HTTP/1.1, part 4: Conditional Requests

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].¶

An implementation is not compliant if it fails to satisfy one or more of the "MUST" or "REQUIRED" level requirements for the protocols it implements. An implementation that satisfies all the "MUST" or "REQUIRED" level and all the "SHOULD" level requirements for its protocols is said to be "unconditionally compliant"; one that satisfies all the "MUST" level requirements but not all the "SHOULD" level requirements for its protocols is said to be "conditionally compliant".¶

This specification uses the ABNF syntax defined in Section 1.2 of [Part1] (which extends the syntax defined in [RFC5234] with a list rule). 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), VCHAR (any visible USASCII character), and WSP (whitespace).¶

>> Request:

GET /index HTTP/1.1
Host: www.example.com
Accept-Encoding: gzip

>> Response:

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!

>> Response:

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...

If the client has performed a conditional GET request and access is allowed, but the document has not been modified, the server SHOULD respond with this status code. 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 9.3 of [Part1]) unless its omission is required by Section 9.3.1 of [Part1]. If a 200 response to the same request would have included any of the header fields Cache-Control, Content-Location, ETag, Expires, Last-Modified, 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 a 304 response includes an entity-tag that indicates a representation not currently cached, then the recipient MUST NOT use the 304 to update its own cache. If that 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, disregard the response and repeat the request without the conditional.¶

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 precondition given in one or more of the request-header fields evaluated to false when it was tested on the server. This response code allows the client to place preconditions on the current resource metadata (header field data) and thus prevent the requested method from being applied to a resource other than the one intended.¶

The "ETag" response-header field provides the current value of the entity-tag (see Section 2) for one representation of the target resource. An entity-tag is intended for use as a resource-local identifier for differentiating between representations of the same resource that vary over time or via content negotiation (see Section 4).¶

Examples:

  ETag: "xyzzy"
  ETag: W/"xyzzy"
  ETag: ""

An entity-tag provides an "opaque" cache validator that allows for more reliable validation than modification dates in situations where it is inconvenient to store modification dates, where the one-second resolution of HTTP date values is not sufficient, or where the origin server wishes to avoid certain paradoxes that might arise from the use of modification dates.¶

The principle behind entity-tags is that only the service author knows the semantics of a resource well enough to select an appropriate cache validation mechanism, and the specification of any validator comparison function more complex than byte-equality would open up a can of worms. Thus, comparisons of any other header fields (except Last-Modified, for compatibility with HTTP/1.0) are never used for purposes of validating a cache entry.¶

The "If-Match" request-header field is used to make a request method conditional. A client that has one or more representations previously obtained from the resource can verify that one of those representations is current by including a list of their associated entity-tags in the If-Match header field.¶

This allows efficient updates of cached information with a minimum amount of transaction overhead. It is also used when updating resources, to prevent inadvertent modification of the wrong version of a resource. As a special case, the value "*" matches any current representation of the resource.¶

If any of the entity-tags match the entity-tag of the representation that would have been returned in the response to a similar GET request (without the If-Match header field) on that resource, or if "*" is given and any current representation exists for that resource, then the server MAY perform the requested method as if the If-Match header field did not exist.¶

If none of the entity-tags match, or if "*" is given and no current representation exists, the server MUST NOT perform the requested method, and MUST return a 412 (Precondition Failed) response. This behavior is most useful when the client wants to prevent an updating method, such as PUT, from modifying a resource that has changed since the client last retrieved it.¶

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.¶

The meaning of "If-Match: *" is that the method SHOULD be performed if the representation selected by the origin server (or by a cache, possibly using the Vary mechanism, see Section 3.5 of [Part6]) exists, and MUST NOT be performed if the representation does not exist.¶

The result of a request having both an If-Match header field and either an If-None-Match or an If-Modified-Since header fields is undefined by this specification.¶

The "If-Modified-Since" request-header field is used to make a request method conditional by date: if the representation that would have been transferred in a 200 response to a GET request has not been modified since the time specified in this field, then do not perform the method; instead, respond as detailed below.¶

A GET method with an If-Modified-Since header field and no Range header field requests that the 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: ¶

1. If the request would normally result in anything other than a 200 (OK) status code, or if the passed If-Modified-Since date is invalid, the response is exactly the same as for a normal GET. A date which is later than the server's current time is invalid.
2. If the representation has been modified since the If-Modified-Since date, the response is exactly the same as for a normal GET.
3. If the representation has not been modified since a valid If-Modified-Since date, the server SHOULD return a 304 (Not Modified) response.

The purpose of this feature is to allow efficient updates of cached information with a minimum amount of transaction overhead. ¶

• Note: The Range request-header field modifies the meaning of If-Modified-Since; see Section 5.4 of [Part5] for full details.
• Note: If-Modified-Since times are interpreted by the server, whose clock might not be synchronized with the client.
• Note: When handling an If-Modified-Since header field, some servers will use an exact date comparison function, rather than a less-than function, for deciding whether to send a 304 (Not Modified) response. To get best results when sending an If-Modified-Since header field for cache validation, clients are advised to use the exact date string received in a previous Last-Modified header field whenever possible.
• Note: If a client uses an arbitrary date in the If-Modified-Since header field instead of a date taken from the Last-Modified header field for the same request, the client needs to be aware that this date is interpreted in the server's understanding of time. Unsynchronized clocks and rounding problems, due to the different encodings of time between the client and server, are concerns. This includes the possibility of race conditions if the document has changed between the time it was first requested and the If-Modified-Since date of a subsequent request, and the possibility of clock-skew-related problems if the If-Modified-Since date is derived from the client's clock without correction to the server's clock. Corrections for different time bases between client and server are at best approximate due to network latency.

