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Versions: 00 01 02 03 04 05 06                                          
Network Working Group                                       Y. Pettersen
Internet-Draft                                        Opera Software ASA
Obsoletes: 2965 (if approved)                           October 16, 2006
Intended status: Standards Track
Expires: April 19, 2007

                   HTTP State Management Mechanism v2

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on April 19, 2007.

Copyright Notice

   Copyright (C) The Internet Society (2006).


   This document specifies a way to create a stateful session with
   Hypertext Transfer Protocol (HTTP) requests and responses.  It
   describes three headers, Cookie, Cookie2, and Set-Cookie2, which
   carry state information between participating origin servers and user
   agents.  The method described here differs from both Netscape's
   Cookie proposal [Netscape], and [RFC2965], but it can, provided some
   requirements are met, interoperate with HTTP/1.1 user agents that use

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   Netscape's method.  (See the HISTORICAL section.)

   This document defines new rules for how cookies can be shared between
   servers within a domain.  These new rules are intended to address
   security and privacy concerns that are difficult to counter for
   clients implementing Netscape's proposed rules or the rules specified
   by RFC 2965.

   This document reflects implementation experience with RFC 2965 and
   obsoletes it.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Description  . . . . . . . . . . . . . . . . . . . . . . . . .  7
     3.1.  Syntax: General  . . . . . . . . . . . . . . . . . . . . .  7
     3.2.  Origin Server Role . . . . . . . . . . . . . . . . . . . .  7
     3.3.  User Agent Role  . . . . . . . . . . . . . . . . . . . . . 11
     3.4.  How an Origin Server Interprets the Cookie Header  . . . . 18
     3.5.  Caching Proxy Role . . . . . . . . . . . . . . . . . . . . 18
   4.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     4.1.  Example 1  . . . . . . . . . . . . . . . . . . . . . . . . 19
     4.2.  Example 2  . . . . . . . . . . . . . . . . . . . . . . . . 20
   5.  Implementation Considerations  . . . . . . . . . . . . . . . . 21
     5.1.  Set-Cookie2 Content  . . . . . . . . . . . . . . . . . . . 21
     5.2.  Stateless Pages  . . . . . . . . . . . . . . . . . . . . . 22
     5.3.  Implementation Limits  . . . . . . . . . . . . . . . . . . 22
     5.4.  Backwards Compatibility  . . . . . . . . . . . . . . . . . 23
   6.  Privacy  . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
     6.1.  User Agent Control . . . . . . . . . . . . . . . . . . . . 23
     6.2.  Origin Server Role . . . . . . . . . . . . . . . . . . . . 24
     6.3.  Clear Text . . . . . . . . . . . . . . . . . . . . . . . . 24
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 25
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 25
     8.1.  Protocol Design  . . . . . . . . . . . . . . . . . . . . . 25
     8.2.  Cookie Spoofing  . . . . . . . . . . . . . . . . . . . . . 25
     8.3.  Unexpected Cookie Sharing  . . . . . . . . . . . . . . . . 26
     8.4.  Cookies for Account Information  . . . . . . . . . . . . . 26
   9.  Historical . . . . . . . . . . . . . . . . . . . . . . . . . . 26
     9.1.  Compatibility with Existing Implementations  . . . . . . . 26
     9.2.  Caching and HTTP/1.0 . . . . . . . . . . . . . . . . . . . 27
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 28
   11. Normative References . . . . . . . . . . . . . . . . . . . . . 28
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 28
   Intellectual Property and Copyright Statements . . . . . . . . . . 30

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1.  Introduction

   HTTP cookies are widely used to maintain state across multiple HTTP
   requests in a wide variety of HTTP based applications, such as
   shopping carts for web shops, login credentials, preferences,
   identity information, etc.  While alternatives exists, they are more
   cumbersome than HTTP cookies, and cookies and their flexibility and
   ease of use may therefore have assisted the rapid spread of the World
   Wide Web.

   Unfortunately, some of the flexibility of cookies, specifically how
   cookies are shared among multiple hosts, is causing possible security
   and privacy concerns.

