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http-state Working Group                                        A. Barth
Internet-Draft                                             U.C. Berkeley
Expires: February 8, 2010                                 August 7, 2009


                    HTTP State Management Mechanism
                         draft-abarth-cookie-00

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   The list of current Internet-Drafts can be accessed at
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   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on February 8, 2010.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.










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Abstract

   This document defines the HTTP Cookie and Set-Cookie headers.

   NOTE:

      This document is currently a "straw-man" cookie proposal.  Much of
      the text herein is completely wrong.  If you have suggestions for
      improving the draft, please send email to http-state@ietf.org.
      Suggestions with test cases are especially appriciated.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  State and Sessions . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Outline  . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     4.1.  Syntax: General  . . . . . . . . . . . . . . . . . . . . .  6
     4.2.  Origin Server Role . . . . . . . . . . . . . . . . . . . .  6
       4.2.1.  General  . . . . . . . . . . . . . . . . . . . . . . .  6
       4.2.2.  Set-Cookie Syntax  . . . . . . . . . . . . . . . . . .  7
       4.2.3.  Controlling Caching  . . . . . . . . . . . . . . . . .  9
     4.3.  User Agent Role  . . . . . . . . . . . . . . . . . . . . . 10
       4.3.1.  Interpreting Set-Cookie  . . . . . . . . . . . . . . . 10
       4.3.2.  Rejecting Cookies  . . . . . . . . . . . . . . . . . . 11
       4.3.3.  Cookie Management  . . . . . . . . . . . . . . . . . . 12
       4.3.4.  Sending Cookies to the Origin Server . . . . . . . . . 12
       4.3.5.  Sending Cookies in Unverifiable Transactions . . . . . 14
     4.4.  How an Origin Server Interprets the Cookie Header  . . . . 14
     4.5.  Caching Proxy Role . . . . . . . . . . . . . . . . . . . . 14
   5.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     5.1.  Example 1  . . . . . . . . . . . . . . . . . . . . . . . . 15
     5.2.  Example 2  . . . . . . . . . . . . . . . . . . . . . . . . 16
   6.  Implementation Considerations  . . . . . . . . . . . . . . . . 18
     6.1.  Set-Cookie Content . . . . . . . . . . . . . . . . . . . . 18
     6.2.  Implementation Limits  . . . . . . . . . . . . . . . . . . 18
       6.2.1.  Denial of Service Attacks  . . . . . . . . . . . . . . 19
   7.  Privacy  . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     7.1.  User Agent Control . . . . . . . . . . . . . . . . . . . . 20
     7.2.  Protocol Design  . . . . . . . . . . . . . . . . . . . . . 21
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 22
     8.1.  Clear Text . . . . . . . . . . . . . . . . . . . . . . . . 22
     8.2.  Cookie Spoofing  . . . . . . . . . . . . . . . . . . . . . 22
     8.3.  Unexpected Cookie Sharing  . . . . . . . . . . . . . . . . 22
   9.  Other, Similar, Proposals  . . . . . . . . . . . . . . . . . . 23
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 24
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 25



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1.  Introduction

   This document defines the HTTP Cookie and Set-Cookie header.
















































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2.  Terminology

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

   Fully-qualified host name (FQHN) means either the fully-qualified
   domain name (FQDN) of a host (i.e., a completely specified domain
   name ending in a top-level domain such as .com or .uk), or the
   numeric Internet Protocol (IP) address of a host.  The fully
   qualified domain name is preferred; use of numeric IP addresses is
   strongly discouraged.  [TODO: What does "strongly discouraged" mean?]

   The terms request-host and request-URI refer to the values the client
   would send to the server as, respectively, the host (but not port)
   and abs_path portions of the absoluteURI (http_URL) of the HTTP
   request line.  Note that request-host must be a FQHN.  Hosts names
   can be specified either as an IP address or a FQHN string.  Sometimes
   we compare one host name with another.  Host A's name domain-matches
   host B's if

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

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

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

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

   Because it was used in Netscape's original implementation of state
   management, we will use the term cookie to refer to the state
   information that passes between an origin server and user agent, and
   that gets stored by the user agent.














