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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 RFC 2965

HTTP Working Group                                      David M. Kristol
INTERNET DRAFT                    Bell Laboratories, Lucent Technologies
Obsoletes: RFC 2109                                         Lou Montulli
                                                      Epinions.com, Inc.
<draft-ietf-http-state-man-mec-12.txt>
August 27, 1999                                Expires February 27, 2000


                    HTTP State Management Mechanism



                          Status of this Memo

     This document is an Internet-Draft and is in full conformance
     with all provisions of Section 10 of RFC2026.

     This document is an Internet-Draft.  Internet-Drafts are
     working documents of the Internet Engineering Task Force
     (IETF), its areas, and its working groups.  Note that other
     groups may also distribute working documents as Internet-
     Drafts.

     Internet-Drafts are draft documents valid for a maximum of six
     months and may be updated, replaced, or obsoleted by other
     documents at any time.  It is inappropriate to use Internet-
     Drafts as reference material or to cite them other than as
     ``work in progress.''

     The list of current Internet-Drafts can be accessed at
     http://www.ietf.org/ietf/1id-abstracts.txt.

     The list of Internet-Draft Shadow Directories can be accessed
     at http://www.ietf.org/shadow.html.

     This is authors' draft 3.11.


1.  ABSTRACT

This document specifies a way to create a stateful session with HTTP
requests and responses.  It describes two new headers, Cookie and Set-
Cookie2, which carry state information between participating origin
servers and user agents.  The method described here differs from
Netscape's Cookie proposal [Netscape], but it can interoperate with
HTTP/1.0 user agents that use Netscape's method.  (See the HISTORICAL
section.)

This document reflects implementation experience with RFC 2109 [RFC2109]
and obsoletes it.









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2.  TERMINOLOGY

The terms user agent, client, server, proxy, origin server, and http_URL
have the same meaning as in the HTTP/1.1 specification [RFC2616].  The
terms abs_path and absoluteURI have the same meaning as in the URI
Syntax specification [RFC2396].

Host name (HN) means either the host domain name (HDN) or the numeric
Internet Protocol (IP) address of a host.  The fully qualified domain
name is preferred; use of numeric IP addresses is strongly discouraged.

The terms request-host and request-URI refer to the values the client
would send to the server as, respectively, the host (but not port) and
abs_path portions of the absoluteURI (http_URL) of the HTTP request
line.  Note that request-host is a HN.

The term effective host name is related to host name.  If a host name
contains no dots, the effective host name is that name with the string
.local appended to it.  Otherwise the effective host name is the same as
the host name.  Note that all effective host names contain at least one
dot.

The term request-port refers to the port portion of the absoluteURI
(http_URL) of the HTTP request line.  If the absoluteURI has no explicit
port, the request-port is the HTTP default, 80.  The request-port of a
cookie is the request-port of the request in which a Set-Cookie2
response header was returned to the user agent.

Host names can be specified either as an IP address or a HDN string.
Sometimes we compare one host name with another.  (Such comparisons
SHALL be case-insensitive.)  Host A's name domain-matches host B's if

   * their host name strings string-compare equal; or

   * A is a HDN string and has the form NB, where N is a non-empty name
     string, B has the form .B', and B' is a HDN string.  (So, x.y.com
     domain-matches .Y.com but not Y.com.)

Note that domain-match is not a commutative operation: a.b.c.com
domain-matches .c.com, but not the reverse.

The reach R of a host name H is defined as follows:

   * If

        - H is the host domain name of a host; and,

        - H has the form A.B; and

        - A has no embedded (that is, interior) dots; and




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        - B has at least one embedded dot, or B is the string ``local''.
     then the reach of H is .B.

   * Otherwise, the reach of H is H.

For two strings that represent paths, P1 and P2, P1 path-matches P2 if
P2 is a prefix of P1 (including the case where P1 and P2 string-compare
equal).  Thus, the string /tec/waldo path-matches /tec.

Because it was used in Netscape's original implementation of state
management, we will use the term cookie to refer to the state
information that passes between an origin server and user agent, and
that gets stored by the user agent.

2.1  Requirements

The key words ``MAY'', ``MUST'', ``MUST NOT'', ``OPTIONAL'',
``RECOMMENDED'', ``REQUIRED'', ``SHALL'', ``SHALL NOT'', ``SHOULD'',
``SHOULD NOT'' in this document are to be interpreted as described in
RFC 2119 [RFC2119].


3.  STATE AND SESSIONS

This document describes a way to create stateful sessions with HTTP
requests and responses.  Currently, HTTP servers respond to each client
request without relating that request to previous or subsequent
requests; the state management mechanism allows clients and servers that
wish to exchange state information to place HTTP requests and responses
within a larger context, which we term a ``session.''  This context
might be used to create, for example, a ``shopping cart,'' in which user
selections can be aggregated before purchase, or a magazine browsing
system, in which a user's previous reading affects which offerings are
presented.

Neither clients nor servers are required to support cookies.  A server
MAY refuse to provide content to a client that does not return the
cookies it sends.


4.  DESCRIPTION

We describe here a way for an origin server to send state information to
the user agent, and for the user agent to return the state information
to the origin server.  The goal is to have a minimal impact on HTTP and
user agents.








