http-state Working GroupA. Barth
Internet-DraftU.C. Berkeley
Intended status: InformationalDecember 22, 2009
Expires: June 25, 2010

HTTP State Management Mechanism


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

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1. Introduction

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

1.1. Syntax Notation

This specification uses the Augmented Backus-Naur Form (ABNF) notation of [RFC5234].

The following core rules are included by reference, as defined in [RFC5234], Appendix B.1: ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit sequence of data), SP (space), VCHAR (any visible [USASCII] character), and WSP (whitespace).

2. Terminology

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

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

The terms request-host and request-URI refer to the values the client would send to the server as, respectively, the host (but not port) and abs_path portions of the absoluteURI (http_URL) of the HTTP request line. Note that request-host must be a FQHN. Hosts names can be specified either as an IP address or a FQHN string.

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. Overview

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

The origin server initiates a session, if it so desires, by including a Set-Cookie header in an HTTP response. (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).

A user agent returns a Cookie request header (see below) to the origin server if it chooses to continue a session. The origin server may ignore it or use it to determine the current state of the session. It may send the client a Set-Cookie response header with the same or different information, or it may send no Set-Cookie header at all.

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

An origin server may include multiple Set-Cookie headers in a response. Note that an intervening gateway MUST NOT fold multiple Set-Cookie headers into a single header.

[TODO: Overview the Set-Cookie and Cookie headers.]

3.1. Examples

[TODO: Put some examples here.

4. Protocol Description

The cookie protocol consists of two HTTP headers: the Set-Cookie header and the Cookie header. The server sends the Set-Cookie header is to the user agent in an HTTP response, causing the user agent to modify the Cookie header it returns to the server.

This section describes the syntax and semantics of the protocol. Detailed conformance requirements for user agents are given in Section [TODO].

4.3. Controlling Caching

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

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

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

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

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

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

HTTP/1.1 servers must send Expires: old-date (where old-date is a date long in the past) on responses containing Set-Cookie response headers unless they know for certain (by out of band means) that there are no downstream 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.

5. User Agent Conformance

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

5.2. Storage Model

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

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

The user agent stores the following fields about each cookie:

  • name (a sequence of bytes)
  • value (a sequence of bytes)
  • expiry (a date)
  • domain (a cookie-domain)
  • path (a sequence of bytes)
  • creation (a date)
  • last-access (a date)
  • persistent (a Boolean)
  • host-only (a Boolean)
  • secure-only (a Boolean)
  • http-only (a Boolean)

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

  1. Create a new cookie based on the parsed Set-Cookie header:
    1. Create a new cookie with the following default field values:
      • name = the cookie-name
      • value = the cookie-value
      • expiry = the latest representable date
      • domain = the request-host
      • path = the cookie's default-path
      • last-access = the date and time the cookie was received
      • persistent = false
      • host-only = true
      • secure-only = false
      • http-only = false
    2. Update the default field values according to the cookie-attributes:
      If the cookie-attributes contains at least one valid Expires attribute, store the expiry-value of the last such attribute in the expiry field. Store the value true in the persistent field. [TODO: Test that this really works when mixing Max-Age and Expires.]
      If the cookie-attributes contains at least one Domain attribute, store the value of the last such attribute in the domain field. Store the value false in the host-only field. [TODO: Reject cookies for unrelated domains.] [TODO: If the URL's host is an IP address, let Domain to be an IP address if it matches the URL's host exactly, but set the host-only flag. ]
      If the cookie-attributes contains at least one Path attribute, store the value of the last such attribute in the path field.
      If the cookie-attributes contains at least one Secure attribute, store the value true in the secure-only field.
      If the cookie-attributes contains at least one HttpOnly attribute, store the value true in the http-only field.
  2. Remove from the cookie store all cookies that have the share the same name, domain, path, and host-only fields as the newly created cookie. [TODO: Validate this list!] [TODO: There's some funny business around http-only here.]
  3. Insert the newly created cookie into the cookie store.

The user agent MUST evict a cookie from the cookie store if A cookie exists in the cookie store with an expiry date in the past.

The user agent MAY evict a cookie from the cookie store if the number of cookies sharing a domain field exceeds some predetermined upper bound (such as 50 cookies). [TODO: Explain where 50 comes from.]

The user agent MAY evict cookies from the cookie store if the cookie store exceeds some maximum storage bound (such as 3000 cookies). [TODO: Explain where 3000 comes from.]

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

  1. Cookies with an expiry date in the past.
  2. Cookies that share a domain field more than a predetermined number of other cookies.
  3. All other cookies.

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

When "the current session is over", the user agent MUST remove from the cookie store all cookies with the persistent field set to false.

  • NOTE: This document does not define when "the current session is over." Many user agents remove non-persistent cookies when they exit. However, other user agent expire non-persistent cookies using other heuristics.

6. Implementation Considerations

6.2. Implementation Limits

Practical user agent implementations have limits on the number and size of cookies that they can store. General-use user agents SHOULD provide each of the following minimum capabilities:

  • At least 4096 bytes per cookie (as measured by the size of the characters that comprise the cookie non-terminal in the syntax description of the Set-Cookie header). [TODO: Validate]
  • At least 50 cookies per domain. [TODO: History lesson]
  • At least 3000 cookies total.

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

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

7. Security Considerations

7.1. Clear Text

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

  1. Any sensitive information that is conveyed in in the headers is exposed to an eavesdropper.
  2. A malicious intermediary could alter the headers as they travel in either direction, with unpredictable results.
  3. A malicious client could alter the Cookie header before transmission, with unpredictable results.

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

7.2. Weak Isolation

[TODO: Weak isolation by port.]

[TODO: Weak isolation by scheme (e.g., ftp, gopher, etc).]

8. Other, Similar, Proposals

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

9. Normative References

Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1”, RFC 2616, June 1999.
Crocker, D., Ed. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF”, STD 68, RFC 5234, January 2008.

Appendix A. Acknowledgements

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

Appendix B. Tabled Items

Tabled items:

Author's Address

Adam Barth
University of California, Berkeley