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GeezerVison is showing datatracker.ietf.org — RFC 7232: Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests Internet Engineering Task Force (IETF) R. Fielding, Ed. Request for Comments: 7232 Adobe Obsoletes: 2616 J. Reschke, Ed. Category: Standards Track greenbytes ISSN: 2070-1721 June 2014 RFC 7232 HTTP/1.1 Conditional Requests June 2014 Copyright Notice Copyright (c) 2014 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English. Fielding & Reschke Standards Track [Page 2] RFC 7232 HTTP/1.1 Conditional Requests June 2014 Table of Contents 1. Introduction ....................................................4 1.1. Conformance and Error Handling .............................4 1.2. Syntax Notation ............................................4 2. Validators ......................................................5 2.1. Weak versus Strong .........................................5 2.2. Last-Modified ..............................................7 2.2.1. Generation ..........................................7 2.2.2. Comparison ..........................................8 2.3. ETag .......................................................9 2.3.1. Generation .........................................10 2.3.2. Comparison .........................................10 2.3.3. Example: Entity-Tags Varying on Content-Negotiated Resources .......................11 2.4. When to Use Entity-Tags and Last-Modified Dates ...........12 3. Precondition Header Fields .....................................13 3.1. If-Match ..................................................13 3.2. If-None-Match .............................................14 3.3. If-Modified-Since .........................................16 3.4. If-Unmodified-Since .......................................17 3.5. If-Range ..................................................18 4. Status Code Definitions ........................................18 4.1. 304 Not Modified ..........................................18 4.2. 412 Precondition Failed ...................................19 5. Evaluation .....................................................19 6. Precedence .....................................................20 7. IANA Considerations ............................................22 7.1. Status Code Registration ..................................22 7.2. Header Field Registration .................................22 8. Security Considerations ........................................22 9. Acknowledgments ................................................23 10. References ....................................................24 10.1. Normative References .....................................24 10.2. Informative References ...................................24 Appendix A. Changes from RFC 2616 .................................25 Appendix B. Imported ABNF .........................................25 Appendix C. Collected ABNF ........................................26 Index .............................................................27 Fielding & Reschke Standards Track [Page 3] RFC 7232 HTTP/1.1 Conditional Requests June 2014 RFC 7232 HTTP/1.1 Conditional Requests June 2014 repetition). Appendix B describes rules imported from other documents. Appendix C shows the collected grammar with all list operators expanded to standard ABNF notation. RFC 7232 HTTP/1.1 Conditional Requests June 2014 There are a variety of strong validators used in practice. The best are based on strict revision control, wherein each change to a representation always results in a unique node name and revision identifier being assigned before the representation is made accessible to GET. A collision-resistant hash function applied to the representation data is also sufficient if the data is available prior to the response header fields being sent and the digest does not need to be recalculated every time a validation request is received. However, if a resource has distinct representations that differ only in their metadata, such as might occur with content negotiation over media types that happen to share the same data format, then the origin server needs to incorporate additional information in the validator to distinguish those representations. In contrast, a "weak validator" is representation metadata that might not change for every change to the representation data. This weakness might be due to limitations in how the value is calculated, such as clock resolution, an inability to ensure uniqueness for all possible representations of the resource, or a desire of the resource owner to group representations by some self-determined set of equivalency rather than unique sequences of data. An origin server SHOULD change a weak entity-tag whenever it considers prior representations to be unacceptable as a substitute for the current representation. In other words, a weak entity-tag ought to change whenever the origin server wants caches to invalidate old responses. For example, the representation of a weather report that changes in content every second, based on dynamic measurements, might be grouped into sets of equivalent representations (from the origin server's perspective) with the same weak validator in order to allow cached representations to be valid for a reasonable period of time (perhaps adjusted dynamically based on server load or weather quality). Likewise, a representation's modification time, if defined with only one-second resolution, might be a weak validator if it is possible for the representation to be modified twice during a single second and retrieved between those modifications. Likewise, a validator is weak if it is shared by two or more representations of a given resource at the same time, unless those representations have identical representation