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4.1  Syntax:  General

The two state management headers, Set-Cookie2 and Cookie, have common
syntactic properties involving attribute-value pairs.  The following
grammar uses the notation, and tokens DIGIT (decimal digits), token
(informally, a sequence of non-special, non-white space characters), and
http_URL from the HTTP/1.1 specification [RFC2616] to describe their
syntax.

av-pairs        =       av-pair *(";" av-pair)
av-pair         =       attr ["=" value]                ; optional value
attr            =       token
value           =       token | quoted-string

Attributes (names) (attr) are case-insensitive.  White space is
permitted between tokens.  Note that while the above syntax description
shows value as optional, most attrs require them.

NOTE: The syntax above allows whitespace between the attribute and the =
sign.

4.2  Origin Server Role

4.2.1  General  The origin server initiates a session, if it so desires.
To do so, it returns an extra response header to the client, Set-
Cookie2.  (The details follow later.)

A user agent returns a Cookie request header (see below) to the origin
server if it chooses to continue a session.  The origin server MAY
ignore it or use it to determine the current state of the session.  It
MAY send back to the client a Set-Cookie2 response header with the same
or different information, or it MAY send no Set-Cookie2 header at all.
The origin server effectively ends a session by sending the client a
Set-Cookie2 header with Max-Age=0.

Servers MAY return Set-Cookie2 response headers with any response.  User
agents SHOULD send Cookie request headers, subject to other rules
detailed below, with every request.

An origin server MAY include multiple Set-Cookie2 headers in a response.
Note that an intervening gateway could fold multiple such headers into a
single header.

4.2.2  Set-Cookie2 Syntax  The syntax for the Set-Cookie2 response
header is









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set-cookie      =       "Set-Cookie2:" cookies
cookies         =       1#cookie
cookie          =       NAME "=" VALUE *(";" set-cookie-av)
NAME            =       attr
VALUE           =       value
set-cookie-av   =       "Comment" "=" value
                |       "CommentURL" "=" <"> http_URL <">
                |       "Discard"
                |       "Domain" "=" value
                |       "Max-Age" "=" value
                |       "Path" "=" value
                |       "Port" [ "=" <"> portlist <"> ]
                |       "Secure"
                |       "Version" "=" 1*DIGIT
portlist        =       1#portnum
portnum         =       1*DIGIT

Informally, the Set-Cookie2 response header comprises the token Set-
Cookie2:, followed by a comma-separated list of one or more cookies.
Each cookie begins with a NAME=VALUE pair, followed by zero or more
semi-colon-separated attribute-value pairs.  The syntax for attribute-
value pairs was shown earlier.  The specific attributes and the
semantics of their values follows.  The NAME=VALUE attribute-value pair
MUST come first in each cookie.  The others, if present, can occur in
any order.  If an attribute appears more than once in a cookie, the
client SHALL use only the value associated with the first appearance of
the attribute; a client MUST ignore values after the first.

The NAME of a cookie MAY be the same as one of the attributes in this
specification.  However, because the cookie's NAME must come first in a
Set-Cookie2 response header, the NAME and its VALUE cannot be confused
with an attribute-value pair.

NAME=VALUE
     REQUIRED.  The name of the state information (``cookie'') is NAME,
     and its value is VALUE.  NAMEs that begin with $ are reserved and
     MUST NOT be used by applications.

     The VALUE is opaque to the user agent and may be anything the
     origin server chooses to send, possibly in a server-selected
     printable ASCII encoding.  ``Opaque'' implies that the content is
     of interest and relevance only to the origin server.  The content
     may, in fact, be readable by anyone that examines the Set-Cookie2
     header.

Comment=value
     OPTIONAL.  Because cookies can be used to derive or store private
     information about a user, the value of the Comment attribute allows
     an origin server to document how it intends to use the cookie.  The
     user can inspect the information to decide whether to initiate or
     continue a session with this cookie.  Characters in value MUST be



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     in UTF-8 encoding. [RFC2279]

CommentURL="http_URL"
     OPTIONAL.  Because cookies can be used to derive or store private
     information about a user, the CommentURL attribute allows an origin
     server to document how it intends to use the cookie.  The user can
     inspect the information identified by the URL to decide whether to
     initiate or continue a session with this cookie.

Discard
     OPTIONAL.  The Discard attribute instructs the user agent to
     discard the cookie unconditionally when the user agent terminates.

Domain=value
     OPTIONAL.  The value of the Domain attribute specifies the domain
     for which the cookie is valid.  If an explicitly specified value
     does not start with a dot, the user agent supplies a leading dot.

Max-Age=value
     OPTIONAL.  The value of the Max-Age attribute is delta-seconds, the
     lifetime of the cookie in seconds, a decimal non-negative integer.
     To handle cached cookies correctly, a client SHOULD calculate the
     age of the cookie according to the age calculation rules in the
     HTTP/1.1 specification [RFC2616].  When the age is greater than
     delta-seconds seconds, the client SHOULD discard the cookie.  A
     value of zero means the cookie SHOULD be discarded immediately.

Path=value
     OPTIONAL.  The value of the Path attribute specifies the subset of
     URLs on the origin server to which this cookie applies.

Port[="portlist"]
     OPTIONAL.  The Port attribute restricts the port to which a cookie
     may be returned in a Cookie request header.  Note that the syntax
     REQUIREs quotes around the OPTIONAL portlist even if there is only
     one portnum in portlist.

Secure
     OPTIONAL.  The Secure attribute (with no value) directs the user
     agent to use only (unspecified) secure means to contact the origin
     server whenever it sends back this cookie, to protect the
     confidentially and authenticity of the information in the cookie.

     The user agent (possibly with user interaction) MAY determine what
     level of security it considers appropriate for ``secure'' cookies.
     The Secure attribute should be considered security advice from the
     server to the user agent, indicating that it is in the session's
     interest to protect the cookie contents.  When it sends a
     ``secure'' cookie back to a server, the user agent SHOULD use no
     less than the same level of security as was used when it received
     the cookie from the server.



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Version=value
     REQUIRED.  The value of the Version attribute, a decimal integer,
     identifies the version of the state management specification to
     which the cookie conforms.  For this specification, Version=1
     applies.

4.2.3  Controlling Caching  An origin server must be cognizant of the
effect of possible caching of both the returned resource and the Set-
Cookie2 header.  Caching ``public'' documents is desirable.  For
example, if the origin server wants to use a public document such as a
``front door'' page as a sentinel to indicate the beginning of a session
for which a Set-Cookie2 response header must be generated, the page
SHOULD be stored in caches ``pre-expired'' so that the origin server
will see further requests.  ``Private documents,'' for example those
that contain information strictly private to a session, SHOULD NOT be
cached in shared caches.

If the cookie is intended for use by a single user, the Set-Cookie2
header SHOULD NOT be cached.  A Set-Cookie2 header that is intended to
be shared by multiple users MAY be cached.

The origin server SHOULD send the following additional HTTP/1.1 response
headers, depending on circumstances:

   * To suppress caching of the Set-Cookie2 header:

     Cache-control: no-cache="set-cookie2"

and one of the following:

   * To suppress caching of a private document in shared caches:

     Cache-control: private

   * To allow caching of a document and require that it be validated
     before returning it to the client:

     Cache-Control: must-revalidate, max-age=0

   * To allow caching of a document, but to require that proxy caches
     (not user agent caches) validate it before returning it to the
     client:

     Cache-Control: proxy-revalidate, max-age=0

   * To allow caching of a document and request that it be validated
     before returning it to the client (by ``pre-expiring'' it):

     Cache-control: max-age=0

     Not all caches will revalidate the document in every case.



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HTTP/1.1 servers MUST send Expires: old-date (where old-date is a date
long in the past) on responses containing Set-Cookie2 response headers
unless they know for certain (by out of band means) that there are no
HTTP/1.0 proxies in the response chain.  HTTP/1.1 servers MAY send other
Cache-Control directives that permit caching by HTTP/1.1 proxies in
addition to the Expires: old-date directive; the Cache-Control directive
will override the Expires: old-date for HTTP/1.1 proxies.

4.3  User Agent Role

4.3.1  Interpreting Set-Cookie2  The user agent keeps separate track of
state information that arrives via Set-Cookie2 response headers from
each origin server (as distinguished by name or IP address and port).
The user agent MUST ignore attribute-value pairs whose attribute it does
not recognize.  The user agent applies these defaults for optional
attributes that are missing:

Discard The default behavior is dictated by the presence or absence of a
        Max-Age attribute.

Domain  Defaults to the effective request-host.  (Note that because
        there is no dot at the beginning of effective request-host, the
        default Domain can only domain-match itself.)

Max-Age The default behavior is to discard the cookie when the user
        agent exits.

Path    Defaults to the path of the request URL that generated the Set-
        Cookie2 response, up to and including the right-most /.

Port    The default behavior is that a cookie MAY be returned to any
        request-port.

Secure  If absent, the user agent MAY send the cookie over an insecure
        channel.

4.3.2  Rejecting Cookies  To prevent possible security or privacy
violations, a user agent rejects a cookie according to rules below.  The
goal of the rules is to try to limit the set of servers for which a
cookie is valid, based on the values of the Path, Domain, and Port
attributes and the request-URI, request-host and request-port.

A user agent rejects (SHALL NOT store its information) if the Version
attribute is missing.  Moreover, a user agent rejects (SHALL NOT store
its information) if any of the following is true of the attributes
explicitly present in the Set-Cookie2 response header:

   * The value for the Path attribute is not a prefix of the request-
     URI.





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   * The value for the Domain attribute contains no embedded dots, and
     the value is not .local.

   * The effective host name that derives from the request-host does not
     domain-match the Domain attribute.

   * The request-host is a HDN (not IP address) and has the form HD,
     where D is the value of the Domain attribute, and H is a string
     that contains one or more dots.

   * The Port attribute has a "port-list", and the request-port was not
     in the list.

Examples:

   * A Set-Cookie2 from request-host y.x.foo.com for Domain=.foo.com
     would be rejected, because H is y.x and contains a dot.

   * A Set-Cookie2 from request-host x.foo.com for Domain=.foo.com would
     be accepted.

   * A Set-Cookie2 with Domain=.com or Domain=.com., will always be
     rejected, because there is no embedded dot.

   * A Set-Cookie2 with Domain=ajax.com will be accepted, and the value
     for Domain will be taken to be .ajax.com, because a dot gets
     prepended to the value.

   * A Set-Cookie2 with Port="80,8000" will be accepted if the request
     was made to port 80 or 8000 and will be rejected otherwise.

   * A Set-Cookie2 from request-host example for Domain=.local will be
     accepted, because the effective host name for the request-host is
     example.local, and example.local domain-matches .local.

4.3.3  Cookie Management  If a user agent receives a Set-Cookie2
response header whose NAME is the same as that of a cookie it has
previously stored, the new cookie supersedes the old when: the old and
new Domain attribute values compare equal, using a case-insensitive
string-compare; and, the old and new Path attribute values string-
compare equal (case-sensitive).  However, if the Set-Cookie2 has a value
for Max-Age of zero, the (old and new) cookie is discarded.  Otherwise a
cookie persists (resources permitting) until whichever happens first,
then gets discarded: its Max-Age lifetime is exceeded; or, if the
Discard attribute is set, the user agent terminates the session.

Because user agents have finite space in which to store cookies, they
MAY also discard older cookies to make space for newer ones, using, for
example, a least-recently-used algorithm, along with constraints on the
maximum number of cookies that each origin server may set.




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If a Set-Cookie2 response header includes a Comment attribute, the user
agent SHOULD store that information in a human-readable form with the
cookie and SHOULD display the comment text as part of a cookie
inspection user interface.

If a Set-Cookie2 response header includes a CommentURL attribute, the
user agent SHOULD store that information in a human-readable form with
the cookie, or, preferably, SHOULD allow the user to follow the http_URL
link as part of a cookie inspection user interface.

The cookie inspection user interface may include a facility whereby a
user can decide, at the time the user agent receives the Set-Cookie2
response header, whether or not to accept the cookie.  A potentially
confusing situation could arise if the following sequence occurs:

   * the user agent receives a cookie that contains a CommentURL
     attribute;

   * the user agent's cookie inspection interface is configured so that
     it presents a dialog to the user before the user agent accepts the
     cookie;

   * the dialog allows the user to follow the CommentURL link when the
     user agent receives the cookie; and,

   * when the user follows the CommentURL link, the origin server (or
     another server, via other links in the returned content) returns
     another cookie.

The user agent SHOULD NOT send any cookies in this context.  The user
agent MAY discard any cookie it receives in this context that the user
has not, through some user agent mechanism, deemed acceptable.

User agents SHOULD allow the user to control cookie destruction, but
they MUST NOT extend the cookie's lifetime beyond that controlled by the
Discard and Max-Age attributes.  An infrequently-used cookie may
function as a ``preferences file'' for network applications, and a user
may wish to keep it even if it is the least-recently-used cookie.  One
possible implementation would be an interface that allows the permanent
storage of a cookie through a checkbox (or, conversely, its immediate
destruction).

Privacy considerations dictate that the user have considerable control
over cookie management.  The PRIVACY section contains more information.

4.3.4  Sending Cookies to the Origin Server  When it sends a request to
an origin server, the user agent includes a Cookie request header if it
has stored cookies that are applicable to the request, based on

   * the request-host and request-port;




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   * the request-URI;

   * the cookie's age.

The syntax for the header is:

cookie          =       "Cookie:" cookie-version 1*((";" | ",") cookie-value)
cookie-value    =       NAME "=" VALUE [";" path] [";" domain] [";" port]
cookie-version  =       "$Version" "=" value
NAME            =       attr
VALUE           =       value
path            =       "$Path" "=" value
domain          =       "$Domain" "=" value
port            =       "$Port" [ "=" <"> value <"> ]

The value of the cookie-version attribute MUST be the value from the
Version attribute of the corresponding Set-Cookie2 response header.
Otherwise the value for cookie-version is 0.  The value for the path
attribute MUST be the value from the Path attribute, if one was present,
of the corresponding Set-Cookie2 response header.  Otherwise the
attribute SHOULD be omitted from the Cookie request header.  The value
for the domain attribute MUST be the value from the Domain attribute, if
one was present, of the corresponding Set-Cookie2 response header.
Otherwise the attribute SHOULD be omitted from the Cookie request
header.

The port attribute of the Cookie request header MUST mirror the Port
attribute, if one was present, in the corresponding Set-Cookie2 response
header.  That is, the port attribute MUST be present if the Port
attribute was present in the Set-Cookie2 header, and it MUST have the
same value, if any.  Otherwise, if the Port attribute was absent from
the Set-Cookie2 header, the attribute likewise MUST be omitted from the
Cookie request header.

Note that there is neither a Comment nor a CommentURL attribute in the
Cookie request header corresponding to the ones in the Set-Cookie2
response header.  The user agent does not return the comment information
to the origin server.

The user agent applies the following rules to choose applicable cookie-
values to send in Cookie request headers from among all the cookies it
has received.

Domain Selection
     The origin server's effective host name MUST domain-match the
     Domain attribute of the cookie.

Port Selection
     There are three possible behaviors, depending on the Port attribute
     in the Set-Cookie2 response header:




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       1.  By default (no Port attribute), the cookie MAY be sent to any
           port.

       2.  If the attribute is present but has no value (e.g., Port),
           the cookie MUST only be sent to the request-port it was
           received from.

       3.  If the attribute has a port-list, the cookie MUST only be
           returned if the new request-port is one of those listed in
           port-list.

Path Selection
     The request-URI MUST path-match the Path attribute of the cookie.

Max-Age Selection
     Cookies that have expired should have been discarded and thus are
     not forwarded to an origin server.

If multiple cookies satisfy the criteria above, they are ordered in the
Cookie header such that those with more specific Path attributes precede
those with less specific.  Ordering with respect to other attributes
(e.g., Domain) is unspecified.

Note: For backward compatibility, the separator in the Cookie header is
semi-colon (;) everywhere.  A server SHOULD also accept comma (,) as the
separator between cookie-values for future compatibility.

4.3.5  Identifying What Version is Understood:  Cookie2  The Cookie2
request header facilitates interoperation between clients and servers
that understand different versions of the cookie specification.  When
the client sends one or more cookies to an origin server, if at least
one of those cookies contains a $Version attribute whose value is
different from the version that the client understands, then the client
MUST also send a Cookie2 request header, the syntax for which is

cookie2 =       "Cookie2:" cookie-version

Here the value for cookie-version is the highest version of cookie
specification (currently 1) that the client understands.  The client
needs to send at most one such request header per request.

4.3.6  Sending Cookies in Unverifiable Transactions  Users MUST have
control over sessions in order to ensure privacy.  (See PRIVACY section
below.)  To simplify implementation and to prevent an additional layer
of complexity where adequate safeguards exist, however, this document
distinguishes between transactions that are verifiable and those that
are unverifiable.  A transaction is verifiable if the user, or a user-
designated agent, has the option to review the request-URI prior to its
use in the transaction.  A transaction is unverifiable if the user does
not have that option.  Unverifiable transactions typically arise when a
user agent automatically requests inlined or embedded entities or when



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it resolves redirection (3xx) responses from an origin server.
Typically the origin transaction, the transaction that the user
initiates, is verifiable, and that transaction may directly or
indirectly induce the user agent to make unverifiable transactions.

An unverifiable transaction is to a third-party host if its request-host
U does not domain-match the reach R of the request-host O in the origin
transaction.

When it makes an unverifiable transaction, a user agent MUST disable all
cookie processing (i.e., MUST NOT send cookies, and MUST NOT accept any
received cookies) if the transaction is to a third-party host.

This restriction prevents a malicious service author from using
unverifiable transactions to induce a user agent to start or continue a
session with a server in a different domain.  The starting or
continuation of such sessions could be contrary to the privacy
expectations of the user, and could also be a security problem.

User agents MAY offer configurable options that allow the user agent, or
any autonomous programs that the user agent executes, to ignore the
above rule, so long as these override options default to ``off.''

(N.B.  Mechanisms may be proposed that will automate overriding the
third-party restrictions under controlled conditions.)

Many current user agents already provide a review option that would
render many links verifiable.  For instance, some user agents display
the URL that would be referenced for a particular link when the mouse
pointer is placed over that link.  The user can therefore determine
whether to visit that site before causing the browser to do so.  (Though
not implemented on current user agents, a similar technique could be
used for a button used to submit a form -- the user agent could display
the action to be taken if the user were to select that button.)
However, even this would not make all links verifiable; for example,
links to automatically loaded images would not normally be subject to
``mouse pointer'' verification.

Many user agents also provide the option for a user to view the HTML
source of a document, or to save the source to an external file where it
can be viewed by another application.  While such an option does provide
a crude review mechanism, some users might not consider it acceptable
for this purpose.

4.4  How an Origin Server Interprets the Cookie Header

A user agent returns much of the information in the Set-Cookie2 header
to the origin server when the request-URI path-matches the Path
attribute of the cookie.  When it receives a Cookie header, the origin
server SHOULD treat cookies with NAMEs whose prefix is $ specially, as
an attribute for the cookie.



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4.5  Caching Proxy Role

One reason for separating state information from both a URL and document
content is to facilitate the scaling that caching permits.  To support
cookies, a caching proxy MUST obey these rules already in the HTTP
specification:

   * Honor requests from the cache, if possible, based on cache validity
     rules.

   * Pass along a Cookie request header in any request that the proxy
     must make of another server.

   * Return the response to the client.  Include any Set-Cookie2
     response header.

   * Cache the received response subject to the control of the usual
     headers, such as Expires,

     Cache-control: no-cache

     and

     Cache-control: private

   * Cache the Set-Cookie2 subject to the control of the usual header,

     Cache-control: no-cache="set-cookie2"

     (The Set-Cookie2 header should usually not be cached.)

Proxies MUST not introduce Set-Cookie2 (Cookie) headers of their own in
proxy responses (requests).


5.  EXAMPLES

5.1  Example 1

Most detail of request and response headers has been omitted.  Assume
the user agent has no stored cookies.

  1.  User Agent -> Server

      POST /acme/login HTTP/1.1
      [form data]

      User identifies self via a form.

  2.  Server -> User Agent




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      HTTP/1.1 200 OK
      Set-Cookie2: Customer="WILE_E_COYOTE"; Version="1"; Path="/acme"

      Cookie reflects user's identity.

  3.  User Agent -> Server

      POST /acme/pickitem HTTP/1.1
      Cookie: $Version="1"; Customer="WILE_E_COYOTE"; $Path="/acme"
      [form data]

      User selects an item for ``shopping basket.''

  4.  Server -> User Agent

      HTTP/1.1 200 OK
      Set-Cookie2: Part_Number="Rocket_Launcher_0001"; Version="1";
              Path="/acme"

      Shopping basket contains an item.

  5.  User Agent -> Server

      POST /acme/shipping HTTP/1.1
      Cookie: $Version="1";
              Customer="WILE_E_COYOTE"; $Path="/acme";
              Part_Number="Rocket_Launcher_0001"; $Path="/acme"
      [form data]

      User selects shipping method from form.

  6.  Server -> User Agent

      HTTP/1.1 200 OK
      Set-Cookie2: Shipping="FedEx"; Version="1"; Path="/acme"

      New cookie reflects shipping method.

  7.  User Agent -> Server

      POST /acme/process HTTP/1.1
      Cookie: $Version="1";
              Customer="WILE_E_COYOTE"; $Path="/acme";
              Part_Number="Rocket_Launcher_0001"; $Path="/acme";
              Shipping="FedEx"; $Path="/acme"
      [form data]

      User chooses to process order.

  8.  Server -> User Agent




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      HTTP/1.1 200 OK

      Transaction is complete.

The user agent makes a series of requests on the origin server, after
each of which it receives a new cookie.  All the cookies have the same
Path attribute and (default) domain.  Because the request-URIs all
path-match /acme, the Path attribute of each cookie, each request
contains all the cookies received so far.

5.2  Example 2

This example illustrates the effect of the Path attribute.  All detail
of request and response headers has been omitted.  Assume the user agent
has no stored cookies.

Imagine the user agent has received, in response to earlier requests,
the response headers

Set-Cookie2: Part_Number="Rocket_Launcher_0001"; Version="1";
        Path="/acme"

and

Set-Cookie2: Part_Number="Riding_Rocket_0023"; Version="1";
        Path="/acme/ammo"

A subsequent request by the user agent to the (same) server for URLs of
the form /acme/ammo/...  would include the following request header:

Cookie: $Version="1";
        Part_Number="Riding_Rocket_0023"; $Path="/acme/ammo";
        Part_Number="Rocket_Launcher_0001"; $Path="/acme"

Note that the NAME=VALUE pair for the cookie with the more specific Path
attribute, /acme/ammo, comes before the one with the less specific Path
attribute, /acme.  Further note that the same cookie name appears more
than once.

A subsequent request by the user agent to the (same) server for a URL of
the form /acme/parts/ would include the following request header:

Cookie: $Version="1"; Part_Number="Rocket_Launcher_0001"; $Path="/acme"

Here, the second cookie's Path attribute /acme/ammo is not a prefix of
the request URL, /acme/parts/, so the cookie does not get forwarded to
the server.







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6.  IMPLEMENTATION CONSIDERATIONS

Here we provide guidance on likely or desirable details for an origin
server that implements state management.

6.1  Set-Cookie2 Content

An origin server's content should probably be divided into disjoint
application areas, some of which require the use of state information.
The application areas can be distinguished by their request URLs.  The
Set-Cookie2 header can incorporate information about the application
areas by setting the Path attribute for each one.

The session information can obviously be clear or encoded text that
describes state.  However, if it grows too large, it can become
unwieldy.  Therefore, an implementor might choose for the session
information to be a key to a server-side resource.  Of course, using a
database creates some problems that this state management specification
was meant to avoid, namely:

  1.  keeping real state on the server side;

  2.  how and when to garbage-collect the database entry, in case the
      user agent terminates the session by, for example, exiting.

6.2  Stateless Pages

Caching benefits the scalability of WWW.  Therefore it is important to
reduce the number of documents that have state embedded in them
inherently.  For example, if a shopping-basket-style application always
displays a user's current basket contents on each page, those pages
cannot be cached, because each user's basket's contents would be
different.  On the other hand, if each page contains just a link that
allows the user to ``Look at My Shopping Basket,'' the page can be
cached.

6.3  Implementation Limits

Practical user agent implementations have limits on the number and size
of cookies that they can store.  In general, user agents' cookie support
should have no fixed limits.  They should strive to store as many
frequently-used cookies as possible.  Furthermore, general-use user
agents SHOULD provide each of the following minimum capabilities
individually, although not necessarily simultaneously:

   * at least 300 cookies

   * at least 4096 bytes per cookie (as measured by the characters that
     comprise the cookie non-terminal in the syntax description of the
     Set-Cookie2 header, and as received in the Set-Cookie2 header)




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   * at least 20 cookies per unique host or domain name

User agents created for specific purposes or for limited-capacity
devices SHOULD provide at least 20 cookies of 4096 bytes, to ensure that
the user can interact with a session-based origin server.

The information in a Set-Cookie2 response header MUST be retained in its
entirety.  If for some reason there is inadequate space to store the
cookie, it MUST be discarded, not truncated.

Applications should use as few and as small cookies as possible, and
they should cope gracefully with the loss of a cookie.

6.3.1  Denial of Service Attacks  User agents MAY choose to set an upper
bound on the number of cookies to be stored from a given host or domain
name or on the size of the cookie information.  Otherwise a malicious
server could attempt to flood a user agent with many cookies, or large
cookies, on successive responses, which would force out cookies the user
agent had received from other servers.  However, the minima specified
above SHOULD still be supported.


