAP operation allow multiple clients to access the same mailbox. Most e-mail clients support either POP3 or IMAP to retrieve messages; however, fewer Internet Service Providers (ISPs) support IMAP. The fundamental difference between POP3 and IMAP4 is that POP3 offers access to a mail drop; the mail exists on the server until it is collected by the client. Even if the client leaves some or all messages on the server, the client's message store is considered authoritative. In contrast, IMAP4 offers access to the mail store; the client may store local copies of the messages, but these are considered to be a temporary cache; the server's store is authoritative. Dial-up access is a form of Internet access through which the client uses a modem connected to a computer and a telephone line to dial into an Internet service providers (ISP) node to establish a modem-to-modem link, which is then routed to the Internet. ... The Internet Message Access Protocol (commonly known as IMAP or IMAP4, and previously called Internet Mail Access Protocol, Interactive Mail Access Protocol (RFC 1064), and Interim Mail Access Protocol[1]) is an application layer Internet protocol operating on port 143 that allows a local client to access e-mail on... “ISP” redirects here. ...


Clients with a leave mail on server option generally use the POP3 UIDL (Unique IDentification Listing) command. Most POP3 commands identify specific messages by their ordinal number on the mail server. This creates a problem for a client intending to leave messages on the server, since these message numbers may change from one connection to the server to another. For example if a mailbox contains five messages at last connect, and a different client then deletes message #3, the next connecting user will find the last two messages' numbers decremented by one. UIDL provides a mechanism to avoid these numbering issues. The server assigns a string of characters as a permanent and unique ID for the message. When a POP3-compatible e-mail client connects to the server, it can use the UIDL command to get the current mapping from these message IDs to the ordinal message numbers. The client can then use this mapping to determine which messages it has yet to download, which saves time when downloading. IMAP has a similar mechanism, using a 32-bit UID (Unique IDentifier) that is required to be strictly ascending. The advantage of the numeric UID is with large mailboxes; a client can request just the UIDs greater than its previously stored "highest UID". In POP, the client must fetch the entire UIDL map. A mail transfer agent or MTA (also called a mail server, or a mail exchange server in the context of the Domain Name System) is a computer program or software agent which transfers electronic mail messages from one computer to another. ...


Whether using POP3 or IMAP to retrieve messages, e-mail clients typically use the SMTP_Submit profile of the SMTP protocol to send messages. E-mail clients are commonly categorized as either POP or IMAP clients, but in both cases the clients also use SMTP. There are extensions to POP3 that allow some clients to transmit outbound mail via POP3 - these are known as "XTND XMIT" extensions. The Qualcomm qpopper and CommuniGate Pro servers and Eudora clients are examples of systems that optionally utilize the XTND XMIT methods of authenticated client-to-server e-mail transmission. Simple Mail Transfer Protocol (SMTP) is the de facto standard for e-mail transmissions across the Internet. ...


MIME serves as the standard for attachments and non-ASCII text in e-mail. Although neither POP3 nor SMTP require MIME-formatted e-mail, essentially all Internet e-mail comes MIME-formatted, so POP clients must also understand and use MIME. IMAP, by design, assumes MIME-formatted e-mail. Multipurpose Internet Mail Extensions (MIME) is an Internet Standard that extends the format of e-mail to support: text in character sets other than US-ASCII; non-text attachments; multi-part message bodies; and header information in non-ASCII character sets. ... Image:ASCII fullsvg There are 95 printable ASCII characters, numbered 32 to 126. ...


Like many other older Internet protocols, POP3 originally supported only an unencrypted login mechanism. Although plain text transmission of passwords in POP3 still commonly occurs, POP3 currently supports several authentication methods to provide varying levels of protection against illegitimate access to a user's e-mail. One such method, APOP, uses the MD5 hash function in an attempt to avoid replay attacks and disclosure of the shared secret. Clients implementing APOP include Mozilla Thunderbird, Opera, Eudora, KMail and Novell Evolution. POP3 clients can also support SASL authentication methods via the AUTH extension. MIT Project Athena also produced a Kerberized version. “Encrypt” redirects here. ... Computer files can be divided into two broad categories: binary and text. ... Authentication (from Greek αυθεντικός; real or genuine, from authentes; author) is the act of establishing or confirming something (or someone) as authentic, that is, that claims made by or about the thing are true. ... In cryptography, MD5 (Message-Digest algorithm 5) is a widely used cryptographic hash function with a 128-bit hash value. ... In cryptography, a cryptographic hash function is a hash function with certain additional security properties to make it suitable for use as a primitive in various information security applications, such as authentication and message integrity. ... A replay attack is a form of network attack in which a valid data transmission is maliciously or fraudulently repeated or delayed. ... Each secret share is a plane, and the secret is the point at which three shares intersect. ... Mozilla Thunderbird is a free, cross-platform e-mail and news client developed by the Mozilla Foundation. ... Opera is an Internet suite which handles common internet-related tasks, including visiting web sites, sending and receiving e-mail messages, managing contacts, and online chat. ... Eudora is an e-mail client used on the Apple Macintosh and Microsoft Windows operating systems. ... KMail is the email client of the KDE Desktop Environment. ... Evolution or Novell Evolution (formerly Ximian Evolution, prior to Novells 2003 acquisition of Ximian) is the official personal information manager and workgroup information management tool for GNOME. It combines e-mail, calendar, address book, and task list management functions. ... Simple Authentication and Security Layer (SASL) is a framework for authentication and authorization in Internet protocols. ... Project Athena was a joint project of MIT, Digital Equipment Corporation, and IBM. It was launched in 1983, and research and development ran through June 30, 1991, eight years after it began. ... Kerberos is the name of a computer network authentication protocol, which allows individuals communicating over an insecure network to prove their identity to one another in a secure manner. ...