data. For example, if the origin server sends the same validator for a representation with a gzip content coding applied as it does for a representation with no content coding, then that validator is weak. However, two simultaneous representations might share the same strong validator if they differ only in the representation metadata, such as when two different media types are available for the same representation data. Fielding & Reschke Standards Track [Page 6] RFC 7232 HTTP/1.1 Conditional Requests June 2014 Strong validators are usable for all conditional requests, including cache validation, partial content ranges, and "lost update" avoidance. Weak validators are only usable when the client does not require exact equality with previously obtained representation data, such as when validating a cache entry or limiting a web traversal to recent changes. RFC 7232 HTTP/1.1 Conditional Requests June 2014 metadata that evaluates to some time in the future, according to the origin server's clock, then the origin server MUST replace that value with the message origination date. This prevents a future modification date from having an adverse impact on cache validation. An origin server without a clock MUST NOT assign Last-Modified values to a response unless these values were associated with the resource by some other system or user with a reliable clock. RFC 7232 HTTP/1.1 Conditional Requests June 2014 This method relies on the fact that if two different responses were sent by the origin server during the same second, but both had the same Last-Modified time, then at least one of those responses would have a Date value equal to its Last-Modified time. The arbitrary 60-second limit guards against the possibility that the Date and Last-Modified values are generated from different clocks or at somewhat different times during the preparation of the response. An implementation MAY use a value larger than 60 seconds, if it is believed that 60 seconds is too short. RFC 7232 HTTP/1.1 Conditional Requests June 2014 An entity-tag can be either a weak or strong validator, with strong being the default. If an origin server provides an entity-tag for a representation and the generation of that entity-tag does not satisfy all of the characteristics of a strong validator (Section 2.1), then the origin server MUST mark the entity-tag as weak by prefixing its opaque value with "W/" (case-sensitive). RFC 7232 HTTP/1.1 Conditional Requests June 2014 The example below shows the results for a set of entity-tag pairs and both the weak and strong comparison function results: +--------+--------+-------------------+-----------------+ | ETag 1 | ETag 2 | Strong Comparison | Weak Comparison | +--------+--------+-------------------+-----------------+ | W/"1" | W/"1" | no match | match | | W/"1" | W/"2" | no match | no match | | W/"1" | "1" | no match | match | | "1" | "1" | match | match | +--------+--------+-------------------+-----------------+ RFC 7232 HTTP/1.1 Conditional Requests June 2014 An alternative representation that does use gzip content coding would be: >> Response: HTTP/1.1 200 OK Date: Fri, 26 Mar 2010 00:05:00 GMT ETag: "123-b" Content-Length: 43 Vary: Accept-Encoding Content-Type: text/plain Content-Encoding: gzip ...binary data... Note: Content codings are a property of the representation data, so a strong entity-tag for a content-encoded representation has to be distinct from the entity tag of an unencoded representation to prevent potential conflicts during cache updates and range requests. In contrast, transfer codings (Section 4 of [RFC7230]) apply only during message transfer and do not result in distinct entity-tags. RFC 7232 HTTP/1.1 Conditional Requests June 2014 o SHOULD send the Last-Modified value in non-subrange cache validation requests (using If-Modified-Since) if only a Last-Modified value has been provided by the origin server. o MAY send the Last-Modified value in subrange cache validation requests (using If-Unmodified-Since) if only a Last-Modified value has been provided by an HTTP/1.0 origin server. The user agent SHOULD provide a way to disable this, in case of difficulty. o SHOULD send both validators in cache validation requests if both an entity-tag and a Last-Modified value have been provided by the origin server. This allows both HTTP/1.0 and HTTP/1.1 caches to respond appropriately. RFC 7232 HTTP/1.1 Conditional Requests June 2014 prevent the "lost update" problem). It can also be used with safe methods to abort a request if the selected representation does not match one already stored (or partially stored) from a prior request. An origin server that receives an If-Match header field MUST evaluate the condition prior to performing the method (Section 5). If the field-value is "*", the condition is false if the origin server does not have a current representation for the target resource. If the field-value is a list of entity-tags, the condition is false if none of the listed tags match the entity-tag of the selected representation. An origin server MUST NOT perform the requested method if a received If-Match condition evaluates to false; instead, the origin server MUST respond with either a) the 412 (Precondition Failed) status code or b) one of the 2xx (Successful) status codes if the origin server has verified that a state change is being requested and the final state is already reflected in the current state of the target resource (i.e., the change requested by the user agent has already succeeded, but the user agent might not be aware of it, perhaps because the prior response was lost or a compatible change was made by some other user agent). In the latter case, the origin server MUST NOT send a validator header field in the response unless it can verify that the request is a duplicate of an