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Obsoleted by: 6265 HISTORIC
Errata Exist
Network Working Group                                         D. Kristol
Request for Comments: 2965        Bell Laboratories, Lucent Technologies
Obsoletes: 2109                                              L. Montulli
Category: Standards Track                             Epinions.com, Inc.
                                                            October 2000


                    HTTP State Management Mechanism

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

IESG Note

   The IESG notes that this mechanism makes use of the .local top-level
   domain (TLD) internally when handling host names that don't contain
   any dots, and that this mechanism might not work in the expected way
   should an actual .local TLD ever be registered.

Abstract

   This document specifies a way to create a stateful session with
   Hypertext Transfer Protocol (HTTP) requests and responses.  It
   describes three new headers, Cookie, Cookie2, 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 and
   obsoletes it.

1.  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].




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   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

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




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      *  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.

1.1  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].

2.  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.

3.  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.

3.1  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





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   (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.

3.2  Origin Server Role

   3.2.1  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.














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

   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.



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   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 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.








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   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.

   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.

   3.2.3  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



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      *  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.

   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.

3.3  User Agent Role

   3.3.1  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 /.



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   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.

   3.3.2  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.

      *  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.




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      *  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.

   3.3.3  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.

   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;





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      *  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.

   3.3.4  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;

      * 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



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   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:

      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.





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   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.

   3.3.5  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.

   3.3.6  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 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.




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   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.

3.4  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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3.5  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).

4.  EXAMPLES

4.1  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.



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      2. Server -> User Agent

        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]



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        User chooses to process order.

      8. Server -> User Agent

        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.

4.2  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:





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   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.

5.  IMPLEMENTATION CONSIDERATIONS

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

5.1  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.

5.2  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.








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5.3  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)

      *  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.

   5.3.1  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.

6.  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.







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6.1  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.

   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.





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   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.

6.2  Origin Server Role

   An 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.

6.3  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
   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.

7.  SECURITY CONSIDERATIONS

7.1  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.




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7.2  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.

7.3  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 encouraged to prevent this sort of
   exchange whenever possible.

7.4  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.

8.  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].




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   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.

9.  HISTORICAL

9.1  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 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.








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   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.

9.2  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.

10.  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 W. Morris.








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11.  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

12.  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. and L. Montulli, "HTTP State Management
              Mechanism", RFC 2109, February 1997.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, 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. and L. Masinter, "Uniform
              Resource Identifiers (URI): Generic Syntax", RFC 2396,
              August 1998.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H. and T.
              Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1",
              RFC 2616, June 1999.






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13.  Full Copyright Statement

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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