HTTP Made Really Easy

HTTP Made Really Easy

HTTP Made Really Easy

A Practical Guide to Writing Clients and Servers

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Table of Contents
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Footnotes

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HTTP is the network protocol of the Web. It is both simple and powerful.
Knowing HTTP enables you to write Web browsers, Web servers,
automatic page downloaders, link-checkers, and other useful tools.

This tutorial explains the simple, English-based structure of HTTP
communication, and teaches you the practical details of writing HTTP clients
and servers. It assumes you know basic socket programming.
HTTP is simple enough for a beginning sockets programmer, so this
page might be a good followup to a
sockets
tutorial. This





Sockets FAQ
focuses on C, but the underlying concepts are language-independent.

Since you're reading this, you probably already use CGI.
If not, it makes sense to
learn that first.

The whole tutorial is about 15 printed pages long, including
examples. The first half explains basic HTTP 1.0, and the second
half explains the new requirements and features of HTTP 1.1. This
tutorial doesn't cover everything about HTTP; it explains the basic
framework, how to comply with the requirements, and where to find out
more when you need it. If you plan to use HTTP extensively, you
should read the specification as well-- see the
end of this document for more details.

Before getting started, understand the following two
paragraphs:

<LECTURE>

Writing HTTP or other network programs requires more care
than programming for a single machine. Of course, you have to
follow standards, or no one will understand you. But even more important
is the burden you place on other machines. Write a bad program for
your own machine, and you waste your own resources (CPU time, bandwidth,
memory). Write a bad network program, and you waste other people's
resources. Write a really bad network program, and you waste
many thousands of people's resources at the same time. Sloppy and
malicious network programming forces network standards to be modified,
made safer but less efficient. So be careful, respectful, and cooperative,
for everyone's sake.

In particular, don't be tempted to write programs that automatically
follow Web links (called robots or spiders)
before you really know what you're doing. They can be useful, but a
badly-written robot is one of the worst kinds of programs on the Web,
blindly following a rapidly increasing number of links and quickly draining
server resources. If you plan to write anything like a robot, please
read
more about them.
There may already be a working program to do what you want. If you really
need to write your own, read these
guidelines.
Definitely support the current
Standard
for Robot Exclusion, and
stay
tuned for further developments.

</LECTURE>

OK, enough of that. Let's get started.

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Table of Contents

Top of Page

Using HTTP 1.0

1. What is HTTP?
   1. What are "Resources"?
2. Structure of HTTP Transactions
   1. Initial Request Line
   2. Initial Response Line (Status Line)
   3. Header Lines
   4. The Message Body
3. Sample HTTP Exchange
4. Other HTTP Methods, Like HEAD and POST
   1. The HEAD Method
   2. The POST Method
5. HTTP Proxies
6. Being Tolerant of Others
7. Conclusion

Upgrading to HTTP 1.1

1. HTTP 1.1
2. HTTP 1.1 Clients
   1. Host: Header
   2. Chunked Transfer-Encoding
   3. Persistent Connections and the
      "Connection: close" Header
   4. The "100 Continue" Response
3. HTTP 1.1 Servers
   1. Requiring the Host: Header
   2. Accepting Absolute URL's
   3. Chunked Transfer-Encoding
   4. Persistent Connections and the
      "Connection: close" Header
   5. Using the "100 Continue" Response
   6. The Date: Header
   7. Handling Requests with If-Modified-Since: or
      If-Unmodified-Since: Headers
   8. Supporting the GET and HEAD methods
   9. Supporting HTTP 1.0 Requests

Appendix

1. The HTTP Specification

Several related topics are discussed on a
"footnotes" page:

1. Sample HTTP Client
2. Using GET to Submit Query or
   Form Data
3. URL-encoding
4. Manually Experimenting with HTTP

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What is HTTP?

HTTP stands for Hypertext Transfer Protocol. It's the network
protocol used to deliver virtually all files and other data (collectively
called resources) on the World Wide Web, whether they're HTML files,
image files, query results, or anything else. Usually, HTTP takes place
through TCP/IP sockets (and this tutorial ignores other possibilities).

