Week14

class: center, middle

# Networking Crash Course

.copyright[
<a rel="license" href="http://creativecommons.org/licenses/by/4.0/"><img alt="Creative Commons License" style="border-width:0" src="https://i.creativecommons.org/l/by/4.0/88x31.png" /></a>
** Matt Terwilliger **
]

---

# Before We Start

## Client/Server Model

__Client__ requests information from __server__ over pre-established
__protocols__.

## TCP/IP Model

Application Layer

Transport Layer

Internet Layer

Link Layer

???

- Keep a running picture of the TCP/IP model on the whiteboard

---

class: middle, center

![Google](img/google.png)

__Client?__

__Server?__

__Protocol?__

???

- Ask class who is the client and who is the server and over what protocol
- ! Whiteboard add HTTP to Application layer

---

# Digging deeper

Let's use a tool we've seen before:

```bash
curl http://www.google.com/
```

```
Connected to www.google.com (172.217.0.36) port 80 (#0)
```

- `www.google.com` - what we typed in
- `172.217.0.36` - ???

???
```bash
curl -v http://www.google.com/ > /dev/null
```

- Have students guess DNS
- Add DNS to L7

---

# DNS

```bash
dig www.google.com
```

```bash
dig +trace www.google.com
```

- Pattern: "I don't know the answer, but I know who does"
- Recursive/distributed approach
    - Limits data each server is required to store
    - No single source of truth for the entire Internet (redundancy)
    - Easier to manage
- Caching

---
# Digging deeper

```
Connected to www.google.com (172.217.0.36) port 80 (#0)
```

- `www.google.com` - what we typed in
- DNS translates `www.google.com` to `172.217.0.36`
- How does my computer (client) get to `172.217.0.36` (server)?

---

# Routing

```bash
traceroute www.google.com
```

![ITS Map](http://its.umich.edu/sites/all/themes/bootstrap_its/images/UMnet-100GE-2016-09-01.png)

---

# Routing (continued)

System maintains routing table

```bash
route -n get www.google.com
```

What is en0?

```bash
system_profiler
```

???

Wi-Fi, ethernet at L1

---
# Digging deeper

```
Connected to www.google.com (172.217.0.36) port 80 (#0)
```

- `www.google.com` - what we typed in
- DNS translates `www.google.com` to `172.217.0.36`
- Computer uses routing table to find `172.217.0.36`
- Missing a layer (Transport)

---
# Ports

An IP uniquely identifies an interface.

- Why do we need ports?

## TCP vs UDP
Two popular Transport Layer protocols (but not the only ones!)

__TCP__: More guarantees (ordering, best-effort delivery attempt)

__UDP__: Less overhead (Fast)

## Different use cases
- Farther down the model we go the dumber the protocols are
- Not everything has the same requirements
    - Gaming service might not care if some data is lost
    - Websites obviously would

## What about what we just saw?

__HTTP__: TCP port 80

__DNS__: UDP port 53

---
# Ports (continued)

__Q__: What makes a server a server?

Client __connects__ to a __listening__ server.

### Simple example

Server:

```bash
nc -l 9999
```

Client:

```bash
nc 127.0.0.1 9999
```

---
# What about HTTP?

### Back to our cURL example

Request:

```
> GET / HTTP/1.1
> Host: www.google.com
> User-Agent: curl/7.49.1
> Accept: */*
```

---

# What about HTTP? (continued)

### Back to our cURL example

Response:
```
< HTTP/1.1 200 OK
< Date: Wed, 07 Dec 2016 06:54:09 GMT
< Expires: -1
< Cache-Control: private, max-age=0
< Content-Type: text/html; charset=ISO-8859-1
< P3P: CP="This is not a P3P policy! See https://www.google.com/support/accounts/answer/151657?hl=en for more info."
< Server: gws
< X-XSS-Protection: 1; mode=block
< X-Frame-Options: SAMEORIGIN
< Set-Cookie: NID=91=srTc7LxMuO_1keewbJvEnV6-ck0Q_GZRtdQmfhGaWQmVCS4L6e2aCuNxky8i2hDPZwdqbZ2PkA9QFsU3GIOAArpsqPp8mBzr3UqOEc8BiD5V_GTYpVXrqnNw9Ew6XZKsNQYaIy6Tbprb-Q; expires=Thu, 08-Jun-2017 06:54:09 GMT; path=/; domain=.google.com; HttpOnly
< Accept-Ranges: none
< Vary: Accept-Encoding
< Transfer-Encoding: chunked
```

---

# Try it

- Open a web browser
- Let's find the minimum set of information in a valid HTTP request

```bash
nc -l 9999
```

- __Note:__ Not port `80`. Why?
- `<C+d>` to signal end of input

#### Request:

```
> GET / HTTP/1.1
```

#### Response:

```
< HTTP/1.1 200 OK
```

---

# Wrap Up

### So much more

- SSL/TLS
- DHCP
- ARP
- IPv6
- NAT
- Firewalls
- etc.

### Friday
![The Cloud](https://imgs.xkcd.com/comics/the_cloud.png)