7 Network Working Group D. Kristol
8 Request for Comments: 2965 Bell Laboratories, Lucent Technologies
9 Obsoletes: 2109 L. Montulli
10 Category: Standards Track Epinions.com, Inc.
14 HTTP State Management Mechanism
18 This document specifies an Internet standards track protocol for the
19 Internet community, and requests discussion and suggestions for
20 improvements. Please refer to the current edition of the "Internet
21 Official Protocol Standards" (STD 1) for the standardization state
22 and status of this protocol. Distribution of this memo is unlimited.
26 Copyright (C) The Internet Society (2000). All Rights Reserved.
30 The IESG notes that this mechanism makes use of the .local top-level
31 domain (TLD) internally when handling host names that don't contain
32 any dots, and that this mechanism might not work in the expected way
33 should an actual .local TLD ever be registered.
37 This document specifies a way to create a stateful session with
38 Hypertext Transfer Protocol (HTTP) requests and responses. It
39 describes three new headers, Cookie, Cookie2, and Set-Cookie2, which
40 carry state information between participating origin servers and user
41 agents. The method described here differs from Netscape's Cookie
42 proposal [Netscape], but it can interoperate with HTTP/1.0 user
43 agents that use Netscape's method. (See the HISTORICAL section.)
45 This document reflects implementation experience with RFC 2109 and
50 The terms user agent, client, server, proxy, origin server, and
51 http_URL have the same meaning as in the HTTP/1.1 specification
52 [RFC2616]. The terms abs_path and absoluteURI have the same meaning
53 as in the URI Syntax specification [RFC2396].
58 Kristol & Montulli Standards Track [Page 1]
60 RFC 2965 HTTP State Management Mechanism October 2000
63 Host name (HN) means either the host domain name (HDN) or the numeric
64 Internet Protocol (IP) address of a host. The fully qualified domain
65 name is preferred; use of numeric IP addresses is strongly
68 The terms request-host and request-URI refer to the values the client
69 would send to the server as, respectively, the host (but not port)
70 and abs_path portions of the absoluteURI (http_URL) of the HTTP
71 request line. Note that request-host is a HN.
73 The term effective host name is related to host name. If a host name
74 contains no dots, the effective host name is that name with the
75 string .local appended to it. Otherwise the effective host name is
76 the same as the host name. Note that all effective host names
77 contain at least one dot.
79 The term request-port refers to the port portion of the absoluteURI
80 (http_URL) of the HTTP request line. If the absoluteURI has no
81 explicit port, the request-port is the HTTP default, 80. The
82 request-port of a cookie is the request-port of the request in which
83 a Set-Cookie2 response header was returned to the user agent.
85 Host names can be specified either as an IP address or a HDN string.
86 Sometimes we compare one host name with another. (Such comparisons
87 SHALL be case-insensitive.) Host A's name domain-matches host B's if
89 * their host name strings string-compare equal; or
91 * A is a HDN string and has the form NB, where N is a non-empty
92 name string, B has the form .B', and B' is a HDN string. (So,
93 x.y.com domain-matches .Y.com but not Y.com.)
95 Note that domain-match is not a commutative operation: a.b.c.com
96 domain-matches .c.com, but not the reverse.
98 The reach R of a host name H is defined as follows:
102 - H is the host domain name of a host; and,
104 - H has the form A.B; and
106 - A has no embedded (that is, interior) dots; and
108 - B has at least one embedded dot, or B is the string "local".
109 then the reach of H is .B.
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116 RFC 2965 HTTP State Management Mechanism October 2000
119 * Otherwise, the reach of H is H.
121 For two strings that represent paths, P1 and P2, P1 path-matches P2
122 if P2 is a prefix of P1 (including the case where P1 and P2 string-
123 compare equal). Thus, the string /tec/waldo path-matches /tec.
125 Because it was used in Netscape's original implementation of state
126 management, we will use the term cookie to refer to the state
127 information that passes between an origin server and user agent, and
128 that gets stored by the user agent.
132 The key words "MAY", "MUST", "MUST NOT", "OPTIONAL", "RECOMMENDED",
133 "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT" in this
134 document are to be interpreted as described in RFC 2119 [RFC2119].
136 2. STATE AND SESSIONS
138 This document describes a way to create stateful sessions with HTTP
139 requests and responses. Currently, HTTP servers respond to each
140 client request without relating that request to previous or
141 subsequent requests; the state management mechanism allows clients
142 and servers that wish to exchange state information to place HTTP
143 requests and responses within a larger context, which we term a
144 "session". This context might be used to create, for example, a
145 "shopping cart", in which user selections can be aggregated before
146 purchase, or a magazine browsing system, in which a user's previous
147 reading affects which offerings are presented.
149 Neither clients nor servers are required to support cookies. A
150 server MAY refuse to provide content to a client that does not return
151 the cookies it sends.
155 We describe here a way for an origin server to send state information
156 to the user agent, and for the user agent to return the state
157 information to the origin server. The goal is to have a minimal
158 impact on HTTP and user agents.
162 The two state management headers, Set-Cookie2 and Cookie, have common
163 syntactic properties involving attribute-value pairs. The following
164 grammar uses the notation, and tokens DIGIT (decimal digits), token
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172 RFC 2965 HTTP State Management Mechanism October 2000
175 (informally, a sequence of non-special, non-white space characters),
176 and http_URL from the HTTP/1.1 specification [RFC2616] to describe
179 av-pairs = av-pair *(";" av-pair)
180 av-pair = attr ["=" value] ; optional value
182 value = token | quoted-string
184 Attributes (names) (attr) are case-insensitive. White space is
185 permitted between tokens. Note that while the above syntax
186 description shows value as optional, most attrs require them.
188 NOTE: The syntax above allows whitespace between the attribute and
191 3.2 Origin Server Role
193 3.2.1 General The origin server initiates a session, if it so
194 desires. To do so, it returns an extra response header to the
195 client, Set-Cookie2. (The details follow later.)
197 A user agent returns a Cookie request header (see below) to the
198 origin server if it chooses to continue a session. The origin server
199 MAY ignore it or use it to determine the current state of the
200 session. It MAY send back to the client a Set-Cookie2 response
201 header with the same or different information, or it MAY send no
202 Set-Cookie2 header at all. The origin server effectively ends a
203 session by sending the client a Set-Cookie2 header with Max-Age=0.
205 Servers MAY return Set-Cookie2 response headers with any response.
206 User agents SHOULD send Cookie request headers, subject to other
207 rules detailed below, with every request.
209 An origin server MAY include multiple Set-Cookie2 headers in a
210 response. Note that an intervening gateway could fold multiple such
211 headers into a single header.
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228 RFC 2965 HTTP State Management Mechanism October 2000
231 3.2.2 Set-Cookie2 Syntax The syntax for the Set-Cookie2 response
234 set-cookie = "Set-Cookie2:" cookies
236 cookie = NAME "=" VALUE *(";" set-cookie-av)
239 set-cookie-av = "Comment" "=" value
240 | "CommentURL" "=" <"> http_URL <">
243 | "Max-Age" "=" value
245 | "Port" [ "=" <"> portlist <"> ]
247 | "Version" "=" 1*DIGIT
251 Informally, the Set-Cookie2 response header comprises the token Set-
252 Cookie2:, followed by a comma-separated list of one or more cookies.
253 Each cookie begins with a NAME=VALUE pair, followed by zero or more
254 semi-colon-separated attribute-value pairs. The syntax for
255 attribute-value pairs was shown earlier. The specific attributes and
256 the semantics of their values follows. The NAME=VALUE attribute-
257 value pair MUST come first in each cookie. The others, if present,
258 can occur in any order. If an attribute appears more than once in a
259 cookie, the client SHALL use only the value associated with the first
260 appearance of the attribute; a client MUST ignore values after the
263 The NAME of a cookie MAY be the same as one of the attributes in this
264 specification. However, because the cookie's NAME must come first in
265 a Set-Cookie2 response header, the NAME and its VALUE cannot be
266 confused with an attribute-value pair.
269 REQUIRED. The name of the state information ("cookie") is NAME,
270 and its value is VALUE. NAMEs that begin with $ are reserved and
271 MUST NOT be used by applications.
273 The VALUE is opaque to the user agent and may be anything the
274 origin server chooses to send, possibly in a server-selected
275 printable ASCII encoding. "Opaque" implies that the content is of
276 interest and relevance only to the origin server. The content
277 may, in fact, be readable by anyone that examines the Set-Cookie2
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284 RFC 2965 HTTP State Management Mechanism October 2000
288 OPTIONAL. Because cookies can be used to derive or store private
289 information about a user, the value of the Comment attribute
290 allows an origin server to document how it intends to use the
291 cookie. The user can inspect the information to decide whether to
292 initiate or continue a session with this cookie. Characters in
293 value MUST be in UTF-8 encoding. [RFC2279]
295 CommentURL="http_URL"
296 OPTIONAL. Because cookies can be used to derive or store private
297 information about a user, the CommentURL attribute allows an
298 origin server to document how it intends to use the cookie. The
299 user can inspect the information identified by the URL to decide
300 whether to initiate or continue a session with this cookie.
303 OPTIONAL. The Discard attribute instructs the user agent to
304 discard the cookie unconditionally when the user agent terminates.
307 OPTIONAL. The value of the Domain attribute specifies the domain
308 for which the cookie is valid. If an explicitly specified value
309 does not start with a dot, the user agent supplies a leading dot.
312 OPTIONAL. The value of the Max-Age attribute is delta-seconds,
313 the lifetime of the cookie in seconds, a decimal non-negative
314 integer. To handle cached cookies correctly, a client SHOULD
315 calculate the age of the cookie according to the age calculation
316 rules in the HTTP/1.1 specification [RFC2616]. When the age is
317 greater than delta-seconds seconds, the client SHOULD discard the
318 cookie. A value of zero means the cookie SHOULD be discarded
322 OPTIONAL. The value of the Path attribute specifies the subset of
323 URLs on the origin server to which this cookie applies.
326 OPTIONAL. The Port attribute restricts the port to which a cookie
327 may be returned in a Cookie request header. Note that the syntax
328 REQUIREs quotes around the OPTIONAL portlist even if there is only
329 one portnum in portlist.
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344 OPTIONAL. The Secure attribute (with no value) directs the user
345 agent to use only (unspecified) secure means to contact the origin
346 server whenever it sends back this cookie, to protect the
347 confidentially and authenticity of the information in the cookie.
349 The user agent (possibly with user interaction) MAY determine what
350 level of security it considers appropriate for "secure" cookies.
