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PERSISTENT
CLIENT
STATE
HTTP COOKIES
Preliminary Specification - Use with caution
INTRODUCTION
Cookies are a general mechanism which server side connections (such as
CGI scripts) can use to both store and retrieve information on the
client side of the connection. The addition of a simple, persistent,
client-side state significantly extends the capabilities of Web-based
client/server applications.
OVERVIEW
A server, when returning an HTTP object to a client, may also send a
piece of state information which the client will store. Included in that
state object is a description of the range of URLs for which that state is
valid. Any future HTTP requests made by the client which fall in that
range will include a transmittal of the current value of the state
object from the client back to the server. The state object is called
a cookie, for no compelling reason.
This simple mechanism provides a powerful new tool which enables a host
of new types of applications to be written for web-based environments.
Shopping applications can now store information about the currently
selected items, for fee services can send back registration information
and free the client from retyping a user-id on next connection,
sites can store per-user preferences on the client, and have the client supply
those preferences every time that site is connected to.
SPECIFICATION
A cookie is introduced to the client by including a Set-Cookie
header as part of an HTTP response, typically this will be generated
by a CGI script.
Syntax of the Set-Cookie HTTP Response Header
This is the format a CGI script would use to add to the HTTP headers
a new piece of data which is to be stored by the client for later retrieval.
This string is a sequence of characters excluding semi-colon, comma and white
space. If there is a need to place such data in the name or value, some
encoding method such as URL style %XX encoding is recommended, though no
encoding is defined or required.
This is the only required attribute
on the Set-Cookie header.
expires=DATE
The expires attribute specifies a date string that
defines the valid life time of that cookie. Once the expiration
date has been reached, the cookie will no longer be stored or
given out.
The date string is formatted as:
Wdy, DD-Mon-YYYY HH:MM:SS GMT
This is based on
RFC 822,
RFC 850,
RFC 1036, and
RFC 1123,
with the variations that the only legal time zone is GMT and
the separators between the elements of the date must be dashes.
expires is an optional attribute. If not specified, the cookie will
expire when the user's session ends.
Note: There is a bug in Netscape Navigator version 1.1 and earlier.
Only cookies whose path attribute is set explicitly to "/" will
be properly saved between sessions if they have an expires
attribute.
domain=DOMAIN_NAME
When searching the cookie list for valid cookies, a comparison of the
domain
attributes of the cookie is made with the Internet domain name of the
host from which the URL will be fetched. If there is a tail match,
then the cookie will go through path matching to see if it
should be sent. "Tail matching" means that domain attribute
is matched against the tail of the fully qualified domain name of
the host. A domain attribute of "acme.com" would match
host names "anvil.acme.com" as well as "shipping.crate.acme.com".
Only hosts within the specified domain
can set a cookie for a domain and domains must have at least two (2)
or three (3) periods in them to prevent domains of the form:
".com", ".edu", and "va.us". Any domain that fails within
one of the seven special top level domains listed below only require
two periods. Any other domain requires at least three. The
seven special top level domains are: "COM", "EDU", "NET", "ORG",
"GOV", "MIL", and "INT".
The default value of domain is the host name of the server
which generated the cookie response.
path=PATH
The path attribute is used to specify the subset of URLs in a
domain for
which the cookie is valid. If a cookie has already passed domain
matching, then the pathname component
of the URL is compared with the path attribute, and if there is
a match, the cookie is considered valid and is sent along with
the URL request. The path "/foo"
would match "/foobar" and "/foo/bar.html". The path "/" is the most
general path.
If the path is not specified, it as assumed to be the same path
as the document being described by the header which contains the cookie.
secure
If a cookie is marked secure, it will only be transmitted if the
communications channel with the host is a secure one. Currently
this means that secure cookies will only be sent to HTTPS (HTTP over SSL)
servers.
If secure is not specified, a cookie is considered safe to be sent
in the clear over unsecured channels.
Syntax of the Cookie HTTP Request Header
When requesting a URL from an HTTP server, the browser will match
the URL against all cookies and if any of them match, a line
containing the name/value pairs of all matching cookies will
be included in the HTTP request. Here is the format of that line:
Multiple Set-Cookie headers can be issued in a single server
response.
Instances of the same path and name will overwrite each other, with the
latest instance taking precedence. Instances of the same path but
different names will add additional mappings.
Setting the path to a higher-level value does not override other more
specific path mappings. If there are multiple matches for a given cookie
name, but with separate paths, all the matching cookies will be sent.
(See examples below.)
The
expires header lets the client know when it is safe to purge the mapping
but the client is not required to do so. A client may also delete a
cookie before it's expiration date arrives if the number of cookies
exceeds its internal limits.
When sending cookies to a server, all cookies with a more specific
path mapping should be sent before cookies with less specific path
mappings. For example, a cookie "name1=foo" with a path mapping
of "/" should be sent after a cookie "name1=foo2" with
a path mapping of "/bar" if they are both to be sent.
There are limitations on the number of cookies that a client
can store at any one time. This is a specification of the minimum
number of cookies that a client should be prepared to receive and
store.
300 total cookies
4 kilobytes per cookie, where the name and the OPAQUE_STRING
combine to form the 4 kilobyte limit.
20 cookies per server or domain. (note that completely
specified hosts and domains are treated as separate entities
and have a 20 cookie limitation for each, not combined)
Servers should not expect clients to be able to exceed these limits.
When the 300 cookie limit or the 20 cookie per server limit
is exceeded, clients should delete the least recently used cookie.
When a cookie larger than 4 kilobytes is encountered the cookie
should be trimmed to fit, but the name should remain intact
as long as it is less than 4 kilobytes.
If a CGI script wishes to delete a cookie, it can do so by
returning a cookie with the same name, and an expires time
which is in the past. The path and name must match exactly
in order for the expiring cookie to replace the valid cookie.
This requirement makes it difficult for anyone but the originator
of a cookie to delete a cookie.
When caching HTTP, as a proxy server might do, the Set-cookie
response header should never be cached.
If a proxy server receives a response which
contains a Set-cookie header, it should propagate the Set-cookie
header to the client, regardless of whether the response was 304
(Not Modified) or 200 (OK).
Similarly, if a client request contains a Cookie: header, it
should be forwarded through a proxy, even if the conditional
If-modified-since request is being made.
EXAMPLES
Here are some sample exchanges which are designed to illustrate the use
of cookies.
First Example transaction sequence:
Client requests a document, and receives in the response:
