Internet Info 1997 December/Internet_Info_CD-ROM_Walnut_Creek_December_1997.iso/drafts/draft__misc/cookie-2.54.txt

Text File | 1997-05-05 | 48KB | 1,387 lines

HTTP Working Group                                      David M. Kristol
INTERNET DRAFT                    Bell Laboratories, Lucent Technologies
Obsoletes: RFC 2109                                         Lou Montulli
                                                 Netscape Communications
<draft-ietf-http-state-man-mec-01.txt>
May 5, 1997                                     Expires November 5, 1997


                 HTTP State Management Mechanism (Rev1)



                          Status of this Memo

     This document is an Internet-Draft.  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.''

     To learn the current status of any Internet-Draft, please
     check the ``1id-abstracts.txt'' listing contained in the
     Internet- Drafts Shadow Directories on ftp.is.co.za (Africa),
     nic.nordu.net (Europe), munnari.oz.au (Pacific Rim),
     ds.internic.net (US East Coast), or ftp.isi.edu (US West
     Coast).

     This is authors' draft 2.54.


1.  ABSTRACT

This document specifies a way to create a stateful session with HTTP
requests and responses.  It describes two new headers, Cookie and Set-
Cookie2, which carry state information between participating origin
servers and user agents.  The method described here differs from
Netscape's Cookie proposal, but it can interoperate with HTTP/1.0 user
agents that use Netscape's method.  (See the HISTORICAL section.)

This document reflects implementation experience with RFC 2109 and
obsoletes it.


2.  TERMINOLOGY

The terms user agent, client, server, proxy, and origin server have the
same meaning as in the HTTP/1.1 specification [RFC 2068].

Fully-qualified host name (FQHN) means either the fully-qualified domain
name (FQDN) of a host (i.e., a completely specified domain name ending



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in a top-level domain such as .com or .uk), or the numeric Internet
Protocol (IP) address of a host.  The fully qualified domain name is
preferred; use of numeric IP addresses is strongly discouraged.

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

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

Host names can be specified either as an IP address or a FQHN string.
Sometimes we compare one host name with another.  Host A's name domain-
matches host B's if

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

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

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

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

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


3.  STATE AND SESSIONS

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





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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.


4.  OUTLINE

We outline here a way for an origin server to send state information to
the user agent, and for the user agent to return the state information
to the origin server.  The goal is to have a minimal impact on HTTP and
user agents.  Only origin servers that need to maintain sessions would
suffer any significant impact, and that impact can largely be confined
to Common Gateway Interface (CGI) programs, unless the server provides
more sophisticated state management support.  (See Implementation
Considerations, below.)

4.1  Syntax:  General

The two state management headers, Set-Cookie2 and Cookie, have common
syntactic properties involving attribute-value pairs.  The following
grammar uses the notation, and tokens DIGIT (decimal digits) and token
(informally, a sequence of non-special, non-white space characters) from
the HTTP/1.1 specification [RFC 2068] to describe their syntax.

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

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

NOTE: The syntax above allows whitespace between the attribute and the =
sign.









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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-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.

4.2.2  Set-Cookie2 Syntax  The syntax for the Set-Cookie2 response
header is

set-cookie      =       "Set-Cookie2:" cookies
cookies         =       1#cookie
cookie          =       NAME "=" VALUE *(";" cookie-av)
NAME            =       attr
VALUE           =       value
cookie-av       =       "Comment" "=" value
                |       "Discard"
                |       "Domain" "=" value
                |       "Max-Age" "=" value
                |       "Path" "=" value
                |       "Port" [ "=" <"> 1#port-list <"> ]
                |       "Secure"
                |       "Version" "=" 1*DIGIT
port-list       |       1#DIGIT

Informally, the Set-Cookie2 response header comprises the token Set-
Cookie2:, followed by a comma-separated list of one or more cookies.
Each cookie begins with a NAME=VALUE pair, followed by zero or more
semi-colon-separated attribute-value pairs.  The syntax for attribute-
value pairs was shown earlier.  The specific attributes and the
semantics of their values follows.  The NAME=VALUE attribute-value pair
must come first in each cookie.  The others, if present, can occur in



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any order.  If an attribute appears more than once in a cookie, the
behavior is undefined.

NAME=VALUE
     Required.  The name of the state information (``cookie'') is NAME,
     and its value is VALUE.  NAMEs that begin with $ are reserved for
     other uses and must not be used by applications.

     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=comment
     Optional.  Because cookies can contain private information about a
     user, 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.