351 The Secure attribute should be considered security advice from the
352 server to the user agent, indicating that it is in the session's
353 interest to protect the cookie contents. When it sends a "secure"
354 cookie back to a server, the user agent SHOULD use no less than
355 the same level of security as was used when it received the cookie
359 REQUIRED. The value of the Version attribute, a decimal integer,
360 identifies the version of the state management specification to
361 which the cookie conforms. For this specification, Version=1
364 3.2.3 Controlling Caching An origin server must be cognizant of the
365 effect of possible caching of both the returned resource and the
366 Set-Cookie2 header. Caching "public" documents is desirable. For
367 example, if the origin server wants to use a public document such as
368 a "front door" page as a sentinel to indicate the beginning of a
369 session for which a Set-Cookie2 response header must be generated,
370 the page SHOULD be stored in caches "pre-expired" so that the origin
371 server will see further requests. "Private documents", for example
372 those that contain information strictly private to a session, SHOULD
373 NOT be cached in shared caches.
375 If the cookie is intended for use by a single user, the Set-Cookie2
376 header SHOULD NOT be cached. A Set-Cookie2 header that is intended
377 to be shared by multiple users MAY be cached.
379 The origin server SHOULD send the following additional HTTP/1.1
380 response headers, depending on circumstances:
382 * To suppress caching of the Set-Cookie2 header:
384 Cache-control: no-cache="set-cookie2"
386 and one of the following:
388 * To suppress caching of a private document in shared caches:
390 Cache-control: private
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396 RFC 2965 HTTP State Management Mechanism October 2000
399 * To allow caching of a document and require that it be validated
400 before returning it to the client:
402 Cache-Control: must-revalidate, max-age=0
404 * To allow caching of a document, but to require that proxy
405 caches (not user agent caches) validate it before returning it
408 Cache-Control: proxy-revalidate, max-age=0
410 * To allow caching of a document and request that it be validated
411 before returning it to the client (by "pre-expiring" it):
413 Cache-control: max-age=0
415 Not all caches will revalidate the document in every case.
417 HTTP/1.1 servers MUST send Expires: old-date (where old-date is a
418 date long in the past) on responses containing Set-Cookie2 response
419 headers unless they know for certain (by out of band means) that
420 there are no HTTP/1.0 proxies in the response chain. HTTP/1.1
421 servers MAY send other Cache-Control directives that permit caching
422 by HTTP/1.1 proxies in addition to the Expires: old-date directive;
423 the Cache-Control directive will override the Expires: old-date for
428 3.3.1 Interpreting Set-Cookie2 The user agent keeps separate track
429 of state information that arrives via Set-Cookie2 response headers
430 from each origin server (as distinguished by name or IP address and
431 port). The user agent MUST ignore attribute-value pairs whose
432 attribute it does not recognize. The user agent applies these
433 defaults for optional attributes that are missing:
435 Discard The default behavior is dictated by the presence or absence
436 of a Max-Age attribute.
438 Domain Defaults to the effective request-host. (Note that because
439 there is no dot at the beginning of effective request-host,
440 the default Domain can only domain-match itself.)
442 Max-Age The default behavior is to discard the cookie when the user
445 Path Defaults to the path of the request URL that generated the
446 Set-Cookie2 response, up to and including the right-most /.
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452 RFC 2965 HTTP State Management Mechanism October 2000
455 Port The default behavior is that a cookie MAY be returned to any
458 Secure If absent, the user agent MAY send the cookie over an
461 3.3.2 Rejecting Cookies To prevent possible security or privacy
462 violations, a user agent rejects a cookie according to rules below.
463 The goal of the rules is to try to limit the set of servers for which
464 a cookie is valid, based on the values of the Path, Domain, and Port
465 attributes and the request-URI, request-host and request-port.
467 A user agent rejects (SHALL NOT store its information) if the Version
468 attribute is missing. Moreover, a user agent rejects (SHALL NOT
469 store its information) if any of the following is true of the
470 attributes explicitly present in the Set-Cookie2 response header:
472 * The value for the Path attribute is not a prefix of the
475 * The value for the Domain attribute contains no embedded dots,
476 and the value is not .local.
478 * The effective host name that derives from the request-host does
479 not domain-match the Domain attribute.
481 * The request-host is a HDN (not IP address) and has the form HD,
482 where D is the value of the Domain attribute, and H is a string
483 that contains one or more dots.
485 * The Port attribute has a "port-list", and the request-port was
490 * A Set-Cookie2 from request-host y.x.foo.com for Domain=.foo.com
491 would be rejected, because H is y.x and contains a dot.
493 * A Set-Cookie2 from request-host x.foo.com for Domain=.foo.com
496 * A Set-Cookie2 with Domain=.com or Domain=.com., will always be
497 rejected, because there is no embedded dot.
499 * A Set-Cookie2 with Domain=ajax.com will be accepted, and the
500 value for Domain will be taken to be .ajax.com, because a dot
501 gets prepended to the value.
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511 * A Set-Cookie2 with Port="80,8000" will be accepted if the
512 request was made to port 80 or 8000 and will be rejected
515 * A Set-Cookie2 from request-host example for Domain=.local will
516 be accepted, because the effective host name for the request-
517 host is example.local, and example.local domain-matches .local.
519 3.3.3 Cookie Management If a user agent receives a Set-Cookie2
520 response header whose NAME is the same as that of a cookie it has
521 previously stored, the new cookie supersedes the old when: the old
522 and new Domain attribute values compare equal, using a case-
523 insensitive string-compare; and, the old and new Path attribute
524 values string-compare equal (case-sensitive). However, if the Set-
525 Cookie2 has a value for Max-Age of zero, the (old and new) cookie is
526 discarded. Otherwise a cookie persists (resources permitting) until
527 whichever happens first, then gets discarded: its Max-Age lifetime is
528 exceeded; or, if the Discard attribute is set, the user agent
529 terminates the session.