A browser is an HTTP client because it sends requests to an
HTTP server (Web server), which then sends responses back to
the client. The standard (and default) port for HTTP servers
to listen on is 80, though they can use any port.

What are "Resources"?

HTTP is used to transmit resources, not just files. A
resource is some chunk of information that can be identified by a URL
(it's the R in URL). The most common kind of resource
is a file, but a resource may also be a dynamically-generated query
result, the output of a CGI script, a document that is available in
several languages, or something else.

While learning HTTP, it may help to think of a resource as similar to
a file, but more general. As a practical matter, almost all HTTP resources
are currently either files or server-side script output.

Return to Table of Contents

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Structure of HTTP Transactions

Like most network protocols, HTTP uses the client-server model:
An HTTP client opens a connection and sends a request message
to an HTTP server; the server then returns a response message,
usually containing the resource that was requested. After delivering
the response, the server closes the connection (making HTTP a
stateless protocol, i.e. not maintaining any connection information
between transactions).

The format of the request and response messages are similar, and
English-oriented. Both kinds of messages consist of:

• an initial line,
• zero or more header lines,
• a blank line (i.e. a CRLF by itself), and
• an optional message body (e.g. a file, or query data, or query output).

Put another way, the format of an HTTP message is:

<initial line, different for request vs. response>
Header1: value1
Header2: value2
Header3: value3

<optional message body goes here, like file contents or query data;
 it can be many lines long, or even binary data $Samp;*%@!^$@>

Initial lines and headers should end in CRLF, though you should
gracefully handle lines ending in just LF. (More exactly, CR and LF
here mean ASCII values 13 and 10, even though some platforms may use
different characters.)

Return to Table of Contents

Initial Request Line

The initial line is different for the request than for the response.
A request line has three parts, separated by spaces: a method name,
the local path of the requested resource, and the version of HTTP being used.
A typical request line is:

GET /path/to/file/index.html HTTP/1.0

Notes:

• GET is the most common HTTP method; it says "give me this resource".
  Other methods include POST and HEAD-- more on those
  later.
  Method names are always uppercase.
• The path is the part of the URL after the host name, also called the
  request URI (a URI is like a URL, but more general).
• The HTTP version always takes the form "HTTP/x.x",
  uppercase.

Return to Table of Contents

Initial Response Line (Status Line)

The initial response line, called the status line, also has
three parts separated by spaces: the HTTP version, a
response status code that gives the result of the request,
and an English reason phrase describing the status code.
Typical status lines are:

HTTP/1.0 200 OK

or

HTTP/1.0 404 Not Found

Notes:

• The HTTP version is in the same format as in the request line,
  "HTTP/x.x".
• The status code is meant to be computer-readable; the reason
  phrase is meant to be human-readable, and may vary.
• The status code is a three-digit integer, and the first digit identifies
  the general category of response:
  • 1xx indicates an informational message only
  • 2xx indicates success of some kind
  • 3xx redirects the client to another URL
  • 4xx indicates an error on the client's part
  • 5xx indicates an error on the server's part

The most common status codes are:

200 OK

The request succeeded, and the resulting resource (e.g. file or script
output) is returned in the message body.

404 Not Found

The requested resource doesn't exist.

301 Moved Permanently

302 Moved Temporarily

303 See Other (HTTP 1.1 only)

The resource has moved to another URL (given by the
Location: response header), and should be automatically
retrieved by the client. This is often used by a CGI script to redirect
the browser to an existing file.

500 Server Error

An unexpected server error. The most common cause is a server-side
script that has bad syntax, fails, or otherwise can't run correctly.

A complete list of status codes is in
the HTTP specification
(section 9 for HTTP 1.0, and section 10 for HTTP 1.1).

Return to Table of Contents

Header Lines

Header lines provide information about the request or response, or
about the object sent in the message body.

The header lines are in the usual text header format, which is: one line
per header, of the form "Header-Name: value", ending
with CRLF. It's the same format used for email and news postings, defined in
RFC 822,
section 3. Details about RFC 822 header lines:

• As noted above, they should end in CRLF, but you should handle LF
  correctly.
• The header name is not case-sensitive (though the value may be).
• Any number of spaces or tabs may be between the ":" and the value.
• Header lines beginning with space or tab are actually part of the
  previous header line, folded into multiple lines for easy reading.