   [RFC2965] specifies that a cookie may be shared with any server
   within the Reach of the host, that is, the parent domain of the host,
   and the [Netscape] proposal allowed, within certain restrictions,
   even wider sharing to servers in the entire second- or third-level
   domain in which the request-host is part.

   In some domain hierarchies, such as the generic Top Level Domain
   (TLD) dotCOM domain this will work well, but in many TLDs such as the
   Country-Code TLD (ccTLD) dotUK, this kind of sharing can cause
   problems, unless severely restricted, because it makes assumptions
   about control and authorization actually granted the request-host .
   The dotUK TLD and many other ccTLDs have numerous subdomains that are
   treated as actual TLDs or registry like domains, similar to dotCOM,
   dotNet and dotORG, such as the co.uk, org.uk and ac.uk domains that
   are used to group otherwise unrelated domains into categories based
   on their intended usage (e.g. commercial, non-commercial,
   governmental, academic).

   Permitting cookies to be shared across such registry-like domains may
   result in undesirable datasharing, denial of service problems, even
   security related problems.

   The original rules in Netscape's proposal, one internal dot in domain
   in the generic TLDs and two in domain name for non-generic TLDs,
   turned out to be not good enough, nor were they properly implemented
   in any client as they severely limited legitimate use of cookies;
   RFC2965's one level up rule restricted the problem somewhat, but not
   enough, as it was still possible to bypass the restrictions.  Clients
   have implemented or proposed various heuristics to limit the impact
   of the problem, some by using a blacklist of second level domains
   that the client is not permitted to accept cookies for, others use
   DNS IP address lookups of the Set-Cookie header's Domain attribute as
   a heuristic metod to determine the appropriateness of permitting a
   cookie to be set, and a large database of domains that should not be

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   able to accept cookies have also been proposed.

   Similarly, but less serious, the ability to set cookies to a parent
   path can, under some circumstances, cause interference between
   different applications in a given environment.  In single application
   environments such sharing is not dangerous, but could be problematic
   when multiple independent administrators share the same service, such
   as in shared hosting environments where all users are located in
   their own path on the same server.  In such environments a malicious
   user can set a cookie that is shared by many users, and since most
   version 0 [Netscape] implementations do not enforce a prefix path
   restriction it is also possible to limit the cookies to a path not
   controlled by the user, but not visible to all the other users on the
   host.  Such cookies can interfere with the function of other
   applications on the host or within the domain.

   This document presents an alternative method for reducing these
   problems by

   o  Removing the Domain attribute that permitted cookies to be
      specified for the parent domain, and instead introduces the
      SubDomain attribute that will permit servers to share cookies, but
      only with servers whose name domain-matches the name of the
      request-host that set the cookie, and not parent domains.

   o  Removing the Path attribute, replacing it by the SubPath attribute
      that may be used to specify which resources under the request-path
      will be allowed to receive the cookie, instead of specifying which
      parent path is allowed to send the cookie.

   This specification will not be able to accept cookies for hosts that
   are using domain specifications for parent domains as defined by the
   previous cookie specifications, but implementations using the older
   specification will be able to accept cookies from hosts following
   this specification.

   This document also introduces new requirements for the contents of
   the Cookie header, specifically that the $Domain and $Path attributes
   must always be sent, even when no domain or path has been specified,
   as this will allow request-hosts to verify the domain of the cookies
   even for cookies received from hosts using the older specifications.

2.  Terminology

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

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   as in the URI Syntax specification [RFC3986].

   Host name (HN) means either the host domain name (HDN) or the numeric
   Internet Protocol (IP) address or IP-literal of a host, as defined by
   [RFC3986].  The fully qualified domain name is preferred; use of
   numeric IP addresses or IP-literals is strongly discouraged.

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

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

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

   Host names can be specified either as an IP address, IP-literal or an
   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

   o  their host name strings string-compare equal; or

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

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

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

   o  If

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

      *  H has the form A.B; and

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

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

   o  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.

3.  Description

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

3.1.  Syntax: General

   The two state management headers, Set-Cookie2 and Cookie, have common
   syntactic properties involving attribute-value pairs.  The following
   grammar uses the notation, and tokens DIGIT (decimal digits), token
   (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

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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.