531 Because user agents have finite space in which to store cookies, they
532 MAY also discard older cookies to make space for newer ones, using,
533 for example, a least-recently-used algorithm, along with constraints
534 on the maximum number of cookies that each origin server may set.
536 If a Set-Cookie2 response header includes a Comment attribute, the
537 user agent SHOULD store that information in a human-readable form
538 with the cookie and SHOULD display the comment text as part of a
539 cookie inspection user interface.
541 If a Set-Cookie2 response header includes a CommentURL attribute, the
542 user agent SHOULD store that information in a human-readable form
543 with the cookie, or, preferably, SHOULD allow the user to follow the
544 http_URL link as part of a cookie inspection user interface.
546 The cookie inspection user interface may include a facility whereby a
547 user can decide, at the time the user agent receives the Set-Cookie2
548 response header, whether or not to accept the cookie. A potentially
549 confusing situation could arise if the following sequence occurs:
551 * the user agent receives a cookie that contains a CommentURL
554 * the user agent's cookie inspection interface is configured so
555 that it presents a dialog to the user before the user agent
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567 * the dialog allows the user to follow the CommentURL link when
568 the user agent receives the cookie; and,
570 * when the user follows the CommentURL link, the origin server
571 (or another server, via other links in the returned content)
572 returns another cookie.
574 The user agent SHOULD NOT send any cookies in this context. The user
575 agent MAY discard any cookie it receives in this context that the
576 user has not, through some user agent mechanism, deemed acceptable.
578 User agents SHOULD allow the user to control cookie destruction, but
579 they MUST NOT extend the cookie's lifetime beyond that controlled by
580 the Discard and Max-Age attributes. An infrequently-used cookie may
581 function as a "preferences file" for network applications, and a user
582 may wish to keep it even if it is the least-recently-used cookie. One
583 possible implementation would be an interface that allows the
584 permanent storage of a cookie through a checkbox (or, conversely, its
585 immediate destruction).
587 Privacy considerations dictate that the user have considerable
588 control over cookie management. The PRIVACY section contains more
591 3.3.4 Sending Cookies to the Origin Server When it sends a request
592 to an origin server, the user agent includes a Cookie request header
593 if it has stored cookies that are applicable to the request, based on
595 * the request-host and request-port;
601 The syntax for the header is:
603 cookie = "Cookie:" cookie-version 1*((";" | ",") cookie-value)
604 cookie-value = NAME "=" VALUE [";" path] [";" domain] [";" port]
605 cookie-version = "$Version" "=" value
608 path = "$Path" "=" value
609 domain = "$Domain" "=" value
610 port = "$Port" [ "=" <"> value <"> ]
612 The value of the cookie-version attribute MUST be the value from the
613 Version attribute of the corresponding Set-Cookie2 response header.
614 Otherwise the value for cookie-version is 0. The value for the path
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623 attribute MUST be the value from the Path attribute, if one was
624 present, of the corresponding Set-Cookie2 response header. Otherwise
625 the attribute SHOULD be omitted from the Cookie request header. The
626 value for the domain attribute MUST be the value from the Domain
627 attribute, if one was present, of the corresponding Set-Cookie2
628 response header. Otherwise the attribute SHOULD be omitted from the
629 Cookie request header.
631 The port attribute of the Cookie request header MUST mirror the Port
632 attribute, if one was present, in the corresponding Set-Cookie2
633 response header. That is, the port attribute MUST be present if the
634 Port attribute was present in the Set-Cookie2 header, and it MUST
635 have the same value, if any. Otherwise, if the Port attribute was
636 absent from the Set-Cookie2 header, the attribute likewise MUST be
637 omitted from the Cookie request header.
639 Note that there is neither a Comment nor a CommentURL attribute in
640 the Cookie request header corresponding to the ones in the Set-
641 Cookie2 response header. The user agent does not return the comment
642 information to the origin server.
644 The user agent applies the following rules to choose applicable
645 cookie-values to send in Cookie request headers from among all the
646 cookies it has received.
649 The origin server's effective host name MUST domain-match the
650 Domain attribute of the cookie.
653 There are three possible behaviors, depending on the Port
654 attribute in the Set-Cookie2 response header:
656 1. By default (no Port attribute), the cookie MAY be sent to any
659 2. If the attribute is present but has no value (e.g., Port), the
660 cookie MUST only be sent to the request-port it was received
663 3. If the attribute has a port-list, the cookie MUST only be
664 returned if the new request-port is one of those listed in
668 The request-URI MUST path-match the Path attribute of the cookie.
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680 Cookies that have expired should have been discarded and thus are
681 not forwarded to an origin server.
683 If multiple cookies satisfy the criteria above, they are ordered in
684 the Cookie header such that those with more specific Path attributes
685 precede those with less specific. Ordering with respect to other
686 attributes (e.g., Domain) is unspecified.
688 Note: For backward compatibility, the separator in the Cookie header
689 is semi-colon (;) everywhere. A server SHOULD also accept comma (,)
690 as the separator between cookie-values for future compatibility.
692 3.3.5 Identifying What Version is Understood: Cookie2 The Cookie2
693 request header facilitates interoperation between clients and servers
694 that understand different versions of the cookie specification. When
695 the client sends one or more cookies to an origin server, if at least
696 one of those cookies contains a $Version attribute whose value is
697 different from the version that the client understands, then the
698 client MUST also send a Cookie2 request header, the syntax for which
701 cookie2 = "Cookie2:" cookie-version
703 Here the value for cookie-version is the highest version of cookie
704 specification (currently 1) that the client understands. The client
705 needs to send at most one such request header per request.
