CS596: Doc 13, HTML and HTTP

CS 596: Client-Server Programming
Spring Semester, 1997
Doc 13, HTML and HTTP

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San Diego State University -- This page last updated Mar 4, 1997

Contents of Doc 13, HTML and HTTP

HTML and HTTP Augmented BNF
URL HTTP URL HTML What is HTML?
HTML 101
Learning HTML HTTP HTTP Message Format
Full-Request
Server Response
Status-Line
Header Fields
Request Methods
Request Methods (continued)
Request Methods Not Always Supported
General Message Header Fields
Request-Header
Response-Header
Entity Header Fields

Doc 13, HTML and HTTP Slide # 2

Augmented BNF

"literal": Quotation marks surround literal text. Unless stated otherwise, the text is case-insensitive
rule1 | rule2: Elements separated by a bar ("|") are alternatives
( rule1 | rule2 ): Parenthesis group elements so they function as one
*rule: "*" preceding an element indicates repetition
<n>*<m>element: At least <n> and at most <m> occurrences of element
[ rule ]: Square brackets enclose optional elements
<n>element: Exactly <n> occurrences of element
<n>#<m>: At least <n> and at most <m> elements, each separated by one or more commas (",") and optional linear white space (LWS)

Doc 13, HTML and HTTP Slide # 3

URL

URL == Uniform Resource Locator

gopher://gopher.yoyodyne.com/
news:rec.gardening
http://www.yoyodyne.com/pub/foobar.html
http://www.yoyodyne.com/pub/foobar.html?hello

Augmented BNF for a URL is defined as:

URL        = scheme ":" *(uchar|reserved)["#" fragment]
uchar      = unreserved | escape
unreserved = ALPHA | DIGIT | safe | extra | national
escape     = "%" HEX HEX
extra      = "!" | "*" | "í" | "(" | ")" | ","
safe       = "$" | "-" | "_" | "."
unsafe     = "CTL | SP | <"> | "#" | "%" | "<" | ">"
national   = <any OCTET excluding ALPHA, DIGIT, reserved,
             extra, safe, and unsafe>
reserved   = ";" | "/" | "?" | ":" | "@" | "&" | "=" | "+"
fragment   = *( uchar | reserved )

Doc 13, HTML and HTTP Slide # 4

HTTP URL

The "http" scheme is used to locate network resources via HTTP

http_URL = "http:" "//" host [ ":" port ][ abs_path ]
host     = <A legal Internet host domain name or IP
           address (in dotted-decimal form), as
           defined by RFC 1123>
port     = *DIGIT
abs_path = "/" rel_path
rel_path = [ path ] [ ";" params ] [ "?" query ]
path     = fsegment *( "/" segment )
fsegment = 1*pchar
segment  = *pchar
params   = param *( ";" param )
param    = *( pchar | "/" )
pchar    = uchar | ":" | "@" | "&" | "=" | "+"
query    = *( uchar | reserved )

Syntax and semantics of URLs can be found in RFC 1738

Doc 13, HTML and HTTP Slide # 5

HTML

Some Buzz Words

WWW: World Wide Web (or Web, for short)
SGML: Standard Generalized Markup Language; This is the standard for describing markup languages
DTD: Document Type Definition; This is a specific markup language, written using SGML
HTML: HyperText Markup Language; HTML is an SMGL DTD; HTML uses markup tags to tell the WWW browser how to display the text

Doc 13, HTML and HTTP Slide # 6

What is HTML?

HTML is a language for describing structured documents

HTML does not describe page layout. (Why not?)

This language is used by web browsers to render the document and display it to a user.

Several versions of HTML:

HTML 1.0 ñ very limited
HTML 2.0 ñ addition of forms
HTML 3.0 ñ failed attempt at standardizing a plethora of features
HTML 3.2 ñ prime candidate for standard

Other developments:

CSS (Cascading Style Sheets)
Feature war between Microsoft and Netscape

Doc 13, HTML and HTTP Slide # 7

HTML 101

HTML files are all ASCII (except for kanji and other alphabets)

Tags are used to markup a document

A tag starts with "<" and ends with ">"

Some tags come in pairs: a start tag and an end tag. End tags have a "/" following the "<".

