[Docs] [txt|pdf|xml|html] [Tracker] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01 02 03 04 05 06 07

http-state Working Group                                        A. Barth
Internet-Draft                                             U.C. Berkeley
Expires: February 16, 2010                               August 15, 2009


                    HTTP State Management Mechanism
                         draft-abarth-cookie-01

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.  This document may contain material
   from IETF Documents or IETF Contributions published or made publicly
   available before November 10, 2008.  The person(s) controlling the
   copyright in some of this material may not have granted the IETF
   Trust the right to allow modifications of such material outside the
   IETF Standards Process.  Without obtaining an adequate license from
   the person(s) controlling the copyright in such materials, this
   document may not be modified outside the IETF Standards Process, and
   derivative works of it may not be created outside the IETF Standards
   Process, except to format it for publication as an RFC or to
   translate it into languages other than English.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on February 16, 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



Barth                   Expires February 16, 2010               [Page 1]


Internet-Draft       HTTP State Management Mechanism         August 2009


   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.
















































Barth                   Expires February 16, 2010               [Page 2]


Internet-Draft       HTTP State Management Mechanism         August 2009


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 appreciated.









































Barth                   Expires February 16, 2010               [Page 3]


Internet-Draft       HTTP State Management Mechanism         August 2009


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.  State and Sessions . . . . . . . . . . . . . . . . . . . . . .  7
   4.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     4.1.  Examples . . . . . . . . . . . . . . . . . . . . . . . . .  8
   5.  Server Conformance . . . . . . . . . . . . . . . . . . . . . .  9
     5.1.  General  . . . . . . . . . . . . . . . . . . . . . . . . .  9
     5.2.  Set-Cookie . . . . . . . . . . . . . . . . . . . . . . . .  9
       5.2.1.  Syntax . . . . . . . . . . . . . . . . . . . . . . . .  9
     5.3.  Semantics  . . . . . . . . . . . . . . . . . . . . . . . . 10
       5.3.1.  Cookie Attributes  . . . . . . . . . . . . . . . . . . 10
     5.4.  Cookie . . . . . . . . . . . . . . . . . . . . . . . . . . 11
       5.4.1.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . 11
       5.4.2.  Semantics  . . . . . . . . . . . . . . . . . . . . . . 12
     5.5.  Controlling Caching  . . . . . . . . . . . . . . . . . . . 12
   6.  User Agent Conformance . . . . . . . . . . . . . . . . . . . . 14
     6.1.  Parsing the Set-Cookie Header  . . . . . . . . . . . . . . 14
     6.2.  Parsing Cookie Dates . . . . . . . . . . . . . . . . . . . 15
     6.3.  Storage Model  . . . . . . . . . . . . . . . . . . . . . . 15
     6.4.  The Cookie Header  . . . . . . . . . . . . . . . . . . . . 18
   7.  Caching Proxy Conformance  . . . . . . . . . . . . . . . . . . 21
   8.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
   9.  Implementation Considerations  . . . . . . . . . . . . . . . . 23
     9.1.  Set-Cookie Content . . . . . . . . . . . . . . . . . . . . 23
     9.2.  Implementation Limits  . . . . . . . . . . . . . . . . . . 23
       9.2.1.  Denial of Service Attacks  . . . . . . . . . . . . . . 24
   10. Privacy  . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
     10.1. User Agent Control . . . . . . . . . . . . . . . . . . . . 25
     10.2. Protocol Design  . . . . . . . . . . . . . . . . . . . . . 26
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 27
     11.1. Clear Text . . . . . . . . . . . . . . . . . . . . . . . . 27
     11.2. Cookie Spoofing  . . . . . . . . . . . . . . . . . . . . . 27
     11.3. Unexpected Cookie Sharing  . . . . . . . . . . . . . . . . 27
   12. Other, Similar, Proposals  . . . . . . . . . . . . . . . . . . 28
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 29
   Appendix B.  Tabled Items  . . . . . . . . . . . . . . . . . . . . 30
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 31












Barth                   Expires February 16, 2010               [Page 4]


Internet-Draft       HTTP State Management Mechanism         August 2009


1.  Introduction

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
















































Barth                   Expires February 16, 2010               [Page 5]


Internet-Draft       HTTP State Management Mechanism         August 2009


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.