7.  PRIVACY

Informed consent should guide the design of systems that use cookies.  A
user should be able to find out how a web site plans to use information
in a cookie and should be able to choose whether or not those policies
are acceptable.  Both the user agent and the origin server must assist
informed consent.

7.1  User Agent Control

An origin server could create a Set-Cookie2 header to track the path of
a user through the server.  Users may object to this behavior as an
intrusive accumulation of information, even if their identity is not
evident.  (Identity might become evident, for example, if a user
subsequently fills out a form that contains identifying information.)
This state management specification therefore requires that a user agent
give the user control over such a possible intrusion, although the
interface through which the user is given this control is left
unspecified.  However, the control mechanisms provided SHALL at least
allow the user

   * to completely disable the sending and saving of cookies.

   * to determine whether a stateful session is in progress.

   * to control the saving of a cookie on the basis of the cookie's
     Domain attribute.





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Such control could be provided, for example, by mechanisms

   * to notify the user when the user agent is about to send a cookie to
     the origin server, to offer the option not to begin a session.

   * to display a visual indication that a stateful session is in
     progress.

   * to let the user decide which cookies, if any, should be saved when
     the user concludes a window or user agent session.

   * to let the user examine and delete the contents of a cookie at any
     time.

A user agent usually begins execution with no remembered state
information.  It SHOULD be possible to configure a user agent never to
send Cookie headers, in which case it can never sustain state with an
origin server.  (The user agent would then behave like one that is
unaware of how to handle Set-Cookie2 response headers.)

When the user agent terminates execution, it SHOULD let the user discard
all state information.  Alternatively, the user agent MAY ask the user
whether state information should be retained; the default should be
``no.''  If the user chooses to retain state information, it would be
restored the next time the user agent runs.

NOTE: User agents should probably be cautious about using files to store
cookies long-term.  If a user runs more than one instance of the user
agent, the cookies could be commingled or otherwise corrupted.

7.2  Origin Server Role

A origin server SHOULD promote informed consent by adding CommentURL or
Comment information to the cookies it sends.  CommentURL is preferred
because of the opportunity to provide richer information in a
multiplicity of languages.

7.3  Clear Text

The information in the Set-Cookie2 and Cookie headers is unprotected.
As a consequence:

  1.  Any sensitive information that is conveyed in them is exposed to
      intruders.

  2.  A malicious intermediary could alter the headers as they travel in
      either direction, with unpredictable results.

These facts imply that information of a personal and/or financial nature
should only be sent over a secure channel.  For less sensitive
information, or when the content of the header is a database key, an



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origin server should be vigilant to prevent a bad Cookie value from
causing failures.

A user agent in a shared user environment poses a further risk.  Using a
cookie inspection interface, User B could examine the contents of
cookies that were saved when User A used the machine.


8.  SECURITY CONSIDERATIONS

8.1  Protocol Design

The restrictions on the value of the Domain attribute, and the rules
concerning unverifiable transactions, are meant to reduce the ways that
cookies can ``leak'' to the ``wrong'' site.  The intent is to restrict
cookies to one host, or a closely related set of hosts.  Therefore a
request-host is limited as to what values it can set for Domain.  We
consider it acceptable for hosts host1.foo.com and host2.foo.com to
share cookies, but not a.com and b.com.

Similarly, a server can set a Path only for cookies that are related to
the request-URI.

8.2  Cookie Spoofing

Proper application design can avoid spoofing attacks from related
domains.  Consider:

  1.  User agent makes request to victim.cracker.edu, gets back cookie
      session_id="1234" and sets the default domain victim.cracker.edu.

  2.  User agent makes request to spoof.cracker.edu, gets back cookie
      session-id="1111", with Domain=".cracker.edu".

  3.  User agent makes request to victim.cracker.edu again, and passes

      Cookie: $Version="1"; session_id="1234",
              $Version="1"; session_id="1111"; $Domain=".cracker.edu"

      The server at victim.cracker.edu should detect that the second
      cookie was not one it originated by noticing that the Domain
      attribute is not for itself and ignore it.

8.3  Unexpected Cookie Sharing

A user agent SHOULD make every attempt to prevent the sharing of session
information between hosts that are in different domains.  Embedded or
inlined objects may cause particularly severe privacy problems if they
can be used to share cookies between disparate hosts.  For example, a
malicious server could embed cookie information for host a.com in a URI
for a CGI on host b.com.  User agent implementors are strongly



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encouraged to prevent this sort of exchange whenever possible.

8.4  Cookies For Account Information

While it is common practice to use them this way, cookies are not
designed or intended to be used to hold authentication information, such
as account names and passwords.  Unless such cookies are exchanged over
an encrypted path, the account information they contain is highly
vulnerable to perusal and theft.


9.  OTHER, SIMILAR, PROPOSALS

Apart from RFC 2109, three other proposals have been made to accomplish
similar goals.  This specification began as an amalgam of Kristol's
State-Info proposal [DMK95] and Netscape's Cookie proposal [Netscape].

