|HTTPbis Working Group||R. Fielding, Editor|
|Obsoletes: 2616 (if approved)||Y. Lafon, Editor|
|Intended status: Standards Track||W3C|
|Expires: December 3, 2012||J. Reschke, Editor|
|June 1, 2012|
HTTP/1.1, part 4: Conditional Requests
The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypertext information systems. HTTP has been in use by the World Wide Web global information initiative since 1990. This document is Part 4 of the seven-part specification that defines the protocol referred to as "HTTP/1.1" and, taken together, obsoletes RFC 2616.
Part 4 defines request header fields for indicating conditional requests and the rules for constructing responses to those requests.
Discussion of this draft should take place on the HTTPBIS working group mailing list (firstname.lastname@example.org), which is archived at <http://lists.w3.org/Archives/Public/ietf-http-wg/>.
The current issues list is at <http://tools.ietf.org/wg/httpbis/trac/report/3> and related documents (including fancy diffs) can be found at <http://tools.ietf.org/wg/httpbis/>.
The changes in this draft are summarized in Appendix C.1.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work in progress”.
This Internet-Draft will expire on December 3, 2012.
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This document defines the HTTP/1.1 conditional request mechanisms, including both metadata for indicating/observing changes in resource representations and request header fields that specify preconditions on that metadata be checked before performing the request method. Conditional GET requests are the most efficient mechanism for HTTP cache updates [Part6]. Conditionals can also be applied to state-changing methods, such as PUT and DELETE, to prevent the "lost update" problem: one client accidentally overwriting the work of another client that has been acting in parallel.
Conditional request preconditions are based on the state of the target resource as a whole (its current value set) or the state as observed in a previously obtained representation (one value in that set). A resource might have multiple current representations, each with its own observable state. The conditional request mechanisms assume that the mapping of requests to corresponding representations will be consistent over time if the server intends to take advantage of conditionals. Regardless, if the mapping is inconsistent and the server is unable to select the appropriate representation, then no harm will result when the precondition evaluates to false.
We use the term "selected representation" to refer to the current representation of the target resource that would have been selected in a successful response if the same request had used the method GET and had excluded all of the conditional request header fields. The conditional request preconditions are evaluated by comparing the values provided in the request header fields to the current metadata for the selected representation.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
This document defines conformance criteria for several roles in HTTP communication, including Senders, Recipients, Clients, Servers, User-Agents, Origin Servers, Intermediaries, Proxies and Gateways. See Section 2 of [Part1] for definitions of these terms.
An implementation is considered conformant if it complies with all of the requirements associated with its role(s). Note that SHOULD-level requirements are relevant here, unless one of the documented exceptions is applicable.
This document also uses ABNF to define valid protocol elements (Section 1.2). In addition to the prose requirements placed upon them, Senders MUST NOT generate protocol elements that are invalid.
Unless noted otherwise, Recipients MAY take steps to recover a usable protocol element from an invalid construct. However, HTTP does not define specific error handling mechanisms, except in cases where it has direct impact on security. This is because different uses of the protocol require different error handling strategies; for example, a Web browser may wish to transparently recover from a response where the Location header field doesn't parse according to the ABNF, whereby in a systems control protocol using HTTP, this type of error recovery could lead to dangerous consequences.
This specification uses the Augmented Backus-Naur Form (ABNF) notation of [RFC5234] with the list rule extension defined in Section 1.2 of [Part1]. Appendix B shows the collected ABNF with the list rule expanded.
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), and VCHAR (any visible US-ASCII character).
The ABNF rules below are defined in [Part1] and [Part2]:
OWS = <OWS, defined in [Part1], Section 3.2.1> obs-text = <obs-text, defined in [Part1], Section 3.2.4> HTTP-date = <HTTP-date, defined in [Part2], Section 5.1>
This specification defines two forms of metadata that are commonly used to observe resource state and test for preconditions: modification dates and opaque entity tags. Additional metadata that reflects resource state has been defined by various extensions of HTTP, such as WebDAV [RFC4918], that are beyond the scope of this specification. A resource metadata value is referred to as a "validator" when it is used within a precondition.