"<" or ">" are reserved for tags only. Tags themselves may not include these characters, either.

In general, all white space (including newlines) is reduced to the equivalent of one space.

Basic HTML document:

<HTML>
<HEAD>
<TITLE>Sample HTML Document</TITLE>
</HEAD>
<BODY>
This is a document
</BODY>
</HTML>

Doc 13, HTML and HTTP Slide # 8

Learning HTML

Learn by example. "View Source" in any browser.

Many books on HTML

Online tutorials (start at http://www.yahoo.com/)

Hyperlinks

<A HREF="http://www.sdsu.edu/">SDSU homepage</A>

will render as

SDSU homepage

Images

will render as

Doc 13, HTML and HTTP Slide # 9

HTTP

HyperText Transfer Protocol

Stateless, object-oriented protocol

The typing and negotiation of data representation, allows systems to be built independently of the data being transferred.

Assigned port 80

Basic Client-Server Interaction

Client: Open connection

Server: Accept/Reject connection

Client: Send request

Server: Send response to request

Connection closed

Doc 13, HTML and HTTP Slide # 10

HTTP Message Format

HTTP-message  = Simple-Request
              | Simple-Response
              | Full-Request
              | Full-Response
Full-Request  = Request-Line
                *( General-Header
                   | Request-Header
                   | Entity-Header )
                CRLF
                [ Entity-Body ]
Full-Response = Status-Line
                *( General-Header
                   | Request-Header
                   | Entity-Header )
                CRLF
                [ Entity-Body ]
HTTP-header   = field-name ":" [ field-value ] CRLF
Entity-Body   = *OCTET

Doc 13, HTML and HTTP Slide # 11

Client Request

Request        = Simple-Request | Full-Request
Simple-Request = "GET" SP Request-URI CRLF

Simple-Request Example

telnet: www.eli.sdsu.edu 80

Server: accepts connection

telnet: GET /index.html<CRLF>

Server:

<!DOCTYPE HTML SYSTEM "html.dtd">
<HTML><HEAD><TITLE>
Roger Whitney
</TITLE></HEAD>
<BODY>
<CENTER><H2>
Roger Whitney<br>
Computer Science (etc...)

Doc 13, HTML and HTTP Slide # 12

Full-Request

Full-Request = Request-Line
               *( General-Header | Request-Header | Entity-Header )
               CRLF
               [ Entity-Body ]
Request-Line = Method SP URI SP HTTP-Version CRLF

Example


telnet: www.eli.sdsu.edu 80

Server: accepts connection

telnet: GET /index.html HTTP/1.0<CRLF>

telnet: <CRLF>

Server:

HTTP/1.0 200 Ok

Server: Netscape-Commerce/1.12

Date: Tuesday, 04-Mar-97 07:58:45 GMT

Last-modified: Thursday, 27-Feb-97 00:19:07 GMT

Content-length: 3949

Content-type: text/html

 

<!DOCTYPE HTML SYSTEM "html.dtd">

<HTML><HEAD><TITLE>

Roger Whitney

Doc 13, HTML and HTTP Slide # 13

Server Response

Response      = Simple-Response | Full-Response
                Simple-Response = [Entity-Body]
Full-Response = Status-Line
                *( General-Header | Response-Header | Entity-Header )
                CRLF
                [ Entity-Body ]

Simple response is sent only in response to simple request

Sample Full-Response:

HTTP/1.0 200 Ok
Server: Netscape-Commerce/1.12
Date: Tuesday, 04-Mar-97 07:58:45 GMT
Last-modified: Thursday, 27-Feb-97 00:19:07 GMT
Content-length: 3949
Content-type: text/html

Doc 13, HTML and HTTP Slide # 14

Status-Line

Status-Line   = HTTP-Version SP Status-Code
                SP Reason-Phrase CRLF
Status-Code   = 3DIGIT
Reason-Phrase = token *( SP token )

Example

HTTP/1.0 200 Ok

Status codes

1xx: Not used, but reserved for future use

2xx: Success ñ The requested action was succesfully received and understood

3xx: Redirection ñ Further action must be taken in order to complete the request

4xx: Client Error ñ The request contains bad syntax or is inherently impossible to fulfill