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"
                      |       "SubDomain"
                      |       "Max-Age" "=" value
                      |       "SubPath" "=" 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

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   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.

   NAME=VALUE  REQUIRED.  The name of the state information ("cookie")
      is NAME, and its value is VALUE.  NAMEs that begin with $ are
      reserved and MUST NOT be used by applications.  The VALUE is
      opaque to the user agent and may be anything the origin server
      chooses to send, possibly in a server-selected printable ASCII
      encoding.  "Opaque" implies that the content is of interest and
      relevance only to the origin server.  The content may, in fact, be
      readable by anyone that examines the Set-Cookie2 header.

   Comment=value  OPTIONAL.  Because cookies can be used to derive or
      store private information about a user, the value of the Comment
      attribute allows an origin server to document how it intends to
      use the cookie.  The user can inspect the information to decide
      whether to initiate or continue a session with this cookie.
      Characters in value MUST be in UTF-8 encoding.  [RFC2279]

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

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

   SubDomain  OPTIONAL.  The SubDomain attribute specifies that the user
      agent should share the cookie with any hosts that domain-matches
      the name of the host sending the cookie

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

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      discard the cookie.  A value of zero means the cookie SHOULD be
      discarded immediately.

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

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

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

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

   Version=value  REQUIRED.  The value of the Version attribute, a
      decimal integer, identifies the version of the state management
      specification to which the cookie conforms.  For this
      specification, Version=2 applies.

3.2.3.  Controlling Caching

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

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

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

   o  To suppress caching of the Set-Cookie2 header:

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

   and one of the following:

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

       Cache-control: private

   o  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

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

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

   o  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 or
   that contain invalid data, and if necessary ignore the entire header.

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   The user agent applies these defaults for optional attributes that
   are missing:

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

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

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

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

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

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

   The user agent MUST ignore the SubDomain attribute if the effective
   request-host is an IP-address or IP-literal.

   If the SubDomain attribute is present the state attribute Domain
   becomes .H where H is the effective request-host.

   If the SubPath attribute is present the state attribute Path becomes
   Px where P is the default path, up to and including the right-most /
   and x is the value of the attribute.

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:

   o  The value for the SubPath attribute appended to the default path
      is not a prefix of the request-URI.

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


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

   o  A Set-Cookie2 from a path /example1/example1 for SubPath=exam will
      be accepted for the path /example/exam

   o  A Set-Cookie2 from a path /example1/example1 for SubPath=exor will
      be rejected because exor is not a prefix of example1.

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:

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   o  the user agent receives a cookie that contains a CommentURL

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

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

   o  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

   o  the request-host and request-port;

   o  the request-URI;

   o  the cookie's age.

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   The syntax for the header is:

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

   The value of the cookie-version attribute MUST be the value from the
   Version attribute of the corresponding Set-Cookie2 response header.
   Otherwise the value for cookie-version is 0.

   The value for the path attribute MUST be the value from the cookie's
   Path state attribute, as determined when the corresponding Set-
   Cookie2 response header was parsed.  If the cookie was set using a
   previous specification this value MUST be the value of the Path
   attribute of the corresponding response header, or the default path
   of the URI setting the cookie.

   The value for the domain attribute MUST be the value from the
   cookie's Domain state attribute, as determined when the corresponding
   Set-Cookie2 response header was parsed.  If the response header used
   the SubDomain attribute the domain value MUST be prefixed by a ".",
   if the Domain attribute is a default domain the domain value MUST NOT
   be prefixed by a ".".  If the cookie was set by host supporting a
   previous version this value MUST be the Domain attribute from the
   correponding header, including a preceding "." if the Domain
   attribute was present; if it was not present the domain value must be
   the name of the host setting the cookie, without being prefixed with
   a ".".

   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

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   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 state 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 state
      attribute of the cookie.

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

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

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

3.3.5.  Identifying What Version is Understood:  Cookie2

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

      cookie2 =       "Cookie2:" cookie-version

   Here the value for cookie-version is the highest version of cookie

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   specification (currently 2) 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.

   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

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   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.