POP3 works over a TCP/IP connection using TCP on network port 110. E-mail clients can encrypt POP3 traffic using TLS or SSL. A TLS or SSL connection is negotiated using the STLS command. Some clients and servers, like Google Gmail, instead use the deprecated alternate-port method, which uses TCP port 995. It has been suggested that this article or section be merged into Computer port (software). ... Transport Layer Security (TLS) and its predecessor, Secure Sockets Layer (SSL), are cryptographic protocols that provide secure communications on the Internet for such things as web browsing, e-mail, Internet faxing, instant messaging and other data transfers. ... Secure Sockets Layer (SSL) and Transport Layer Security (TLS), its successor, are cryptographic protocols which provide secure communications on the Internet. ... Transport Layer Security (TLS) and its predecessor, Secure Sockets Layer (SSL), are cryptographic protocols that provide secure communications on the Internet for such things as web browsing, e-mail, Internet faxing, instant messaging and other data transfers. ... Secure Sockets Layer (SSL) and Transport Layer Security (TLS), its successor, are cryptographic protocols which provide secure communications on the Internet. ...


Dialog example

RFC 1939 APOP support indicated by <1896.697170952@dbc.mtview.ca.us> here:

 S: <wait for connection on TCP port 110> C: <open connection> S: +OK POP3 server ready <1896.697170952@dbc.mtview.ca.us> C: APOP mrose c4c9334bac560ecc979e58001b3e22fb S: +OK mrose's maildrop has 2 messages (320 octets) C: STAT S: +OK 2 320 C: LIST S: +OK 2 messages (320 octets) S: 1 120 S: 2 200 S: . C: RETR 1 S: +OK 120 octets S: <the POP3 server sends message 1> S: . C: DELE 1 S: +OK message 1 deleted C: RETR 2 S: +OK 200 octets S: <the POP3 server sends message 2> S: . C: DELE 2 S: +OK message 2 deleted C: QUIT S: +OK dewey POP3 server signing off (maildrop empty) C: <close connection> S: <wait for next connection> 

POP3 servers without the optional APOP command expect you to log in with the USER and PASS commands:

 C: USER mrose S: +OK User accepted C: PASS mrosepass S: +OK Pass accepted 

POP4

While not yet an official standardized mail protocol, a proposal has been outlined for a POP4 specification, complete with a working server implementation.


The proposed POP4 extension adds basic folder management, multipart message support, as well as message flag management, allowing for a light protocol which supports some popular IMAP features which POP3 currently lacks.


No progress has been observed in the POP4 specification since 2003.


SDPS

Demon Internet introduced extensions to POP3 that allow multiple accounts per domain, and has become known as Standard Dial-up POP3 Service (SDPS).[1] Demon Internet is a British Internet Service Provider. ...


To access each account, the username includes the hostname, as john@hostname or john+hostname.


See also

The Internet Message Access Protocol (commonly known as IMAP or IMAP4, and previously called Internet Mail Access Protocol, Interactive Mail Access Protocol (RFC 1064), and Interim Mail Access Protocol[1]) is an application layer Internet protocol operating on port 143 that allows a local client to access e-mail on... Simple Mail Transfer Protocol (SMTP) is the de facto standard for e-mail transmissions across the Internet. ... This article does not cite any references or sources. ... Webmail is a class of web applications that allow users to read and write e-mail using a web browser, or in a more general sense, an e-mail account accessed through such an application. ... Getmail is a simple replacement to fetchmail, implemented in Python. ... fetchmail is a Free software utility for POSIX-compliant operating systems which is used to retrieve e-mail from a remote POP3, IMAP, ETRN or ODMR mail server to the users local system. ... There are very few or no other articles that link to this one. ...

External links

POP3 RFCs

  • RFC 1939 - "Post Office Protocol - Version 3"
  • RFC 2195 - "IMAP/POP AUTHorize Extension for Simple Challenge/Response"
  • RFC 2449 - "POP3 Extension Mechanism"
  • RFC 1734 - "POP3 AUTHentication command"
  • RFC 2222 - "Simple Authentication and Security Layer (SASL)"
  • RFC 3206 - "The SYS and AUTH POP Response Codes"
  • RFC 2595 - "Using TLS with IMAP, POP3 and ACAP"
  • RFC 937 - "POST OFFICE PROTOCOL - VERSION 2"
  • RFC 918 - "POST OFFICE PROTOCOL"

In internetworking and computer network engineering, Request for Comments (RFC) documents are a series of memoranda encompassing new research, innovations, and methodologies applicable to Internet technologies. ...

Server implementations


  Results from FactBites:
 
RFC 1939 (rfc1939) - Post Office Protocol - Version 3 (5166 words)
The Post Office Protocol - Version 3 (POP3) is intended to permit a workstation to dynamically access a maildrop on a server host in a useful fashion.
Usually, this means that the POP3 protocol is used to allow a workstation to retrieve mail that the server is holding for it.
Such message deletions are outside the scope of the POP3 protocol and are not considered a protocol violation.
  More results at FactBites »

 
 

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