Understanding Networks :: Fall2013 / September 3

Understanding Networks: Week 1

Network Topologies

Definition of “network?":

nodes connected by links
networks tend to exist to distribute something within the network
Example: power network (e.g. “grid”)
- some networks exist in order to deal with things that are difficult to store
- there is a concept of “stocks” and “flows”
- in a well functioning dynamic network, there is a balance between stocks and flows
- power networks are interesting because they must find ways to distribute power without storing it, because storing energy is difficult and dangerous
Ben: what do you call the “waste” of a network, e.g. the power lost by the energy grid? Tom: waste is a good term
all networks exist in a condition of vulnerability--all networks can be disrupted, all nodes within any network become vulnerable by their participation in the network
Tom: vulnerability is a condition, not a bug
networks do break

The big points:

distribution, how stocks flow
vulnerability, networks break
resilience: the ability to heal, in some way
protecting a network tends to limit the network’s operation
- e.g. ping of death, ping flooding
- ITS network policy limits access to some network tools like ICMP ping and traceroute

Network topologies show how the nodes are linked

a network of 3 nodes, with 3 links
a network of 5 nodes with 4 links
a network of 6 nodes with 5 links, in a star topology
- this is a totally centralized network
a network of 5 nodes with 5 links, in a ring topology
- e.g. token ring network, avoiding collisions by agreeing to have only node speaking at once
- no matter the topology, good network nodes must work out how to listen, in order to avoid collisions where more than one node is communicating at the same time
a network of 5 nodes with 10 links, maximum hop distance of 1
- this is a totally decentralized network
- Tom, quoting Clay: "the trick is to practice Satanism so you know how to draw pentagrams"
- one typical topology is several tightly connected networks that are sparsely connected to other tightly connected networks
- there are special nodes, which you could call “hubs”
- hub nodes have special vulnerabilities, sometimes mitigated by traffic management
- the Internet is roughly organized like this, as a “distributed network” rather than a “decentralized network”
formula for link count of totally connected network: D = (N^2 - N) / 2
the greater a network’s link density, the greater its robustness

“Bus network”

e.g. Arduino SPI, USB
centralized controller, connected on a single communication line
communication is one at a time
typically, the controller node tends to provide a clock signal, and the controller sends data on the uptick of the clock signal, while the rest of the nodes send data on the downtick of the clock signal
bus networks tend to be synchronous rather than asynchronous

“Mesh” networks

e.g. ad hoc wifi networks
mesh networks tends to be protocols that assign addresses among cooperating nodes
- e.g. DARPA’s smart minefield project that physically redistribute themselves when a mine is detonated or disabled
ITP should teach you how to gracefully interoperate between incompatible protocols
example of peer to peer networks
example of protocols layered on top of each other (e.g. HTTP is implemented on top of TCP/IP)

The “network metaphor” is one way to understand the world, but not the only one

other examples of useful metaphors are the Enlightenment era “universe as a clockworks” metaphor
question: difference between bus and decentralized network
- e.g. I2C protocol
- e.g. RGB LEDs in Adafruit’s Flora (like a LilyPad) allows control of multiple LEDs
- e.g. SPI, serial peripheral interface, “synchronous serial”
difference between synchronous serial and asynchronous serial
- synchronous: single clock
- asynchronous: multiple clocks with a protocol defining agreement on baud rate, and agreement on who should be talking and who should be listening
- e.g. hardware handshaking using CTS and RTS lines
- e.g. software handshaking

OSI Model

pneumonic: "Please Do Not Throw Sausage Pizza Away," for "Physical Datalink Network Transport Session Presentation Application"
this is the OSI Model, AKA the “Reference Lie” (Clay’s term)
See http://en.wikipedia.org/wiki/OSI_model
Physical layer
- e.g. Ethernet cables, wifi adapters
Datalink
- e.g. Ethernet packets, addressing (MAC, media access control, addresses)
- format of data to be transferred, possibly containing addressing, checksums, metadata, and data
Network
- this is where an Internet network address is assigned
- “IP address” is an address on the Internet
Transport
- defines how we’re going to exchange data over our network as a whole
- e.g. streaming media vs. email
- in streaming media, loss of a small amount of data tends to not have a huge impact on the usability of the data
- in email, loss of a small amount of data can have a much more significant impact on the usability of the data
- UDP versus TCP
- streaming media tends to use UDP, email tends to use TCP
- TCP does error checking and can resend data if the data was corrupted or lost in transit
- UDP just blasts the data across without regard to whether it was received or corrupted
- there are many other transport protocols, e.g. ICMP which is used for the Ping protocol
Session
- allows multiple kinds of data to be transmitted or received on a single IP address
- e.g. you can SSH to a server and get web pages from the same server--how does the server know to send the SSH related data to your SSH client, but the HTTP data to your web browser?
Presentation
- data format that is agreed on within a session or transport
- e.g. ASCII
Application
- e.g. HTTP, SMTP, POP, IMAP, SSH, FTP, SFTP
- this is where you’ve been living within ITP so far

Sessions: HTTP Communication on the Command Line

Example: telnet directly to a web server, in Terminal (on Mac):

telnet tigoe.net 80
GET / HTTP/1.1
Host: tigoe.net

HTTP/1.1 200 OK
Date: Tue, 03 Sep 2013 21:48:37 GMT
Server: Apache
MS-Author-Via: DAV
Vary: Accept-Encoding
Content-Length: 1241
Content-Type: text/html



<html>

	<head>
		<meta http-equiv="content-type" content="text/html;charset=iso-8859-1">
		<title>Tom Igoe</title>
		<link rel="stylesheet" href="css/tomstyle.css" type="text/css">

		<script src="http://www.google-analytics.com/urchin.js" type="text/javascript">
</script>
<script type="text/javascript">
_uacct = "UA-1541944-1";
urchinTracker();
</script>

	</head>

	<body>
		<div align="right">
			<h2>
			<table border="0" cellpadding="0" cellspacing="10" width="594" align="center">
				<tr>
					<td colspan="4" valign="top">
						<div align="right">
							<h2>Tom Igoe</h2>
						</div>
					</td>
				</tr>
				<tr>
					<td valign="top">
							<div align="right">
							<span class="margin" align = "right">  
<P><A href="/pcomp/index.php">physical computing</A>
<P><A href="/consulting.php">consulting</A> 
<P><A href="/resume.php">r?sum?</A>
<P><A href="/projects.php">projects</A>
<P><A href="/photo/">images</A>
<P><A href="/blog/">blog</A>
</span>						</div>
						</td>
					<td align="left" valign="middle" colspan="2">
						<div align="right">
								<img src="img/sil8.jpg"><p>Disciplined self-indulgence
							&nbsp;</div>
					</td>
					<td></td>
				</tr>
			</table>
				</h2>
		</div>
	</body>

</html>
Connection closed by foreign host.

Sessions: Serial Communication on the Command Line

Example: serial communication on the command line (in Mac), connecting to the Arduino Yun, the Beaglebone, and the Pi

List of all serial devices:

ls /dev/tty.*

You can use screen to access those serial ports:

screen <device> <baud>
screen /dev/tty.udbmodem621 115200

From here you’ll see a Linux command line session.

In screen, control-A control-K will disconnect the serial session.

The Raspberry Pi requires a special serial cable to communicate with it over USB. Adafruit has a cable that you can use to wire the Pi serial pins to a USB cable.

Understanding Networks