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:

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

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

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

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

      Cache-control: no-cache

      Cache-control: private

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

      Cache-control: no-cache="set-cookie2"
      (The Set-Cookie2 header should usually not be cached.)

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   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, and that the hostname is

   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-Cookie2: Customer="WILE_E_COYOTE"; Version="1";

   Cookie reflects user's identity.

   3.  User Agent -> Server

       POST /acme/pickitem HTTP/1.1
       Cookie: $Version="1"; Customer="WILE_E_COYOTE";
               $Domain="www.example.com"; $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

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       POST /acme/shipping HTTP/1.1
       Cookie: $Version="1";
                        $Domain="www.example.com"; $Path="/acme/";
                        $Domain="www.example.com"; $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"

   New cookie reflects shipping method.

   7.  User Agent -> Server

       POST /acme/process HTTP/1.1
       Cookie: $Version="1";
                        $Domain="www.example.com"; $Path="/acme/";
                        $Domain="www.example.com"; $Path="/acme/";
                        $Domain="www.example.com"; $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-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.

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   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";
                          $Domain="www.example.com"; $Path="/acme/ammo";
                          $Domain="www.example.com"; $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";
                            $Domain="www.example.com"; $Path="/acme"

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

5.  Implementation Considerations

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

5.1.  Set-Cookie2 Content

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

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   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

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:

   o  at least 300 cookies

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

   o  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

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   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.

5.4.  Backwards Compatibility

   Servers sending cookies according to this specification and that
   wishes to send cookies with the same properties to a client following
   the RFC2965 specification MAY send Domain and Path attributes in the
   same header as the version 2 arguments.  Clients following this
   specification MUST ignore these attributes.

6.  Privacy

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

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

   o  to completely disable the sending and saving of cookies.

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   o  to determine whether a stateful session is in progress.

   o  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

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

   o  to display a visual indication that a stateful session is in

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

   o  to let the user examine and delete the contents of a cookie at any

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

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

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

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:

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

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

   These facts imply that information of a personal and/or financial
   nature should only be sent over a secure channel.  For less sensitive
   information, or when the content of the header is a database key, an
   origin server should be vigilant to prevent a bad Cookie value from
   causing failures.

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

7.  IANA Considerations

   This document makes no request of IANA.

   Note to RFC Editor: this section may be removed on publication as an

8.  Security Considerations

8.1.  Protocol Design

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

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

8.2.  Cookie Spoofing

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

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

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

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

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

8.4.  Cookies for Account Information

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

9.  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.

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   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 state 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.

   A user agent that supports both this specification and Netscape-style
   cookies SHOULD still send a Cookie request header that follows the
   format specified in this document, as the benefit of adding domain
   and path information to each cookie and thus providing even older
   server with the ability to detect incorrectly set cookies outweigh
   the potential problems unknown cookienames may cause.

   The client should also send this header in requests to servers that
   receive cookies that are not of the version specified by this

      Cookie2: $Version="2"

   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

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   being sent to another user.  The Set-Cookie2 and Set-Cookie headers
   are stored in the cache, and, although the document is stale
   (expired), the cache returns the document in response to later
   requests, including cached headers.

10.  Acknowledgements

   This document is based on [RFC2965] by David Kristol and Lou Montulli
   and the collective efforts of the HTTP Working Group of the IETF and,
   particularly, the following people, in addition to the authors of RFC
   2965: Roy Fielding, Yaron Goland, Marc Hedlund, Ted Hardie, Koen
   Holtman, Shel Kaphan, Rohit Khare, Foteos Macrides, David W. Morris.

11.  Normative References

              "Persistent Client State -- HTTP Cookies",

              available at

   [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 ISO
              10646", RFC 2279, January 1998.

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

   [RFC2965]  Kristol, D. and L. Montulli, "HTTP State Management
              Mechanism", RFC 2965, October 2000.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

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Author's Address

   Yngve N Pettersen
   Opera Software ASA
   Waldemar Thranes gate 98
   N-0175 OSLO,

   Email: yngve@opera.com

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Full Copyright Statement

   Copyright (C) The Internet Society (2006).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an

Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
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