Electronic Mail (SMTP)

Transcription

1 Electronic Mail (SMTP) Nowadays is more popular than the paper letters called snail-mails. It is a form of network communication. Some of the other forms of network communication being voice-over-internet, instant messaging. Over the years protocols have evolved. The first system simply consisted on file transfer protocol, more features were added to it like sending to multiple recipients, multimedia capability, GUI, spam its Architecture and Service The architecture of system as seen in figure consist of two kinds of subsystems: i. User Agents : To allow people to read and send ii. Message Transfer agents (or Mail Servers): This moves the message from source to destination. User Agent: It is a program that: i. User agent runs on the same machine on which a user reads the

2 ii. Provide GUI or Character-based interface, TUI (text based interface) iii. Means to compose message, reply to message, display, searching, discarding and mail submission etc. iv. Some user processing may be automatic, filter message v. Autometic response Message Transfer Agent I. The transfer of message through the network is done with SMTP (Simple Mail Transfer Protocol) II. SMTP originally specified in RFC 821 and the current RFC is RFC 5321 III. It sends mail over the connections and reports back the delivery status and report any error. IV. MTA also implement mailing list where the identical copy of message is delivered to everyone on a list of mail address. Other facility being carbon copy(cc), bling carbon copy (bcc), secrecy, ability to read and reply to boss mail. V. Linking user agents and message transfer agents are the concept of mailboxes. Mailboxes stores the received for a user and are maintained by servers. VI. Mail is transferred between MTA in a standard format, the key idea in message format is the envelop and its content. Message inside the envelop consist of two separate parts header and the body. The header contain information for user agent and the body is for the human to interpret. Message format: Below we discuss two message formats: Internet Message Format(IMF) Mumtipurpose Internet Mail Extensions (MIME) Internet Message Format (IMF) The message format is defined in RFC 5321 and is given below: Header TO: CC: BCC: From: Sender: Received: Return-Path: Meaning address(es) of primary recipient(s) address(es) of secondary recipient(s) address(es) for blind carbon copies Person or people who created this message address of the actual sender Line added by each transfer agent along the route Can be used to identify a path back to the sender

3 Mumtipurpose Internet Mail Extensions (MIME) The over internet evolved from simple txt based message to multiningual(french, German), No alphabet (Russian, Chinese) to rich contents with no text containing only video, audio, binary document/programs. Mumtipurpose Internet Mail Extensions (MIME) helps in these kind of messages. Th MIME header as defined in RFC , 4288,4289,2049 is given below that supports RFC 822 and RFC5322 format. Header MIME-Version: Content-Description: Content-Id: Meeaning Identifies the MIME version Human-readable string telling what is in the message Unique identifier Content-Transfer-Encoding: How the body is wrapped for transmission Content-Type: Type and format of the content MIME provide five type of encoding scheme, these are: i. 7-bit ASCII Text Encoding : Message with a limit of 1000 character per line ii. 8-bit ASCII simple text Encoding : Message with a limit of 1000 character per line iii. True Binary Encoding : same as 8-bit ASCII, but has no upper iv. limit of 1000 character in a line. Base64 Encoding: groups of 24 bits are broken up into four 6-bit units, with each unit being sent as a legal ASCII character. The coding is A for 0, B for 1, and so on, followed by the 26 lowercase letters, the 10 digits, and finally + and / for 62 and 63, respectively. The == and = sequences indicate that the last group contained only 8 or 16 bits, respectively. Carriage returns and line feeds are ignored, so they can be inserted at will in the encoded character stream to keep the lines short enough. v. quoted-printable encoding: For messages that are almost entirely ASCII but with a few non-ascii character base64 encoding is little inefficient, so this scheme The last header Content Type is important, It specifies the nature of the message body and has had an impact well beyond . For instance, content downloaded from the Web is labeled with MIME types so that the

4 browser knows how to present it. The types, along with example of commonly used subtypes, are shown in table below: MIME Type and Example SubType Type Example Subtype Description Text Plain, html, xml, css Text in various formats Image Gif, jpeg, tiff Pictures Audio Basic, mpeg, mp4 Sound Video Mpeg, mp4, quicktime Movies Model vrml 3D Model Application octet-stream, pdf, Data produced by javascript, zip applications Message http, rfc822 Encapsulated message Multipart mixed, alternative, Combination of multiple parallel, digest types Message Transfer: The mail transfer is done through SMTP protocol. The simplest way to move messages is to establish a transport connection from the source machine to the destination machine and then just transfer the message. Over the years, however, two different uses of SMTP have been differentiated. The first use is mail submission, by which user agents send messages into the mail system for delivery. The second use is to transfer messages between message transfer agents SMTP (Simple Mail Transfer Protocol) i. Within the Internet, is delivered by having the sending machine establish a TCP connection to port 25 of th receiving machine. ii. Listening to this port is a mail server that speaks SMTP (Simple Mail Transfer Protocol) iii. The sender machine waits for the server to talk. iv. The server sends a line of text with its identity and that it is ready to accept the mail. Or the client can retry again v. If server is willing to receive, the client announces whom the mail is coming from and to whom it meant for. vi. If such a recipient exists at the destination, the server gives the client go-ahead to send the mail. vii. The client sends the message and server acknowledges it, no checksum is done as TCP provide reliable byte stream. viii. When all the have been exchanged between them, and there is no more mail, the connection is released Mail Submission:

5 User agent run on user machines e.g. laptop, desktop etc and are not always connected to Internet. Mail transfer agent (MTA) run on ISP and company server and are always connected to Internet. An ISP or company usually does not want any remote user to be able to submit messages to its mail server to be delivered elsewhere. The ISP or company is not running the server as a public service. In addition, this kind of open mail relay attracts spammers. This is because it provides a way to launder the original sender and thus make the message more difficult to identify as spam. Given these considerations, SMTP is normally used for mail submission with the AUTH extension. This extension lets the server check the credentials (username and password) of the client Mail Transfer: i. Once the sending MTA receives a message from user agent, it will deliver it to receiving MTA using SMTP. ii. Fo some destination ee.uwa.edu.au, to what mail server the message should be delivered. iii. For this DNS is consulted. In this DNS query is made for MX (mail iv. exchanger) record of the domain ee.uwa.edu.au. This query returns an ordered list of the names and IP addresses of one or more mail servers. v. The sending MTA then makes a TCP connection to port 25 to the IP address of mail srver to reach the receiving MTA, and uses SMTP to relay the message. vi. vii. viii. The receiving MTA then place the mail for the user bob in the correct mailbox for bob to read at a later time. This may involve moving message among computers if there is large mail infrastructure. With this process, mail travels from intial to final MTA in a single step. Final Delivery: The mail for bob is now lying in his mailbox, the final step left is to transfer a copy of it to bob s user agent for diplay. The job of the user agent is to present a view of the contents of the mailbox, and to allow the mailbox to be remotely manipulated.