    Network Working Group                                         D. Kristol
    Request for Comments: 2109        Bell Laboratories, Lucent Technologies
    Category: Standards Track                                    L. Montulli
                                                     Netscape Communications
                                                               February 1997

                        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.

    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-Cookie, which carry state information between participating
       origin servers and user agents.  The method described here differs
       from Netscape's Cookie proposal, but it can interoperate with
       HTTP/1.0 user agents that use Netscape's method.  (See the HISTORICAL
       section.)

    2.  TERMINOLOGY

       The terms user agent, client, server, proxy, and origin server have
       the same meaning as in the HTTP/1.0 specification.

       Fully-qualified host name (FQHN) means either the fully-qualified
       domain name (FQDN) of a host (i.e., a completely specified domain
       name ending in a top-level domain such as .com or .uk), 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 must be a FQHN.

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       Hosts names can be specified either as an IP address or a FQHN
       string.  Sometimes we compare one host name with another.  Host A's
       name domain-matches host B's if

       * both host names are IP addresses and their host name strings match
         exactly; or

       * both host names are FQDN strings and their host name strings match
         exactly; or

       * A is a FQDN string and has the form NB, where N is a non-empty name
         string, B has the form .B', and B' is a FQDN 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.

       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.

    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 technique 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.

       There are, of course, many different potential contexts and thus many
       different potential types of session.  The designers' paradigm for
       sessions created by the exchange of cookies has these key attributes:

          1.  Each session has a beginning and an end.

          2.  Each session is relatively short-lived.

          3.  Either the user agent or the origin server may terminate a
              session.

          4.  The session is implicit in the exchange of state information.

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    4.  OUTLINE

       We outline 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.  Only origin servers that need to
       maintain sessions would suffer any significant impact, and that
       impact can largely be confined to Common Gateway Interface (CGI)
       programs, unless the server provides more sophisticated state
       management support.  (See Implementation Considerations, below.)

    4.1  Syntax:  General

       The two state management headers, Set-Cookie and Cookie, have common
       syntactic properties involving attribute-value pairs.  The following
       grammar uses the notation, and tokens DIGIT (decimal digits) and
       token (informally, a sequence of non-special, non-white space
       characters) from the HTTP/1.1 specification [RFC 2068] to describe
       their syntax.

       av-pairs        =       av-pair *(";" av-pair)
       av-pair         =       attr ["=" value]        ; optional value
       attr            =       token
       value           =       word
       word            =       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.  (Note that
       "session" here does not refer to a persistent network connection but
       to a logical session created from HTTP requests and responses.  The
       presence or absence of a persistent connection should have no effect
       on the use of cookie-derived sessions).  To initiate a session, the
       origin server returns an extra response header to the client, Set-
       Cookie.  (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

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       session.  It may send back to the client a Set-Cookie response header
       with the same or different information, or it may send no Set-Cookie
       header at all.  The origin server effectively ends a session by
       sending the client a Set-Cookie header with Max-Age=0.

       Servers may return a Set-Cookie 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-Cookie headers in a
       response.  Note that an intervening gateway could fold multiple such
       headers into a single header.

    4.2.2  Set-Cookie Syntax

       The syntax for the Set-Cookie response header is

       set-cookie      =       "Set-Cookie:" cookies
       cookies         =       1#cookie
       cookie          =       NAME "=" VALUE *(";" cookie-av)
       NAME            =       attr
       VALUE           =       value
       cookie-av       =       "Comment" "=" value
                       |       "Domain" "=" value
                       |       "Max-Age" "=" value
                       |       "Path" "=" value
                       |       "Secure"
                       |       "Version" "=" 1*DIGIT

       Informally, the Set-Cookie response header comprises the token Set-
       Cookie:, 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 behavior is undefined.

       NAME=VALUE
          Required.  The name of the state information ("cookie") is NAME,
          and its value is VALUE.  NAMEs that begin with $ are reserved for
          other uses and must not be used by applications.

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          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-Cookie
          header.

       Comment=comment
          Optional.  Because cookies can contain private information about a
          user, the Cookie attribute allows an origin server to document its
          intended use of a cookie.  The user can inspect the information to
          decide whether to initiate or continue a session with this cookie.

       Domain=domain
          Optional.  The Domain attribute specifies the domain for which the
          cookie is valid.  An explicitly specified domain must always start
          with a dot.