Validators come in two flavors: strong or weak. Weak validators are easy to generate but are far less useful for comparisons. Strong validators are ideal for comparisons but can be very difficult (and occasionally impossible) to generate efficiently. Rather than impose that all forms of resource adhere to the same strength of validator, HTTP exposes the type of validator in use and imposes restrictions on when weak validators can be used as preconditions.
A "strong validator" is a representation metadata value that MUST be changed to a new, previously unused or guaranteed unique, value whenever a change occurs to the representation data such that a change would be observable in the payload body of a 200 response to GET. A strong validator MAY be changed for other reasons, such as when a semantically significant part of the representation metadata is changed (e.g., Content-Type), but it is in the best interests of the origin server to only change the value when it is necessary to invalidate the stored responses held by remote caches and authoring tools. A strong validator MUST be unique across all representations of a given resource, such that no two representations of that resource share the same validator unless their payload body would be identical.
Cache entries might persist for arbitrarily long periods, regardless of expiration times. Thus, a cache might attempt to validate an entry using a validator that it obtained in the distant past. A strong validator MUST be unique across all versions of all representations associated with a particular resource over time. However, there is no implication of uniqueness across representations of different resources (i.e., the same strong validator might be in use for representations of multiple resources at the same time and does not imply that those representations are equivalent).
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 cryptographic 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 a server SHOULD incorporate additional information in the validator to distinguish those representations and avoid confusing cache behavior.
In contrast, a "weak validator" is a representation metadata value that might not be changed for every change to the representation data. This weakness might be due to limitations in how the value is calculated, such as clock resolution or an inability to ensure uniqueness for all possible representations of the resource, or due to a desire by the resource owner to group representations by some self-determined set of equivalency rather than unique sequences of data. A weak entity-tag SHOULD change whenever the origin server considers prior representations to be unacceptable as a substitute for the current representation. In other words, a weak entity-tag SHOULD 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.
A "use" of a validator occurs when either a client generates a request and includes the validator in a precondition or when a server compares two validators. Weak validators are only usable in contexts that do not depend on exact equality of a representation's payload body. Strong validators are usable and preferred for all conditional requests, including cache validation, partial content ranges, and "lost update" avoidance.
The "Last-Modified" header field indicates the date and time at which the origin server believes the selected representation was last modified.
Last-Modified = HTTP-date
An example of its use is
Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
Origin servers SHOULD send Last-Modified for any selected representation for which a last modification date can be reasonably and consistently determined, since its use in conditional requests and evaluating cache freshness ([Part6]) results in a substantial reduction of HTTP traffic on the Internet and can be a significant factor in improving service scalability and reliability.
A representation is typically the sum of many parts behind the resource interface. The last-modified time would usually be the most recent time that any of those parts were changed. How that value is determined for any given resource is an implementation detail beyond the scope of this specification. What matters to HTTP is how recipients of the Last-Modified header field can use its value to make conditional requests and test the validity of locally cached responses.
An origin server SHOULD obtain the Last-Modified value of the representation as close as possible to the time that it generates the Date field-value for its response. This allows a recipient to make an accurate assessment of the representation's modification time, especially if the representation changes near the time that the response is generated.
An origin server with a clock MUST NOT send a Last-Modified date that is later than the server's time of message origination (Date). If the last modification time is derived from implementation-specific 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.
A Last-Modified time, when used as a validator in a request, is implicitly weak unless it is possible to deduce that it is strong, using the following rules:
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.
The ETag header field provides the current entity-tag for the selected representation. An entity-tag is an opaque validator for differentiating between multiple representations of the same resource, regardless of whether those multiple representations are due to resource state changes over time, content negotiation resulting in multiple representations being valid at the same time, or both. An entity-tag consists of an opaque quoted string, possibly prefixed by a weakness indicator.
ETag = entity-tag entity-tag = [ weak ] opaque-tag weak = %x57.2F ; "W/", case-sensitive opaque-tag = DQUOTE *etagc DQUOTE etagc = %x21 / %x23-7E / obs-text ; VCHAR except double quotes, plus obs-text
Note: Previously, opaque-tag was defined to be a quoted-string ([RFC2616], Section 3.11), thus some recipients might perform backslash unescaping. Servers therefore ought to avoid backslash characters in entity tags.