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3.  State and Sessions

   This document describes a way to create stateful sessions with HTTP
   requests and responses.  HTTP servers respond to each client request
   without relating that request to previous or subsequent requests; the
   technique allows clients and servers that wish to exchange state
   information to place HTTP requests and responses within a larger
   context, which we term a "session".  This context might be used to
   create, for example, a "shopping cart", in which user selections can
   be aggregated before purchase, or a magazine browsing system, in
   which a user's previous reading affects which offerings are
   presented.

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

   1.  Each session has a beginning and an end.

   2.  Each session is relatively short-lived.

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

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


























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4.  Outline

   We outline here a way for an origin server to send state information
   to the user agent, and for the user agent to return the state
   information to the origin server.

4.1.  Syntax: General

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

   [TODO: Test this grammar.  I think there are many, many issue with
   this grammer.  For example, this grammar seems to permit whitespace
   around the "=", but I don't think that actually works.]

      av-pairs        =       av-pair *(";" av-pair)
      av-pair         =       attr ["=" value]        ; optional value
      attr            =       token
      value           =       word
      word            =       token | quoted-string

   Attributes (names) (attr) are case-insensitive.  White space is
   permitted between tokens.  Note that while the above syntax
   description shows value as optional, most attrs require them.

   NOTE: The syntax above allows whitespace between the attribute and
   the = sign.  [TODO: This is probably wrong, however.]

4.2.  Origin Server Role

4.2.1.  General

   The origin server initiates a session, if it so desires.  (Note that
   "session" here does not refer to a persistent network connection but
   to a logical session created from HTTP requests and responses.  The
   presence or absence of a persistent connection should have no effect
   on the use of cookie-derived sessions).  To initiate a session, the
   origin server returns an extra response header to the client, Set-
   Cookie.  (The details follow later.)

   A user agent returns a Cookie request header (see below) to the
   origin server if it chooses to continue a session.  The origin server
   may ignore it or use it to determine the current state of the
   session.  It may send the client a Set-Cookie response header with



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   the same or different information, or it may send no Set-Cookie
   header at all.  The origin server effectively ends a session by
   sending the client a Set-Cookie header with Max-Age=0.  [TODO: Need
   to say something about Expires here.]

   Servers may return a Set-Cookie response headers with any response.
   User agents should send Cookie request headers, subject to other
   rules detailed below, with every request.

   An origin server may include multiple Set-Cookie headers in a
   response.  Note that an intervening gateway could fold multiple such
   headers into a single header.  [TODO: Investigate how UAs cope with
   such folded headers.]

4.2.2.  Set-Cookie Syntax

   The syntax for the Set-Cookie response header is

   [TODO: Valdiate this syntax.]

      set-cookie      =       "Set-Cookie:" cookies
      cookies         =       1#cookie
      cookie          =       NAME "=" VALUE *(";" cookie-av)
      NAME            =       attr
      VALUE           =       value
      cookie-av       =       "Comment" "=" value
                      |       "Domain" "=" value
                      |       "Max-Age" "=" value
                              [TODO: Expires is clearly missing.]
                      |       "Path" "=" value
                      |       "Secure"
                              [TODO: HTTPOnly is also missing.]
                      |       "Version" "=" 1*DIGIT
                              [TODO: Version is likely a fantasy.]

   Informally, the Set-Cookie response header comprises the token Set-
   Cookie:, followed by a comma-separated list of one or more cookies.
   Each cookie begins with a NAME=VALUE pair, followed by zero or more
   semi-colon-separated attribute-value pairs.  The specific attributes
   and the semantics of their values follows.  The NAME=VALUE attribute-
   value pair must come first in each cookie.  The others, if present,
   can occur in any order.  If an attribute appears more than once in a
   cookie, the behavior is undefined.  [TODO: Test what happens when
   attributes are multiply defined.]

      NAME=VALUE





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         Required.  The name of the state information ("cookie") is
         NAME, and its value is VALUE.  NAMEs that begin with $ are
         reserved for other uses and must not be used by applications.
         [TODO: I suspect the $ rule is a fantasy.]  The VALUE is opaque
         to the user agent and may be anything the origin server chooses
         to send, possibly in a server-selected printable ASCII
         encoding.  "Opaque" implies that the content is of interest and
         relevance only to the origin server.  The content may, in fact,
         be readable by anyone that examines the Set-Cookie header.

