% lec 17: network programming

# today's plan

DNS & HTTP

programming abstraction: socket

(today: client-side socket)

(later this week: server-side socket & case study - node.js)

# big picture

what happens when browsing <http://www.cs.uw.edu/>

. . .

protocol: HTTP

domain name: `www.cs.uw.edu`

(IP address: `128.208.3.118`)

port: 80 (default)

q: can we directly use `128.208.3.118` in browser?

# Internet protocol suite (TCP/IP)

``` {.ditaa #l17/tcp-ip}
+-------------+             +-------------+
| application |             | application |
+-------------+             +-------------+
       |                           ^
       v                           |
+-------------+             +-------------+
|  transport  |             |  transport  |
+-------------+             +-------------+
       |                           ^
       v                           |
+-------------+             +-------------+
|   internet  |             |   internet  |
+-------------+             +-------------+
       |                           ^
       v                           |
+-------------+             +-------------+
|     link    |             |     link    |
+-------------+             +-------------+
       |                           ^
       v                           |
-------------------------------------------
      ethernet/wifi/fiber/satellite/...
```

[see also: OSI model]

# protocol examples

application layer: DNS, HTTP, SSH, ...

transport layer: TCP, UDP, ...

internet layer: IPv4, IPv6, ...

# overview

translate `www.cs.uw.edu` to `128.208.3.118` (DNS)

connect to `128.208.3.118` (TCP)

send request to fetch data (HTTP)

# DNS

a naming system (recall file system)

map domain names to IP addresses

example: `www.cs.uw.edu` → `128.208.3.118`

. . .

q: can one domain name map to multiple IP addresses?

q: can multiple domain names map to the same IP address?

# DNS resolution

a single file (`HOSTS.TXT` or `/etc/hosts`)? scalability?

. . .

hierarchy

root: `.`

generic top-level domain (gTLD): `edu`

[draw google.com, mit.edu]

# root servers

[13 logical root servers](http://www.root-servers.org/)

run: `dig . ns`

demo: resolve `www.cs.uw.edu`

. . .

q: where's `mit.edu` (from Seattle)?

# programming interface

manpage: `getaddrinfo`, `freeaddrinfo`, `getnameinfo`

[example: dns.c](l17/dns.c.html)

# more on DNS

MIT 6.033's [DNS hands-on](http://web.mit.edu/6.033/www/assignments/handson-dns.html)

take [CSE 461](https://courses.cs.washington.edu/courses/cse461/)

# what's next

now we have the IP address

how to connect to the server and fetch data?

# socket programming

``` {.ditaa #l17/syscall}
+---------------+                         +---------------+
|  application  |                         |  application  |
+---------------+ <---socket syscalls---> +---------------+
|   OS kernel   |                         |   OS kernel   |
+---------------+                         +---------------+
|      NIC      |                         |      NIC      |
+-------+-------+                         +-------+-------+
        |                                         |
        +-----------------------------------------+
```

# client-side socket

```c
// return a socket (file descriptor)
int socket(int domain, int type, int protocol);
// connect to a server
int connect(int fd, const struct sockaddr *addr, socklen_t addrlen);
```

use `socket`/`connect` to start a client-side socket (not `open`)

you can use `read`/`write` on sockets (or `recv`/`send`)

`close` when done

[example: http.c](l17/http.c.html)

# HTTP protocol

request/response protocol, over TCP, default port 80

client (e.g., browser) sends a request

server produces a response

[HTTP/1.1 textual](http://www.w3.org/Protocols/rfc2616/rfc2616.html)
(HTTP/2 binary)

# request format

```c
[method] [uri] HTTP/[version]\r\n
[name]: [value]\r\n
...
[name]: [value]\r\n
\r\n
[body]
```

methods: GET, POST, HEAD, ...

. . .

demo (chrome & httpie): `http -v www.cs.uw.edu`

# response format

```c
HTTP/[version] [status code] [reasons]\r\n
[name]: [value]\r\n
...
[name]: [value]\r\n
\r\n
[body]
```

status code

1xx: some kind of informational message

2xx: success of some kind

3xx: redirects the client to a different URL

4xx: the client’s request contained some error

5xx: the server experienced an error

# see you on Wednesday

server-side socket