COMS 3157 Advanced Programming

14 - Sockets and HTTP

Preparation

UNIX I/O using file descriptors

review lecture note 11 - Introduction to UNIX

Big-endian vs. Little-endian

endian-demo.c

Sockets API

IPv4 address structures

struct sockaddr {
    sa_family_t sa_family;
    char        sa_data[14];
}

struct sockaddr_in {
    sa_family_t    sin_family; /* address family: AF_INET */
    in_port_t      sin_port;   /* port in network byte order */
    struct in_addr sin_addr;   /* internet address */
};

struct in_addr {
    uint32_t       s_addr;     /* address in network byte order */
};

Sockets API Functions

send(int socket, const void *buffer, size_t length, int flags)

normally, send() blocks until it sends all bytes requested
returns num bytes sent or -1 for error
send(sock, buf, len, 0) is equivalent to write(sock, buf, len)

recv(int socket, void *buffer, size_t length, int flags)

normally, recv() blocks until it has received at least 1 byte
returns num bytes received, 0 if connection closed, -1 if error
recv(sock, buf, len, 0) is equivalent to read(sock, buf, len)
With TCP sockets, we can receive less data than we requested; MSG_WAITALL flag changes this behavior – it requests that the operation block until the full request is satisfied.

HTTP

Recommended reading & reference:

HTTP Made Really Easy
Lecture slides: Sockets API and HTTP 1.0

HTTP protocol in action:

An example of HTTP exchange using netcat.

HTTP 1.0 v. HTTP 1.1

persistent connection

Dynamic web page

HTML form:

<form method=GET action="/mdb-lookup">
    Lookup: <input type="text" name="key">
    <p>
    <input type="submit" value="Lookup">
</form>

when the text box is filled with “abc” and button clicked, the browser sends:

GET /mdb-lookup?key=abc HTTP/1.1

How do dynamic web sites manage sessions?

Server “feeds” cookie at the start of session:
```
HTTP/1.1 200 OK
Content-type: text/html
Set-Cookie: name=value
```
Browser stores the cookies in a local file, indexed by web sites, and on subsequent visits, it brings the corresponding cookie along:
```
GET /index.html HTTP/1.1
Cookie: name=value
```