4.1 TCP/IP Mail System works as a DTN Mail System

4.1  TCP/IP Mail System works as a DTN Mail System

Email is a method of exchanging digital messages from a sender to one or more recipients. Email system commonly operates across the internet and is based on a store and forward operation. Users on both sides are not required to be online at the same time. User only needs to connect to mail server briefly for them to read, write or send messages. Unique of email system is it tolerant to delay and disruption. Email applications are asynchronous in nature. No need for "always on" connectivity. It does not communicate most of the time and does not require end-to-end connectivity. Since email system briefly is a DTN system, TCP/IP email system can be implemented as a DTN email system [14]. Table 4.1 shows a Store, Carry and Forward Scenario for Internet (TCP/IP), Email and DTN. 

Scenario	Data Delivery Paradigm
Internet (TCP/IP)	Independently handled packets with immediate forward or discard
Email (RFC 5322)	Unitary Transfer of complete emails with Store and Forward at each intermediate hop
DTN (RFC 5050)	Unitary Transfer of ‘Bundles’ with Store, Carry and Forward at each intermediate hop node; nodes may be mobile.

Table 4.1: Store, Carry and Forward Scenario

There are two current internet standard protocols for email retrieval which are Post Office Protocol (POP) and Internet Message Application Protocol (IMAP). There is a difference in a way of IMAP and POP control the email Inbox. When using IMAP to access email Inbox, IMAP actually does not move messages into the computer even though the message appears on the computer. The message still remains in the mail server. When using POP to access email Inbox, POP checks the server for new messages and download them to the computer and then deletes the messages from the mail server. Figure 4.1 shows the difference between IMAP and POP. In this thesis, IMAP is used to ensure the message remains on the mail server for data synchronization between infomediary device and both side mail servers.

Figure 4.1: IMAP vs POP

There are several commercials and free mail server available such as hMailServer, AtMail etc. HMailServer is a free mail server for Microsoft Windows. It supports the common email protocols such as IMAP, SMTP and POP3. ) HMailServer can easily be integrated with many existing webmail systems. It has a flexible score-based spam protection using Bayesian spam filtering which is an email filtering based on statistical technique. This technique classified collection of words to identify spam email. It also can attach email to the virus scanner to scan all incoming and outgoing email [15]. AtMail is a webmail client written in PHP. It provides an elegant Ajax webmail client for existing IMAP mail servers, with less bloat and a focus on an intuitive, simple user interface. Atmail supports a variety of platforms. It offers a web interface feature [16]. In this thesis work, hMailServer is used for a mail server since it is easy to use, open source, free and Windows based application.

There are also several commercials and free mail client available such as SquirrelMail, Rmail, roundcube. SquirrelMail is a standards-based webmail package written in PHP. It includes built-in pure PHP support for the IMAP and SMTP protocols, and all pages render in pure HTML 4.0 (with no JavaScript required) for maximum compatibility across browsers. It has very few requirements and is very easy to configure and install. SquirrelMail has all the functionality as an email client, including strong MIME support, address books, and folder manipulation. Rmail is a Java[TM] library for sending and receiving e-mail messages with Mime extensions (attachments, html), SMTP (sending) and POP3 (receiving) protocols [17].

Roundcube webmail is a browser-based multilingual IMAP client with an application-like user interface. It provides full functionality as an e-mail client, including MIME support, address book, folder management, message searching and spell checking. Roundcube webmail is written in PHP and requires the MySQL, PostgreSQL or SQLite database. Its plugin API is easily extendable and the user interface is fully customizable using skins which are pure XHTML and CSS 2.Roundcube webmail is used in this thesis work since it supports IMAP and browser based which mean of portability [18].