Introduction to HTTP

The HyperText Transport Protocol is an ‘application-layer’ protocol for the ‘client/server’ paradigm

‘Request’ and ‘Response’

client server

HTTPRequestmessage

HTTPResponsemessage

timeline

Built on TCP/IP

• Application programmers will need to be aware that HTTP relies on TCP’s reliable, stream-oriented and connection-based transport-layer facilities when specifying the socket types, functions, and options

socket()

bind()

listen()

accept()

read()

write()

close()

socket()

connect()

write()

read()

close()

server client

HTTP Request

Request line

Headers

Empty line

Body(may be absent)

HTTP Response

Status line

Headers

Empty line

Body(may be absent)

Sample Request line

“GET /home/web/cs336/syllabus.s09 HTTP/1.0\r\n”

command (one word - all capitals)

space

resource pathname (UNIX filename syntax)

space

protocol and version-number

carriage-return and line-feed

Sample Request header-lines

• The header-lines must be followed by an ‘empty’ line (carriage-return and line-feed)

“Connection: close\r\n”“User-agent: Mozilla 4.0\r\n”“Accept-language: en\r\n”

carriage-return and line-feed

Sample Response line

“HTTP/1.0 200 OK\r\n”

space space

protocol and version-number

carriage-return and line-feed

response phrase

status code

Sample Response header-lines

• The header-lines must be followed by an ‘empty’ line (carriage-return and line-feed)

“Connection: close\r\n”“Date: Tue, 15 March 2009\r\n”“Server: Apache/1.3 *Unix)\r\n”“Content-Type: text/html\r\n”

carriage-return and line-feed

Demo: ‘grabfile.cpp’

• We shall construct a simple HTTP client which will allow a user to obtain a named internet object by typing its URL (Uniform Resource Locator) on the command-line:

$ grabfile http://www.cs.usfca.edu/index.html

The URL concept

• URL means ‘Uniform Resource Locator’

• It’s a standard way of specifying any kind of information available on the Internet

• Four elements of a URL specification: – Method (i.e., the protocol for object retrieval)– Host (i.e., location hostname or IP-address)– Port (i.e., port-number for contacting server)– Path (i.e., pathname of the resource’s file)

The URL Format

method :// host port: path/

EXAMPLE: http://cs.usfca.edu:80/~cruse/cs336/syllabus.pdf

Note: The port-number is often omitted in cases where the ‘method’ is an internet protocol (like HPPT) which uses a ‘well-known port’

Application’s organizationParse the URL entered on the command-line

to determine the server’s hostname and port-number and the pathname to the desired file-obsect

Open a stream-oriented TCP internet socket and establish a connection with the server

Form the HTTP Request message and write it to the socket

Read from the socket to receive the HTTP Response message (and echo it to the display)

Close the socket to terminate the TCP connection

Parsing the URL

• The most challenging part of this program concerns the parsing of the command-line argument, allowing for some ‘degenerate’ cases and some malformed specifications

• Several standard string-functions from the UNIX runtime-library are put to good use, including ‘strlen()’, ‘strncpy()’, ‘strtok()’ and ‘strtok_r()’, plus ‘strspn()’ and ‘strcspn()’

‘strlen()’

• This function calculates the length of the null-terminated string whose address is supplied as the function-argument

size_t strlen( const char *s );

#include <string.h> char message[ ] = “Hello”; int main( void ) {

int len = strlen( message );printf( “\’%s\’ has %d characters\n”, len );

}

OUTPUT: ‘Hello’ has 5 characters

‘strncpy()’

• This function copies at most n characters from the ‘src’ string into the ‘dst’ string, so provides a ‘safe’ way to copy from a string that might be too long to fit the destination

char *strncpy( char *dst,const char *src, size_t n );

int main( int argc, char *argv[] ) { char param[ 64 ];

if ( argc == 1 ) { fprintf( stderr, “ param? \n” ); exit(1); } strncpy( param, argv[ 1 ], 63 ); // source string has unknown length …

}

‘strtok()’

• This function extracts tokens from a string, but after being called once, it remembers where it stopped in case the caller wants to extract more tokens from that string

char *strtok( char *s, const char *delim );

char sentence[ ] = “Hello, world!\n”; char *word1 = strtok( sentence, “ ,!\n” ); char *word2 = strtok( NULL, “ ,!\n” ); char *word3 = strtok( NULL, “ ,!\n” );

printf( “ \’%s\’ \’%s\’ \’%s\’ \n”, word1, word2, word3 );

OUTPUT: ‘Hello’ ‘world’ ‘<nul>’

‘strtok_r()’

• This function is a ‘reentrant’ version of the ‘strtok()’ function, placing the address of the character where a subsequent search for another token to extract would begin

char *strtok_r( char *s, const char *delim, char **saveptr );

char sentence[ ] = “Hello, world!\n”;char *word1, *word2, *word3;

word1 = strtok( sentence, “ ,!\n”, word2 ); strtok( word2, “ ,!\n”, word3 ); printf( “ \’%s\’ \’%s\’ \’%s\’ \n”, word1, word2, word3 );

OUTPUT: ‘Hello’ ‘world’ ‘<nul>’

‘strspn()’

• This function searches a string for a set of characters, and returns the length of the initial segment which consists entirely of characters that are in the ‘accept’ string

size_t strspn( const char *s, const char *accept );

char vowels[ ] = “aeiou”; char word[ ] = “eating”; int len = strspn( word, vowels ); printf( “\’%s\’ has %d vowels before any consonant \n”, word, vowels );

OUTPUT: ‘eating’ has 2 vowels before any consonant

‘strcspn()’

• This function searches a string for a set of characters, and returns the length of the initial segment which consists entirely of characters that are not in the ‘reject’ string

size_t strcspn( const char *s, const char *reject );

char vowels[ ] = “aeiou”; char word[ ] = “shout”; int len = strcspn( word, vowels ); printf( “\’%s\’ has %d consonants before any vowel \n”, word, vowels );

OUTPUT: ‘shout’ has 2 consonants before any vowel

Examples

• Here are a few examples of ‘malformed’ and ‘degenerate’ URL parameter-strings

http://:54321/index.html # no server hostname

http://yahoo.com:/index.html # missing port

http://usfca.edu:::54321/index.html # excess ‘:’s

www.sfmuni.com/index.html # no ‘method’

http://www.bart.gov/ # no pathname

www.sfsu.edu:80:57/index.html # extra chars

In-class exercise

• Download our ‘grabfile.cpp’ application and see whether you are able to retrieve any files by typing a URL as an argument

• HINT: You can use some of the same IP-addresses and hostnames that you tried successfully while you were testing your earlier ‘showpath.cpp’ project