MCUs Application Note Rev. 4120A-8051-04/02 E-mail Delivery:

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This application note explains microcontrollers send e-mail using common protocol Internet SMTP. Currently, most Atmel microcontrollers capable sending e-mail worldwide. This application note describes efficient implementation Simple Mail Transfer Protocol (SMTP) with @Web TCP/IP library. This application note describes interface SMTP client firmware through simple e-mail API. full source code SMTP client e-mail given SMTP.html file available dowloaded (Zip) package.
MCUs Application Note
Rev. 4120A-8051-04/02
E-mail Delivery: SMTP Internet
TCP/IP Based Protocols
TCP/IP provides reliable data transfer through networks (such Internet). TCP/IP universal method establish full duplex communication channel between hosts network. Simple Mail Transfer Protocol, like most TCP/IP based protocols, defines simple ASCII commands replies between server (that will receive transfer e-mail) client (that intends communicate e-mail server). sender, posting e-mail worldwide means communicating with only SMTP server. e-mail delivery ensured SMTP server. Figure Overview Internet Environment SMTP server SMTP client
TCP/IP modem 8-bit microcontroller TCP/IP TCP/IP
Internet
TCP/IP
Internet host
Internet host
Internet host
SMTP protocol described 2821 (see http://www.rfc-editor.org), which replaces (obsolete). message format e-mail, after DATA command) specified 2822, this document explains send attachments complex messages. previous figure example configuration overview: three Internet hosts, SMTP server 8-bit microcontroller communicate e-mail through TCP/IP network (Internet). Each peer physically distant from each other, several thousands kilometers.
@Web SMTP
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@Web SMTP
More About SMTP
SMTP uses Internet standard reply codes: first three ASCII digits have special significance. instance `xyz' reply code, first digit gives following information: means Positive Preliminary reply Positive Completion reply Positive Intermediate reply Transient Negative Completion reply Permanent Negative Completion reply Syntax Information Connections etc, 2821 more information
second digit encodes responses specific categories:
following scenario, reply from Server, command from Client. text bold format sent microcontroller, normal text received italic text just highlight transaction direction. Typical SMTP client/server transaction scenario:
<The client establishes connection SMTP server port> mail.atmel-nantes.fr Simple Mail Transfer Service Ready HELO c51uweb mail.atmel-nantes.fr MAIL FROM:<c51uweb@atmel-nantes.fr> RCPT TO:<tom.jones@atmel-nantes.fr> DATA Start mail input; with Date: Thu, 2001 05:33:22 -0700 From: C51µWeb Subject: Next Meeting Board Jones Tom: next meeting board directors will Tuesday. C51µWeb. QUIT mail.atmel-nantes.fr Service closing transmission channel <The client server close connection>
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E-mail Client
Sending this type e-mail will probably take seconds SMTP client/server transaction about seconds complete delivery worldwide through Internet. Figure Screenshot Email Client seconds after e-mail sent)
Tom:
Often, e-mail will arrive directly computer recipient address. main reasons this are: this computer on-line this computer does have working SMTP server
Usually, SMTP server company) stores e-mail dedicated mailbox clients employees). These people another protocol read mailbox, mainly: Post Office Protocol (nowadays version POP3) Internet Message Access Protocol (nowadays version 4rev1 IMAP4)
described following section, there usually SMTP client (the sender), several SMTP clients/servers (the relay hosts) POP3 client (the recipient) email delivery process.
@Web SMTP
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@Web SMTP
Network
network (Internet, intranet other network) used "data packet transfer" service. following figure, each circle network host computer, router, server, etc.). Data packets circulate this network step step (hop over network links). Figure Network Overview
Embedded SMTP client
above figure shows that data path over networks unique change between consecutive packets. distance over network increases number hops, speed links quality transmission, rather than number kilometers between client server. Some nodes this network have seen part SMTP transactions. following figure shows first SMTP transactions between microcontroller first SMTP server (here, there "data packets", 3.2K byte e-mail).
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Figure Network Activity
E-mail
Startup
