HTTP State Management Mechanism

The origin server initiates a session, if it so desires. To do so, it returns an extra response header to the client, Set-Cookie2. (The details follow later.)¶

A user agent returns a Cookie request header (see below) to the origin server if it chooses to continue a session. The origin server MAY ignore it or use it to determine the current state of the session. It MAY send back to the client a Set-Cookie2 response header with the same or different information, or it MAY send no Set-Cookie2 header at all. The origin server effectively ends a session by sending the client a Set-Cookie2 header with Max-Age=0.¶

Servers MAY return Set-Cookie2 response headers with any response. User agents SHOULD send Cookie request headers, subject to other rules detailed below, with every request.¶

An origin server MAY include multiple Set-Cookie2 headers in a response. Note that an intervening gateway could fold multiple such headers into a single header.¶

The syntax for the Set-Cookie2 response header is¶

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 any order. If an attribute appears more than once in a cookie, the client SHALL use only the value associated with the first appearance of the attribute; a client MUST ignore values after the first.¶

The NAME of a cookie MAY be the same as one of the attributes in this specification. However, because the cookie's NAME must come first in a Set-Cookie2 response header, the NAME and its VALUE cannot be confused with an attribute-value pair.¶

NAME=VALUE

REQUIRED. The name of the state information ("cookie") is NAME, and its value is VALUE. NAMEs that begin with $ are reserved and MUST NOT be used by applications.

The VALUE is opaque to the user agent and may be anything the origin server chooses to send, possibly in a server-selected printable ASCII encoding. "Opaque" implies that the content is of interest and relevance only to the origin server. The content may, in fact, be readable by anyone that examines the Set-Cookie2 header.

Comment=value

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

CommentURL="http_URL"

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

Discard

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

Domain=value

OPTIONAL. The value of the Domain attribute specifies the domain for which the cookie is valid. If an explicitly specified value does not start with a dot, the user agent supplies a leading dot.

Max-Age=value

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

Path=value

OPTIONAL. The value of the Path attribute specifies the subset of URLs on the origin server to which this cookie applies.

Port[="portlist"]

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

Secure

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

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

Version=value

REQUIRED. The value of the Version attribute, a decimal integer, identifies the version of the state management specification to which the cookie conforms. For this specification, Version=1 applies.

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 every case.

HTTP/1.1 servers MUST send Expires: old-date (where old-date is a date long in the past) on responses containing Set-Cookie2 response headers unless they know for certain (by out of band means) that there are no HTTP/1.0 proxies in the response chain. HTTP/1.1 servers MAY send other Cache-Control directives that permit caching by HTTP/1.1 proxies in addition to the Expires: old-date directive; the Cache-Control directive will override the Expires: old-date for HTTP/1.1 proxies.¶

Discard

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

Domain

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

Max-Age

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

Path

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

Port

The default behavior is that a cookie MAY be returned to any request-port.

Secure

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

To prevent possible security or privacy violations, a user agent rejects a cookie according to rules below. The goal of the rules is to try to limit the set of servers for which a cookie is valid, based on the values of the Path, Domain, and Port attributes and the request-URI, request-host and request-port.¶

A user agent rejects (SHALL NOT store its information) if the Version attribute is missing. Moreover, a user agent rejects (SHALL NOT store its information) if any of the following is true of the attributes explicitly present in the Set-Cookie2 response header: ¶

• The value for the Path attribute is not a prefix of the request-URI.
• The value for the Domain attribute contains no embedded dots, and the value is not .local.
• The effective host name that derives from the request-host does not domain-match the Domain attribute.
• The request-host is a HDN (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 accepted, and the value for Domain will be taken to be .ajax.com, because a dot gets prepended to the value.
• 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.
• A Set-Cookie2 from request-host example for Domain=.local will be accepted, because the effective host name for the request- host is example.local, and example.local domain-matches .local.

If a user agent receives a Set-Cookie2 response header whose NAME is the same as that of a cookie it has previously stored, the new cookie supersedes the old when: the old and new Domain attribute values compare equal, using a case-insensitive string-compare; and, the old and new Path attribute values string-compare equal (case-sensitive). However, if the Set-Cookie2 has a value for Max-Age of zero, the (old and new) cookie is discarded. Otherwise a cookie persists (resources permitting) until whichever happens first, then gets discarded: its Max-Age lifetime is exceeded; or, if the Discard attribute is set, the user agent terminates the session.¶

Because user agents have finite space in which to store cookies, they MAY also discard older cookies to make space for newer ones, using, for example, a least-recently-used algorithm, along with constraints on the maximum number of cookies that each origin server may set.¶