Brian Behlendorf proposed a Session-ID header that would be user-agent-
initiated and could be used by an origin server to track
``clicktrails.''  It would not carry any origin-server-defined state,
however.  Phillip Hallam-Baker has proposed another client-defined
session ID mechanism for similar purposes.

While both session IDs and cookies can provide a way to sustain stateful
sessions, their intended purpose is different, and, consequently, the
privacy requirements for them are different.  A user initiates session
IDs to allow servers to track progress through them, or to distinguish
multiple users on a shared machine.  Cookies are server-initiated, so
the cookie mechanism described here gives users control over something
that would otherwise take place without the users' awareness.
Furthermore, cookies convey rich, server-selected information, whereas
session IDs comprise user-selected, simple information.


10.  HISTORICAL

10.1  Compatibility with Existing Implementations

Existing cookie implementations, based on the Netscape specification,
use the Set-Cookie (not Set-Cookie2) header.  User agents that receive
in the same response both a Set-Cookie and Set-Cookie2 response header
for the same cookie MUST discard the Set-Cookie information and use only
the Set-Cookie2 information.  Furthermore, a user agent MUST assume, if
it received a Set-Cookie2 response header, that the sending server
complies with this document and will understand Cookie request headers
that also follow this specification.

New cookies MUST replace both equivalent old- and new-style cookies.
That is, if a user agent that follows both this specification and
Netscape's original specification receives a Set-Cookie2 response
header, and the NAME and the Domain and Path attributes match (per the



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Cookie Management section) a Netscape-style cookie, the Netscape-style
cookie MUST be discarded, and the user agent MUST retain only the cookie
adhering to this specification.

Older user agents that do not understand this specification, but that do
understand Netscape's original specification, will not recognize the
Set-Cookie2 response header and will receive and send cookies according
to the older specification.

A user agent that supports both this specification and Netscape-style
cookies SHOULD send a Cookie request header that follows the older
Netscape specification if it received the cookie in a Set-Cookie
response header and not in a Set-Cookie2 response header.  However, it
SHOULD send the following request header as well:

        Cookie2: $Version="1"

The Cookie2 header advises the server that the user agent understands
new-style cookies.  If the server understands new-style cookies, as
well, it SHOULD continue the stateful session by sending a Set-Cookie2
response header, rather than Set-Cookie.  A server that does not
understand new-style cookies will simply ignore the Cookie2 request
header.

10.2  Caching and HTTP/1.0

Some caches, such as those conforming to HTTP/1.0, will inevitably cache
the Set-Cookie2 and Set-Cookie headers, because there was no mechanism
to suppress caching of headers prior to HTTP/1.1.  This caching can lead
to security problems.  Documents transmitted by an origin server along
with Set-Cookie2 and Set-Cookie headers usually either will be
uncachable, or will be ``pre-expired.''  As long as caches obey
instructions not to cache documents (following Expires: <a date in the
past> or Pragma: no-cache (HTTP/1.0), or Cache-control: no-cache
(HTTP/1.1)) uncachable documents present no problem.  However, pre-
expired documents may be stored in caches.  They require validation (a
conditional GET) on each new request, but some cache operators loosen
the rules for their caches, and sometimes serve expired documents
without first validating them.  This combination of factors can lead to
cookies meant for one user later being sent to another user.  The Set-
Cookie2 and Set-Cookie headers are stored in the cache, and, although
the document is stale (expired), the cache returns the document in
response to later requests, including cached headers.


11.  ACKNOWLEDGEMENTS

This document really represents the collective efforts of the HTTP
Working Group of the IETF and, particularly, the following people, in
addition to the authors: Roy Fielding, Yaron Goland, Marc Hedlund, Ted
Hardie, Koen Holtman, Shel Kaphan, Rohit Khare, Foteos Macrides, David



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W. Morris.


12.  AUTHORS' ADDRESSES

David M. Kristol
Bell Laboratories, Lucent Technologies
600 Mountain Ave.  Room 2A-333
Murray Hill, NJ  07974

Phone: (908) 582-2250
FAX: (908) 582-1239
Email: dmk@bell-labs.com

Lou Montulli
Epinions.com, Inc.
2037 Landings Dr.
Mountain View, CA  94301
Email: lou@montulli.org


13.  REFERENCES

[DMK95]Kristol, D.M., ``Proposed HTTP State-Info Mechanism'', available
       at <http://portal.research.bell-labs.com/~dmk/state-info.html>,
       September, 1995.

[Netscape] ``Persistent Client State -- HTTP Cookies'', available at
       <http://www.netscape.com/newsref/std/cookie_spec.html>, undated.

[RFC2109] Kristol, D.M., Montulli, L., ``HTTP State Management
       Mechanism'', RFC 2109, February, 1997.

[RFC2119] Bradner, S., ``Key words for use in RFCs to Indicate
       Requirement Levels'', RFC 2119, March 1997.

[RFC2279] Yergeau, F., ``UTF-8, a transformation format of Unicode and
       ISO-10646'', RFC 2279, January 1998.

[RFC2396] Berners-Lee, T., Fielding, R., Masinter, L., ``Uniform
       Resource Identifiers (URI): Generic Syntax'', RFC 2396, August,
       1998.

[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Berners-Lee,
       T., ``Hypertext Transfer Protocol -- HTTP/1.1'', RFC 2616, June,
       1999.



                                               Expires February 27, 2000




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