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Network Working Group D. Kristol Request for Comments: 2965 Bell Laboratories, Lucent Technologies Obsoletes: 2109 L. Montulli Category: Standards Track, 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.


 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.


 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.


 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].

Kristol & Montulli Standards Track [Page 1] RFC 2965 HTTP State Management Mechanism October 2000

 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
 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, domain-matches but not
 Note that domain-match is not a commutative operation:
 domain-matches, but not the reverse.
 The reach R of a host name H is defined as follows:
  • If
  1. H is the host domain name of a host; and,
  1. H has the form A.B; and
  1. A has no embedded (that is, interior) dots; and
  1. B has at least one embedded dot, or B is the string "local".

then the reach of H is .B.

Kristol & Montulli Standards Track [Page 2] RFC 2965 HTTP State Management Mechanism October 2000

  • 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",
 document are to be interpreted as described in RFC 2119 [RFC2119].


 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.


 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

Kristol & Montulli Standards Track [Page 3] RFC 2965 HTTP State Management Mechanism October 2000

 (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.

Kristol & Montulli Standards Track [Page 4] RFC 2965 HTTP State Management Mechanism October 2000

 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
 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.
    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

Kristol & Montulli Standards Track [Page 5] RFC 2965 HTTP State Management Mechanism October 2000

    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]
    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.
    OPTIONAL.  The Discard attribute instructs the user agent to
    discard the cookie unconditionally when the user agent terminates.
    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.
    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
    OPTIONAL.  The value of the Path attribute specifies the subset of
    URLs on the origin server to which this cookie applies.
    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.

Kristol & Montulli Standards Track [Page 6] RFC 2965 HTTP State Management Mechanism October 2000

    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.
    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
 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

Kristol & Montulli Standards Track [Page 7] RFC 2965 HTTP State Management Mechanism October 2000

  • 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 /.

Kristol & Montulli Standards Track [Page 8] RFC 2965 HTTP State Management Mechanism October 2000

 Port    The default behavior is that a cookie MAY be returned to any
 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


  • 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.

  • A Set-Cookie2 from request-host for

would be rejected, because H is y.x and contains a dot.

  • A Set-Cookie2 from request-host for

would be accepted.

  • A Set-Cookie2 with or, will always be

rejected, because there is no embedded dot.

  • A Set-Cookie2 with will be accepted, and the

value for Domain will be taken to be, because a dot

       gets prepended to the value.

Kristol & Montulli Standards Track [Page 9] RFC 2965 HTTP State Management Mechanism October 2000

  • A Set-Cookie2 with Port="80,8000" will be accepted if the

request was made to port 80 or 8000 and will be rejected

  • 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


  • 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;

Kristol & Montulli Standards Track [Page 10] RFC 2965 HTTP State Management Mechanism October 2000

  • 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
 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*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
Kristol & Montulli Standards Track [Page 11] RFC 2965 HTTP State Management Mechanism October 2000
 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
    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
    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
 Path Selection
    The request-URI MUST path-match the Path attribute of the cookie.
Kristol & Montulli Standards Track [Page 12] RFC 2965 HTTP State Management Mechanism October 2000
 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
 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.
Kristol & Montulli Standards Track [Page 13] RFC 2965 HTTP State Management Mechanism October 2000
 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
 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.
Kristol & Montulli Standards Track [Page 14] RFC 2965 HTTP State Management Mechanism October 2000 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
       Cache-control: private
  • Cache the Set-Cookie2 subject to the control of the usual
       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.
Kristol & Montulli Standards Track [Page 15] RFC 2965 HTTP State Management Mechanism October 2000
    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";
      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]
Kristol & Montulli Standards Track [Page 16] RFC 2965 HTTP State Management Mechanism October 2000
      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";
 Set-Cookie2: Part_Number="Riding_Rocket_0023"; Version="1";
 A subsequent request by the user agent to the (same) server for URLs
 of the form /acme/ammo/...  would include the following request
 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:
Kristol & Montulli Standards Track [Page 17] RFC 2965 HTTP State Management Mechanism October 2000
 Cookie: $Version="1"; Part_Number="Rocket_Launcher_0001";
 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.
 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
 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
Kristol & Montulli Standards Track [Page 18] RFC 2965 HTTP State Management Mechanism October 2000 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
  • 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.
 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.
Kristol & Montulli Standards Track [Page 19] RFC 2965 HTTP State Management Mechanism October 2000 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
  • to display a visual indication that a stateful session is in
  • 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.
Kristol & Montulli Standards Track [Page 20] RFC 2965 HTTP State Management Mechanism October 2000
 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
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.
 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 and to share cookies, but not and
 Similarly, a server can set a Path only for cookies that are related
 to the request-URI.
Kristol & Montulli Standards Track [Page 21] RFC 2965 HTTP State Management Mechanism October 2000 7.2 Cookie Spoofing
 Proper application design can avoid spoofing attacks from related
 domains.  Consider:
    1. User agent makes request to, gets back
       cookie session_id="1234" and sets the default domain
    2. User agent makes request to, gets back cookie
       session-id="1111", with Domain="".
    3. User agent makes request to again, and
       Cookie: $Version="1"; session_id="1234",
               $Version="1"; session_id="1111"; $Domain=""
       The server at 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 in a URI for a CGI on host  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.
 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
Kristol & Montulli Standards Track [Page 22] RFC 2965 HTTP State Management Mechanism October 2000
 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
 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.
Kristol & Montulli Standards Track [Page 23] RFC 2965 HTTP State Management Mechanism October 2000
 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.
 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.
Kristol & Montulli Standards Track [Page 24] RFC 2965 HTTP State Management Mechanism October 2000 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
 Lou Montulli, Inc.
 2037 Landings Dr.
 Mountain View, CA  94301
 [DMK95]    Kristol, D.M., "Proposed HTTP State-Info Mechanism",
            available at <http://portal.research.bell-
  >, September, 1995.
 [Netscape] "Persistent Client State -- HTTP Cookies", available at
 [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.
Kristol & Montulli Standards Track [Page 25] RFC 2965 HTTP State Management Mechanism October 2000 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
 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
 Funding for the RFC Editor function is currently provided by the
 Internet Society.
Kristol & Montulli Standards Track [Page 26]
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