HTTP/1.1, part 4: Conditional Requests

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The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia 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.¶

This document defines HTTP/1.1 response metadata for indicating potential changes to payload content, including modification time stamps and opaque entity-tags, and the HTTP conditional request mechanisms that allow preconditions to be placed on a request method. Conditional GET requests allow for efficient cache updates. Other conditional request methods are used to protect against overwriting or misunderstanding the state of a resource that has been changed unbeknownst to the requesting client.¶

This document is currently disorganized in order to minimize the changes between drafts and enable reviewers to see the smaller errata changes. The next draft will reorganize the sections to better reflect the content. In particular, the sections on resource metadata will be discussed first and then followed by each conditional request-header, concluding with a definition of precedence and the expectation of ordering strong validator checks before weak validator checks. It is likely that more content from [Part6] will migrate to this part, where appropriate. The current mess reflects how widely dispersed these topics and associated requirements had become in [RFC2616].¶

This specification uses the ABNF syntax defined in Section 2.1 of [Part1] and the core rules defined in Section 2.2 of [Part1]: [abnf.dep: ABNF syntax and basic rules will be adopted from RFC 5234, see <http://tools.ietf.org/wg/httpbis/trac/ticket/36>.] ¶

Entity tags are used for comparing two or more entities from the same requested resource. HTTP/1.1 uses entity tags in the ETag (Section 7.1), If-Match (Section 7.2), If-None-Match (Section 7.4), and If-Range (Section 6.3 of [Part5]) header fields. The definition of how they are used and compared as cache validators is in Section 5. An entity tag consists of an opaque quoted string, possibly prefixed by a weakness indicator.¶

A "strong entity tag" MAY be shared by two entities of a resource only if they are equivalent by octet equality.¶

A "weak entity tag," indicated by the "W/" prefix, MAY be shared by two entities of a resource only if the entities are equivalent and could be substituted for each other with no significant change in semantics. A weak entity tag can only be used for weak comparison.¶

An entity tag MUST be unique across all versions of all entities associated with a particular resource. A given entity tag value MAY be used for entities obtained by requests on different URIs. The use of the same entity tag value in conjunction with entities obtained by requests on different URIs does not imply the equivalence of those entities.¶

Since both origin servers and caches will compare two validators to decide if they represent the same or different entities, one normally would expect that if the entity (the entity-body or any entity-headers) changes in any way, then the associated validator would change as well. If this is true, then we call this validator a "strong validator."¶

However, there might be cases when a server prefers to change the validator only on semantically significant changes, and not when insignificant aspects of the entity change. A validator that does not always change when the resource changes is a "weak validator."¶

Entity tags are normally "strong validators," but the protocol provides a mechanism to tag an entity tag as "weak." One can think of a strong validator as one that changes whenever the bits of an entity changes, while a weak value changes whenever the meaning of an entity changes. Alternatively, one can think of a strong validator as part of an identifier for a specific entity, while a weak validator is part of an identifier for a set of semantically equivalent entities. ¶

• Note: One example of a strong validator is an integer that is incremented in stable storage every time an entity is changed.
• An entity's modification time, if represented with one-second resolution, could be a weak validator, since it is possible that the resource might be modified twice during a single second.
• Support for weak validators is optional. However, weak validators allow for more efficient caching of equivalent objects; for example, a hit counter on a site is probably good enough if it is updated every few days or weeks, and any value during that period is likely "good enough" to be equivalent.

A "use" of a validator is either when a client generates a request and includes the validator in a validating header field, or when a server compares two validators.¶

Strong validators are usable in any context. Weak validators are only usable in contexts that do not depend on exact equality of an entity. For example, either kind is usable for a conditional GET of a full entity. However, only a strong validator is usable for a sub-range retrieval, since otherwise the client might end up with an internally inconsistent entity.¶

Clients MAY issue simple (non-subrange) GET requests with either weak validators or strong validators. Clients MUST NOT use weak validators in other forms of request.¶

The only function that HTTP/1.1 defines on validators is comparison. There are two validator comparison functions, depending on whether the comparison context allows the use of weak validators or not: ¶

• The strong comparison function: in order to be considered equal, both validators MUST be identical in every way, and both MUST NOT be weak.
• The weak comparison function: in order to be considered equal, both validators MUST be identical in every way, but either or both of them MAY be tagged as "weak" without affecting the result.