      Comment=comment

         Optional.  Because cookies can contain private information
         about a user, the Cookie attribute allows an origin server to
         document its intended use of a cookie.  The user can inspect
         the information to decide whether to initiate or continue a
         session with this cookie.  [TODO: Does this actually exist?]

      Domain=domain

         Optional.  The Domain attribute specifies the domain for which
         the cookie is valid.  An explicitly specified domain must
         always start with a dot.  [TODO: Test what happens without a
         dot.]

      Max-Age=delta-seconds

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

      Path=path

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

      Secure

         Optional.  The Secure attribute (with no value) directs the
         user agent to use only (unspecified) secure means to contact
         the origin server whenever it sends back this cookie.  [TODO:
         We should give better implementation advice than this.]

         The user agent (possibly under the user's control) may
         determine what level of security it considers appropriate for



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         "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
         confidentiality of the cookie's value.

      Version=version

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

4.2.3.  Controlling Caching

   [TODO: Should we go into this much detail here?  This seems redudant
   with the HTTP specs.]

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

   If the cookie is intended for use by a single user, the Set-Cookie
   header should not be cached.  A Set-Cookie header that is intended to
   be shared by multiple users may be cached.

   The origin server should send the following additional HTTP/1.1
   response headers, depending on circumstances: [TODO: Is this good
   advice?]

   o  To suppress caching of the Set-Cookie header: Cache-control: no-
      cache="set-cookie".

   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.





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   o  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.

   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-Cookie response
   headers unless they know for certain (by out of band means) that
   there are no downsteam HTTP/1.0 proxies.  HTTP/1.1 servers may send
   other Cache-Control directives that permit caching by HTTP/1.1
   proxies in addition to the Expires: old-date directive; the Cache-
   Control directive will override the Expires: old-date for HTTP/1.1
   proxies.

4.3.  User Agent Role

4.3.1.  Interpreting Set-Cookie

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

   Version  Defaults to "old cookie" behavior as originally specified by
      Netscape.  See the HISTORICAL section.  [TODO: Unlikely.]

   Domain  Defaults to the request-host.  (Note that there is no dot at
      the beginning of request-host.)  [TODO: This is important to
      test!]

   Max-Age  The default behavior is to discard the cookie when the user
      agent exits.  [TODO: Interaction with Expires.]

   Expires  The default behavior is to discard the cookie when the user
      agent exits.  [TODO: Interaction with Max-Age.]

   Path  Defaults to the path of the request URL that generated the Set-
      Cookie response, up to, but not including, the right-most /.
      [TODO: Test!  This seems wrong for paths that are just a single
      slash]







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

4.3.2.  Rejecting Cookies

   To prevent possible security or privacy violations, a user agent must
   reject a cookie (shall not store its information) if any of the
   following is true:

   o  The value of the Path attribute is not a prefix of the request-
      URI.  [TODO: This is a lie.]

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

   o  The value for the request-host does not domain-match the Domain
      attribute.  [TODO: Test whether you can set a cookie for a
      subdomain of yourself.]

   o  The request-host is a FQDN (not IP address) and has the form HD,
      where D is the value of the Domain attribute, and H is a string
      that contains one or more dots.  [TODO: I don't think this is
      right. foo.bar.baz.com can set a cookie for .baz.com]

   o  [TODO: Need to interact with public suffix list!]

   Examples:

   o  A Set-Cookie from request-host y.x.foo.com for Domain=.foo.com
      would be rejected, because H is y.x and contains a dot.  [TODO: I
      don't think this is right.]

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

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

   o  A Set-Cookie with Domain=foo.com will be rejected because the
      value for Domain does not begin with a dot.  [TODO: This seems
      unlikely, but test!]

   o  A Set-Cookie with Domain=.co.uk will be rejected because .co.uk is
      a public suffix.







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4.3.3.  Cookie Management

   If a user agent receives a Set-Cookie response header whose NAME is
   the same as a pre-existing cookie, and whose Domain and Path
   attribute values exactly (string) match those of a pre-existing
   cookie, the new cookie supersedes the old.  However, if the Set-
   Cookie has a value for Max-Age of zero, the (old and new) cookie is
   discarded.  Otherwise cookies accumulate until they expire (resources
   permitting), at which time they are discarded.  [TODO: Do cookies
   really accumulate like this?  Also, need to talk about Expires]

   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.
   [TODO: Consider recommending a cookie eviction strategy that works in
   practice.]