Barth                   Expires February 16, 2010               [Page 6]


Internet-Draft       HTTP State Management Mechanism         August 2009


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.


























Barth                   Expires February 16, 2010               [Page 7]


Internet-Draft       HTTP State Management Mechanism         August 2009


4.  Overview

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

   The 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.

4.1.  Examples





































Barth                   Expires February 16, 2010               [Page 8]


Internet-Draft       HTTP State Management Mechanism         August 2009


5.  Server Conformance

5.1.  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
   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 MUST NOT fold multiple
   Set-Cookie headers into a single header.  [TODO: Investigate how UAs
   cope with folded headers.]

5.2.  Set-Cookie

5.2.1.  Syntax

   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 NAME=VALUE
   attribute-value pair must come first in each cookie.


            set-cookie-header = "Set-Cookie:" name-value-pairs
            name-value-pairs  = name-value-pair *(";" name-value-pair)
            name-value-pair   = name ["=" value]        ; optional value
            name              = token
            value             = token





Barth                   Expires February 16, 2010               [Page 9]


Internet-Draft       HTTP State Management Mechanism         August 2009


   [TODO: Investigate what token actually means.]

   Attributes names are case-insensitive.  White space is permitted
   between tokens.  Note that although the above syntax description
   shows value as optional, some attributes require values.

   The cookie-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 who examines the Set-Cookie header.

   NOTE: The syntax above allows whitespace between the attribute and
   the = sign.  Servers wishing to interoperate with some legacy user
   agents might wish to elide this extra white space to maximize
   compatibility.

5.3.  Semantics

   When the user agent receives a Set-Cookie header, the user agent
   stores the cookie in its cookie store.  When the user agent makes
   another HTTP request to the origin server, the user agent will return
   the cookie in the Cookie header.

   The server can override the default handling of cookies by specifying
   a number of cookie attributes.  User agents ignore unrecognized
   cookie attributes.

5.3.1.  Cookie Attributes

   This section describes the semantics of a number of cookie
   attributes.

5.3.1.1.  Max-Age

   Syntax  A sequence of ASCII numerals.

   Semantics  The value of the Max-Age attribute represents the maximum
      lifetime of the cookie, measured in seconds from the moment the
      user agent receives the cookie.  If the server does not supply an
      Expires or a Max-Age attribute, the lifetime of the cookie is
      limited to the current session (as defined by the user agent).

5.3.1.2.  Expires







Barth                   Expires February 16, 2010              [Page 10]


Internet-Draft       HTTP State Management Mechanism         August 2009


   Syntax  An RFC 1123 date [cite].  (User agents use a very forgiving
      date parers; see Section [TODO]).

   Semantics  The value of the Expires attribute represents the maximum
      lifetime of the cookie, represented as the point in time at which
      the cookie expires.  If the server does not supply an Expires or a
      Max-Age attribute, the lifetime of the cookie is limited to the
      current session (as defined by the user agent).

5.3.1.3.  Domain

   [TODO: Test Domain.]  The Domain attribute specifies the domain for
   which the cookie is valid.  The leading dot isn't required.  If there
   is no Domain attribute, the default is to return the cookie only to
   the origin server.  [TODO: You can only set cookies for related
   domains.]

5.3.1.4.  Path

   [TODO: Test path.]  The Path attribute specifies the subset of URLs
   to which this cookie applies.

5.3.1.5.  Secure

   Syntax  The empty string.

   Semantics  The user agent SHOULD protect the confidentiality of
      cookies with the Secure attribute.

5.3.1.6.  HttpOnly

   Syntax  The empty string.

   Semantics  The user agent SHOULD protect confidentiality of cookies
      with the HttpOnly attribute by including HttpOnly cookies only
      when generating cookie strings for use in HTTP requests.

5.4.  Cookie

5.4.1.  Syntax

   The user agent returns stored cookies to the origin server in the
   cookie header.  The Cookie header shares a common syntax with the
   Set-Cookie header, but the semantics of the header differ
   dramatically.