       Max-Age=delta-seconds
          Optional.  The Max-Age attribute defines the lifetime of the
          cookie, in seconds.  The delta-seconds value is a decimal non-
          negative integer.  After delta-seconds seconds elapse, the client
          should discard the cookie.  A value of zero means the cookie
          should be discarded immediately.

       Path=path
          Optional.  The Path attribute specifies the subset of URLs to
          which this cookie applies.

       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.

          The user agent (possibly under the user's control) 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.

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

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    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-Cookie 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-
       Cookie 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-cookie
       header should not be cached.  A Set-cookie 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-Cookie header: Cache-control: no-
         cache="set-cookie".

       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.

       * 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.

       * 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-Cookie response
       headers unless they know for certain (by out of band means) that
       there are no downsteam HTTP/1.0 proxies.  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.

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    4.3  User Agent Role

    4.3.1  Interpreting Set-Cookie

       The user agent keeps separate track of state information that arrives
       via Set-Cookie response headers from each origin server (as
       distinguished by name or IP address and port).  The user agent
       applies these defaults for optional attributes that are missing:

       VersionDefaults to "old cookie" behavior as originally specified by
              Netscape.  See the HISTORICAL section.

       Domain Defaults to the request-host.  (Note that there is no dot at
              the beginning of request-host.)

       Max-AgeThe 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-Cookie response, up to, but not including, the
              right-most /.

       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 (shall not store its information) if any of the
       following is true:

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

       * The value for the Domain attribute contains no embedded dots or
         does not start with a dot.

       * The value for the request-host does not domain-match the Domain
         attribute.

       * The request-host is a FQDN (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.

       Examples:

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

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

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

       * A Set-Cookie with Domain=ajax.com will be rejected because the
         value for Domain does not begin with a dot.

    4.3.3  Cookie Management

       If a user agent receives a Set-Cookie response header whose NAME is
       the same as a pre-existing cookie, and whose Domain and Path
       attribute values exactly (string) match those of a pre-existing
       cookie, the new cookie supersedes the old.  However, if the Set-
       Cookie has a value for Max-Age of zero, the (old and new) cookie is
       discarded.  Otherwise cookies accumulate until they expire (resources
       permitting), at which time they are discarded.

       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-Cookie 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.

       User agents should allow the user to control cookie destruction.  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 sends a
       Cookie request header to the origin server if it has cookies that are
       applicable to the request, based on

       * the request-host;

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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]
       cookie-version  =       "$Version" "=" value
       NAME            =       attr
       VALUE           =       value
       path            =       "$Path" "=" value
       domain          =       "$Domain" "=" value

       The value of the cookie-version attribute must be the value from the
       Version attribute, if any, of the corresponding Set-Cookie 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 any,
       of the corresponding Set-Cookie 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 any, of the corresponding Set-Cookie response header.
       Otherwise the attribute should be omitted from the Cookie request
       header.

       Note that there is no Comment attribute in the Cookie request header
       corresponding to the one in the Set-Cookie response header.  The user
       agent does not return the comment information to the origin server.

       The following rules apply to choosing applicable cookie-values from
       among all the cookies the user agent has.

       Domain Selection
            The origin server's fully-qualified host name must domain-match
            the Domain attribute of the cookie.

       Path Selection
            The Path attribute of the cookie must match a prefix of the
            request-URI.

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

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       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  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 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.

       When it makes an unverifiable transaction, a user agent must enable a
       session only if a cookie with a domain attribute D was sent or
       received in its origin transaction, such that the host name in the
       Request-URI of the unverifiable transaction domain-matches D.

       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".

       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

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       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-Cookie header
       to the origin server when the Path attribute matches that of a new
       request.  When it receives a Cookie header, the origin server should
       treat cookies with NAMEs whose prefix is $ specially, as an attribute
       for the adjacent cookie.  The value for such a NAME is to be
       interpreted as applying to the lexically (left-to-right) most recent
       cookie whose name does not have the $ prefix.  If there is no
       previous cookie, the value applies to the cookie mechanism as a
       whole.  For example, consider the cookie

       Cookie: $Version="1"; Customer="WILE_E_COYOTE";
               $Path="/acme"

       $Version applies to the cookie mechanism as a whole (and gives the
       version number for the cookie mechanism).  $Path is an attribute
       whose value (/acme) defines the Path attribute that was used when the
       Customer cookie was defined in a Set-Cookie response header.