If a Set-Cookie2 response header includes a Comment attribute, the user agent SHOULD store that information in a human-readable form with the cookie and SHOULD display the comment text as part of a cookie inspection user interface.¶

If a Set-Cookie2 response header includes a CommentURL attribute, the user agent SHOULD store that information in a human-readable form with the cookie, or, preferably, SHOULD allow the user to follow the http_URL link as part of a cookie inspection user interface.¶

The cookie inspection user interface may include a facility whereby a user can decide, at the time the user agent receives the Set-Cookie2 response header, whether or not to accept the cookie. A potentially confusing situation could arise if the following sequence occurs: ¶

• the user agent receives a cookie that contains a CommentURL attribute;
• the user agent's cookie inspection interface is configured so that it presents a dialog to the user before the user agent accepts the cookie;
• the dialog allows the user to follow the CommentURL link when the user agent receives the cookie; and,
• when the user follows the CommentURL link, the origin server (or another server, via other links in the returned content) returns another cookie.

The user agent SHOULD NOT send any cookies in this context. The user agent MAY discard any cookie it receives in this context that the user has not, through some user agent mechanism, deemed acceptable.¶

User agents SHOULD allow the user to control cookie destruction, but they MUST NOT extend the cookie's lifetime beyond that controlled by the Discard and Max-Age attributes. An infrequently-used cookie may function as a "preferences file" for network applications, and a user may wish to keep it even if it is the least-recently-used cookie. One possible implementation would be an interface that allows the permanent storage of a cookie through a checkbox (or, conversely, its immediate destruction).¶

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

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

• the request-host and request-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] [";" port]
cookie-version  =  "$Version" "=" value
NAME            =  attr
VALUE           =  value
path            =  "$Path" "=" value
domain          =  "$Domain" "=" value
port            =  "$Port" [ "=" <"> value <"> ]

The value of the cookie-version attribute MUST be the value from the Version attribute of the corresponding Set-Cookie2 response header. Otherwise the value for cookie-version is 0. The value for the path attribute MUST be the value from the Path attribute, if one was present, 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 one was present, of the corresponding Set-Cookie2 response header. Otherwise the attribute SHOULD be omitted from the Cookie request header.¶

The port attribute of the Cookie request header MUST mirror the Port attribute, if one was present, in the corresponding Set-Cookie2 response header. That is, the port attribute MUST be present if the Port attribute was present in the Set-Cookie2 header, and it MUST have the same value, if any. Otherwise, if the Port attribute was absent from the Set-Cookie2 header, the attribute likewise MUST be omitted from the Cookie request header.¶

Note that there is neither a Comment nor a CommentURL attribute in the Cookie request header corresponding to the ones in the Set-Cookie2 response header. The user agent does not return the comment information to the origin server.¶

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

Domain Selection

The origin server's effective host name MUST domain-match the Domain attribute of the cookie.

Port Selection

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

1. By default (no Port attribute), the cookie MAY be sent to any port.
2. If the attribute is present but has no value (e.g., Port), the cookie MUST only be sent to the request-port it was received from.
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 request-URI MUST path-match the Path attribute of the cookie.

Max-Age Selection

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

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

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

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

Here the value for cookie-version is the highest version of cookie specification (currently 1) that the client understands. The client needs to send at most one such request header per request.¶

Users MUST have control over sessions in order to ensure privacy. (See PRIVACY section below.) To simplify implementation and to prevent an additional layer of complexity where adequate safeguards exist, however, this document distinguishes between transactions that are verifiable and those that are unverifiable. A transaction is verifiable if the user, or a user-designated agent, has the option to review the request-URI prior to its use in the transaction. A transaction is unverifiable if the user does not have that option. Unverifiable transactions typically arise when a user agent automatically requests inlined or embedded entities or when it resolves redirection (3xx) responses from an origin server. Typically the origin transaction, the transaction that the user initiates, is verifiable, and that transaction may directly or indirectly induce the user agent to make unverifiable transactions.¶

An unverifiable transaction is to a third-party host if its request-host U does not domain-match the reach R of the request-host O in the origin transaction.¶

When it makes an unverifiable transaction, a user agent MUST disable all cookie processing (i.e., MUST NOT send cookies, and MUST NOT accept any received cookies) if the transaction is to a third-party host.¶

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

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

(N.B. Mechanisms may be proposed that will automate overriding the third-party restrictions under controlled conditions.)¶

Many current user agents already provide a review option that would render many links verifiable. For instance, some user agents display the URL that would be referenced for a particular link when the mouse pointer is placed over that link. The user can therefore determine whether to visit that site before causing the browser to do so. (Though not implemented on current user agents, a similar technique 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.¶

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.¶