707 3.3.6 Sending Cookies in Unverifiable Transactions Users MUST have
708 control over sessions in order to ensure privacy. (See PRIVACY
709 section below.) To simplify implementation and to prevent an
710 additional layer of complexity where adequate safeguards exist,
711 however, this document distinguishes between transactions that are
712 verifiable and those that are unverifiable. A transaction is
713 verifiable if the user, or a user-designated agent, has the option to
714 review the request-URI prior to its use in the transaction. A
715 transaction is unverifiable if the user does not have that option.
716 Unverifiable transactions typically arise when a user agent
717 automatically requests inlined or embedded entities or when it
718 resolves redirection (3xx) responses from an origin server.
719 Typically the origin transaction, the transaction that the user
720 initiates, is verifiable, and that transaction may directly or
721 indirectly induce the user agent to make unverifiable transactions.
723 An unverifiable transaction is to a third-party host if its request-
724 host U does not domain-match the reach R of the request-host O in the
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735 When it makes an unverifiable transaction, a user agent MUST disable
736 all cookie processing (i.e., MUST NOT send cookies, and MUST NOT
737 accept any received cookies) if the transaction is to a third-party
740 This restriction prevents a malicious service author from using
741 unverifiable transactions to induce a user agent to start or continue
742 a session with a server in a different domain. The starting or
743 continuation of such sessions could be contrary to the privacy
744 expectations of the user, and could also be a security problem.
746 User agents MAY offer configurable options that allow the user agent,
747 or any autonomous programs that the user agent executes, to ignore
748 the above rule, so long as these override options default to "off".
750 (N.B. Mechanisms may be proposed that will automate overriding the
751 third-party restrictions under controlled conditions.)
753 Many current user agents already provide a review option that would
754 render many links verifiable. For instance, some user agents display
755 the URL that would be referenced for a particular link when the mouse
756 pointer is placed over that link. The user can therefore determine
757 whether to visit that site before causing the browser to do so.
758 (Though not implemented on current user agents, a similar technique
759 could be used for a button used to submit a form -- the user agent
760 could display the action to be taken if the user were to select that
761 button.) However, even this would not make all links verifiable; for
762 example, links to automatically loaded images would not normally be
763 subject to "mouse pointer" verification.
765 Many user agents also provide the option for a user to view the HTML
766 source of a document, or to save the source to an external file where
767 it can be viewed by another application. While such an option does
768 provide a crude review mechanism, some users might not consider it
769 acceptable for this purpose.
771 3.4 How an Origin Server Interprets the Cookie Header
773 A user agent returns much of the information in the Set-Cookie2
774 header to the origin server when the request-URI path-matches the
775 Path attribute of the cookie. When it receives a Cookie header, the
776 origin server SHOULD treat cookies with NAMEs whose prefix is $
777 specially, as an attribute for the cookie.
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791 3.5 Caching Proxy Role
793 One reason for separating state information from both a URL and
794 document content is to facilitate the scaling that caching permits.
795 To support cookies, a caching proxy MUST obey these rules already in
796 the HTTP specification:
798 * Honor requests from the cache, if possible, based on cache
801 * Pass along a Cookie request header in any request that the
802 proxy must make of another server.
804 * Return the response to the client. Include any Set-Cookie2
807 * Cache the received response subject to the control of the usual
808 headers, such as Expires,
810 Cache-control: no-cache
814 Cache-control: private
816 * Cache the Set-Cookie2 subject to the control of the usual
819 Cache-control: no-cache="set-cookie2"
821 (The Set-Cookie2 header should usually not be cached.)
823 Proxies MUST NOT introduce Set-Cookie2 (Cookie) headers of their own
824 in proxy responses (requests).
830 Most detail of request and response headers has been omitted. Assume
831 the user agent has no stored cookies.
833 1. User Agent -> Server
835 POST /acme/login HTTP/1.1
838 User identifies self via a form.
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847 2. Server -> User Agent
850 Set-Cookie2: Customer="WILE_E_COYOTE"; Version="1"; Path="/acme"
852 Cookie reflects user's identity.
854 3. User Agent -> Server
856 POST /acme/pickitem HTTP/1.1
857 Cookie: $Version="1"; Customer="WILE_E_COYOTE"; $Path="/acme"
860 User selects an item for "shopping basket".
862 4. Server -> User Agent
865 Set-Cookie2: Part_Number="Rocket_Launcher_0001"; Version="1";
868 Shopping basket contains an item.
870 5. User Agent -> Server
872 POST /acme/shipping HTTP/1.1
873 Cookie: $Version="1";
874 Customer="WILE_E_COYOTE"; $Path="/acme";
875 Part_Number="Rocket_Launcher_0001"; $Path="/acme"
878 User selects shipping method from form.
880 6. Server -> User Agent
883 Set-Cookie2: Shipping="FedEx"; Version="1"; Path="/acme"
885 New cookie reflects shipping method.
887 7. User Agent -> Server
889 POST /acme/process HTTP/1.1
890 Cookie: $Version="1";
891 Customer="WILE_E_COYOTE"; $Path="/acme";
892 Part_Number="Rocket_Launcher_0001"; $Path="/acme";
893 Shipping="FedEx"; $Path="/acme"
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900 RFC 2965 HTTP State Management Mechanism October 2000
903 User chooses to process order.
905 8. Server -> User Agent
909 Transaction is complete.
911 The user agent makes a series of requests on the origin server, after
912 each of which it receives a new cookie. All the cookies have the
913 same Path attribute and (default) domain. Because the request-URIs
914 all path-match /acme, the Path attribute of each cookie, each request
915 contains all the cookies received so far.