    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-Cookie response
         header.

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       * 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-Cookie subject to the control of the usual header,
         Cache-control: no-cache="set-cookie".  (The Set-Cookie header
         should usually not be cached.)

       Proxies must not introduce Set-Cookie (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

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

             Shopping basket contains an item.

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         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-Cookie: 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

             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 URLs
       all have /acme as a prefix, and that matches the Path attribute, 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

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

       and

       Set-Cookie: 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.

    6.  IMPLEMENTATION CONSIDERATIONS

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

    6.1  Set-Cookie 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-Cookie 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

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       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 size of the
            characters that comprise the cookie non-terminal in the syntax
            description of the Set-Cookie 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-Cookie 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.

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    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

    7.1  User Agent Control

       An origin server could create a Set-Cookie 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 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 by, for example, mechanisms

          * to notify the user when the user agent is about to send a cookie
            to the origin server, offering 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 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

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       an origin server.  (The user agent would then behave like one that is
       unaware of how to handle Set-Cookie 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 messed
       up.

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

    8.  SECURITY CONSIDERATIONS

    8.1  Clear Text

       The information in the Set-Cookie and Cookie headers is unprotected.
       Two consequences are:

       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.

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    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";
                             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 encouraged to prevent this sort of
       exchange whenever possible.

    9.  OTHER, SIMILAR, PROPOSALS

       Three other proposals have been made to accomplish similar goals.
       This specification is an amalgam of Kristol's State-Info proposal and
       Netscape's Cookie proposal.

       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.

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       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 Netscape's Implementation

       HTTP/1.0 clients and servers may use Set-Cookie and Cookie headers
       that reflect Netscape's original cookie proposal.  These notes cover
       inter-operation between "old" and "new" cookies.

    10.1.1  Extended Cookie Header

       This proposal adds attribute-value pairs to the Cookie request header
       in a compatible way.  An "old" client that receives a "new" cookie
       will ignore attributes it does not understand; it returns what it
       does understand to the origin server.  A "new" client always sends
       cookies in the new form.

       An "old" server that receives a "new" cookie will see what it thinks
       are many cookies with names that begin with a $, and it will ignore
       them.  (The "old" server expects these cookies to be separated by
       semi-colon, not comma.)  A "new" server can detect cookies that have
       passed through an "old" client, because they lack a $Version
       attribute.

    10.1.2  Expires and Max-Age

       Netscape's original proposal defined an Expires header that took a
       date value in a fixed-length variant format in place of Max-Age:

       Wdy, DD-Mon-YY HH:MM:SS GMT

       Note that the Expires date format contains embedded spaces, and that
       "old" cookies did not have quotes around values.  Clients that
       implement to this specification should be aware of "old" cookies and
       Expires.

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    10.1.3  Punctuation

       In Netscape's original proposal, the values in attribute-value pairs
       did not accept "-quoted strings.  Origin servers should be cautious
       about sending values that require quotes unless they know the
       receiving user agent understands them (i.e., "new" cookies).  A
       ("new") user agent should only use quotes around values in Cookie
       headers when the cookie's version(s) is (are) all compliant with this
       specification or later.

       In Netscape's original proposal, no whitespace was permitted around
       the = that separates attribute-value pairs.  Therefore such
       whitespace should be used with caution in new implementations.

    10.2  Caching and HTTP/1.0

       Some caches, such as those conforming to HTTP/1.0, will inevitably
       cache the Set-Cookie header, 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-Cookie headers will usually either be uncachable, or
       will be "pre-expired".  As long as caches obey instructions not to
       cache documents (following Expires:  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-Cookie header is 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
       following people, in addition to the authors: Roy Fielding, Marc
       Hedlund, Ted Hardie, Koen Holtman, Shel Kaphan, Rohit Khare.

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    12.  AUTHORS' ADDRESSES

       David M. Kristol
       Bell Laboratories, Lucent Technologies
       600 Mountain Ave.  Room 2A-227
       Murray Hill, NJ  07974

       Phone: (908) 582-2250
       Fax: (908) 582-5809
       EMail: dmk@bell-labs.com

       Lou Montulli
       Netscape Communications Corp.
       501 E. Middlefield Rd.
       Mountain View, CA  94043

       Phone: (415) 528-2600
       EMail: montulli@netscape.com

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