The result of a request having both an If-Modified-Since header field and either an If-Match or an If-Unmodified-Since header fields is undefined by this specification.¶

The "If-None-Match" request-header field is used to make a request method conditional. A client that has one or more representations previously obtained from the resource can verify that none of those representations is current by including a list of their associated entity-tags in the If-None-Match header field.¶

This allows efficient updates of cached information with a minimum amount of transaction overhead. It is also used to prevent a method (e.g., PUT) from inadvertently modifying an existing resource when the client believes that the resource does not exist.¶

As a special case, the value "*" matches any current representation of the resource.¶

If any of the entity-tags match the entity-tag of the representation that would have been returned in the response to a similar GET request (without the If-None-Match header field) on that resource, or if "*" is given and any current representation exists for that resource, then the server MUST NOT perform the requested method, unless required to do so because the resource's modification date fails to match that supplied in an If-Modified-Since header field in the request. Instead, if the request method was GET or HEAD, the server SHOULD respond with a 304 (Not Modified) response, including the cache-related header fields (particularly ETag) of one of the representations that matched. 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 5 for a discussion of server behavior when both If-Modified-Since and If-None-Match appear in the same request.)¶

The meaning of "If-None-Match: *" is that the method MUST NOT be performed if the representation selected by the origin server (or by a cache, possibly using the Vary mechanism, see Section 3.5 of [Part6]) exists, and SHOULD be performed if the representation does not exist. This feature is intended to be useful in preventing races between PUT operations.¶

The result of a request having both an If-None-Match header field and either an If-Match or an If-Unmodified-Since header fields is undefined by this specification.¶

The "If-Unmodified-Since" request-header field is used to make a request method conditional. If the representation that would have been transferred in a 200 response to a GET request on the same resource has not been modified since the time specified in this field, the server SHOULD perform the requested operation as if the If-Unmodified-Since header field were not present.¶

If the representation has been modified since the specified time, the server MUST NOT perform the requested operation, and MUST return a 412 (Precondition Failed).¶

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 is 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 fields is undefined by this specification.¶

The "Last-Modified" header field indicates the date and time at which the origin server believes the representation was last modified.¶

An example of its use is¶

The exact meaning of this header field depends on the implementation of the origin server and the nature of the original resource. For files, it might be just the file system last-modified time. For representations with dynamically included parts, it might be the most recent of the set of last-modify times for its component parts. For database gateways, it might be the last-update time stamp of the record. For virtual objects, it might be the last time the internal state changed.¶

An origin server MUST NOT send a Last-Modified date which is later than the server's time of message origination. In such cases, where the resource's last modification would indicate some time in the future, the server MUST replace that date with the message origination date.¶

An origin server SHOULD obtain the Last-Modified value of the representation as close as possible to the time that it generates the Date value of 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.¶

HTTP/1.1 servers SHOULD send Last-Modified whenever feasible.¶

The Last-Modified header field value is often used as a cache validator. In simple terms, a cache entry is considered to be valid if the representation has not been modified since the Last-Modified value.¶

The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".¶

10.1. Normative References

[Part1]

Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., and J. Reschke, Ed., “HTTP/1.1, part 1: URIs, Connections, and Message Parsing”, Internet-Draft draft-ietf-httpbis-p1-messaging-12 (work in progress), October 2010.

[Part3]

Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., and J. Reschke, Ed., “HTTP/1.1, part 3: Message Payload and Content Negotiation”, Internet-Draft draft-ietf-httpbis-p3-payload-12 (work in progress), October 2010.

[Part5]

Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., and J. Reschke, Ed., “HTTP/1.1, part 5: Range Requests and Partial Responses”, Internet-Draft draft-ietf-httpbis-p5-range-12 (work in progress), October 2010.

[Part6]

Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., Nottingham, M., Ed., and J. Reschke, Ed., “HTTP/1.1, part 6: Caching”, Internet-Draft draft-ietf-httpbis-p6-cache-12 (work in progress), October 2010.

[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.

10.2. Informative References

[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.

Extracted relevant partitions from [RFC2616].¶

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/35>: "Normative and Informative references"

Other changes: ¶

• Move definitions of 304 and 412 condition codes from Part2.

Ongoing work on ABNF conversion (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>): ¶

• Add explicit references to BNF syntax and rules imported from other parts of the specification.

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/116>: "Weak ETags on non-GET requests"

Ongoing work on IANA Message Header Field Registration (<http://tools.ietf.org/wg/httpbis/trac/ticket/40>): ¶

• Reference RFC 3984, and update header field registrations for header fields defined in this document.

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/71>: "Examples for ETag matching"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/124>: "'entity value' undefined"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/126>: "bogus 2068 Date header reference"

Ongoing work on ABNF conversion (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>): ¶

• Use "/" instead of "|" for alternatives.
• Introduce new ABNF rules for "bad" whitespace ("BWS"), optional whitespace ("OWS") and required whitespace ("RWS").
• Rewrite ABNFs to spell out whitespace rules, factor out header field value format definitions.

Final work on ABNF conversion (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>): ¶

• Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/153>: "case-sensitivity of etag weakness indicator"

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/116>: "Weak ETags on non-GET requests" (If-Match still was defined to require strong matching)
• <http://tools.ietf.org/wg/httpbis/trac/ticket/198>: "move IANA registrations for optional status codes"

No significant changes.¶

No significant changes.¶

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/69>: "Clarify 'Requested Variant'"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/109>: "Clarify entity / representation / variant terminology"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/220>: "consider removing the 'changes from 2068' sections"

None.¶