Barth                   Expires February 16, 2010              [Page 11]


Internet-Draft       HTTP State Management Mechanism         August 2009


             cookie-header     = "Cookie:" name-value-pairs
             name-value-pairs  = name-value-pair *(";" name-value-pair)
             name-value-pair   = name "=" value
             name              = token
             value             = token


   NOTE: If the server supplies a Set-Cookie header that does not
   conform to the grammar in Section TODO, the user agent might not
   supply a Cookie header that conforms to the grammar in this Section.

5.4.2.  Semantics

   Each name-value-pair represents a cookie stored by the user agent.
   The cookie name is returned in as the name and the cookie value is
   returned as the value.

5.5.  Controlling Caching

   [TODO: Should we go into this much detail here?  This seems redundant
   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.



Barth                   Expires February 16, 2010              [Page 12]


Internet-Draft       HTTP State Management Mechanism         August 2009


   o  To allow caching of a document and require that it be validated
      before returning it to the client: Cache-Control: must-revalidate.

   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.































Barth                   Expires February 16, 2010              [Page 13]


Internet-Draft       HTTP State Management Mechanism         August 2009


6.  User Agent Conformance

   Not all origin servers conform to the behavior specified in the
   previous section.  To ensure interoperability, user agents MUST
   process cookies in a manner that is "black-box" indistinguishable
   from the requirements in this section.

6.1.  Parsing the Set-Cookie Header

   Let an LWS character be either a U+20 (SPACE) or a U+09 (TAB)
   character.

   A user agent MUST use the following algorithm to parse the Set-Cookie
   header:

   1.  [TODO: Deal with ',' characters.]

   2.  If the header contains a ';' character:

          the name-value string is characters up to, but not including,
          the first ';', and the unparsed-cookie-attributes are the
          remainder of the header (including the ';' in question).

       Otherwise:

          the name-value string is all the character contained in the
          header, and the unparsed-cookie-attributes is the empty
          string.

   3.  If the first non-LWS character of the name-value string is '=',
       remove it.

   4.  If the name-value string contains a '=' character:

          the name string is the characters up to, but not including,
          the first '=' character, and the value string is the
          characters after the first '=' character .

       Otherwise:

          the name string is empty, and the value string is the entire
          name-value string.

   5.  Remove any leading or trailing space from the name string.

   6.  Remove any leading or trailing space from the value string.





Barth                   Expires February 16, 2010              [Page 14]


Internet-Draft       HTTP State Management Mechanism         August 2009


   7.  The cookie-name is the name string.

   8.  The cookie-value is the value string.

   The user agent MUST use the following algorithm to parse the
   unparsed-attributes:

   1.  [TODO: Figure out how to parse cookie attributes.]

   [TODO: Can parsing a cookie ever fail?]

   [TODO: Convert Max-Age to a date during parsing.]

   When the user agent finishes parsing the Set-Cookie header, the user
   agent *receives a cookie* from the origin server with name cookie-
   name, value cookie-value, and attributes cookie-attributes.

6.2.  Parsing Cookie Dates

   Basically, cookie dates are a mess for historical reasons.

   To be compatible with legacy servers, however, user agents should
   accept dates formated according to this grammar:


      cookie-date       = rfc1123-like-date / mystery-date
      rfc1123-like-date = weekday "," SP rfc1123-like-dmy SP time SP "GMT"
      weekday           = "Monday" / "Mon" / "Tuesday" / "Tue" / ...
      rfc1123-like-dmy  = day dmy-div month dmy-div year
      dmy-div           = SP / "-"
      day               = 2DIGIT / *1SP DIGIT
      month             = "Jan" / "Feb" / ...
      year              = 2DIGIT / 4DIGIT
      time              = 2DIGIT ":" 2DIGIT ":" 2DIGIT

      mystery-date      = *CHAR ; see below


   [TODO: More information about mystery-date.]

6.3.  Storage Model

   When the user agent receives a cookie, the user agent SHOULD record
   the cookie in its cookie store as follows.

   A user agent MAY ignore received cookies in their entirety if the
   user agent is configured to block receiving cookie for a particular
   response.  For example, the user agent might wish to block receiving



Barth                   Expires February 16, 2010              [Page 15]


Internet-Draft       HTTP State Management Mechanism         August 2009


   cookies from "third-party" responses.

