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Namespace: System. Headers Assembly: System. An empty string is a string with zero length or no characters, whereas NULL values represent an absence of data. An empty string is typically represented by a quoted empty string e. The following file format options enable you to differentiate between empty strings and NULL values when unloading or loading data. Use this option to enclose strings in the specified character: single quote ' , double quote " , or NONE.

Enclosing string values in quotes while unloading data is not required. In this case, the Content-Language would properly only include "en". Content-Language MAY be applied to any media type -- it is not limited to textual documents. Applications SHOULD use this field to indicate the transfer-length of the message-body, unless this is prohibited by the rules in section 4.

Any Content-Length greater than or equal to zero is a valid value. Section 4. The Content-Location entity-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.

A server SHOULD provide a Content-Location for the variant corresponding to the response entity; especially in the case where a resource has multiple entities associated with it, and those entities actually have separate locations by which they might be individually accessed, the server SHOULD provide a Content-Location for the particular variant which is returned.

The Content-Location value is not a replacement for the original requested URI; it is only a statement of the location of the resource corresponding to this particular entity at the time of the request. However, the Content- Location can be used to differentiate between multiple entities retrieved from a single requested resource, as described in section Note: a MIC is good for detecting accidental modification of the entity-body in transit, but is not proof against malicious attacks.

The Content-MD5 header field MAY be generated by an origin server or client to function as an integrity check of the entity-body. Any recipient of the entity- body, including gateways and proxies, MAY check that the digest value in this header field matches that of the entity-body as received. The MD5 digest is computed based on the content of the entity-body, including any content-coding that has been applied, but not including any transfer-encoding applied to the message-body.

If the message is received with a transfer-encoding, that encoding MUST be removed prior to checking the Content-MD5 value against the received entity. This has the result that the digest is computed on the octets of the entity-body exactly as, and in the order that, they would be sent if no transfer-encoding were being applied. There are several consequences of this. If a body-part has a Content-Transfer- Encoding or Content-Encoding header, it is assumed that the content of the body-part has had the encoding applied, and the body-part is included in the Content-MD5 digest as is -- i.

The Transfer-Encoding header field is not allowed within body-parts. The Content-Range entity-header is sent with a partial entity-body to specify where in the full entity-body the partial body should be applied.

Range units are defined in section 3. The header SHOULD indicate the total length of the full entity-body, unless this length is unknown or difficult to determine. Unlike byte-ranges-specifier values see section A byte-content-range-spec with a byte-range-resp-spec whose last- byte-pos value is less than its first-byte-pos value, or whose instance-length value is less than or equal to its last-byte-pos value, is invalid.

The recipient of an invalid byte-content-range- spec MUST ignore it and any content transferred along with it. The instance-length specifies the current length of. Examples of byte-content-range-spec values, assuming that the entity contains a total of bytes:. When an HTTP message includes the content of a single range for example, a response to a request for a single range, or to a request for a set of ranges that overlap without any holes , this content is transmitted with a Content-Range header, and a Content-Length header showing the number of bytes actually transferred.

When an HTTP message includes the content of multiple ranges for example, a response to a request for multiple non-overlapping ranges , these are transmitted as a multipart message. See appendix When a client requests multiple byte-ranges in one request, the server SHOULD return them in the order that they appeared in the request.

If the server ignores a byte-range-spec because it is syntactically invalid, the server SHOULD treat the request as if the invalid Range header field did not exist. Normally, this means return a response containing the full entity. If the server receives a request other than one including an If- Range request-header field with an unsatisfiable Range request- header field that is, all of whose byte-range-spec values have a first-byte-pos value greater than the current length of the selected resource , it SHOULD return a response code of Requested range not satisfiable section The Content-Type entity-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.

Media types are defined in section 3. An example of the field is. Further discussion of methods for identifying the media type of an entity is provided in section 7.

The Date general-header field represents the date and time at which the message was originated, having the same semantics as orig-date in RFC The field value is an HTTP-date, as described in section 3. A received message that does not have a Date header field MUST be assigned one by the recipient if the message will be cached by that recipient or gatewayed via a protocol which requires a Date. It SHOULD represent the best available approximation of the date and time of message generation, unless the implementation has no means of generating a reasonably accurate date and time.

In theory, the date ought to represent the moment just before the entity is generated. In practice, the date can be generated at any time during the message origination without affecting its semantic value.

Some origin server implementations might not have a clock available. An origin server without a clock MUST NOT assign Expires or Last- Modified values to a response, unless these values were associated with the resource by a system or user with a reliable clock.

It MAY assign an Expires value that is known, at or before server configuration time, to be in the past this allows "pre-expiration" of responses without storing separate Expires values for each resource. The ETag response-header field provides the current value of the entity tag for the requested variant. The headers used with entity tags are described in sections The entity tag MAY be used for comparison with other entities from the same resource see section The Expect request-header field is used to indicate that particular server behaviors are required by the client.