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

Domain=domain
     Optional.  The Domain attribute specifies the domain for which the
     cookie is valid.  An explicitly specified domain must always start
     with a dot.

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

Path=path
     Optional.  The Path attribute specifies the subset of URLs on the
     origin server to which this cookie applies.

Port[="port-list"]
     Optional.  The Port attribute restricts the port to which a cookie
     may be returned in a Cookie request header.

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
     confidentially and authenticity of the information in the cookie.



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     The user agent (possibly under the user's control) may determine
     what level of security it considers appropriate for ``secure''
     cookies.  The Secure attribute should be considered security advice
     from the server to the user agent, indicating that it is in the
     session's interest to protect the cookie contents.  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=version
     Required.  The Version attribute, a decimal integer, identifies to
     which version of the state management specification the cookie
     conforms.  For this specification, Version=1 applies.

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

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

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

   * To suppress caching of the Set-Cookie2 header: Cache-control: no-
     cache="set-cookie2".

and one of the following:

   * To suppress caching of a private document in shared caches: Cache-
     control: private.

   * To allow caching of a document and require that it be validated
     before returning it to the client: Cache-Control: must-revalidate,
     max-age=0.

   * To allow caching of a document, but to require that proxy caches
     (not user agent caches) validate it before returning it to the
     client: Cache-Control: proxy-revalidate, max-age=0.

   * To allow caching of a document and request that it be validated
     before returning it to the client (by ``pre-expiring'' it): Cache-
     control: max-age=0.  Not all caches will revalidate the document in



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     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
upstream 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-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-values pairs whose attribute it
does not recognize.  The user agent applies these defaults for optional
attributes that are missing:

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

Domain Defaults to the request-host.  (Note that there is no dot at the
       beginning of request-host.)

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

Path   Defaults to the path of the request URL that generated the Set-
       Cookie2 response, up to, but not including, the right-most /.

Port   The default behavior is that a cookie may be returned only to its
       request-port.

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

4.3.2  Rejecting Cookies  To prevent possible security or privacy
violations, a user agent rejects a cookie (shall not store its
information) if any of the following is true of the attributes
explicitly present in the Set-Cookie2 response header:

   * The value for the Path attribute is not a prefix of the request-
     URI.

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

   * The value for the request-host does not domain-match the Domain
     attribute.




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   * 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.

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

Examples:

   * A Set-Cookie2 from request-host y.x.foo.com for Domain=.foo.com
     would be rejected, because H is y.x and contains a dot.

   * A Set-Cookie2 from request-host x.foo.com for Domain=.foo.com would
     be accepted.

   * A Set-Cookie2 with Domain=.com or Domain=.com., will always be
     rejected, because there is no embedded dot.

   * A Set-Cookie2 with Domain=ajax.com will be rejected because the
     value for Domain does not begin with a dot.

   * 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.

4.3.3  Cookie Management  If a user agent receives a Set-Cookie2
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-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.

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



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destruction).

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

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

   * the request-host and port;

   * the request-URI;

   * the cookie's age.

The syntax for the header is:

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

The value of the cookie-version attribute must be the value from the
Version attribute 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 Path attribute, if any, of the
corresponding Set-Cookie2 response header.  Otherwise the attribute
should be omitted from the Cookie request header.  The value for the
domain attribute must be the value from the Domain attribute, if any, of
the corresponding Set-Cookie2 response header.  Otherwise the attribute
should be omitted from the Cookie request header.

Note that there is no Comment attribute in the Cookie request header
corresponding to the one in the Set-Cookie2 response header.  The user
agent does not return the comment information to the origin server.

The user agent applies the following rules to choose applicable cookie-
values to send in Cookie request headers from among all the cookies it
has received.

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

Port Selection
     There are three possible behaviors, depending on the Port attribute
     in the Set-Cookie2 response header:



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       1.  By default (no Port attribute), the cookie may be sent to any
           port.

       2.  If the attribute is present but has no value (e.g., Port),
           the cookie must only be sent to its request port.

       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
           port-list.

Path Selection
     The Path attribute of the cookie must match a prefix of the
     request-URI.

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

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

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

4.3.5  Sending Cookies in Unverifiable Transactions  Users must have
control over sessions in order to ensure privacy.  (See PRIVACY section
below.)  To simplify implementation and to prevent an additional layer
of complexity where adequate safeguards exist, however, this document
distinguishes between transactions that are verifiable and those that
are unverifiable.  A transaction is verifiable if the user, 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.