   The user agent stores the following fields about each cookie:

   o  name (a sequence of bytes)

   o  value (a sequence of bytes)

   o  expiry (a date)

   o  domain (a cookie-domain)

   o  path (a cookie-path)

   o  creation (a date)

   o  last-access (a date)

   o  persistent (a Boolean)

   o  host-only (a Boolean)

   o  secure-only (a Boolean)

   o  http-only (a Boolean)

   When the user agent receives a cookie, the user agent MUST follow the
   following algorithm:

   1.  Create a new cookie based on the parsed Set-Cookie header:

       1.  Create a new cookie with the following default field values:

           +  name = the cookie-name

           +  value = the cookie-value

           +  expiry = the latest representable date

           +  domain = the request-host

           +  path = 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]

           +  last-access = the date and time the cookie was received




Barth                   Expires February 16, 2010              [Page 16]


Internet-Draft       HTTP State Management Mechanism         August 2009


           +  last-access = the date and time the cookie was received

           +  persistent = false

           +  host-only = true

           +  secure-only = false

           +  http-only = false

       2.  Update the default field values according to the cookie-
           attributes:

           expiry  If the cookie-attributes contains at least one
              Expires or a Max-Age attribute, store the value of the
              [TODO: first] such attribute in the expiry field.  Store
              the value true in the persistent field.

           domain  If the cookie-attributes contains at least one Domain
              attribute, store the value of the [TODO: first] such
              attribute in the domain field.  Store the value false in
              the host-only field.  [TODO: Reject cookies for unrelated
              domains.]  [TODO: If the URL's host is an IP address, let
              Domain to be an IP address if it matches the URL's host
              exactly, but set the host-only flag. ]

           path  If the cookie-attributes contains at least one Path
              attribute, store the value of the [TODO: first] such
              attribute in the path field.

           secure-only  If the cookie-attributes contains at least one
              Secure attribute, store the value true in the secure-only
              field.

           http-only  If the cookie-attributes contains at least one
              HttpOnly attribute, store the value true in the http-only
              field.

   2.  Remove from the cookie store all cookies that have the share the
       same name, domain, path, and host-only fields as the newly
       created cookie.  [TODO: Valiate this list!]  [TODO: There's some
       funny business around http-only here.]

   3.  Insert the newly created cookie into the cookie store.

   The user agent MUST evict a cookie from the cookie store if either of
   the following conditions are met:




Barth                   Expires February 16, 2010              [Page 17]


Internet-Draft       HTTP State Management Mechanism         August 2009


   o  A cookie exists in the cookie store with an expiry date in the
      past.

   o  More than 50 cookies exist in the cookie store with the same
      domain field.

   The user agent MAY evict cookies from the cookie store if the cookie
   store exceeds some maximum storage bound (such as 3000 cookies).

   When the user agent evicts cookies from the cookie store, the user
   agent MUST evict cookies in the following priority order:

   1.  A cookie with an expiry date in the past.

   2.  A cookie that shares a domain field with more than 50 other
       cookies in the cookie store.

   3.  All other cookies.

   If two cookies have the same removal priority, the user agent MUST
   evict the cookie with the least recent last-access date first.

   When the user agent exits, the user agent MUST remove from the cookie
   store all cookies with the persistent field set to false.

6.4.  The Cookie Header

   When the user agent generates an HTTP request for a particular URI,
   the user agent SHOULD attach exactly one HTTP named Cookie if the
   cookie-string (defined below) for that URI is non-empty.

   A user agent MAY elide the Cookie header in its entirety if the user
   agent is configured to block sending cookie for a particular request.
   For example, the user agent might wish to block sending cookies
   during "third-party" requests.

   When generating a cookie-string from a URI with a "secure" scheme,
   the user agent MUST set the SECURE flag to true.  Otherwise, the user
   agent MUST set the SECURE flag to false.

      NOTE: The notion of an "secure" scheme is not defined by this
      document.  Typically, user agents consider a scheme secure if the
      scheme refers to a protocol that makes use of transport-layer
      security, such as TLS.  For example, most user agents consider
      "https" to be a secure scheme.

