IST362: Network Protocols

Simple Mail Transfer Protocol

transport layer service used: TCP

port number: 25

Alice is sending letter to Bob what happens to the letter:

Alice mail user-agent forms an SMTP message and send it to Alice's mail server.
Alice's mail server realizes that the destination address does not belong to this server and sends a corresponding message to Bob's mail server
Bob invokes his mail user agent to get his letters from the server.

Warning: Step 3 can not be done using SMTP!

The SMTP commands define the mail transfer or the mail system function requested by the user. SMTP commands are character strings terminated by CR+LF. The commands themselves are alphabetic characters terminated by space if parameters follow and CR+LF otherwise.

MAIL FROM: <reverse-path>
This command tells the SMTP-receiver that a new mail transaction is starting and to reset all its state tables and buffers, including any recipients or mail data. The reverse-path portion of the first or only argument contains the source mailbox (between "<" and ">" brackets), which can be used to report errors (see section 4.2 for a discussion of error reporting). If accepted, the SMTP server returns a 250 OK reply. If the mailbox specification is not acceptable for some reason, the server must return a reply indicating whether the failure is permanent (i.e., will occur again if the client tries to send the same address again) or temporary (i.e., the address might be accepted if the client tries again later).
```
 MAIL FROM:<userx@y.foo.org> 
```
RCPT TO: <forward-path>
The first or only argument to this command includes a forward-path(normally a mailbox and domain, always surrounded by "<" and ">" brackets) identifying one recipient. If accepted, the SMTP server returns a 250 OK reply and stores the forward-path. If the recipient is known not to be a deliverable address, the SMTP server returns a 550 reply, typically with a string such as "no such user - " and the mailbox name (other circumstances and reply codes are possible). This step of the procedure can be repeated any number of times.
```
 RCPT TO:<userc@d.bar.org> 
```
DATA
If accepted, the SMTP server returns a 354 Intermediate reply and considers all succeeding lines up to but not including the end of mail data indicator to be the message text. When the end of text is successfully received and stored the SMTP-receiver sends a 250 OK reply.
Since the mail data is sent on the transmission channel, the end of mail data must be indicated so that the command and reply dialog can be resumed. SMTP indicates the end of the mail data by sending a line containing only a "." (period or full stop).
RSET
This command specifies that the current mail transaction will be aborted. Any stored sender, recipients, and mail data must be discarded, and all buffers and state tables cleared. The receiver must send a 250 OK reply to a RSET command with no arguments. A reset command may be issued by the client at any time.
NOOP
This command does not affect any parameters or previously entered commands. It specifies no action other than that the receiver send an OK reply.
EHLO or HELO
These commands are used to identify the SMTP client to the SMTP server. The argument field contains the fully-qualified domain name of the SMTP client if one is available. In situations in which the SMTP client system does not have a meaningful domain name (e.g., when its address is dynamically allocated and no reverse mapping record is available), the client SHOULD send an address literal, optionally followed by information that will help to identify the client system. The SMTP server identifies itself to the SMTP client in the connection greeting reply and in the response to this command. A client SMTP SHOULD start an SMTP session by issuing the EHLO command.
```
 EHLO marshall.edu 
```
QUIT
This command specifies that the receiver must send an OK reply, and then close the transmission channel.

Sending letter scenario

HELO marshall.edu
MAIL FROM: <dementiev@marshall.edu>
RCPT TO: <danil@mathcs.emory.edu>
RCPT TO: <dementiev@marshall.edu>
DATA
This is my test e-mail from SMTP server.
Dan.
.
QUIT

Official information about Simple Mail Transfer Protocol can be found in RFC2821.

Comparison with HTTP.

push vs. pull
7-bit content vs. 8-bit content
multipart messages (images & attachments). One message vs. sequence of messages.

Q: How to send different kind of information in one message?
A: MIME (Multipurpose Internet Mail Extension)

Header of the mail

Format is described in standard of the internet text messages.

From:
To:
Subject:
Do not forget an empty line between the header and the body of the message.

HELO mathcs.emory.edu
MAIL FROM: <danil@mathcs.emory.edu>
RCPT TO: <dementiev@marshall.edu>
RCPT TO: <_dementiev@marshall.edu>
DATA
From: someaddr@somewhere.edu
To: dementiev@marshall.edu
Subject: IST362 MIME encoding
MIME-Version: 1.0

Hello,
this is a test message for IST362 class sent via direct TCP connection to Marshall SMTP server.
Sincerely,
Daniel
.
QUIT

The MIME extension for non-ASCII data

MIME (Multipurpose Internet Mail Extension) is a special extension to RFC 822 - Standard for the format of ARPA Internet text messages.

Content-Transfer-Encoding:
Content-Type: (text/plain | image/jpeg | multipart/mixed)

MIME is defined in RFC 2045 and RFC 2046. Base 64 is one of the standards to convert 8-bit data into 7-bit data. Windows version of the program that does it is here. And this is a scenario that send a picture:

HELO mathcs.emory.edu
MAIL FROM: <danil@mathcs.emory.edu>
RCPT TO: <dementiev@marshall.edu>
RCPT TO: <_dementiev@marshall.edu>
DATA
From: someaddr@somewhere.edu
To: dementiev@marshall.edu
Subject: IST362 MIME encoding
MIME-Version: 1.0
Content-Transfer-Encoding: base64
Content-Type: image/gif

R0lGODlhDwAPAKL/AP+9AP/GAP/OAP/eCP/vEMDAwAAAAAAAACH5BAEAAAUALAAAAAAPAA8A
QANGWKrWvSyIKYa9g5higDPCN2wMZhEoyQBs4FLkJ8/cNIopxHz60rSuh48FcEkoIw7rE3JY
GkGn6AYzGTIajumCyvquWI0qAQA7
.
QUIT

Using MIME we can text and binary data in on letter:

HELO mathcs.emory.edu
MAIL FROM: <danil@mathcs.emory.edu>
RCPT TO: <dementiev@marshall.edu>
RCPT TO: <_dementiev@marshall.edu>
DATA
From: someaddr@somewhere.edu
To: dementiev@marshall.edu
Subject: IST362 MIME encoding
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary=gc0p4Jq0M2Yt08j34c0p

--gc0p4Jq0M2Yt08j34c0p
Content-Type: text/plain

Hello,
this is a test message for IST362 class sent via direct TCP connection to Marshall SMTP server.
This message also includes this image:
--gc0p4Jq0M2Yt08j34c0p
Content-Transfer-Encoding: base64
Content-Type: image/gif

R0lGODlhDwAPAKL/AP+9AP/GAP/OAP/eCP/vEMDAwAAAAAAAACH5BAEAAAUALAAAAAAPAA8A
QANGWKrWvSyIKYa9g5higDPCN2wMZhEoyQBs4FLkJ8/cNIopxHz60rSuh48FcEkoIw7rE3JY
GkGn6AYzGTIajumCyvquWI0qAQA7
--gc0p4Jq0M2Yt08j34c0p

Sincerely,
Daniel
--gc0p4Jq0M2Yt08j34c0p--
.
QUIT