immediately prior change made by the same user agent. The If-Match header field can be ignored by caches and intermediaries because it is not applicable to a stored response. RFC 7232 HTTP/1.1 Conditional Requests June 2014 Examples: If-None-Match: "xyzzy" If-None-Match: W/"xyzzy" If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz" If-None-Match: * If-None-Match is primarily used in conditional GET requests to enable efficient updates of cached information with a minimum amount of transaction overhead. When a client desires to update one or more stored responses that have entity-tags, the client SHOULD generate an If-None-Match header field containing a list of those entity-tags when making a GET request; this allows recipient servers to send a 304 (Not Modified) response to indicate when one of those stored responses matches the selected representation. If-None-Match can also be used with a value of "*" to prevent an unsafe request method (e.g., PUT) from inadvertently modifying an existing representation of the target resource when the client believes that the resource does not have a current representation (Section 4.2.1 of [RFC7231]). This is a variation on the "lost update" problem that might arise if more than one client attempts to create an initial representation for the target resource. An origin server that receives an If-None-Match header field MUST evaluate the condition prior to performing the method (Section 5). If the field-value is "*", the condition is false if the origin server has a current representation for the target resource. If the field-value is a list of entity-tags, the condition is false if one of the listed tags match the entity-tag of the selected representation. An origin server MUST NOT perform the requested method if the condition evaluates to false; instead, the origin server MUST respond with either a) the 304 (Not Modified) status code if the request method is GET or HEAD or b) the 412 (Precondition Failed) status code for all other request methods. Requirements on cache handling of a received If-None-Match header field are defined in Section 4.3.2 of [RFC7234]. Fielding & Reschke Standards Track [Page 15] RFC 7232 HTTP/1.1 Conditional Requests June 2014 RFC 7232 HTTP/1.1 Conditional Requests June 2014 When used for limiting the scope of retrieval to a recent time window, a user agent will generate an If-Modified-Since field value based on either its own local clock or a Date header field received from the server in a prior response. Origin servers that choose an exact timestamp match based on the selected representation's Last-Modified field will not be able to help the user agent limit its data transfers to only those changed during the specified window. An origin server that receives an If-Modified-Since header field SHOULD evaluate the condition prior to performing the method (Section 5). The origin server SHOULD NOT perform the requested method if the selected representation's last modification date is earlier than or equal to the date provided in the field-value; instead, the origin server SHOULD generate a 304 (Not Modified) response, including only those metadata that are useful for identifying or updating a previously cached response. Requirements on cache handling of a received If-Modified-Since header field are defined in Section 4.3.2 of [RFC7234]. RFC 7232 HTTP/1.1 Conditional Requests June 2014 If-Unmodified-Since is most often used with state-changing methods (e.g., POST, PUT, DELETE) to prevent accidental overwrites when multiple user agents might be acting in parallel on a resource that does not supply entity-tags with its representations (i.e., to prevent the "lost update" problem). It can also be used with safe methods to abort a request if the selected representation does not match one already stored (or partially stored) from a prior request. An origin server that receives an If-Unmodified-Since header field MUST evaluate the condition prior to performing the method (Section 5). The origin server MUST NOT perform the requested method if the selected representation's last modification date is more recent than the date provided in the field-value; instead the origin server MUST respond with either a) the 412 (Precondition Failed) status code or b) one of the 2xx (Successful) status codes if the origin server has verified that a state change is being requested and the final state is already reflected in the current state of the target resource (i.e., the change requested by the user agent has already succeeded, but the user agent might not be aware of that because the prior response message was lost or a compatible change was made by some other user agent). In the latter case, the origin server MUST NOT send a validator header field in the response unless it can verify that the request is a duplicate of an immediately prior change made by the same user agent. The If-Unmodified-Since header field can be ignored by caches and intermediaries because it is not applicable to a stored response. RFC 7232 HTTP/1.1 Conditional Requests June 2014 conditional, already has a valid representation; the server is therefore redirecting the client to make use of that stored representation as if it were the payload of a 200 (OK) response. The server generating a 304 response MUST generate any of the following header fields that would have been sent in a 200 (OK) response to the same request: Cache-Control, Content-Location, Date, ETag, Expires, and Vary. Since the goal of a 304 response is to minimize information transfer when the recipient already has one or more cached representations, a sender SHOULD NOT generate representation metadata other than the above listed fields unless said metadata exists for the purpose of guiding cache updates (e.g., Last-Modified might be useful