An entity tag is strong unless it is explicitly tagged as weak. Section 3 gives the syntax for entity tags.¶

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: ¶

• The validator is being compared by an origin server to the actual current validator for the entity and,
• That origin server reliably knows that the associated entity did not change twice during the second covered by the presented validator.

or ¶

• The validator is about to be used by a client in an If-Modified-Since or If-Unmodified-Since header, because the client has a cache entry for the associated entity, and
• That cache entry includes a Date value, which gives the time when the origin server sent the original response, and
• The presented Last-Modified time is at least 60 seconds before the Date value.

or ¶

• The validator is being compared by an intermediate cache to the validator stored in its cache entry for the entity, and
• That cache entry includes a Date value, which gives the time when the origin server sent the original response, and
• The presented Last-Modified time is at least 60 seconds before the Date value.

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.¶

If a client wishes to perform a sub-range retrieval on a value for which it has only a Last-Modified time and no opaque validator, it MAY do this only if the Last-Modified time is strong in the sense described here.¶

A cache or origin server receiving a conditional request, other than a full-body GET request, MUST use the strong comparison function to evaluate the condition.¶

These rules allow HTTP/1.1 caches and clients to safely perform sub-range retrievals on values that have been obtained from HTTP/1.0 servers.¶

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: ¶

• SHOULD send an entity tag validator unless it is not feasible to generate one.
• MAY send a weak entity tag instead of a strong entity tag, if performance considerations support the use of weak entity tags, or if it is unfeasible to send a strong entity tag.
• SHOULD send a Last-Modified value if it is feasible to send one, unless the risk of a breakdown in semantic transparency that could result from using this date in an If-Modified-Since header would lead to serious problems.

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.¶

In order to be legal, a strong entity tag MUST change whenever the associated entity value changes in any way. A weak entity tag SHOULD change whenever the associated entity changes in a semantically significant way. ¶

• Note: in order to provide semantically transparent caching, an origin server must avoid reusing a specific strong entity tag value for two different entities, or reusing a specific weak entity tag value for two semantically different entities. Cache entries might persist for arbitrarily long periods, regardless of expiration times, so it might be inappropriate to expect that a cache will never again attempt to validate an entry using a validator that it obtained at some point in the past.

HTTP/1.1 clients: ¶

• If an entity tag has been provided by the origin server, MUST use that entity tag in any cache-conditional request (using If-Match or If-None-Match).
• If only a Last-Modified value has been provided by the origin server, SHOULD use that value in non-subrange cache-conditional requests (using If-Modified-Since).
• If only a Last-Modified value has been provided by an HTTP/1.0 origin server, MAY use that value in subrange cache-conditional requests (using If-Unmodified-Since:). The user agent SHOULD provide a way to disable this, in case of difficulty.
• If both an entity tag and a Last-Modified value have been provided by the origin server, SHOULD use both validators in cache-conditional requests. This allows both HTTP/1.0 and HTTP/1.1 caches to respond appropriately.

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 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. ¶

• Note: The general principle behind these rules is that HTTP/1.1 servers and clients should transmit as much non-redundant information as is available in their responses and requests. HTTP/1.1 systems receiving this information will make the most conservative assumptions about the validators they receive.
• HTTP/1.0 clients and caches will ignore entity tags. Generally, last-modified values received or used by these systems will support transparent and efficient caching, and so HTTP/1.1 origin servers should provide Last-Modified values. In those rare cases where the use of a Last-Modified value as a validator by an HTTP/1.0 system could result in a serious problem, then HTTP/1.1 origin servers should not provide one.

This section defines the syntax and semantics of HTTP/1.1 header fields related to conditional requests.¶

For entity-header fields, both sender and recipient refer to either the client or the server, depending on who sends and who receives the entity.¶

[rfc.comment.1: TBD.] ¶

No additional security considerations have been identified beyond those applicable to HTTP in general [Part1].¶

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