Thus, the following two headers are equivalent:

Header1: some-long-value-1a, some-long-value-1b

HEADER1:    some-long-value-1a,
            some-long-value-1b

HTTP 1.0 defines 16 headers, though none are required.
HTTP 1.1 defines 46 headers, and one (Host:) is required in
requests.
For Net-politeness, consider including these headers in your requests:

• The From: header gives the email address of
  whoever's making the request, or running the program doing so.
  (This must be user-configurable, for privacy concerns.)
• The User-Agent: header identifies the program that's
  making the request, in the form "Program-name/x.xx", where
  x.xx is the (mostly) alphanumeric version of the program.
  For example, Netscape 3.0 sends the header
  "User-agent: Mozilla/3.0Gold".

These headers help webmasters troubleshoot problems. They also
reveal information about the user. When you decide which headers to
include, you must balance the webmasters' logging needs against your users'
needs for privacy.

If you're writing servers, consider including these headers in your
responses:

• The Server: header is analogous to the
  User-Agent: header: it identifies the server software
  in the form "Program-name/x.xx". For example, one beta version
  of Apache's server returns
  "Server: Apache/1.2b3-dev".
• The Last-Modified: header gives the modification date
  of the resource that's being returned. It's used in caching and
  other bandwidth-saving activities. Use Greenwich Mean Time, in the format

      Last-Modified: Fri, 31 Dec 1999 23:59:59 GMT
      

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The Message Body

An HTTP message may have a body of data sent after the header
lines. In a response, this is where the requested resource is returned
to the client (the most common use of the message body), or perhaps
explanatory text if there's an error. In a request, this is where
user-entered data or uploaded files are sent to the server.

If an HTTP message includes a body, there are usually header lines in the
message that describe the body. In particular,

• The Content-Type: header gives the MIME-type of the data in
  the body, such as text/html or image/gif.
• The Content-Length: header gives the number of bytes
  in the body.

Return to Table of Contents

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Sample HTTP Exchange

To retrieve the file at the URL

http://www.somehost.com/path/file.html

first open a socket to the host www.somehost.com, port 80
(use the default port of 80 because none is specified in the URL).
Then, send something like the following through the socket:

GET /path/file.html HTTP/1.0
From: someuser@jmarshall.com
User-Agent: HTTPTool/1.0
[blank line here]

The server should respond with something like the following, sent
back through the same socket:

HTTP/1.0 200 OK
Date: Fri, 31 Dec 1999 23:59:59 GMT
Content-Type: text/html
Content-Length: 1354

<html>
<body>
<h1>Happy New Millennium!</h1>
(more file contents)
  .
  .
  .
</body>
</html>

After sending the response, the server closes the socket.

To familiarize yourself with requests and responses,
manually experiment with HTTP
using telnet.

Return to Table of Contents

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Other HTTP Methods, Like HEAD and POST

Besides GET, the two most commonly used methods are HEAD and POST.

The HEAD Method

A HEAD request is just like a GET request, except it asks the server
to return the response headers only, and not the actual resource (i.e. no
message body). This is useful to check characteristics of a resource
without actually downloading it, thus saving bandwidth. Use HEAD when
you don't actually need a file's contents.

The response to a HEAD request must never contain a message body,
just the status line and headers.

Return to Table of Contents

The POST Method

A POST request is used to send data to the server to be processed
in some way, like by a CGI script. A POST request is different from a
GET request in the following ways:

• There's a block of data sent with the request, in the message body.
  There are usually extra headers to describe this message body, like
  Content-Type: and Content-Length:.
• The request URI is not a resource to retrieve; it's usually a
  program to handle the data you're sending.
• The HTTP response is normally program output, not a static file.

The most common use of POST, by far, is to submit HTML form data to CGI
scripts. In this case, the Content-Type: header is usually
application/x-www-form-urlencoded, and the
Content-Length: header gives the length of the URL-encoded
form data (here's a
note on URL-encoding).
The CGI script receives the message body through STDIN, and decodes it.
Here's a typical form submission, using POST:

POST /path/script.cgi HTTP/1.0
From: frog@jmarshall.com
User-Agent: HTTPTool/1.0
Content-Type: application/x-www-form-urlencoded
Content-Length: 32

home=Cosby&favorite+flavor=flies

You can use a POST request to send whatever data you want, not just
form submissions. Just make sure the sender and the receiving program
agree on the format.