When it makes an unverifiable transaction, a user agent must enable a
session only if a cookie with a domain attribute D was sent or accepted
in its origin transaction, such that the host name in the Request-URI of
the unverifiable transaction domain-matches D.

This restriction prevents a malicious service author from using
unverifiable transactions to induce a user agent to start or continue a
session with a server in a different domain.  The starting or
continuation of such sessions could be contrary to the privacy



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expectations of the user, and could also be a security problem.

User agents may offer configurable options that allow the user agent, or
any autonomous programs that the user agent executes, to ignore the
above rule, so long as these override options default to ``off.''

Many current user agents already provide a review option that would
render many links verifiable.  For instance, some user agents display
the URL that would be referenced for a particular link when the mouse
pointer is placed over that link.  The user can therefore determine
whether to visit that site before causing the browser to do so.  (Though
not implemented on current user agents, a similar technique could be
used for a button used to submit a form -- the user agent could display
the action to be taken if the user were to select that button.)
However, even this would not make all links verifiable; for example,
links to automatically loaded images would not normally be subject to
``mouse pointer'' verification.

Many user agents also provide the option for a user to view the HTML
source of a document, or to save the source to an external file where it
can be viewed by another application.  While such an option does provide
a crude review mechanism, some users might not consider it acceptable
for this purpose.

4.4  How an Origin Server Interprets the Cookie Header

A user agent returns much of the information in the Set-Cookie2 header
to the origin server when the Path attribute matches that of a new
request.  When it receives a Cookie header, the origin server should
treat cookies with NAMEs whose prefix is $ specially, as an attribute
for the cookie.

4.5  Caching Proxy Role

One reason for separating state information from both a URL and document
content is to facilitate the scaling that caching permits.  To support
cookies, a caching proxy must obey these rules already in the HTTP
specification:

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

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

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

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



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   * 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.)

Proxies must not introduce Set-Cookie2 (Cookie) headers of their own in
proxy responses (requests).


5.  EXAMPLES

5.1  Example 1

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

  1.  User Agent -> Server

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

      User identifies self via a form.

  2.  Server -> User Agent

      HTTP/1.1 200 OK
      Set-Cookie2: Customer="WILE_E_COYOTE"; Version="1"; Path="/acme"

      Cookie reflects user's identity.

  3.  User Agent -> Server

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

      User selects an item for ``shopping basket.''

  4.  Server -> User Agent

      HTTP/1.1 200 OK
      Set-Cookie2: Part_Number="Rocket_Launcher_0001"; Version="1";
              Path="/acme"

      Shopping basket contains an item.

  5.  User Agent -> Server








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

      User selects shipping method from form.

  6.  Server -> User Agent

      HTTP/1.1 200 OK
      Set-Cookie2: Shipping="FedEx"; Version="1"; Path="/acme"

      New cookie reflects shipping method.

  7.  User Agent -> Server

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

      User chooses to process order.

  8.  Server -> User Agent

      HTTP/1.1 200 OK

      Transaction is complete.

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

5.2  Example 2

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

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

Set-Cookie2: Part_Number="Rocket_Launcher_0001"; Version="1";
        Path="/acme"

and



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Set-Cookie2: Part_Number="Riding_Rocket_0023"; Version="1";
        Path="/acme/ammo"

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

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

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

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

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

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


6.  IMPLEMENTATION CONSIDERATIONS

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

6.1  Set-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 one.

The session information can obviously be clear or encoded text that
describes state.  However, if it grows too large, it can become
unwieldy.  Therefore, an implementor might choose for the session
information to be a key to a server-side resource.  Of course, using 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.





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6.2  Stateless Pages

Caching benefits the scalability of WWW.  Therefore it is important to
reduce the number of documents that have state embedded in them
inherently.  For example, if a shopping-basket-style application always
displays a user's current basket contents on each page, those pages
cannot be cached, because each user's basket's contents would be
different.  On the other hand, if each page contains just a link that
allows the user to ``Look at My Shopping Basket,'' the page can be
cached.

6.3  Implementation Limits

Practical user agent implementations have limits on the number and size
of cookies that they can store.  In general, user agents' cookie support
should have no fixed limits.  They should strive to store as many
frequently-used cookies as possible.  Furthermore, general-use user
agents should provide each of the following minimum capabilities
individually, although not necessarily simultaneously:

   * at least 300 cookies

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

   * at least 20 cookies per unique host or domain name

User agents created for specific purposes or for limited-capacity
devices should provide at least 20 cookies of 4096 bytes, to ensure that
the user can interact with a session-based origin server.