A server that does not understand or is unable to comply with any of the expectation values in the Expect field of a request MUST respond with appropriate error status. The server MUST respond with a Expectation Failed status if any of the expectations cannot be met or, if there are other problems with the request, some other 4xx status.

This header field is defined with extensible syntax to allow for future extensions. If a server receives a request containing an Expect field that includes an expectation-extension that it does not support, it MUST respond with a Expectation Failed status. Comparison of expectation values is case-insensitive for unquoted tokens including the continue token , and is case-sensitive for quoted-string expectation-extensions.

However, the Expect request-header itself is end-to-end; it MUST be forwarded if the request is forwarded. See section 8. A stale cache entry may not normally be returned by a cache either a proxy cache or a user agent cache unless it is first validated with the origin server or with an intermediate cache that has a fresh copy of the entity.

The presence of an Expires field does not imply that the original resource will change or cease to exist at, before, or after that time. The format is an absolute date and time as defined by HTTP-date in section 3. To mark a response as "already expired," an origin server sends an Expires date that is equal to the Date header value. See the rules for expiration calculations in section To mark a response as "never expires," an origin server sends an Expires date approximately one year from the time the response is sent.

The presence of an Expires header field with a date value of some time in the future on a response that otherwise would by default be non-cacheable indicates that the response is cacheable, unless indicated otherwise by a Cache-Control header field section This header field MAY be used for logging purposes and as a means for identifying the source of invalid or unwanted requests.

The interpretation of this field is that the request is being performed on behalf of the person given, who accepts responsibility for the method performed. In particular, robot agents SHOULD include this header so that the person responsible for running the robot can be contacted if problems occur on the receiving end.

The Internet e-mail address in this field MAY be separate from the Internet host which issued the request. It is strongly recommended that the user be able to disable, enable, and modify the value of this field at any time prior to a request. A "host" without any trailing port information implies the default port for the service requested e. See sections 5. The If-Match request-header field is used with a method to make it conditional. A client that has one or more entities previously obtained from the resource can verify that one of those entities is current by including a list of their associated entity tags in the If-Match header field.

Entity tags are defined in section 3. The purpose of this feature is to allow efficient updates of cached information with a minimum amount of transaction overhead. It is also used, on updating requests, to prevent inadvertent modification of the wrong version of a resource. A server MUST use the strong comparison function see section 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.

A request intended to update a resource e. Following is the simple syntax for using connection header:. By default, HTTP 1. HTTP 1. No new Pragma directives will be defined in the future. The Trailer general field value indicates that the given set of header fields is present in the trailer of a message encoded with chunked transfer-coding. Following is the syntax of Trailer header field:.

Message header fields listed in the Trailer header field must not include the following header fields:. The Transfer-Encoding general-header field indicates what type of transformation has been applied to the message body in order to safely transfer it between the sender and the recipient.

This is not the same as content-encoding because transfer-encodings are a property of the message, not of the entity-body. The syntax of Transfer-Encoding header field is as follows:. The Upgrade general-header allows the client to specify what additional communication protocols it supports and would like to use if the server finds it appropriate to switch protocols. The Via general-header must be used by gateways and proxies to indicate the intermediate protocols and recipients.

The request received by www. The Warning general-header is used to carry additional information about the status or transformation of a message which might not be reflected in the message.

A response may carry more than one Warning header. The Accept request-header field can be used to specify certain media types which are acceptable for the response. The general syntax is as follows:. Multiple media types can be listed separated by commas and the optional qvalue represents an acceptable quality level for accept types on a scale of 0 to 1. Following is an example:. The Accept-Charset request-header field can be used to indicate what character sets are acceptable for the response.

Following is the general syntax:. Multiple character sets can be listed separated by commas and the optional qvalue represents an acceptable quality level for nonpreferred character sets on a scale of 0 to 1. The Accept-Encoding request-header field is similar to Accept, but restricts the content-codings that are acceptable in the response. The general syntax is:. The Accept-Language request-header field is similar to Accept, but restricts the set of natural languages that are preferred as a response to the request.

Multiple languages can be listed separated by commas and the optional qvalue represents an acceptable quality level for non preferred languages on a scale of 0 to 1. The Authorization request-header field value consists of credentials containing the authentication information of the user agent for the realm of the resource being requested. The value decodes into is guest:guest where guest is user ID and guest is the password. The Expect request-header field is used to indicate that a particular set of server behaviors is required by the client.

If a server receives a request containing an Expect field that includes an expectation-extension that it does not support, it must respond with a Expectation Failed status. The From request-header field contains an Internet e-mail address for the human user who controls the requesting user agent. Following is a simple example:.