The GET method can also be used to submit forms. The form data is
URL-encoded
and appended to the request URI. Here are
more details.

If you're writing HTTP servers that support CGI scripts, you should
read the
NCSA's CGI definition
if you haven't already, especially which
environment variables
you need to pass to the scripts.

Return to Table of Contents

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HTTP Proxies

An HTTP proxy is a program that acts as an intermediary between
a client and a server. It receives requests from clients, and forwards
those requests to the intended servers. The responses pass back through it
in the same way. Thus, a proxy has functions of both a client and a server.

Proxies are commonly used in firewalls, for LAN-wide caches, or in other
situations. If you're writing proxies, read the
HTTP specification;
it contains details about proxies not covered in this tutorial.

When a client uses a proxy, it typically sends all requests to
that proxy, instead of to the servers in the URLs. Requests to a proxy
differ from normal requests in one way: in the first line, they use the
complete URL of the resource being requested, instead of just the path.
For example,

GET http://www.somehost.com/path/file.html HTTP/1.0

That way, the proxy knows which server to forward the request to (though
the proxy itself may use another proxy).

Return to Table of Contents

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Being Tolerant of Others

As the saying goes (in network programming, anyway), "Be strict in what
you send and tolerant in what you receive." Other clients and servers you
interact with may have minor flaws in their messages, but you should try to
work gracefully with them. In particular, the
HTTP specification suggests the
following:

• Even though header lines should end with CRLF, someone might use a
  single LF instead. Accept either CRLF or LF.
• The three fields in the initial message line should be separated by
  a single space, but might instead use several spaces, or tabs. Accept
  any number of spaces or tabs between these fields.

The specification has other suggestions too, like how to handle varying
date formats. If your program interprets dates from other programs, read
the "Tolerant Applications" section of the specification.

Return to Table of Contents

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Conclusion

That's the basic structure of HTTP. If you understand everything so far,
you have a good overview of HTTP communication, and should be able to write
simple HTTP 1.0 programs. See this
example to get started.
Again, before you do anything heavy-duty, read
the specification.

The rest of this document tells how to upgrade your clients and servers
to use HTTP 1.1. There is a list of new client requirements, and a
list of new server requirements. You can stop here if HTTP 1.0
satisfies your current needs (though you'll probably need HTTP 1.1 in
the future).

Note: As of early 1997, the Web is moving from HTTP 1.0 to
HTTP 1.1. Whenever practical, use HTTP 1.1. It's more efficient
overall, and by using it, you'll help the Web perform better for everyone.

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HTTP 1.1

Like many protocols, HTTP is constantly evolving. HTTP 1.1 has
recently been defined, to address new needs and overcome shortcomings
of HTTP 1.0. Generally speaking, it is a superset of HTTP 1.0.
Improvements include:

• Faster response, by allowing multiple transactions to take place
  over a single persistent connection.
• Faster response and great bandwidth savings, by adding cache support.
• Faster response for dynamically-generated pages, by supporting
  chunked encoding, which allows a response to be sent before
  its total length is known.
• Efficient use of IP addresses, by allowing multiple domains to be
  served from a single IP address.

HTTP 1.1 requires a few extra things from both clients and servers.
The next two sections detail how to make clients
and servers comply with HTTP 1.1. If
you're only writing clients, you can skip the section on servers. If you're
writing servers, read both sections.

Only requirements for HTTP 1.1 compliance are described here.
HTTP 1.1 has many optional features you may find useful; read
the specification to learn more.

Return to Table of Contents

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HTTP 1.1 Clients

To comply with HTTP 1.1, clients must

• include the Host: header with each
  request
• accept responses with chunked data
• either support persistent connections, or include the
  "Connection: close" header with each request
• handle the "100 Continue"
  response

Return to Table of Contents

Host: Header

Starting with HTTP 1.1, one server at one IP address can be
multi-homed, i.e. the home of several Web domains. For example,
"www.host1.com" and "www.host2.com" can live on the same server.