   If a Set-Cookie response header includes a Comment attribute, the
   user agent should store that information in a human-readable form
   with the cookie and should display the comment text as part of a
   cookie inspection user interface.  [TODO: I think the Comment
   attribute is a fantasy.]

   User agents should allow the user to control cookie destruction.  An
   infrequently-used cookie may function as a "preferences file" for
   network applications, and a user may wish to keep it even if it is
   the least-recently-used cookie.  One possible implementation would be
   an interface that allows the permanent storage of a cookie through a
   checkbox (or, conversely, its immediate destruction).  [TODO:
   Remove?]

   Privacy considerations dictate that the user have considerable
   control over cookie management.  The PRIVACY section contains more
   information.

4.3.4.  Sending Cookies to the Origin Server

   When it sends a request to an origin server, the user agent sends a
   Cookie request header to the origin server if it has cookies that are
   applicable to the request, based on

   o  the request-host,

   o  the request-URI, and

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


   [TODO: This syntax is entirely wrong.]

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

      Domain Selection

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

      Path Selection

         The Path attribute of the cookie must match a prefix of the
         request-URI.  [TODO: Need a more complex algorithm here
         involving the / character.]

      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.  [TODO: Figure out the
   correct ordering.]

   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.
   [TODO: Test whether servers actually do this.]







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4.3.5.  Sending Cookies in Unverifiable Transactions

   [TODO: This entire section seems like a fantasy.]

   [TODO: Consider explaining how third-party cookie blocking works.]

4.4.  How an Origin Server Interprets the Cookie Header

   [TODO: This section appears to be nonsense.]

4.5.  Caching Proxy Role

   One reason for separating state information from both a URL and
   document content is to facilitate the scaling that caching permits.
   To support cookies, a caching proxy must obey these rules already in
   the HTTP specification [TODO: If they're already in the HTTP
   specification, aren't they redundant here?]:

   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-Cookie
      response header.

   o  Cache the received response subject to the control of the usual
      headers, such as Expires, Cache-Control: no-cache, and Cache-
      Control: private.

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

   Proxies must not introduce Set-Cookie (Cookie) headers of their own
   in proxy responses (requests).














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5.  Examples

5.1.  Example 1

   Most detail of request and response headers has been omitted.  Assume
   the user agent has no stored cookies.

   1.  User Agent -> Server

       POST /acme/login HTTP/1.1
       [form data]

       User identifies self via a form.

   2.  Server -> User Agent

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

       Cookie reflects user's identity.  [TODO: This is insecure.]

   3.  User Agent -> Server

       POST /acme/pickitem HTTP/1.1
       Cookie: $Version="1"; Customer="WILE_E_COYOTE"; $Path="/acme"
       [form data]

       User selects an item for "shopping basket."

   4.  Server -> User Agent

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

       Shopping basket contains an item.

   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




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

       New cookie reflects shipping method.

   7.  User Agent -> Server

       POST /acme/process HTTP/1.1
       Cookie: $Version="1";
               Customer="WILE_E_COYOTE"; $Path="/acme";
               Part_Number="Rocket_Launcher_0001"; $Path="/acme";
               Shipping="FedEx"; $Path="/acme"
       [form data]

       User chooses to process order.

   8.  Server -> User Agent

       HTTP/1.1 200 OK

       Transaction is complete.

   [TODO: This example is really silly.  We shouldn't be recommending
   this at all.]

   The user agent makes a series of requests on the origin server, after
   each of which it receives a new cookie.  All the cookies have the
   same Path attribute and (default) domain.  Because the request URLs
   all have /acme as a prefix, and that matches the Path attribute, each
   request contains all the cookies received so far.

5.2.  Example 2

   This example illustrates the effect of the Path attribute.  All
   detail of request and response headers has been omitted.  Assume the
   user agent has no stored cookies.