919 This example illustrates the effect of the Path attribute. All
920 detail of request and response headers has been omitted. Assume the
921 user agent has no stored cookies.
923 Imagine the user agent has received, in response to earlier requests,
926 Set-Cookie2: Part_Number="Rocket_Launcher_0001"; Version="1";
931 Set-Cookie2: Part_Number="Riding_Rocket_0023"; Version="1";
934 A subsequent request by the user agent to the (same) server for URLs
935 of the form /acme/ammo/... would include the following request
938 Cookie: $Version="1";
939 Part_Number="Riding_Rocket_0023"; $Path="/acme/ammo";
940 Part_Number="Rocket_Launcher_0001"; $Path="/acme"
942 Note that the NAME=VALUE pair for the cookie with the more specific
943 Path attribute, /acme/ammo, comes before the one with the less
944 specific Path attribute, /acme. Further note that the same cookie
945 name appears more than once.
947 A subsequent request by the user agent to the (same) server for a URL
948 of the form /acme/parts/ would include the following request header:
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956 RFC 2965 HTTP State Management Mechanism October 2000
959 Cookie: $Version="1"; Part_Number="Rocket_Launcher_0001";
962 Here, the second cookie's Path attribute /acme/ammo is not a prefix
963 of the request URL, /acme/parts/, so the cookie does not get
964 forwarded to the server.
966 5. IMPLEMENTATION CONSIDERATIONS
968 Here we provide guidance on likely or desirable details for an origin
969 server that implements state management.
971 5.1 Set-Cookie2 Content
973 An origin server's content should probably be divided into disjoint
974 application areas, some of which require the use of state
975 information. The application areas can be distinguished by their
976 request URLs. The Set-Cookie2 header can incorporate information
977 about the application areas by setting the Path attribute for each
980 The session information can obviously be clear or encoded text that
981 describes state. However, if it grows too large, it can become
982 unwieldy. Therefore, an implementor might choose for the session
983 information to be a key to a server-side resource. Of course, using
984 a database creates some problems that this state management
985 specification was meant to avoid, namely:
987 1. keeping real state on the server side;
989 2. how and when to garbage-collect the database entry, in case the
990 user agent terminates the session by, for example, exiting.
994 Caching benefits the scalability of WWW. Therefore it is important
995 to reduce the number of documents that have state embedded in them
996 inherently. For example, if a shopping-basket-style application
997 always displays a user's current basket contents on each page, those
998 pages cannot be cached, because each user's basket's contents would
999 be different. On the other hand, if each page contains just a link
1000 that allows the user to "Look at My Shopping Basket", the page can be
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1012 RFC 2965 HTTP State Management Mechanism October 2000
1015 5.3 Implementation Limits
1017 Practical user agent implementations have limits on the number and
1018 size of cookies that they can store. In general, user agents' cookie
1019 support should have no fixed limits. They should strive to store as
1020 many frequently-used cookies as possible. Furthermore, general-use
1021 user agents SHOULD provide each of the following minimum capabilities
1022 individually, although not necessarily simultaneously:
1024 * at least 300 cookies
1026 * at least 4096 bytes per cookie (as measured by the characters
1027 that comprise the cookie non-terminal in the syntax description
1028 of the Set-Cookie2 header, and as received in the Set-Cookie2
1031 * at least 20 cookies per unique host or domain name
1033 User agents created for specific purposes or for limited-capacity
1034 devices SHOULD provide at least 20 cookies of 4096 bytes, to ensure
1035 that the user can interact with a session-based origin server.
1037 The information in a Set-Cookie2 response header MUST be retained in
1038 its entirety. If for some reason there is inadequate space to store
1039 the cookie, it MUST be discarded, not truncated.
1041 Applications should use as few and as small cookies as possible, and
1042 they should cope gracefully with the loss of a cookie.
1044 5.3.1 Denial of Service Attacks User agents MAY choose to set an
1045 upper bound on the number of cookies to be stored from a given host
1046 or domain name or on the size of the cookie information. Otherwise a
1047 malicious server could attempt to flood a user agent with many
1048 cookies, or large cookies, on successive responses, which would force
1049 out cookies the user agent had received from other servers. However,
1050 the minima specified above SHOULD still be supported.
1054 Informed consent should guide the design of systems that use cookies.
1055 A user should be able to find out how a web site plans to use
1056 information in a cookie and should be able to choose whether or not
1057 those policies are acceptable. Both the user agent and the origin
1058 server must assist informed consent.
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1068 RFC 2965 HTTP State Management Mechanism October 2000
1071 6.1 User Agent Control
1073 An origin server could create a Set-Cookie2 header to track the path
1074 of a user through the server. Users may object to this behavior as
1075 an intrusive accumulation of information, even if their identity is
1076 not evident. (Identity might become evident, for example, if a user
1077 subsequently fills out a form that contains identifying information.)
1078 This state management specification therefore requires that a user
1079 agent give the user control over such a possible intrusion, although
1080 the interface through which the user is given this control is left
1081 unspecified. However, the control mechanisms provided SHALL at least
1084 * to completely disable the sending and saving of cookies.
1086 * to determine whether a stateful session is in progress.
1088 * to control the saving of a cookie on the basis of the cookie's
1091 Such control could be provided, for example, by mechanisms
1093 * to notify the user when the user agent is about to send a
1094 cookie to the origin server, to offer the option not to begin a
1097 * to display a visual indication that a stateful session is in
1100 * to let the user decide which cookies, if any, should be saved
1101 when the user concludes a window or user agent session.