An entity-tag can be more reliable for validation than a modification date in situations where it is inconvenient to store modification dates, where the one-second resolution of HTTP date values is not sufficient, or where modification dates are not consistently maintained.
ETag: "xyzzy" ETag: W/"xyzzy" ETag: ""
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 the requirements for a strong validator (Section 2.1), then that entity-tag MUST be marked as weak by prefixing its opaque value with "W/" (case-sensitive).
The principle behind entity-tags is that only the service author knows the implementation of a resource well enough to select the most accurate and efficient validation mechanism for that resource, and that any such mechanism can be mapped to a simple sequence of octets for easy comparison. Since the value is opaque, there is no need for the client to be aware of how each entity-tag is constructed.
For example, a resource that has implementation-specific versioning applied to all changes might use an internal revision number, perhaps combined with a variance identifier for content negotiation, to accurately differentiate between representations. Other implementations might use a stored hash of representation content, a combination of various filesystem attributes, or a modification timestamp that has sub-second resolution.
Origin servers SHOULD send ETag for any selected representation for which detection of changes can be reasonably and consistently determined, since the entity-tag's use in conditional requests and evaluating cache freshness ([Part6]) can result in a substantial reduction of HTTP network traffic and can be a significant factor in improving service scalability and reliability.
There are two entity-tag comparison functions, depending on whether the comparison context allows the use of weak validators or not:
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/"2"||no match||no match|
Consider a resource that is subject to content negotiation (Section 8 of [Part2]), and where the representations returned upon a GET request vary based on the Accept-Encoding request header field (Section 9.3 of [Part2]):
GET /index HTTP/1.1 Host: www.example.com Accept-Encoding: gzip
In this case, the response might or might not use the gzip content coding. If it does not, the response might look like:
HTTP/1.1 200 OK Date: Thu, 26 Mar 2010 00:05:00 GMT ETag: "123-a" Content-Length: 70 Vary: Accept-Encoding Content-Type: text/plain Hello World! Hello World! Hello World! Hello World! Hello World!
An alternative representation that does use gzip content coding would be:
HTTP/1.1 200 OK Date: Thu, 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, so therefore an entity-tag of an encoded representation has to be distinct from an unencoded representation to prevent conflicts during cache updates and range requests. In contrast, transfer codings (Section 4 of [Part1]) apply only during message transfer and do not require distinct entity-tags.
We adopt a set of rules and recommendations for origin servers, clients, and caches regarding when various validator types ought to be used, and for what purposes.
HTTP/1.1 origin servers:
In other words, the preferred behavior for an HTTP/1.1 origin server is to send both a strong entity-tag and a Last-Modified value.
An HTTP/1.1 origin server, upon receiving a conditional request that includes both a Last-Modified date (e.g., in an If-Modified-Since or If-Unmodified-Since header field) and one or more entity-tags (e.g., in an If-Match, If-None-Match, or If-Range header field) as cache validators, MUST NOT return a response status code of 304 (Not Modified) unless doing so is consistent with all of the conditional header fields in the request.
An HTTP/1.1 caching proxy, upon receiving a conditional request that includes both a Last-Modified date and one or more entity-tags as cache validators, MUST NOT return a locally cached response to the client unless that cached response is consistent with all of the conditional header fields in the request.
This section defines the syntax and semantics of HTTP/1.1 header fields for applying preconditions on requests.
The "If-Match" header field MAY be used to make a request method conditional on the current existence or value of an entity-tag for one or more representations of the target resource. If-Match is generally useful for resource update requests, such as PUT requests, as a means for protecting against accidental overwrites when multiple clients are acting in parallel on the same resource (i.e., the "lost update" problem). An If-Match field-value of "*" places the precondition on the existence of any current representation for the target resource.
If-Match = "*" / 1#entity-tag
If any of the entity-tags listed in the If-Match field value match (as per Section 2.3.2) the entity-tag of the selected representation for the target resource, or if "*" is given and any current representation exists for the target resource, then the server MAY perform the request method as if the If-Match header field was not present.
If none of the entity-tags match, or if "*" is given and no current representation exists, the server MUST NOT perform the requested method. Instead, the server MUST respond with the 412 (Precondition Failed) status code.
