Comparing POP vs. IMAP

Processing of non−e−mail data, like NetNews or documents; this is very handy for uniformly accessing different classes of information • Offline access mode for minimum connect time and server resources; useful in situations where the only access to the mail server is via expensive dialup connections, and multiplatform access to the mailboxes is not needed • Permits online performance optimization, especially over low−speed links • IMAP was originally developed in 1986 at Stanford University. However, it garnered wide attention almost a decade later, and today IMAP is implemented in more and more software products. It is still not as well−known as earlier−released and less−capable alternatives such as POP. There is a companion protocol to IMAP, called Internet message support protocol IMSP, defined for user configuration management. IMSP permits the same location−independent multiplatform access to personal configuration data such as address books, bookmark lists, etc. that IMAP offers for mailboxes.

20.5.2.3 Comparing POP vs. IMAP

The basic characteristics of IMAP and POP reflect the characteristics of the online and offline access paradigms; their differences also determine the main differences between the two protocols. The summarized differences between the two paradigms are: Two distinct modes of use: offline = On−demand retrieval to a single client machine ♦ online = Interactive access to multiple mailboxes from multiple clients ♦ • Offline paradigm advantages: Minimum use of connect time ♦ Minimum use of server resources ♦ • Online paradigm advantages: Ability to use different computers at different times ♦ Ability to use dataless client machines, as in labs ♦ Platform−independent access to multiple mailboxes ♦ Tossibility of concurrent access to shared mailboxes ♦ • A brief comparison of POP and IMAP shows: Characteristics common to both POP and IMAP: Both can support offline operation ♦ E−mail is delivered to a shared active mail server ♦ New e−mail is accessible from a variety of client platform types ♦ New mail is accessible from anywhere in network ♦ Both protocols are open ♦ Both implementations are freely available including source ♦ Clients available for PCs, MACs, and UNIX ♦ Commercial implementations available ♦ • 498 Both protocols deal with access only; both rely on SMTP to send ♦ Both protocols support persistent message IDs for disconnected operation ♦ POP protocol advantages: A simpler protocol; easier to implement ♦ More client software currently available ♦ • IMAP protocol advantages: Manipulates persistent message status flags ♦ Stores messages as well as fetches them ♦ Can access and manage multiple mailboxes ♦ Supports concurrent updates and access to shared mailboxes ♦ Suitable for accessing non−e−mail data; e.g. NetNews or documents ♦ Can also use offline paradigm for minimum connect time and disk use ♦ Companion protocol defined for user configuration management IMSP ♦ Constructs to permit online performance optimization, especially over low−speed links ♦ • In summary, IMAP offers advantages over POP in three areas: richer functionality in manipulating the users inbox, the ability to manage mail folders other than the users inbox, and primitives to allow optimization of online performance, especially when dealing with large MIME messages. Because there are freely available IMAP development libraries, its additional complexity over POP should not be a significant obstacle to use. Therefore, a reasonable conclusion is that the only advantage of POP over IMAP is that there is currently more POP software available. However, this is changing rapidly, and IMAPs functional advantages over POP are nothing less than overwhelming. 499

Chapter 21: UNIX Network Support

21.1 Common UNIX Network Applications

The majority of network applications are not strictly required for the network to operate, but they provide user services that are central to the networks implementation; without them, the network serves no real purpose. Many of these applications require no special configuration. Once the UNIX system is configured properly and the network is set up including the setup of the Internet super daemon inetd, a number of network applications can be used immediately; other network applications require some administration. Among the most common network applications, we will briefly discuss three: telnet 1. The network terminal protocol, which provides remote login over the network ftp 2. The file transfer protocol, which is used for file transfers over the network finger 3. Provides information about remote users These applications are instrumental in daily UNIX administration. Because of their inherent interactive nature, they are primarily used from the command line, but they can also be a part of shell scripts and other programs. All three applications are based on the clientserver model. On the client side, the corresponding application program is supposed to be started from the command line, script, or any other program on an as−needed basis. The server side is handled by the corresponding daemons telnetd, ftpd, and fingerd that are invoked by the Internet super server the inetd daemon once a client request is received at the corresponding port. None of the three applications require a lot of work to be properly set; they just need to be enabled or disabled on the server side. These actions are provided through the inetd configuration file etcinetd.conf. Since the inetd daemon and the etcinetd.conf file were covered in Chapter 15, only the etcinetd.conf entries related to these applications are presented in the following example: cat etcinetd.conf ..... ..... ARPABerkeley services ftp stream tcp nowait root etcftpd ftpd −l telnet stream tcp nowait root etctelnetd telnetd finger stream tcp nowait bin etcfingerd fingerd ..... In this example, ftp and telnet are enabled, while finger is disabled the entry is commented−out and deactivated.

21.1.1 Telnet

Telnet provides a user interface to a remote system using the TELNET protocol. If telnet is invoked without arguments, it enters command mode, indicated by its own prompt telnet. In this mode, it accepts and executes the telnet commands these will be listed later. When invoked with arguments, it performs an open command with those arguments. The format of the command is: 500