HTTP 1.0 (draft 7)

References:

Lincoln D. Stein, How to Set Up and Maintain a WWW Site, Addison Wesley, 1995, pp. 45-53.
Richard Stevens, TCP/IP Illustrated, Vol. 3, Addison-Wesley, 1996, Ch. 13-14.

Introduction

A Web client transfers a URL by opening a connection to the well-known HTTPD port on a Web server (port 80, usually).

HTTP requires any reliable transport protocol; almost universally TCP is used.
HTTP messages are requests and responses.
HTTP is a stateless protocol: Servers retain no information about past requests that they received.

Protocol Version Numbers

The old HTTP protocol (Sep. 1996) is HTTP/1.0. Recently the Web switched to HTTP/1.1. To accommodate use of multiple protocol versions in the Internet, proxies and gateways never send a message with a version number higher than its version number.

HTTP/1.0 Details

Servers just return documents via HTTP, while clients must present documents to users. Thus HTTP servers are simpler than clients (6,500 vs. 80,000 lines of C code for NCSA httpd 1.3 and Mosaic 2.5).

HTTP/1.0 is a very simple-minded protocol. It uses ASCII commands terminated by CR/LF. So you can simulate an HTTP client with a telnet session:

daphne.cs.vt.edu> telnet www 80
Trying 128.173.40.201... Connected to info.cs.vt.edu.
Escape character is '^]'.
GET / HTTP/1.0

<HTML>
<HEAD>
<TITLE>
Virginia Tech Computer Science Department Home Page
</TITLE>
</HEAD>

<BODY>

<CENTER>
<A HREF=/htbin/imagemap/image/maps/vtcs.gif.map>
<IMG BORDER=0 SRC=/image/new_headers/vtcs.gif 
 ALT="Virginia Tech Department of Computer Science" ISMAP></A>
<P>
660 McBryde Hall<BR>
Blacksburg, VA 24061<BR>
Phone: (540)231-6931<BR>
FAX: (540)231-6075<P>
Department Head: Dr. John M. Carroll
</CENTER>

<HR SIZE=4>

<P> Welcome to the Virginia Tech Department of Computer Science Home
Page. 

...

<H6>
Last updated August 28, 1996<BR>
http://www.cs.vt.edu/
</H6>
</BODY>
</HTML>
Connection closed by foreign host.

In the example above, we fetched the home page of host www.cs.vt.edu.

HTTP Message Types

HTTP/1.0 request format:

request-line:  reqest request-URI HTTP-version
headers (0 or more lines)
<blank line>
body (only if a POST command)

Possible requests:

GET: Request a document named by a URI (uniform resource identifier - superclass of URL)
HEAD: Return only header of specified URI (e.g., test link for validity, recent modification)
POST: Post a document: elelctronic mail, form input, news.

HTTP/1.0 response format:

status-line:  HTTP-version response-code human-readable-phrase
headers (0 or more)
<blank line>
body

Example: Header in Response

daphne.cs.vt.edu> telnet ei 80
Trying 128.173.40.129... Connected to ei.cs.vt.edu.
Escape character is '^]'.
GET /~wwwbtb/fall.96/ClassNotes/Protocols/MIMEscreen.gif HTTP/1.0

HTTP/1.0 200 Document follows
Date: Tue, 10 Sep 1996 14:34:06 GMT
Server: NCSA/1.4.2
Content-type: image/gif
Last-modified: Tue, 10 Sep 1996 13:25:26 GMT
Content-length: 9755

GIF87<9750 non-ascii characters>Connection closed by foreign host.

Example: If-Modified-Since

A client that has a copy of a document in its cache may send a GET with the If-Modified-Since header to check whether the cached copy is up-to-date.

daphne.cs.vt.edu> telnet ei 80
Trying 128.173.40.129... Connected to ei.cs.vt.edu.
Escape character is '^]'.
GET /~wwwbtb/fall.96/ClassNotes/Protocols/MIMEscreen.gif HTTP/1.0
If-Modified-Since: Saturday, 10-Sep-96 20:20:14

HTTP/1.0 304 Not modified
Date: Tue, 10 Sep 1996 14:40:58 GMT
Server: NCSA/1.4.2

Connection closed by foreign host.

Example: Server Redirect

daphne.cs.vt.edu> telnet ei 80
Trying 128.173.40.129... Connected to ei.cs.vt.edu.
Escape character is '^]'.
GET /~wwwbtb HTTP/1.0

HTTP/1.0 302 Found
Date: Tue, 10 Sep 1996 14:42:49 GMT
Server: NCSA/1.4.2
Location: http://ei.cs.vt.edu/~wwwbtb/
Content-type: text/html

<HEAD><TITLE>Document moved</TITLE></HEAD>
<BODY><H1>Document moved</H1>
This document has moved <A
HREF="http://ei.cs.vt.edu/~wwwbtb/">here</A>.<P>
</BODY>
Connection closed by foreign host.

HTTP Header Fields

From Figure 13.3 in Stevens:

Header Name	Meaning	Request?	Response?	Both?
Allow				yes
Authorization	Send userid/password	yes
Content-Encoding				yes
Content-Length	How many bytes of data?			yes
Content-Type	MIME-type of data			yes
Date	Current time/date	yes	yes
Expires	When to discard from cache			yes
From		yes
If-Modified-Since	For conditional GET	yes
Last-Modified	Date data last modified			yes
Location			yes
MIME-Version	So MIME names can change	yes	yes
Pragma	No-cache, etc.	yes	yes
Referer	URL previously visited	yes
Server			yes
User-Agent	Web browser name	yes
WWW Authenticate			yes

Return Codes

Classification of return codes

Response Code	Meaning
1xx	Not used
2xxx	Request succeeded
3xxx	Client error
4xxx	Server error

From Figure 13.4 in Stevens:

Response	Meaning
200	OK, request succeeded
202	Request accepted, but processing incomplete
301	Requested URL has been assigned a new, permanent URL
302	Reqeusted URL has been temporarily assigned a new URL
304	Document not modified (response to conditional `GET`
400	Bad request
401	Request not accepted because user authentication required
403	Forbidden for unspecified reason
404	Not found
500	Internal server error
501	Not implemented
502	Invalid response from gateway or upstream server
503	Service temporarily unavailable

Last modified on 27 Jan 1998.

Send comments to abrams@vt.edu.
[This is http://ei.cs.vt.edu/~wwwbtb/Notes/Protocols/http1_0.html.]