If the request would, without the If-Match header field, result in anything other than a 2xx or 412 status code, then the If-Match header field MUST be ignored.
If-Match: "xyzzy" If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" If-Match: *
The result of a request having both an If-Match header field and either an If-None-Match or an If-Modified-Since header field is undefined by this specification.
The "If-None-Match" header field MAY be used to make a request method conditional on not matching any of the current entity-tag values for representations of the target resource. If-None-Match is primarily used in conditional GET requests to enable efficient updates of cached information with a minimum amount of transaction overhead. A client that has one or more representations previously obtained from the target resource can send If-None-Match with a list of the associated entity-tags in the hope of receiving a 304 response if at least one of those representations matches the selected representation.
If-None-Match MAY 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. 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.
If-None-Match = "*" / 1#entity-tag
If any of the entity-tags listed in the If-None-Match field-value match (as per Section 2.3.2) the entity-tag of the selected representation, or if "*" is given and any current representation exists for that resource, then the server MUST NOT perform the requested method. Instead, if the request method was GET or HEAD, the server SHOULD respond with a 304 (Not Modified) status code, including the cache-related header fields (particularly ETag) of the selected representation that has a matching entity-tag. For all other request methods, the server MUST respond with a 412 (Precondition Failed) status code.
If none of the entity-tags match, then the server MAY perform the requested method as if the If-None-Match header field did not exist, but MUST also ignore any If-Modified-Since header field(s) in the request. That is, if no entity-tags match, then the server MUST NOT return a 304 (Not Modified) response.
If the request would, without the If-None-Match header field, result in anything other than a 2xx or 304 status code, then the If-None-Match header field MUST be ignored. (See Section 2.4 for a discussion of server behavior when both If-Modified-Since and If-None-Match appear in the same request.)
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: *
The result of a request having both an If-None-Match header field and either an If-Match or an If-Unmodified-Since header field is undefined by this specification.
The "If-Modified-Since" header field MAY be used to make a request method conditional by modification date: if the selected representation has not been modified since the time specified in this field, then do not perform the request method; instead, respond as detailed below.
If-Modified-Since = HTTP-date
An example of the field is:
If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
A GET method with an If-Modified-Since header field and no Range header field requests that the selected representation be transferred only if it has been modified since the date given by the If-Modified-Since header field. The algorithm for determining this includes the following cases:
The purpose of this feature is to allow efficient updates of cached information with a minimum amount of transaction overhead.
The result of a request having both an If-Modified-Since header field and either an If-Match or an If-Unmodified-Since header field is undefined by this specification.
The "If-Unmodified-Since" header field MAY be used to make a request method conditional by modification date: if the selected representation has been modified since the time specified in this field, then the server MUST NOT perform the requested operation and MUST instead respond with the 412 (Precondition Failed) status code. If the selected representation has not been modified since the time specified in this field, the server SHOULD perform the request method as if the If-Unmodified-Since header field were not present.
If-Unmodified-Since = HTTP-date
An example of the field is:
If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
If the request normally (i.e., without the If-Unmodified-Since header field) would result in anything other than a 2xx or 412 status code, the If-Unmodified-Since header field SHOULD be ignored.
If the specified date is invalid, the header field MUST be ignored.
The result of a request having both an If-Unmodified-Since header field and either an If-None-Match or an If-Modified-Since header field is undefined by this specification.
The If-Range header field provides a special conditional request mechanism that is similar to If-Match and If-Unmodified-Since but specific to HTTP range requests. If-Range is defined in Section 5.3 of [Part5].
The 304 status code indicates that a conditional GET request has been received and would have resulted in a 200 (OK) response if it were not for the fact that the condition has evaluated to false. In other words, there is no need for the server to transfer a representation of the target resource because the client's request indicates that it already has a valid representation, as indicated by the 304 response header fields, and is therefore redirecting the client to make use of that stored representation as if it were the payload of a 200 response. The 304 response MUST NOT contain a message-body, and thus is always terminated by the first empty line after the header fields.
A 304 response MUST include a Date header field (Section 9.10 of [Part2]) unless the origin server does not have a clock that can provide a reasonable approximation of the current time. If a 200 response to the same request would have included any of the header fields Cache-Control, Content-Location, ETag, Expires, or Vary, then those same header fields MUST be sent in a 304 response.