The information in a Set-Cookie2 response header must be retained in its
entirety.  If for some reason there is inadequate space to store the
cookie, it must be discarded, not truncated.

Applications should use as few and as small cookies as possible, and
they should cope gracefully with the loss of a cookie.

6.3.1  Denial of Service Attacks  User agents may choose to set an upper
bound on the number of cookies to be stored from a given host or domain
name or on the size of the cookie information.  Otherwise a malicious
server could attempt to flood a user agent with many cookies, or large
cookies, on successive responses, which would force out cookies the user
agent had received from other servers.  However, the minima specified
above should still be supported.








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7.  PRIVACY

7.1  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 if a user subsequently fills
out a form that contains identifying information.)  This state
management specification therefore requires that a user agent give the
user control over such a possible intrusion, although the interface
through which the user is given this control is left unspecified.
However, the control mechanisms provided shall at least allow the user

   * to completely disable the sending and saving of cookies.

   * to determine whether a stateful session is in progress.

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

Such control could be provided by, for example, mechanisms

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

   * to display a visual indication that a stateful session is in
     progress.

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

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

A user agent usually begins execution with no remembered state
information.  It should be possible to configure a user agent never to
send Cookie headers, in which case it can never sustain state with 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 corrupted.





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7.2  Protocol Design

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

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


8.  SECURITY CONSIDERATIONS

8.1  Clear Text

The information in the Set-Cookie2 and Cookie headers is unprotected.
Two consequences are:

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

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

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

8.2  Cookie Spoofing

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

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

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

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

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

      The server at victim.cracker.edu should detect that the second
      cookie was not one it originated by noticing that the Domain



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      attribute is not for itself and ignore it.

8.3  Unexpected Cookie Sharing

A user agent should make every attempt to prevent the sharing of session
information between hosts that are in different domains.  Embedded or
inlined objects may cause particularly severe privacy problems if they
can be used to share cookies between disparate hosts.  For example, a
malicious server could embed cookie information for host a.com in a URI
for a CGI on host b.com.  User agent implementors are strongly
encouraged to prevent this sort of exchange whenever possible.


9.  OTHER, SIMILAR, PROPOSALS

Apart from RFC 2109, three other proposals have been made to accomplish
similar goals.  This specification is an amalgam of Kristol's State-Info
proposal and Netscape's Cookie proposal.

Brian Behlendorf proposed a Session-ID header that would be user-agent-
initiated and could be used by an origin server to track
``clicktrails.''  It would not carry any origin-server-defined state,
however.  Phillip Hallam-Baker has proposed another client-defined
session ID mechanism for similar purposes.

While both session IDs and cookies can provide a way to sustain stateful
sessions, their intended purpose is different, and, consequently, the
privacy requirements for them are different.  A user initiates session
IDs to allow servers to track progress through them, or to distinguish
multiple users on a shared machine.  Cookies are server-initiated, so
the cookie mechanism described here gives users control over something
that would otherwise take place without the users' awareness.
Furthermore, cookies convey rich, server-selected information, whereas
session IDs comprise user-selected, simple information.


10.  HISTORICAL

10.1  Compatibility with Existing Implementations

Existing cookie implementations, based on a Netscape specification, use
Set-Cookie (not Set-Cookie2) and Cookie headers.  User agents that
receive both a Set-Cookie and Set-Cookie2 response header in the same
response must combine the Set-Cookie (``old'' cookie) and Set-Cookie2
(``new'' cookie) information as described below.  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 specification.

New cookies must replace both equivalent old and new cookies.  That is,
if a user agent that follows both this specification and Netscape's



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original specification receives a Set-Cookie2 response header, and the
NAME and the Domain and Path attributes match (per the Cookie Management
section) a Netscape-style cookie, the Netscape-style cookie must be
discarded, and the user agent must retain only the cookie adhering to
this specification.

An origin server that supports user agents that are compatible both with
Netscape's original proposal and this one must, for a transition period,
send two response headers.  Set-Cookie contains the ``old'' cookie
information.  Set-Cookie2 contains the cookie information that is new to
this specification.  The rules below ensure that the two pieces get
combined correctly.  Eventually, when the majority of user agents follow
this specification, the Set-Cookie response header can be phased out,
and all cookie information can be carried in the Set-Cookie2 response
header.

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 old cookies.

Once a server receives a new cookie from a client, it may continue a
session by sending only Set-Cookie2 response headers.