   Imagine the user agent has received, in response to earlier requests,
   the response headers

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

   and

   Set-Cookie: Part_Number="Riding_Rocket_0023"; Version="1";
           Path="/acme/ammo"




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   A subsequent request by the user agent to the (same) server for URLs
   of the form /acme/ammo/... would include the following request
   header:

   Cookie: $Version="1";
           Part_Number="Riding_Rocket_0023"; $Path="/acme/ammo";
           Part_Number="Rocket_Launcher_0001"; $Path="/acme"

   Note that the NAME=VALUE pair for the cookie with the more specific
   Path attribute, /acme/ammo, comes before the one with the less
   specific Path attribute, /acme.  Further note that the same cookie
   name appears more than once.

   A subsequent request by the user agent to the (same) server for a URL
   of the form /acme/parts/ would include the following request header:

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

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






























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6.  Implementation Considerations

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

6.1.  Set-Cookie Content

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

   The session information can obviously be clear or encoded text that
   describes state.  However, if it grows too large, it can become
   unwieldy.  Therefore, an implementor might choose for the session
   information to be a key to a server-side resource.  [TODO: Describe
   briefly how to generate a decent session key.]

   [TODO: We could recommend that servers encrypt and mac their cookie
   data.]

   [TODO: Mention issues that arise from having multiple concurrent
   sessions.]

6.2.  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.  [TODO: Why not?]  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: [TODO: Where do these numbers come from?]

   o  at least 300 cookies

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

   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.




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   The information in a Set-Cookie response header must be retained in
   its entirety.  If for some reason there is inadequate space to store
   the cookie, it must be discarded, not truncated.

   Applications should use as few and as small cookies as possible, and
   they should cope gracefully with the loss of a cookie.  [TODO: Could
   mention latency issues that arise from having tons of cookies.]

6.2.1.  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.  [TODO: These minima still let an attacker exhaust the
   entire cookie store.  There's not much we can do about it though.]

































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7.  Privacy

7.1.  User Agent Control

   An origin server could create a Set-Cookie header to track the path
   of a user through the server.  Users may object to this behavior as
   an intrusive accumulation of information, even if their identity is
   not evident.  (Identity might become evident if a user subsequently
   fills out a form that contains identifying information.)  This state
   management specification therefore requires that a user agent give
   the user control over such a possible intrusion, although the
   interface through which the user is given this control is left
   unspecified.  However, the control mechanisms provided shall at least
   allow the user

   o  to completely disable the sending and saving of cookies,

   o  to determine whether a stateful session is in progress, and

   o  to control the saving of a cookie on the basis of the cookie's
      Domain attribute.

   Such control could be provided by, for example, mechanisms

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

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

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

   o  to let the user examine the contents of a cookie at any time.

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

   When the user agent terminates execution, it should let the user
   discard all state information.  Alternatively, the user agent may ask
   the user whether state information should be retained.  If the user
   chooses to retain state information, it would be restored the next
   time the user agent runs.





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7.2.  Protocol Design

   The restrictions on the value of the Domain attribute are meant to
   reduce the ways that cookies can "leak" to the "wrong" site.  The
   intent is to restrict cookies to one, or a closely related set of
   hosts.  Therefore a request-host is limited as to what values it can
   set for Domain.












































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8.  Security Considerations

8.1.  Clear Text

   The information in the Set-Cookie and Cookie headers is transmitted
   in the clear.  Three consequences are:

   1.  Any sensitive information that is conveyed in in the headers is
       exposed to an easedropper.

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

   3.  A malicious client could alter the Cookie header before
       transmission, with unpredictable results.

   These facts imply that information of a personal and/or financial
   nature should 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.

8.2.  Cookie Spoofing

   [TODO: Mention integrity issue where a sibling domain can inject
   cookies.]

   [TODO: Mention integrity issue where a HTTP can inject cookies into
   HTTPS.]

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 host b.com.  User agent
   implementors are strongly encouraged to prevent this sort of exchange
   whenever possible.  [TODO: How are they supposed to do this?  This
   section makes little sense.]










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9.  Other, Similar, Proposals

   [TODO: Describe relation to the Netscape Cookie Spec, RFC 2109, RFC
   2629, and cookie-v2.]















































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Appendix A.  Acknowledgements

   This document borrows heavily from RFC 2109.  [TODO: Figure out the
   proper way to credit the authors of RFC 2109.]















































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Author's Address

   Adam Barth
   University of California, Berkeley

   Email: abarth@eecs.berkeley.edu
   URI:   http://www.adambarth.com/












































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