if the response does not have an ETag field). Requirements on a cache that receives a 304 response are defined in Section 4.3.4 of [RFC7234]. If the conditional request originated with an outbound client, such as a user agent with its own cache sending a conditional GET to a shared proxy, then the proxy SHOULD forward the 304 response to that client. A 304 response cannot contain a message-body; it is always terminated by the first empty line after the header fields. RFC 7232 HTTP/1.1 Conditional Requests June 2014 A server that is not the origin server for the target resource and cannot act as a cache for requests on the target resource MUST NOT evaluate the conditional request header fields defined by this specification, and it MUST forward them if the request is forwarded, since the generating client intends that they be evaluated by a server that can provide a current representation. Likewise, a server MUST ignore the conditional request header fields defined by this specification when received with a request method that does not involve the selection or modification of a selected representation, such as CONNECT, OPTIONS, or TRACE. Conditional request header fields that are defined by extensions to HTTP might place conditions on all recipients, on the state of the target resource in general, or on a group of resources. For instance, the "If" header field in WebDAV can make a request conditional on various aspects of multiple resources, such as locks, if the recipient understands and implements that field ([RFC4918], Section 10.4). Although conditional request header fields are defined as being usable with the HEAD method (to keep HEAD's semantics consistent with those of GET), there is no point in sending a conditional HEAD because a successful response is around the same size as a 304 (Not Modified) response and more useful than a 412 (Precondition Failed) response. RFC 7232 HTTP/1.1 Conditional Requests June 2014 2. When recipient is the origin server, If-Match is not present, and If-Unmodified-Since is present, evaluate the If-Unmodified-Since precondition: * if true, continue to step 3 * if false, respond 412 (Precondition Failed) unless it can be determined that the state-changing request has already succeeded (see Section 3.4) 3. When If-None-Match is present, evaluate the If-None-Match precondition: * if true, continue to step 5 * if false for GET/HEAD, respond 304 (Not Modified) * if false for other methods, respond 412 (Precondition Failed) 4. When the method is GET or HEAD, If-None-Match is not present, and If-Modified-Since is present, evaluate the If-Modified-Since precondition: * if true, continue to step 5 * if false, respond 304 (Not Modified) 5. When the method is GET and both Range and If-Range are present, evaluate the If-Range precondition: * if the validator matches and the Range specification is applicable to the selected representation, respond 206 (Partial Content) [RFC7233] 6. Otherwise, * all conditions are met, so perform the requested action and respond according to its success or failure. Any extension to HTTP/1.1 that defines additional conditional request header fields ought to define its own expectations regarding the order for evaluating such fields in relation to those defined in this document and other conditionals that might be found in practice. Fielding & Reschke Standards Track [Page 21] RFC 7232 HTTP/1.1 Conditional Requests June 2014 RFC 7232 HTTP/1.1 Conditional Requests June 2014 The validators defined by this specification are not intended to ensure the validity of a representation, guard against malicious changes, or detect man-in-the-middle attacks. At best, they enable more efficient cache updates and optimistic concurrent writes when all participants are behaving nicely. At worst, the conditions will fail and the client will receive a response that is no more harmful than an HTTP exchange without conditional requests. An entity-tag can be abused in ways that create privacy risks. For example, a site might deliberately construct a semantically invalid entity-tag that is unique to the user or user agent, send it in a cacheable response with a long freshness time, and then read that entity-tag in later conditional requests as a means of re-identifying that user or user agent. Such an identifying tag would become a persistent identifier for as long as the user agent retained the original cache entry. User agents that cache representations ought to ensure that the cache is cleared or replaced whenever the user performs privacy-maintaining actions, such as clearing stored cookies or changing to a private browsing mode. RFC 7232 HTTP/1.1 Conditional Requests June 2014 RFC 7232 HTTP/1.1 Conditional Requests June 2014 RFC 7232 HTTP/1.1 Conditional Requests June 2014 RFC 7232 HTTP/1.1 Conditional Requests June 2014 Index 3 304 Not Modified (status code) 19 4 412 Precondition Failed (status code) 18 E ETag header field 9 G Grammar entity-tag 9 ETag 9 etagc 9 If-Match 13 If-Modified-Since 15 If-None-Match 14 If-Unmodified-Since 17 Last-Modified 7 opaque-tag 9 weak 9 I If-Match header field 13 If-Modified-Since header field 16 If-None-Match header field 14 If-Unmodified-Since header field 17 L Last-Modified header field 7 M metadata 5 S selected representation 4 V validator 5 strong 5 weak 5 Fielding & Reschke Standards Track [Page 27] RFC 7232 HTTP/1.1 Conditional Requests June 2014 Authors' Addresses Roy T. Fielding (editor) Adobe Systems Incorporated 345 Park Ave San Jose, CA 95110 USA EMail: [email protected] URI: http://roy.gbiv.com/ Julian F. Reschke (editor) greenbytes GmbH Hafenweg 16 Muenster, NW 48155 Germany EMail: [email protected] URI: http://greenbytes.de/tech/webdav/ Fielding & Reschke Standards Track [Page 28]
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