initialization function init_smtp_client. This function must called before other function e-mail initialize client) after error registered resume this error). This function sets some global variables e-mail state machine. This main part e-mail's sending service. This simple client fits unique function called smtp_client. send e-mail, this function must called regularly, about five times second, about five seconds simple e-mail. This function (and entire API) compliant with cooperative multitasking environment. Sending simple e-mail (without image attachments) overloading task, network microcontroller. state variables allow know exactly what SMTP clients smtp_internal_state, state variable main state machine smtp_client smtp_state, used additional information
SMTP Client
@Web SMTP
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@Web SMTP
Recipient E-mail
function send_email_to sets recipient e-mail. This uses only recipient easily upgraded multiple recipients. following part SMTP client/server transaction scenario shows several recipients same e-mail. Each command RCPT adds recipient. Scenario with more recipients:
mail.atmel-nantes.fr Simple Mail Transfer Service Ready HELO c51uweb mail.atmel-nantes.fr MAIL FROM:<c51uweb@atmel-nantes.fr> RCPT TO:<tom.jones@atmel-nantes.fr> RCPT RCPT DATA Start mail input; with
Content Transfer
default, this sends pre-defined e-mail. However, there templates rapidly customize final application: change pre-defined e-mail send send types e-mail, including dynamic information
e-mail content must 2822 compliant. Some work usually done send complex e-mail such base64 encoding. This doesn't provide general functions this because software designer drastically simplify content formating particular application. change pre-defined e-mail, variable smtp_test activate your choice. Example:
#define I_HAVE_smtp_test_VARIABLE smtp_test[]="Simple test SMTP client with pre-defined e-mail.";
change pre-defined function that sends e-mail content default, this funct sends _test content write func tion send_email_content, based TCP/IP API. Example (send empty e-mail):
#define send_email_content (void) return TRUE; Tell SMTP client that e-mail content sent
This function called smtp_client when e-mail content must sent. Until return value TRUE, this function called often smtp_client called.
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Minimal Source Code
#include "tcpip.h" #include "smtp.h" void main (void) Minimal prog which send pre-defined e-mail name@company.org init_network(); init_smtp_client(); send_email_to ("name@company.org") while smtp_internal_state SMTP_WAIT_APPLI) smtp_client (smtp_state SMTP_ERROR_SERVER) Sending error else Mail sent
Implementation Notes
TCP/IP
SMTP client implementation uses following calls TCP/IP API: tcp_open, initiate full-duplex TCP/IP connection with SMTP server tcp_send, send commands tcp_receive, receive replies tcp_close, close connection parse_internet_code, parse server reply
Multi-line Reply
Internet Code Parser provided parse_internet_code compliant with re-packetization multi-line replies, according 2821. Here example multiline reply:
123-First line 123-Second line 123-234 text beginning with numbers last line
equivalent line:
Same information, terminated <CRLF>
@Web SMTP
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@Web SMTP
State Machine smtp_client
Figure SMTP Client State Machine
init_smtp_client()
Wait Appli send_email_to("addr") smtp_to:="addr" Open Port Client/Server Ready disconnection tcp_close() init_smtp_client() Wait Ready tcp_receive() small segment multiline reply Update Internet Code Parser Protocol Parser Typical scenario Client/Server Ready mail transfer Send Info tcp_open(SMTP_param) Reset Internet Code Parser
tcp_send()
Internet Code Parser Error atypical Internet Code reply tcp_close() smtp_state:=SMTP_ERROR_ SERVER Wait Appli
send_email_content()()
Mail Transfer
Note:
smtp_state SMTP_WAIT_SERVER init_smtp_client.
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References
Atmel @Web TCP/IP Stack Documentation (included with source code downloadable package) TCP/IP Illustrated, Volume Protocols Richard Stevens TCP/IP Tutorial Technical Overview Rodriguez ibm.com/redbooks 1049: Content-type Header Field Internet Messages 1652: SMTP Service Extension 8bit-MIME Transport 1939: Post Office Protocol Version 2060: Internet Message Access Protocol Version4rev1 2821: Simple Mail Transfer Protocol 2822: Internet Message Format
Acronyms
API: Application Program Interface ASCII: American Standard Code Information Interchange CRLF: Cariage-Return; Linefeed IETF: Internet Engineering Task Force Internet Protocol MIME: Multipurpose Internet Mail Extensions RFC: Request Comments SMTP: Simple Mail Transfer Protocol TCP: Transfer Control Protocol
@Web SMTP
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4120A-8051-04/02

MCUs Application Note Rev. 4120A-8051-04/02 E-mail Delivery:

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