   When generating a cookie-string for use in an HTTP request, the user
   agent MUST set the HTTP flag to true.  Otherwise, the user agent MUST



Barth                   Expires February 16, 2010              [Page 18]


Internet-Draft       HTTP State Management Mechanism         August 2009


   set the HTTP flag to false.

   The user agent MUST use the following algorithm to compute the
   cookie-string from a cookie store and from a URI:

   1.  Let cookie-list be the set of cookies from the cookie store that
       meet the following requirements:

       *  The cookie's domain field must domain-match the URI's host.
          [TODO: Spec me]

       *  The cookie's path field must path-match the URI's path.
          [TODO: Spec me]

       *  If the cookie's host-only flag is set, the cookie's domain
          field must denote exactly the same FQDN as the URI's host.
          [TODO: Internet Explorer does not implement this requirement
          but most other major implementations do.]

       *  If the cookie's secure-only field is true, then the SECURE
          flag must be true.

       *  If the cookie's http-only field is true, then the HTTP flag
          must be true.

       NOTE: The Cookie header will not contain any expired cookies
       because cookies past their expiry date are removed from the
       cookie store immediately.

   2.  Sort the cookie-list in the following order:

       *  Cookies with longer path fields are listed before cookies with
          shorter path field.

       *  Among cookies that have equal length path fields, cookies with
          earlier creation dates are listed before cookies with later
          creation dates.

   3.  Update the last-access field of each cookie in the cookie-list to
       the current date.

   4.  Serialize the cookie-list into a cookie-string by processing each
       cookie in the cookie-list in order:

       1.  Output the cookie's name field.

       2.  Output the character U+3D ("=")




Barth                   Expires February 16, 2010              [Page 19]


Internet-Draft       HTTP State Management Mechanism         August 2009


       3.  Output the cookie's value field.

       4.  If there is an unprocessed cookie in the cookie-list, output
           the characters U+3B and U+20 ("; ")















































Barth                   Expires February 16, 2010              [Page 20]


Internet-Draft       HTTP State Management Mechanism         August 2009


7.  Caching Proxy Conformance

   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).
























Barth                   Expires February 16, 2010              [Page 21]


Internet-Draft       HTTP State Management Mechanism         August 2009


8.  Examples

   [TODO: Write sensible examples.]
















































Barth                   Expires February 16, 2010              [Page 22]


Internet-Draft       HTTP State Management Mechanism         August 2009


9.  Implementation Considerations

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

9.1.  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.]

9.2.  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 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)

   User agents created for specific purposes or for limited-capacity
   devices should provide at least 50 cookies of 4096 bytes, to ensure
   that the user can interact with a session-based origin server.

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




Barth                   Expires February 16, 2010              [Page 23]


Internet-Draft       HTTP State Management Mechanism         August 2009


   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.]

9.2.1.  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.]





































Barth                   Expires February 16, 2010              [Page 24]


Internet-Draft       HTTP State Management Mechanism         August 2009


10.  Privacy

10.1.  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.





Barth                   Expires February 16, 2010              [Page 25]


Internet-Draft       HTTP State Management Mechanism         August 2009


10.2.  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.












































Barth                   Expires February 16, 2010              [Page 26]


Internet-Draft       HTTP State Management Mechanism         August 2009


11.  Security Considerations

11.1.  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.

11.2.  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.]

11.3.  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.]










Barth                   Expires February 16, 2010              [Page 27]


Internet-Draft       HTTP State Management Mechanism         August 2009


12.  Other, Similar, Proposals

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















































Barth                   Expires February 16, 2010              [Page 28]


Internet-Draft       HTTP State Management Mechanism         August 2009


Appendix A.  Acknowledgements

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















































Barth                   Expires February 16, 2010              [Page 29]


Internet-Draft       HTTP State Management Mechanism         August 2009


Appendix B.  Tabled Items

   Tabled items:

   o  Public suffix.














































Barth                   Expires February 16, 2010              [Page 30]


Internet-Draft       HTTP State Management Mechanism         August 2009


Author's Address

   Adam Barth
   University of California, Berkeley

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












































Barth                   Expires February 16, 2010              [Page 31]


Html markup produced by rfcmarkup 1.129c, available from https://tools.ietf.org/tools/rfcmarkup/