10.1.1  Combining Set-Cookie and Set-Cookie2  Because it may not be sure
whether a user agent understands new cookies, an origin server should
send both a Set-Cookie and Set-Cookie2 header.  The user agent
interprets the combined headers as follows.  First, it must establish a
one-to-one correspondence between the cookies in the Set-Cookie and
Set-Cookie2 headers.  It then combines the corresponding parts (as
separated by a comma) into (the equivalent of) a single Set-Cookie2
header, adding a semi-colon separator, if necessary, between the
attributes in the Set-Cookie header and the attributes in the
corresponding Set-Cookie2 header.  Finally, the user agent interprets
the resulting Set-Cookie2 header according to this specification.  (Note
that in this case the Set-Cookie2 response header that the origin server
sends does not, by itself, conform to this specification.)

For example, assume the origin server sends n cookies.  After header
folding (to simplify the explanation), the user agent sees:

        Set-Cookie:  O1, O2, ..., On
        Set-Cookie2: N1, N2, ..., Nn

where each Oi is an old cookie and each Ni is (attributes of) a new
cookie.  The user agent must treat these two headers as if only

        Set-Cookie2: O1; N1, O2; N2, ..., On; Nn

had been sent.  (Obviously the Set-Cookie and Set-Cookie2 headers must
both contain the same number of cookies.)




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The Expires attribute (part of Netscape's specification) requires
special care.

  1.  Because the Expires attribute is defined as a date whose format
      has a comma embedded in it, user agents must be careful to turn
      the value for Expires into a quoted string.  Otherwise the comma
      could be interpreted as a separator between successive cookies.

  2.  If a Max-Age attribute is present in the Set-Cookie2 header, it
      must override any Expires attribute in the Set-Cookie header.

  3.  A user agent may choose to honor the Expires attribute if there is
      no Max-Age attribute.

10.1.2  An Example  Suppose an origin server sends the following
response headers:

Set-Cookie: Customer="WILE_E_COYOTE"; Path="/acme"
Set-Cookie2: Version="1"
Set-Cookie: Part_Number="Rocket_Launcher_0001+price=USD1000";
        Path="/acme"; Expires=Thu, 01-May-97 00:00:00 GMT
Set-Cookie2:    Max-Age=604800; Version="1"

After folding like headers together and quoting Expires, these become:

Set-Cookie: Customer="WILE_E_COYOTE"; Path="/acme",
        Part_Number="Rocket_Launcher_0001+price=USD1000";
        Path="/acme"; Expires="Thu, 01-May-97 00:00:00 GMT"
Set-Cookie2: Version="1",
        Max-Age=604800; Version="1"

After matching up and combining corresponding attributes of cookies, and
inserting semi-colons, these become:

Set-Cookie2: Customer="WILE_E_COYOTE"; Path="/acme"; Version="1",
        Part_Number="Rocket_Launcher_0001+price=USD1000";
        Path="/acme"; Expires="Thu, 01-May-97 00:00:00 GMT";
        Max-Age=604800; Version="1"

Thus, there are two cookies to be interpreted according to this
specification, one named Customer and the other named Part_Number.  The
latter has both an Expires and a Max-Age attribute; Expires must be
ignored.

10.2  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



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uncachable, or will be ``pre-expired.''  As long as caches obey
instructions not to cache documents (following Expires: <a date in the
past> or Pragma: no-cache (HTTP/1.0), or Cache-control: no-cache
(HTTP/1.1)) uncachable documents present no problem.  However, pre-
expired documents may be stored in caches.  They require validation (a
conditional GET) on each new request, but some cache operators loosen
the rules for their caches, and sometimes serve expired documents
without first validating them.  This combination of factors can lead to
cookies meant for one user later being sent to another user.  The Set-
Cookie2 and Set-Cookie headers are stored in the cache, and, although
the document is stale (expired), the cache returns the document in
response to later requests, including cached headers.


11.  ACKNOWLEDGEMENTS

This document really represents the collective efforts of the following
people, in addition to the authors: Roy Fielding, Yaron Goland, Marc
Hedlund, Ted Hardie, Koen Holtman, Shel Kaphan, Rohit Khare.


12.  AUTHORS' ADDRESSES

David M. Kristol
Bell Laboratories, Lucent Technologies
600 Mountain Ave.  Room 2A-227
Murray Hill, NJ  07974

Phone: [phone]
FAX: [phone]
Email: [e-mail]

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

Phone: [phone]
Email: [e-mail]




                                                Expires November 5, 1997










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