5xx: Server Error ñ The server could not fulfill the request

Doc 13, HTML and HTTP Slide # 15

Header Fields

HTTP-header   = Field-name ":" [Field-value ] CRLF
Field-name    = 1*<any CHAR, excluding CTLs, SP,
                and ":">
Field-value   = *( Field-content | comment | LWS )
Field-content = <the OCTETs making up the field-value
                and sinsisting of either *text or
                combinations of token, tspecials, and 
                quoted-string>

Sample Full-Response:

HTTP/1.0 200 Ok
Server: Netscape-Commerce/1.12
Date: Tuesday, 04-Mar-97 07:58:45 GMT
Last-modified: Thursday, 27-Feb-97 00:19:07 GMT
Content-length: 3949
Content-type: text/html

Doc 13, HTML and HTTP Slide # 16

Request Methods

Method = "GET" | "HEAD" | "PUT" | "POST" 
         | "DELETE" | "LINK" | "UNLINK" 
         | extension-method

GET and HEAD must be supported by all HTTP/1.0 servers

Servers should return Status-Code

"501 Not Implemented"

if the method is unknown.

GET

Retrieves whatever item is identified by the URI

The URI can refer to a data-producing process, or a script

The produced data shall be returned as the Entity-Body

HEAD

Identical to GET except that the server must not return any Entity-Body in the response.

Doc 13, HTML and HTTP Slide # 17

Request Methods (continued)

POST

Request that the origin server accept the item enclosed in the request as a new subordinate of the resource identified by the URI.

Allows a uniform function to:

Annotation of existing documents;
Posting a message to a bulletin board topic, newsgroup, mailing list, or similar group of articles;
Provides a block of data (usually a form) to a data-handling process, or a script which can be run by such a process;
Extending a document during authorship

Doc 13, HTML and HTTP Slide # 18

Request Methods Not Always Supported

Why?

PUT

The enclosed item in the request is to be stored under the supplied URI

DELETE

Request that the server delete the resource identified by the given URI.

LINK

Establishes one or more Link relationships between the existing resource identified by the URI and other existing resources

UNLINK

Removes one or more Link relationships from the existing resource identified by the URI

Doc 13, HTML and HTTP Slide # 19

General Message Header Fields

General-Header = Connection
                 | Data
                 | Forwarded
                 | Mandatory
                 | Message-ID
                 | MIME-Version
Connection       = "Connection" ":" 1#connect-option
connect-option   = token [ "=" word ]

Doc 13, HTML and HTTP Slide # 20

Request-Header

Request        = Simple-Request | Full-Request
Full-Request   = Request_Line
                 *( General-Header | Request-Header | Entity-Header)
                 CRLF
                 [ Entity-Body ]
Request-Header = User-Agent
               | If-Modified-Since
               | Pragma
               | Authorization
               | Proxy-Authorization
               | Referer
               | From
               | Accept
               | Accept-Encoding
               | Accept-Language

Doc 13, HTML and HTTP Slide # 21

Response-Header

Full-Response   = Status-Line
                  *( General-Header | Response-Header | Entity-Header )
                  CRLF
                  [ Entity-Body ]

Response-Header = Server
                | WWW-Authenticate
                | Proxy-Authenticate
                | Retry-After

Doc 13, HTML and HTTP Slide # 22

Entity Header Fields

Unknown header fields should be considered Entity-Header fields.

Entity-Header    = Allow
                 | Content-Length
                 | Content-Type
                 | Content-Encoding
                 | Content-Transfer-Encoding
                 | Content-Language
                 | Expires
                 | Last-Modified
                 | URI-header
                 | Location
                 | Version
                 | Derived-From
                 | Title
                 | Link
                 | extension-header
extension-header = HTTP-header

CS 596: Client-Server Programming Spring Semester, 1997 Doc 13, HTML and HTTP

Contents of Doc 13, HTML and HTTP

Client Request

CS 596: Client-Server Programming
Spring Semester, 1997
Doc 13, HTML and HTTP