Several domains living on the same server is like several people
sharing one phone: a caller knows who they're calling for, but whoever
answers the phone doesn't. Thus, every HTTP request must specify which
host name (and possibly port) the request is intended for, with the
Host: header. A complete HTTP 1.1 request might be

GET /path/file.html HTTP/1.1
Host: www.host1.com:80
[blank line here]

except the ":80" isn't required, since that's the default HTTP port.

Host: is the only required header in an HTTP 1.1 request.
It's also the most urgently needed new feature in HTTP 1.1. Without it,
each host name requires a unique IP address, and we're
quickly running out of IP addresses with the explosion of new domains.

Return to Table of Contents

Chunked Transfer-Encoding

If a server wants to start sending a response before knowing its total
length (like with long script output), it might use the simple
chunked transfer-encoding, which breaks the complete response into
smaller chunks and sends them in series. You can identify such a response
because it contains the "Transfer-Encoding: chunked" header.
All HTTP 1.1 clients must be able to receive chunked messages.

A chunked message body contains a series of chunks, followed by a
line with "0" (zero), followed by optional footers (just like headers), and a
blank line. Each chunk consists of two parts:

• a line with the size of the chunk data, in hex, possibly followed
  by a semicolon and extra parameters you can ignore (none are currently
  standard), and ending with CRLF.
• the data itself, followed by CRLF.

So a chunked response might look like the following:

HTTP/1.1 200 OK
Date: Fri, 31 Dec 1999 23:59:59 GMT
Content-Type: text/plain
Transfer-Encoding: chunked

1a; ignore-stuff-here
abcdefghijklmnopqrstuvwxyz
10
1234567890abcdef
0
some-footer: some-value
another-footer: another-value
[blank line here]

Note the blank line after the last footer. The length of the
text data is 42 bytes (1a + 10, in hex), and the data itself is
abcdefghijklmnopqrstuvwxyz1234567890abcdef. The footers should be
treated like headers, as if they were at the top of the response.

The chunks can contain any binary data, and may be much larger than the
examples here. The size-line parameters are rarely used, but you
should at least ignore them correctly. Footers are also rare, but might
be appropriate for things like checksums or digital signatures.

For comparison, here's the equivalent to the above response, without
using chunked encoding:

HTTP/1.1 200 OK
Date: Fri, 31 Dec 1999 23:59:59 GMT
Content-Type: text/plain
Content-Length: 42
some-footer: some-value
another-footer: another-value

abcdefghijklmnopqrstuvwxyz1234567890abcdef

Return to Table of Contents

Persistent Connections and the "Connection: close" Header

In HTTP 1.0 and before, TCP connections are closed after each request
and response, so each resource to be retrieved requires its own connection.
Opening and closing TCP connections takes a substantial amount of CPU time,
bandwidth, and memory. In practice, most Web pages consist of several
files on the same server, so much can be saved by allowing several requests
and responses to be sent through a single persistent connection.

Persistent connections are the default in HTTP 1.1, so nothing special
is required to use them. Just open a connection and send several requests
in series (called pipelining), and read the responses in the same
order as the requests were sent. If you do this, be very careful to read
the correct length of each response, to separate them correctly.

If a client includes the "Connection: close" header
in the request, then the connection will be closed after the corresponding
response. Use this if you don't support persistent connections,
or if you know a request will be the last on its connection. Similarly,
if a response contains this header, then the server will close the
connection following that response, and the client shouldn't send any
more requests through that connection.

A server might close the connection before all responses are sent, so
a client must keep track of requests and resend them as needed. When
resending, don't pipeline the requests until you know the connection is
persistent. Don't pipeline at all if you know the server won't support
persistent connections (like if it uses HTTP 1.0, based on a previous
response).

Return to Table of Contents

The "100 Continue" Response

During the course of an HTTP 1.1 client sending a request to a server,
the server might respond with an interim "100 Continue"
response. This means the server has received the first part of the request,
and can be used to aid communication over slow links. In any case, all
HTTP 1.1 clients must handle the 100 response correctly (perhaps
by just ignoring it).

