RFC 7232 - Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests

The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext information systems. This document defines HTTP/1.1 conditional requests, including metadata header fields for indicating state changes, request header fields for making preconditions on such state, and rules for constructing the responses to a conditional request when one or more preconditions evaluate to false.¶

2. Validators

This specification defines two forms of metadata that are commonly used to observe resource state and test for preconditions: modification dates (Section 2.2) and opaque entity tags (Section 2.3). Additional metadata that reflects resource state has been defined by various extensions of HTTP, such as Web Distributed Authoring and Versioning (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.¶

2.1. Weak versus Strong

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 representation metadata that changes value whenever a change occurs to the representation data that would be observable in the payload body of a 200 (OK) response to GET.¶

A strong validator might change for reasons other than a change to the representation data, 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.¶

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 is 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 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.¶

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.¶

2.2. Last-Modified

The "Last-Modified" header field in a response provides a timestamp indicating the date and time at which the origin server believes the selected representation was last modified, as determined at the conclusion of handling the request.¶

  Last-Modified = HTTP-date

An example of its use is¶

  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT

2.2.1. Generation

An origin server 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 ([RFC7234]) 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.¶

2.2.2. Comparison

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 representation and,
That origin server reliably knows that the associated representation 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, If-Unmodified-Since, or If-Range header field, because the client has a cache entry for the associated representation, 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 representation, 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.¶

2.3. ETag

The "ETag" header field in a response provides the current entity-tag for the selected representation, as determined at the conclusion of handling the request. 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

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.¶

Examples:

  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 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).¶

2.3.1. Generation

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 collision-resistant hash of representation content, a combination of various file attributes, or a modification timestamp that has sub-second resolution.¶

An origin server SHOULD send an 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 ([RFC7234]) can result in a substantial reduction of HTTP network traffic and can be a significant factor in improving service scalability and reliability.¶

2.3.2. Comparison

There are two entity-tag comparison functions, depending on whether or not the comparison context allows the use of weak validators: ¶

Strong comparison: two entity-tags are equivalent if both are not weak and their opaque-tags match character-by-character.
Weak comparison: two entity-tags are equivalent if their opaque-tags match character-by-character, regardless of either or both being tagged as "weak".

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

2.3.3. Example: Entity-Tags Varying on Content-Negotiated Resources

Consider a resource that is subject to content negotiation (Section 3.4 of [RFC7231]), and where the representations sent in response to a GET request vary based on the Accept-Encoding request header field (Section 5.3.4 of [RFC7231]):¶

>> Request:

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:¶

>> Response:

HTTP/1.1 200 OK
Date: Fri, 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:¶

>> 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...

2.4. When to Use Entity-Tags and Last-Modified Dates

In 200 (OK) responses to GET or HEAD, an origin server: ¶

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.

In other words, the preferred behavior for an origin server is to send both a strong entity-tag and a Last-Modified value in successful responses to a retrieval request.¶

A client: ¶

MUST send that entity-tag in any cache validation request (using If-Match or If-None-Match) if an entity-tag has been provided by the origin server.
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.
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.
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.

Value	Description	Reference
304	Not Modified	Section 4.1
412	Precondition Failed	Section 4.2

Header Field Name	Protocol	Status	Reference
ETag	http	standard	Section 2.3
If-Match	http	standard	Section 3.1
If-Modified-Since	http	standard	Section 3.3
If-None-Match	http	standard	Section 3.2
If-Unmodified-Since	http	standard	Section 3.4
Last-Modified	http	standard	Section 2.2