Connect to the Internet for Free . . . Well, Almost Free

1
Connect to the Internet for Free . . . Well,
Almost Free

• Mike Willey
• Embedded Systems Conference Boston 2001
• Class No. 302

2
Introduction

• Mike Willey
• Vice President, Paragon Innovations, Inc.
• Specializing in Embedded Internet Devices
• 23 Years experience designing embedded systems
• willey_at_paragoninnovations.com
• 972-680-2900 x222

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Course Outline

• Why do we need an inexpensive internet
  connection?
• What do we need to know about TLA?
• How do you cram 5 lbs of stuff in a 3 lb bag?
• What does an inexpensive PPP implementation look
  like?
• Conclusions

4
Why do we need an inexpensive internet connection?

• Remote control and monitoring
• Industrial automation
• Home automation
• CONSUMER DEMAND!

5
What do we need to know about TLA?

• Internet Layer
• IP - Internet Protocol
• ARP - Address Resolution Protocol
• Transport Layer
• TCP - Transmission Control Protocol
• UDP - User Datagram Protocol
• Application Layer
• HTTP - Hypertext Transfer Protocol
• SMTP - Simple Mail Transport Protocol
• POP3 - Post Office Protocol

6
How do you cram 5 lbs of stuff in a 3 lb bag?

• 5 lbs of stuff
• Typical Internet Stack
• Define the Problem
• Implement what you need

7
What does an inexpensive PPP implementation look
like?

• Remote meter reading
• Counts pulses on the sensors
• Emails totals once a month

8
Required Protocols

• SMTP - Transfers mail message to internet
• TCP - Estabishes connection, ensures end-end
  transmission
• IP - Establishes machine-machine connection
• PPP - Wraps everything up for asynch transmission

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SMTP
220 linus.paragon-tx.com ESMTP Sendmail 8.9.3/8.9.
3/Debian/GNU HELO meter.goofynet.net 250
linus.paragon-tx.com Hello p84-max14.akl.linus.par
agon-tx.com MAIL FROM 2
50 ... Sender ok RCPT
TO 250 COM... Recipient ok DATA 354 Enter mail, end
with "." on a line by it self Subject Meter data
from Mikes house Mike has used 1252 KWH of
electricity in the last 28 days. . 250 OAA09988
Message accepted for delivery QUIT
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SMTP (cont.)

• Server sends 220
• Reply HELO my.url.com
• Server sends 250
• Reply MAIL FROM myid_at_my.url.com
• Server sends 250
• Reply RCPT TO destid_at_your.url.com
• Server sends 354
• Reply with Subject, CC, BCC, etc message
• End with a line containing only a period

11
TCP Stack

• Source port - Anything, Dest Port - 25 (SMTP)
• Sequence / Acknowledgement Numbers
• Window - how many unacked packets can be sent

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TCP (cont.)

• Offset (4 bits) - 32 bit words in header
• Reserved (6 bits) - always zero
• Flags
• URG - Message is urgent
• ACK - Acknowledgement field is valid
• PSH - Flush or push all data so far to the
  receiving app.
• RST - Reset the connection
• SYN - Synchronize sequence numbers
• FIN - Close connection

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TCP (cont.)

• Checksum - 16 bit ones complement of packet and
  the source and destination addresses, protocol
  type, and length from the IP header
• Must deal with limited RAM

14
Exchanging Sequence Numbers
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IP Stack

• Source IP Address
• Destination IP Address

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PPP Stack

• Wrapper for the rest of the data

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State Diagram for the Bag

• Application called in-state
• NOT a General Purpose Application

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More Information

• Sams Teach Yourself TCP/IP in 24 Hours, Sams
  Publishing, Indianapolis, Indiana, 1998
• RFC 1025 - Testing TCP and IP
• RFC 1071 - Computing Checksums
• RFC 1122 - Internet protocol stacks
• RFC 1172 - PPP Configuration Options
• RFC 1180 - TCP/IP Tutorial

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More Information (cont.)

• RFC 1331 - PPP Overview
• RFC 1332 - PPP Definition
• RFC 1661 - PPP Standard
• RFC 1661 - HDLC Framing for PPP
• RFC 1962 - PPP Compression Control Protocol
• RFC 1994 - PPP Chalange Handshake Authenticaion
  Protocol (CHAP)

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More Information (cont.)

• RFC 2131 - Dynamic Host Configuration Protocol
  (DHCP)
• RFC 768 - User Datagram Protocol (UDP)
• RFC 791 - Internet Protocol (IP)
• RFC 729 - Internet Control Message Protocol
  (ICMP)
• RFC 793 - Transport Control Protocol (TCP)
• RFC 813 - Windows and Acknowledgement in TCP

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More Information (cont.)

• RFC 879 - Datagram sizes
• Whew!

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Hardware Implementation

• Required ICs

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Conclusions

• Implementation for
• Source code available for SMTP, HTTP, and POP3 on
  www.ubicom.com
• Principles are easily transferred to other
  platforms