Entity Header Fields

Entity header fields define meta information about the entity body or, if no body is present, about the resource identified by the request. Some of this meta information is optional; some might be required.

The entity header fields defined in RFC 2616 are as follows:

• The Allow header field lists the set of methods supported by the resource identified by the Request-URI. The purpose of this field is strictly to inform the recipient of valid methods associated with the resource.

• The Content-Encoding header field is used as a modifier to the media type. When present, its value indicates what additional content codings have been applied to the entity body, and thus what decoding mechanisms must be applied in order to obtain the media type referenced by the Content-Type header field. Content-Encoding is primarily used to allow a document to be compressed without losing the identity of its underlying media type.

• The Content-Language header field describes the natural language(s) of the intended audience for the enclosed entity. Note that this might not be equivalent to all the languages used within the entity-body.

• The Content-Length header field indicates the size of the entity-body, in decimal number of octets, sent to the recipient or, in the case of the HEAD method, the size of the entity-body that would have been sent had the request been a GET.

• The Content-Location header field may be used to supply the resource location for the entity enclosed in the message when that entity is accessible from a location separate from the requested resource's URI.

• The Content-MD5 header field contains an MD5 digest of the entity-body for the purpose of providing an end-to-end message integrity check (MIC) of the entity-body. (Note: An MIC is good for detecting accidental modification of the entity-body in transit but is not proof against malicious attacks.)

• The Content-Type header field indicates the media type of the entity body sent to the recipient or, in the case of the HEAD method, the media type that would have been sent had the request been a GET.

• The Content-Range header field is sent with a partial entity body to specify where in the full entity body the partial body should be applied.

• The Date header field contains the date and time at which the message was originated.

• The Expires header field contains the date and time after which the response is considered stale.

• The Last-Modified header field indicates the date and time at which the server believes the document being sent to the browser was last modified.