1103 * to let the user examine and delete the contents of a cookie at
1106 A user agent usually begins execution with no remembered state
1107 information. It SHOULD be possible to configure a user agent never
1108 to send Cookie headers, in which case it can never sustain state with
1109 an origin server. (The user agent would then behave like one that is
1110 unaware of how to handle Set-Cookie2 response headers.)
1112 When the user agent terminates execution, it SHOULD let the user
1113 discard all state information. Alternatively, the user agent MAY ask
1114 the user whether state information should be retained; the default
1115 should be "no". If the user chooses to retain state information, it
1116 would be restored the next time the user agent runs.
1122 Kristol & Montulli Standards Track [Page 20]
1124 RFC 2965 HTTP State Management Mechanism October 2000
1127 NOTE: User agents should probably be cautious about using files to
1128 store cookies long-term. If a user runs more than one instance of
1129 the user agent, the cookies could be commingled or otherwise
1132 6.2 Origin Server Role
1134 An origin server SHOULD promote informed consent by adding CommentURL
1135 or Comment information to the cookies it sends. CommentURL is
1136 preferred because of the opportunity to provide richer information in
1137 a multiplicity of languages.
1141 The information in the Set-Cookie2 and Cookie headers is unprotected.
1144 1. Any sensitive information that is conveyed in them is exposed
1147 2. A malicious intermediary could alter the headers as they travel
1148 in either direction, with unpredictable results.
1150 These facts imply that information of a personal and/or financial
1151 nature should only be sent over a secure channel. For less sensitive
1152 information, or when the content of the header is a database key, an
1153 origin server should be vigilant to prevent a bad Cookie value from
1156 A user agent in a shared user environment poses a further risk.
1157 Using a cookie inspection interface, User B could examine the
1158 contents of cookies that were saved when User A used the machine.
1160 7. SECURITY CONSIDERATIONS
1164 The restrictions on the value of the Domain attribute, and the rules
1165 concerning unverifiable transactions, are meant to reduce the ways
1166 that cookies can "leak" to the "wrong" site. The intent is to
1167 restrict cookies to one host, or a closely related set of hosts.
1168 Therefore a request-host is limited as to what values it can set for
1169 Domain. We consider it acceptable for hosts host1.foo.com and
1170 host2.foo.com to share cookies, but not a.com and b.com.
1172 Similarly, a server can set a Path only for cookies that are related
1178 Kristol & Montulli Standards Track [Page 21]
1180 RFC 2965 HTTP State Management Mechanism October 2000
1185 Proper application design can avoid spoofing attacks from related
1188 1. User agent makes request to victim.cracker.edu, gets back
1189 cookie session_id="1234" and sets the default domain
1192 2. User agent makes request to spoof.cracker.edu, gets back cookie
1193 session-id="1111", with Domain=".cracker.edu".
1195 3. User agent makes request to victim.cracker.edu again, and
1198 Cookie: $Version="1"; session_id="1234",
1199 $Version="1"; session_id="1111"; $Domain=".cracker.edu"
1201 The server at victim.cracker.edu should detect that the second
1202 cookie was not one it originated by noticing that the Domain
1203 attribute is not for itself and ignore it.
1205 7.3 Unexpected Cookie Sharing
1207 A user agent SHOULD make every attempt to prevent the sharing of
1208 session information between hosts that are in different domains.
1209 Embedded or inlined objects may cause particularly severe privacy
1210 problems if they can be used to share cookies between disparate
1211 hosts. For example, a malicious server could embed cookie
1212 information for host a.com in a URI for a CGI on host b.com. User
1213 agent implementors are strongly encouraged to prevent this sort of
1214 exchange whenever possible.
1216 7.4 Cookies For Account Information
1218 While it is common practice to use them this way, cookies are not
1219 designed or intended to be used to hold authentication information,
1220 such as account names and passwords. Unless such cookies are
1221 exchanged over an encrypted path, the account information they
1222 contain is highly vulnerable to perusal and theft.
1224 8. OTHER, SIMILAR, PROPOSALS
1226 Apart from RFC 2109, three other proposals have been made to
1227 accomplish similar goals. This specification began as an amalgam of
1228 Kristol's State-Info proposal [DMK95] and Netscape's Cookie proposal
1234 Kristol & Montulli Standards Track [Page 22]
1236 RFC 2965 HTTP State Management Mechanism October 2000
1239 Brian Behlendorf proposed a Session-ID header that would be user-
1240 agent-initiated and could be used by an origin server to track
1241 "clicktrails". It would not carry any origin-server-defined state,
1242 however. Phillip Hallam-Baker has proposed another client-defined
1243 session ID mechanism for similar purposes.
1245 While both session IDs and cookies can provide a way to sustain
1246 stateful sessions, their intended purpose is different, and,
1247 consequently, the privacy requirements for them are different. A
1248 user initiates session IDs to allow servers to track progress through
1249 them, or to distinguish multiple users on a shared machine. Cookies
1250 are server-initiated, so the cookie mechanism described here gives
1251 users control over something that would otherwise take place without
1252 the users' awareness. Furthermore, cookies convey rich, server-
1253 selected information, whereas session IDs comprise user-selected,
1258 9.1 Compatibility with Existing Implementations
1260 Existing cookie implementations, based on the Netscape specification,
1261 use the Set-Cookie (not Set-Cookie2) header. User agents that
1262 receive in the same response both a Set-Cookie and Set-Cookie2
1263 response header for the same cookie MUST discard the Set-Cookie
1264 information and use only the Set-Cookie2 information. Furthermore, a
1265 user agent MUST assume, if it received a Set-Cookie2 response header,
1266 that the sending server complies with this document and will
1267 understand Cookie request headers that also follow this
1270 New cookies MUST replace both equivalent old- and new-style cookies.
1271 That is, if a user agent that follows both this specification and
1272 Netscape's original specification receives a Set-Cookie2 response
1273 header, and the NAME and the Domain and Path attributes match (per
1274 the Cookie Management section) a Netscape-style cookie, the
1275 Netscape-style cookie MUST be discarded, and the user agent MUST
1276 retain only the cookie adhering to this specification.
