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Chapter: Computer Networks : Application Layer

SMTP –Simple Mail Transfer Protocol

Next we look at SMTP- the protocol used to transfer messages from one host to another. To place SMTP in the right context, we need to identify the key players.



Next we look at SMTP- the protocol used to transfer messages from one host to another. To place SMTP in the right context, we need to identify the key players. First, users interact with a mail reader when they compose ,file ,search, and read their email. There are countless mail readers available ,just like there are many web browsers now include a mail reader. Second ,there is a mail daemon running on each host. You can think of this process as playing the role of a post office :mail readers give the daemon messages they want to send to others users, the daemon uses SMTP running over TCP to transmit the message into a daemon running on another machine, and the daemon puts incoming messages into the users mailbox. Since SMTP is a protocol that anyone could implement , in theory there could be many different implementations of the mail daemon. It runs out, though that the mail daemon running on most hosts is derived from the sendma il program originally implemented on berkely u nix.



While it is certainly possible that the sendmail program on a s enders machine establishes an SMTP/TCP connection to the sendmail program on the recipients machine, in many cases the mail traverses o ne or more mail gateways on its route from the senders host to the receivers host. Like the end hosts, these gateways also run a send-mail pr ocess. Its not an accident that these intermediatee nodes are called “gateways” since their job is to store and forward email messages.


Mail Reader:


The final step is for the user to actually receive her messages from th e mail box, read them ,reply to them, and possib ly save a copy for future reference .The user performs all the actions by interacting with a ma il reader. In many cases ,this reader is just a pro gram running on the same machine as the users mailbox resides, in which case it simply reads and writes the file that implements the mailbox .I n other cases ,the user accesses her mailbox from a remote machine using yet another protocol, such as the Post Office Protocol(POP ) or the Internet Message Access Control(IMAP).It is beyond the scope of this book to discuss the user interface aspects of the mail reader but it is definitely within our scope to talk about the access protocol. We consider IMAP, in particular.


IMAP is similar to S MTP in many ways .It is a client/server protocol running over TCP, where the client (running on the users desktop machine) issues commands in the form of <CRLF> terminated ASCII text lines and the mail server(running on the machine that maintains the users mailbox) responds in-kind. The exchange begins with the client authenticating herself, an d identifying the mailbox she wants to acc ess. This can be represented by the simple state tr ansaction diagram shown in the figure. In this d iagram, LOGIN, AUTHENTICATE, SELECT, EXAMINE, CLOSE and LOGOUT are exampl e commands that the client can issue, while OK is one possible server response. Other common c ommands include FETCH, STORE, DELETE, an d EXPUNGE, with the obvious meanings. Additional server responds include NO (client d oes not have permission to perform that operation) and BAD (command is ill-formed).


When the user asks to FETCH a message, the server returns it in MIM E format and the mail reader decodes it. In addition to the message itself, IMAP also defines a set of message attributes that are exchanged as part of other commands, independent of transfer ring the message itself. Message attributes include information like the size of the message, but more interestingly, various flags associated with a message, such as Seen, Answered, Deleted and Recent. These flags are used to keep the client and server synchronized, that is, when the user deletes a message in the mail reader, the client needs to report this fact to the mail server. Later, should the user decide to expunge all deleted messages, the client issues an EXPUNGE command to the server, which knows to actually remove all earlier deleted messages from the mail box.


Finally, note that when the user replies to a message,or sends a new message, the mail reader does not forward the message from the client to the mail server using IMAP ,but it instead uses SMTP .This means that the users mail server is effectively the first mail gateway traversed along the path from the desktop to the recipients mail box.


TCP/IP protocol suite specifies a standard for the exchange of mail between machines. It was derived from the (MTP) Mail Transfer Protocol. it deals with how the underlying mail delivery system passes messages across a link from one machine to another. The mail is enclosed in what is called an envelope. The envelope contains t he To and From fields and these are followed by the mail . The mail consists of two parts namely the Header and the Data. The Header has the To and From fields. If Headers are defined by us they should start with X. The standard headers do not start with X. In SMTP data portion can contain only printable ASCII characters The old method of sending a binary file was to send it in uuencoded form but there was no way to distinguish between the many types of binary files possible eg. .tar , .gz , .dvi etc.


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