Since the goal of a 304 response is to minimize information transfer when the recipient already has one or more cached representations, the response SHOULD NOT include representation metadata other than the above listed fields unless said metadata exists for the purpose of guiding cache updates (e.g., future HTTP extensions).
If the recipient of a 304 response does not have a cached representation corresponding to the entity-tag indicated by the 304 response, then the recipient MUST NOT use the 304 to update its own cache. If this 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 304 response MAY be forwarded to the outbound client. Otherwise, the recipient MUST disregard the 304 response and repeat the request without any preconditions.
If a cache uses a received 304 response to update a cache entry, the cache MUST update the entry to reflect any new field values given in the response.
The 412 status code indicates that one or more preconditions given in the request header fields evaluated to false when tested on the server. This response code allows the client to place preconditions on the current resource state (its current representations and metadata) and thus prevent the request method from being applied if the target resource is in an unexpected state.
The HTTP Status Code Registry located at <http://www.iana.org/assignments/http-status-codes> shall be updated with the registrations below:
The Message Header Field Registry located at <http://www.iana.org/assignments/message-headers/message-header-index.html> shall be updated with the permanent registrations below (see [RFC3864]):
|Header Field Name||Protocol||Status||Reference|
The change controller is: "IETF (email@example.com) - Internet Engineering Task Force".
No additional security considerations have been identified beyond those applicable to HTTP in general [Part1].
See Section 9 of [Part1].
|[Part1]||Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed., “HTTP/1.1, part 1: URIs, Connections, and Message Parsing”, Internet-Draft draft-ietf-httpbis-p1-messaging-latest (work in progress), June 2012.|
|[Part2]||Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed., “HTTP/1.1, part 2: Message Semantics”, Internet-Draft draft-ietf-httpbis-p2-semantics-latest (work in progress), June 2012.|
|[Part5]||Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed., “HTTP/1.1, part 5: Range Requests and Partial Responses”, Internet-Draft draft-ietf-httpbis-p5-range-latest (work in progress), June 2012.|
|[Part6]||Fielding, R., Ed., Lafon, Y., Ed., Nottingham, M., Ed., and J. Reschke, Ed., “HTTP/1.1, part 6: Caching”, Internet-Draft draft-ietf-httpbis-p6-cache-latest (work in progress), June 2012.|
|[RFC2119]||Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels”, BCP 14, RFC 2119, March 1997.|
|[RFC5234]||Crocker, D., Ed. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF”, STD 68, RFC 5234, January 2008.|
|[RFC2616]||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.|
|[RFC3864]||Klyne, G., Nottingham, M., and J. Mogul, “Registration Procedures for Message Header Fields”, BCP 90, RFC 3864, September 2004.|
|[RFC4918]||Dusseault, L., Ed., “HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)”, RFC 4918, June 2007.|
Allow weak entity-tags in all requests except range requests (Sections 2.1 and 3.2).
Change ETag header field ABNF not to use quoted-string, thus avoiding escaping issues. (Section 2.3)
Change ABNF productions for header fields to only define the field value. (Section 3)
ETag = entity-tag HTTP-date = <HTTP-date, defined in [Part2], Section 5.1> If-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS entity-tag ] ) ) If-Modified-Since = HTTP-date If-None-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS entity-tag ] ) ) If-Unmodified-Since = HTTP-date Last-Modified = HTTP-date OWS = <OWS, defined in [Part1], Section 3.2.1> entity-tag = [ weak ] opaque-tag etagc = "!" / %x23-7E ; '#'-'~' / obs-text obs-text = <obs-text, defined in [Part1], Section 3.2.4> opaque-tag = DQUOTE *etagc DQUOTE weak = %x57.2F ; W/
; ETag defined but not used ; If-Match defined but not used ; If-Modified-Since defined but not used ; If-None-Match defined but not used ; If-Unmodified-Since defined but not used ; Last-Modified defined but not used
Changes up to the first Working Group Last Call draft are summarized in <http://tools.ietf.org/html/draft-ietf-httpbis-p4-conditional-19#appendix-C>.