1278 Older user agents that do not understand this specification, but that
1279 do understand Netscape's original specification, will not recognize
1280 the Set-Cookie2 response header and will receive and send cookies
1281 according to the older specification.
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1292 RFC 2965 HTTP State Management Mechanism October 2000
1295 A user agent that supports both this specification and Netscape-style
1296 cookies SHOULD send a Cookie request header that follows the older
1297 Netscape specification if it received the cookie in a Set-Cookie
1298 response header and not in a Set-Cookie2 response header. However,
1299 it SHOULD send the following request header as well:
1301 Cookie2: $Version="1"
1303 The Cookie2 header advises the server that the user agent understands
1304 new-style cookies. If the server understands new-style cookies, as
1305 well, it SHOULD continue the stateful session by sending a Set-
1306 Cookie2 response header, rather than Set-Cookie. A server that does
1307 not understand new-style cookies will simply ignore the Cookie2
1310 9.2 Caching and HTTP/1.0
1312 Some caches, such as those conforming to HTTP/1.0, will inevitably
1313 cache the Set-Cookie2 and Set-Cookie headers, because there was no
1314 mechanism to suppress caching of headers prior to HTTP/1.1. This
1315 caching can lead to security problems. Documents transmitted by an
1316 origin server along with Set-Cookie2 and Set-Cookie headers usually
1317 either will be uncachable, or will be "pre-expired". As long as
1318 caches obey instructions not to cache documents (following Expires:
1319 <a date in the past> or Pragma: no-cache (HTTP/1.0), or Cache-
1320 control: no-cache (HTTP/1.1)) uncachable documents present no
1321 problem. However, pre-expired documents may be stored in caches.
1322 They require validation (a conditional GET) on each new request, but
1323 some cache operators loosen the rules for their caches, and sometimes
1324 serve expired documents without first validating them. This
1325 combination of factors can lead to cookies meant for one user later
1326 being sent to another user. The Set-Cookie2 and Set-Cookie headers
1327 are stored in the cache, and, although the document is stale
1328 (expired), the cache returns the document in response to later
1329 requests, including cached headers.
1331 10. ACKNOWLEDGEMENTS
1333 This document really represents the collective efforts of the HTTP
1334 Working Group of the IETF and, particularly, the following people, in
1335 addition to the authors: Roy Fielding, Yaron Goland, Marc Hedlund,
1336 Ted Hardie, Koen Holtman, Shel Kaphan, Rohit Khare, Foteos Macrides,
1346 Kristol & Montulli Standards Track [Page 24]
1348 RFC 2965 HTTP State Management Mechanism October 2000
1351 11. AUTHORS' ADDRESSES
1354 Bell Laboratories, Lucent Technologies
1355 600 Mountain Ave. Room 2A-333
1356 Murray Hill, NJ 07974
1358 Phone: (908) 582-2250
1360 EMail: dmk@bell-labs.com
1366 Mountain View, CA 94301
1368 EMail: lou@montulli.org
1372 [DMK95] Kristol, D.M., "Proposed HTTP State-Info Mechanism",
1373 available at <http://portal.research.bell-
1374 labs.com/~dmk/state-info.html>, September, 1995.
1376 [Netscape] "Persistent Client State -- HTTP Cookies", available at
1377 <http://www.netscape.com/newsref/std/cookie_spec.html>,
1380 [RFC2109] Kristol, D. and L. Montulli, "HTTP State Management
1381 Mechanism", RFC 2109, February 1997.
1383 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
1384 Requirement Levels", BCP 14, RFC 2119, March 1997.
1386 [RFC2279] Yergeau, F., "UTF-8, a transformation format of Unicode
1387 and ISO-10646", RFC 2279, January 1998.
1389 [RFC2396] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
1390 Resource Identifiers (URI): Generic Syntax", RFC 2396,
1393 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H. and T.
1394 Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1",
1395 RFC 2616, June 1999.
1402 Kristol & Montulli Standards Track [Page 25]
1404 RFC 2965 HTTP State Management Mechanism October 2000
1407 13. Full Copyright Statement
1409 Copyright (C) The Internet Society (2000). All Rights Reserved.
1411 This document and translations of it may be copied and furnished to
1412 others, and derivative works that comment on or otherwise explain it
1413 or assist in its implementation may be prepared, copied, published
1414 and distributed, in whole or in part, without restriction of any
1415 kind, provided that the above copyright notice and this paragraph are
1416 included on all such copies and derivative works. However, this
1417 document itself may not be modified in any way, such as by removing
1418 the copyright notice or references to the Internet Society or other
1419 Internet organizations, except as needed for the purpose of
1420 developing Internet standards in which case the procedures for
1421 copyrights defined in the Internet Standards process must be
1422 followed, or as required to translate it into languages other than
1425 The limited permissions granted above are perpetual and will not be
1426 revoked by the Internet Society or its successors or assigns.
1428 This document and the information contained herein is provided on an
1429 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
1430 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
1431 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
1432 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
1433 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
1437 Funding for the RFC Editor function is currently provided by the
1458 Kristol & Montulli Standards Track [Page 26]