The "100 Continue" response is structured like
any HTTP response, i.e. consists of a status line, optional headers,
and a blank line. Unlike other responses, it is always followed by
another complete, final response.

So, further extending the last example, the full data that comes
back from the server might consist of two responses in series, like

HTTP/1.1 100 Continue

HTTP/1.1 200 OK
Date: Fri, 31 Dec 1999 23:59:59 GMT
Content-Type: text/plain
Content-Length: 42
some-footer: some-value
another-footer: another-value

abcdefghijklmnoprstuvwxyz1234567890abcdef

To handle this, a simple HTTP 1.1 client might read one response from the
socket; if the status code is 100, discard the first response and read the
next one instead.

Return to Table of Contents

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HTTP 1.1 Servers

To comply with HTTP 1.1, servers must:

• require the Host: header from
  HTTP 1.1 clients
• accept absolute URL's in a request
• accept requests with chunked data
• either support persistent connections, or
  include the "Connection: close" header with each
  response
• use the "100 Continue" response
  appropriately
• include the Date: header in each
  response
• handle requests with If-Modified-Since:
  or If-Unmodified-Since: headers
• support at least the GET and HEAD methods
• support HTTP 1.0 requests

Return to Table of Contents

Requiring the Host: Header

Because of the urgency of implementing the new Host:
header, servers are not allowed to tolerate HTTP 1.1 requests without it.
If a server receives such a request, it must return a
"400 Bad Request" response, like

HTTP/1.1 400 Bad Request
Content-Type: text/html
Content-Length: 111

<html><body>
<h2>No Host: header received</h2>
HTTP 1.1 requests must include the Host: header.
</body></html>

This requirement applies only to clients using HTTP 1.1, not
any future version of HTTP.
If the request uses an HTTP version later than 1.1, the server can
accept an absolute URL instead of a Host: header (see
next section).
If the request uses HTTP 1.0, the server may accept the request without
any host identification.

Return to Table of Contents

Accepting Absolute URL's

The Host: header is actually an interim solution to
the problem of host identification. In future versions of HTTP,
requests will use an absolute URL instead of a pathname, like

GET http://www.somehost.com/path/file.html HTTP/1.2

To enable this protocol transition, HTTP 1.1 servers must accept this
form of request, even though HTTP 1.1 clients won't send them. The server
must still report an error if an HTTP 1.1 client leaves out
the Host: header, as described in the
previous section.

Return to Table of Contents

Chunked Transfer-Encoding

Just as HTTP 1.1 clients must accept chunked responses, servers must
accept chunked requests (an unlikely scenario, but possible). See the
earlier section on
HTTP 1.1 Clients
for details of the chunked data format.

Servers aren't required to generate chunked messages; they just have to
be able to receive them.

Return to Table of Contents

Persistent Connections and the "Connection: close" Header

If an HTTP 1.1 client sends multiple requests through a single connection,
the server should send responses back in the same order as the requests--
this is all it takes for a server to support persistent connections.

If a request includes the "Connection: close" header,
that request is the final one for the connection and the server should
close the connection after sending the response. Also, the server should
close an idle connection after some timeout period (can be anything;
10 seconds is fine).

If you don't want to support persistent connections, include the
"Connection: close" header in the response. Use this
header whenever you want to close the connection, even if not all requests
have been fulfilled. The header says that the connection will be closed
after the current response, and a valid HTTP 1.1 client will handle it
correctly.

Return to Table of Contents

Using the "100 Continue" Response

As described in the section on
HTTP 1.1 Clients,
this response exists to help deal with slow links.

When an HTTP 1.1 server receives the first line of an HTTP 1.1
(or later) request, it must respond with either
"100 Continue" or an error. If it sends the
"100 Continue" response, it must also send another,
final response, once the request has been processed. The
"100 Continue" response requires no headers, but must
be followed by the usual blank line, like:

HTTP/1.1 100 Continue
[blank line here]
[another HTTP response will go here]

Don't send "100 Continue" to HTTP 1.0 clients,
since they don't know how to handle it.

Return to Table of Contents

The Date: Header

Caching is an important improvement in HTTP 1.1, and can't work
without timestamped responses. So, servers must timestamp every response
with a Date: header containing the current time, in the form

Date: Fri, 31 Dec 1999 23:59:59 GMT

All responses except those with 100-level status (but including error
responses) must include the Date: header.

