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: <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-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: [email protected]


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

  Phone: (415) 528-2600
  EMail: [email protected]
































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