Before we get into the meat of our discussions we're going to take just a moment to list some of the terms and acronyms we'll be using. Experienced users will probably want to skip this section but because of the popularity of Linux many inexperienced users are starting to use complex systems so it will probably be helpful if we cover a few of the more important background material before we get too deep into our discussions.
protocol - A set of formal rules describing how to transmit data, especially across a network. Low level protocols define the electrical and physical standards to be observed, bit- and byte-ordering and the transmission and error detection and correction of the bit stream. High level protocols deal with the data formatting, including the syntax of messages, the terminal to computer dialogue, character sets, sequencing of messages etc.
Many protocols are defined by RFCs or by OSI.
MTA - Message Transfer Agent <messaging> (MTA) The program responsible for delivering e-mail messages. Upon receiving a message from a Mail User Agent or another MTA it stores it temporarily locally and analyses the recipients and either delivers it (local addressee) or forwards it to another MTA (routing). In either case it may edit and/or add to the message headers. The most widely used MTA for Unix is sendmail.
MUA - Mail User Agent <messaging> (MUA) The program that allows the user to compose and read electronic mail messages. The MUA provides the interface between the user and the Message Transfer Agent. Outgoing mail is eventually handed over to an MTA for delivery while the incoming messages are picked up from where the MTA left it (although MUA's running on single-user machines may pick up mail using POP).
Popular MUAs for Unix include elm, mush, pine, and RMAIL.
MDA - Mail Delivery Agent. <messaging> (MDA) A software device used to handle local delivery of inbound mail. It is sometimes called the Local Delivery Agent (LDA). The default MDA on Red Hat Linux is procmail. MDA's are often locally defined.
The process works this way:
Example 2-1. Mail process description
Message sent via MUA ----> SMTP ----> MTA Message transferred via MTA ----> SMTP ----> MTA Message received via MTA ----> SMTP ----> MTA Message "receipt-to" verified ----> SMTP ----> MTA Message handed off to MDA ----> SMTP ----> MDA Message delivered to user ----> MDA ----> spool Message read by user ----> MUA
There might be better ways to define this process but, quite simply, one process hands off to the other, in series, until the message is received by the destination recipient.
RFC - Request For Comments. <standard> (RFC) One of a series, begun in 1969, of numbered Internet informational documents and standards widely followed by commercial software and freeware in the Internet and Unix communities. Few RFCs are standards but all Internet standards are recorded in RFCs. Perhaps the single most influential RFC has been RFC 822, the Internet electronic mail format standard.
The RFCs are unusual in that they are floated by technical experts acting on their own initiative and reviewed by the Internet at large, rather than formally promulgated through an institution such as ANSI. For this reason, they remain known as RFCs even once adopted as standards. The RFC tradition of pragmatic, experience-driven, after-the-fact standard writing done by individuals or small working groups has important advantages over the more formal, committee-driven process typical of ANSI or ISO.
Emblematic of some of these advantages is the existence of a flourishing tradition of "joke" RFCs; usually at least one a year is published, usually on April 1st. Well-known joke RFCs have included 527 ("ARPAWOCKY", R. Merryman, UCSD; 22 June 1973), 748 ("Telnet Randomly-Lose Option", Mark R. Crispin; 1 April 1978), and 1149 ("A Standard for the Transmission of IP Datagrams on Avian Carriers", D. Waitzman, BBN STC; 1 April 1990). The first was a Lewis Carroll pastiche; the second a parody of the TCP/IP documentation style, and the third a deadpan skewering of standards-document legalese, describing protocols for transmitting Internet data packets by carrier pigeon. The RFCs are most remarkable for how well they work - they manage to have neither the ambiguities that are usually rife in informal specifications, nor the committee-perpetrated misfeatures that often haunt formal standards, and they define a network that has grown to truly worldwide proportions.
SMTP - Simple Mail Transfer Protocol. A protocol defined in STD 10, RFC 821, used to transfer electronic mail between computers, usually over Ethernet or Internet. It is a server to server protocol, so other protocols are used to access the messages. The SMTP dialog usually happens in the background under the control of the message transport system, e.g. sendmail but it is possible to interact with an SMTP server using telnet to connect to the normal SMTP port, 25. E.g.
Example 2-2. connect to the remote host:
telnet mhs-relay.ac.uk 25
Example 2-3. identifying the local host:
Example 2-4. issue commands to verify an address or expand an alias:
VRFY email@example.com VRFY postmaster
Example 2-5. expand a mailing list:
Example 2-6. send a message:
MAIL From:<firstname.lastname@example.org> RCPT To:<email@example.com> DATA Having fun yet? . QUIT
This is useful if you want to find out exactly what is happening to your message at a certain point. See also Post Office Protocol, RFC 822, and sendmail.
POP - Post Office Protocol. <messaging, protocol> (POP) A protocol designed to allow single user computers to retrieve electronic mail from a POP server. The POP server might be a computer with a permanent Internet connection whereas its clients might only connect to it occasionally, e.g. by modem. There are (in 1994) three versions: POP, POP2, and POP3. Later versions are NOT compatible with earlier ones.
IMAP - Internet Message Access Protocol. <protocol, messaging> (IMAP) A protocol allowing a client to access and manipulate electronic mail messages on a server. It permits manipulation of remote message folders (mailboxes), in a way that is functionally equivalent to local mailboxes.
IMAP includes operations for creating, deleting, and renaming mailboxes; checking for new messages; permanently removing messages; searching; and selective fetching of message attributes, texts, and portions thereof. It does not specify a means of posting mail; this function is handled by a mail transfer protocol such as SMTP.
CIDR - Classless Inter-Domain Routing <networking> (CIDR) /sid*r/ A scheme which allocates blocks of Internet addresses in a way that allows summarisation into a smaller number of routing table entries.
A CIDR block is a block of Internet addresses assigned to an Internet Service Provider (ISP) by the Internic.
CIDR was introduced to enable the use of more sophisticated routing protocols such as OSPF (Open Shortest Path First).
ETRN - "Extended TURN". <messaging, protocol> (ETRN) An ESMTP command (first defined in RFC 1985) with which a client asks the server to deliver queued mail to the client via a new ESMTP connection.
ETRN supercedes the SMTP "TURN" command in the same way that ESMTP's "EHLO" supercedes SMTP's "HELO".
ESMTP - Extended SMTP. <messaging, protocol> (ESMTP) Initially defined in RFC 1869 and extended thereafter.
See also ETRN.
Note: All of these definitions were obtained from Free On-Line Dictionary of Computing (FOLDOC).