All time values in HTTP use Greenwich Mean Time.

Return to Table of Contents

Handling Requests with If-Modified-Since: or If-Unmodified-Since:
Headers

To avoid sending resources that don't need to be sent, thus saving
bandwidth, HTTP 1.1 defines the If-Modified-Since: and
If-Unmodified-Since: request headers.
The former says "only send the resource if it has changed since this date";
the latter says the opposite. Clients aren't required to use them, but
HTTP 1.1 servers are required to honor requests that do use them.

Unfortunately, due to earlier HTTP versions,
the date value may be in any of three possible formats:

If-Modified-Since:  Fri, 31 Dec 1999 23:59:59 GMT
If-Modified-Since:  Friday, 31-Dec-99 23:59:59 GMT
If-Modified-Since:  Fri Dec 31 23:59:59 1999

Again, all time values in HTTP use Greenwich Mean Time (though try to be
tolerant of non-GMT times). If a date with a two-digit year seems to be more
than 50 years in the future, treat it as being in the past-- this
helps with the millennium bug. In fact, do this with any date handling in
HTTP 1.1.

Although servers must accept all three date formats, HTTP 1.1
clients and servers must only generate the first kind.

If the date in either of these headers is invalid, or is in the future,
ignore the header.

If, without the header, the request would result in an unsuccessful
(non-200-level) status code, ignore the header and send the non-200-level
response. In other words, only apply these headers when you know the
resource would otherwise be sent.

The If-Modified-Since: header is used with a GET
request. If the requested resource has been modified since the given date,
ignore the header and return the resource as you normally would.
Otherwise, return a "304 Not Modified" response,
including the Date: header and no message body, like

HTTP/1.1 304 Not Modified
Date: Fri, 31 Dec 1999 23:59:59 GMT
[blank line here]

The If-Unmodified-Since: header is similar, but can be
used with any method. If the requested resource has not been
modified since the given date, ignore the header and return the resource as
you normally would. Otherwise, return a
"412 Precondition Failed" response, like

HTTP/1.1 412 Precondition Failed
[blank line here]

Return to Table of Contents

Supporting the GET and HEAD methods

To comply with HTTP 1.1, a server must support at least the GET and HEAD
methods. If you're handling CGI scripts, you should probably support the
POST method too.

Four other methods (PUT, DELETE, OPTIONS, and TRACE) are defined in
HTTP 1.1, but are rarely used. If a client requests a method you don't
support, respond with "501 Not Implemented", like

HTTP/1.1 501 Not Implemented
[blank line here]

Return to Table of Contents

Supporting HTTP 1.0 Requests

To be compatible with older browsers, HTTP 1.1 servers must support
HTTP 1.0 requests. In particular, when a request uses HTTP 1.0
(as identified in the initial request line),

• don't require the Host: header, and
• don't send the "100 Continue" response.

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The HTTP Specification

If you plan to do anything elaborate in HTTP, read the
official specification. HTTP 1.0 was never made an official
Internet standard, but the de facto standard is described in
RFC 1945.
HTTP 1.1 was developed by a
working group of the
IETF, openly gathering input from many
sources before reaching an approximate consensus. The resulting specification
is RFC 2616,
released in June 1999.

These documents are available in various formats:

• HTTP 1.0 (RFC 1945)--
  HTML,
  text, and
  gzip'd PostScript
• HTTP 1.1 (RFC 2616)--
  HTML,
  text,
  PostScript, and
  PDF.

Download and print the version you'll be using, for reference and
bedtime reading.

The World Wide Web Consortium
has a
page devoted to HTTP
including news and updates, and a page listing
HTTP specifications,
drafts, and reports.

Other RFC's you might find useful:

• RFC 822--
  structure of Internet text messages, including header fields
• RFC 2396--
  definition of URL/URI syntax (replaces
  RFC 1738
  and
  RFC 1808)
• RFC 1521--
  definition of MIME and of MIME types

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© 1997
James Marshall
(comments welcome)

Last (significantly) modified: August 15, 1997