MCS 260 Fall 2020
Emily Dumas
A computer network is a group of computers that are connected to one another in a way that allows them to exchange data.
There is a vast worldwide computer network called the internet to which we are all connected right now.
I open a web browser, enter
http://example.com/
and press Enter. Soon, a web page is displayed.
Today we'll discuss the layers of networking technology that are used in this process.
First we need some additional concepts and terminology.
Computers in a network are hosts or nodes.
Each host contains a device to send and receive data over the network. This is a network interface controller or NIC.
e.g. ethernet adapters (wired) and wifi adapters (wireless) are NICs.
Most modern networks (including the internet) are based on packets, i.e. groups of bits that move from one device to another as a unit.
The internet supports many ways to communicate between hosts for different purposes. These are protocols, i.e. rules defining how the communication takes place.
Protocols are a relatively high level concept. At a low level, everything comes down to packets that move from one NIC to another.
The string http://example.com/ is a URL or Uniform Resource Locator. It has several parts:
The name example.com is looked up in a directory, yielding a 4-byte numeric IP address like 93.184.216.34 (byte values separated by dots).
My computer opens a channel to talk to 93.184.216.34. The channel is a bit like a file.
By writing to this channel, my computer asks for "/".
By reading from this channel, my computer receives the content of the web page (HTML).
Focus on one step where HTTP is used:
By writing to this channel, my computer asks for "/".
This is a complex operation. Let's dig into the details a bit more.
HTTP is a protocol based on sending text commands. The command
GET /
will ask for the contents of / (which is really http://example.com/ since we're talking to example.com.)
So we send this text over the channel.
This is a complex operation. Let's dig into the details a bit more.
GET / is translated into a packet of data to send to 93.184.216.34. The string itself is in this packet, along with a bunch of control data.
This packet is sent, it passes through a number of intermediate hosts along its way, and it is received by the web server at example.com.
(The web server sends a packet back to acknowledge receipt, so we determine there is no need to resend.)
This is a complex operation. Let's dig into the details a bit more.
A packet needs to go to IP address 93.184.216.34.
But the NIC in my computer can only send packets to other NICs on the local network, each of which is identified by a hardware address (or MAC).
This IP address is not on our local network. Therefore, we send this packet to the router, whose MAC the OS knows.
The router will figure out what to do next (e.g. forward the packet to some other part of the internet).
This is a complex operation. Let's dig into the details a bit more.
We have a packet ready to go to the router.
It is a sequence of bytes, including lots of control data:
11 11 11 11 11 11 22 22 22 22 22 22 08 00 45 00 ......""""""..E.
02 49 7e f8 40 00 40 06 79 c3 0a 00 00 21 5d b8 .I~.@.@.y....!].
d8 22 ba a4 00 50 90 19 f9 ae ae e9 d2 ea 80 18 ."...P..........
01 f6 42 2f 00 00 01 01 08 0a 03 1e 08 54 d9 98 ..B/.........T..
ab d5 47 45 54 20 2f 20 48 54 54 50 2f 31 2e 31 ..GET / HTTP/1.1
0d 0a 48 6f 73 74 3a 20 65 78 61 6d 70 6c 65 2e ..Host: example.
63 6f 6d 0d 0a 43 6f 6e 6e 65 63 74 69 6f 6e 3a com..Connection:
20 6b 65 65 70 2d 61 6c 69 76 65 0d 0a 43 61 63 keep-alive..Cac
68 65 2d 43 6f 6e 74 72 6f 6c 3a 20 6d 61 78 2d he-Control: max-
61 67 65 3d 30 0d 0a 44 4e 54 3a 20 31 0d 0a 55 age=0..DNT: 1..U
The NIC changes the voltage on certain wires of the network cable in a pattern corresponding to the bits of this packet. The router at the other end of the cable monitors these wires and reconstructs the packet.
Application layer network operations in Python:
Making HTTP requests with the urllib module.