14.12 Content-Encoding The Content-Encoding entity-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. Content-Encoding = "Content-Encoding" ":" 1#content-coding Content codings are defined in section 3.5. An example of its use is Content-Encoding: gzip The Content-Encoding is a characteristic of the entity identified by the Request-URI. Typically, the entity-body is stored with this encoding and is only decoded before rendering or analogous usage. If multiple encodings have been applied to an entity, the content codings MUST be listed in the order in which they were applied. Additional information about the encoding parameters MAY be provided by other entity-header fields not defined by this specification. 14.13 Content-Language The Content-Language entity-header field describes the natural language(s) of the intended audience for the enclosed entity. Note that this may not be equivalent to all the languages used within the entity-body. Content-Language = "Content-Language" ":" 1#language-tag Language tags are defined in section 3.10. The primary purpose of Content-Language is to allow a user to identify and differentiate entities according to the user's own preferred language. Thus, if the body content is intended only for a Danish-literate audience, the appropriate field is Content-Language: da If no Content-Language is specified, the default is that the content is intended for all language audiences. This may mean that the sender
does not consider it to be specific to any natural language, or that the sender does not know for which language it is intended. Multiple languages MAY be listed for content that is intended for multiple audiences. For example, a rendition of the "Treaty of Waitangi," presented simultaneously in the original Maori and English versions, would call for Content-Language: mi, en However, just because multiple languages are present within an entity does not mean that it is intended for multiple linguistic audiences. An example would be a beginner's language primer, such as "A First Lesson in Latin," which is clearly intended to be used by an English-literate audience. In this case, the Content-Language should only include "en". Content-Language may be applied to any media type -- it is not limited to textual documents. 14.14 Content-Length The Content-Length entity-header field indicates the size of the message-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. Content-Length = "Content-Length" ":" 1*DIGIT An example is Content-Length: 3495 Applications SHOULD use this field to indicate the size of the message-body to be transferred, regardless of the media type of the entity. It must be possible for the recipient to reliably determine the end of HTTP/1.1 requests containing an entity-body, e.g., because the request has a valid Content-Length field, uses Transfer-Encoding: chunked or a multipart body. Any Content-Length greater than or equal to zero is a valid value. Section 4.4 describes how to determine the length of a message-body if a Content-Length is not given.
Note: The meaning of this field is significantly different from the corresponding definition in MIME, where it is an optional field used within the "message/external-body" content-type. In HTTP, it SHOULD be sent whenever the message's length can be determined prior to being transferred. 14.15 Content-Location The Content-Location entity-header field may be used to supply the resource location for the entity enclosed in the message. 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. In addition, a server SHOULD provide a Content-Location for the resource corresponding to the response entity. Content-Location = "Content-Location" ":" ( absoluteURI | relativeURI ) If no Content-Base header field is present, the value of Content- Location also defines the base URL for the entity (see section 14.11). 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. Future requests MAY use the Content-Location URI if the desire is to identify the source of that particular entity. A cache cannot assume that an entity with a Content-Location different from the URI used to retrieve it can be used to respond to later requests on that Content-Location URI. However, the Content- Location can be used to differentiate between multiple entities retrieved from a single requested resource, as described in section 13.6. If the Content-Location is a relative URI, the URI is interpreted relative to any Content-Base URI provided in the response. If no Content-Base is provided, the relative URI is interpreted relative to the Request-URI.
14.16 Content-MD5 The Content-MD5 entity-header field, as defined in RFC 1864 , is 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: a MIC is good for detecting accidental modification of the entity-body in transit, but is not proof against malicious attacks.) Content-MD5 = "Content-MD5" ":" md5-digest md5-digest = <base64 of 128 bit MD5 digest as per RFC 1864> The Content-MD5 header field may be generated by an origin server to function as an integrity check of the entity-body. Only origin servers may generate the Content-MD5 header field; proxies and gateways MUST NOT generate it, as this would defeat its value as an end-to-end integrity check. 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-Encoding that has been applied, but not including any Transfer-Encoding that may have been 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. HTTP extends RFC 1864 to permit the digest to be computed for MIME composite media-types (e.g., multipart/* and message/rfc822), but this does not change how the digest is computed as defined in the preceding paragraph. Note: There are several consequences of this. The entity-body for composite types may contain many body-parts, each with its own MIME and HTTP headers (including Content-MD5, Content-Transfer-Encoding, and Content-Encoding headers). 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.e., after the application. The Transfer-Encoding header field is not allowed within body-parts. Note: while the definition of Content-MD5 is exactly the same for HTTP as in RFC 1864 for MIME entity-bodies, there are several ways
in which the application of Content-MD5 to HTTP entity-bodies differs from its application to MIME entity-bodies. One is that HTTP, unlike MIME, does not use Content-Transfer-Encoding, and does use Transfer-Encoding and Content-Encoding. Another is that HTTP more frequently uses binary content types than MIME, so it is worth noting that, in such cases, the byte order used to compute the digest is the transmission byte order defined for the type. Lastly, HTTP allows transmission of text types with any of several line break conventions and not just the canonical form using CRLF. Conversion of all line breaks to CRLF should not be done before computing or checking the digest: the line break convention used in the text actually transmitted should be left unaltered when computing the digest. 14.17 Content-Range 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 inserted. It also indicates the total size of the full entity-body. When a server returns a partial response to a client, it must describe both the extent of the range covered by the response, and the length of the entire entity-body. Content-Range = "Content-Range" ":" content-range-spec content-range-spec = byte-content-range-spec byte-content-range-spec = bytes-unit SP first-byte-pos "-" last-byte-pos "/" entity-length entity-length = 1*DIGIT Unlike byte-ranges-specifier values, a byte-content-range-spec may only specify one range, and must contain absolute byte positions for both the first and last byte of the range. A byte-content-range-spec whose last-byte-pos value is less than its first-byte-pos value, or whose entity-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.
Examples of byte-content-range-spec values, assuming that the entity contains a total of 1234 bytes: o The first 500 bytes: bytes 0-499/1234 o The second 500 bytes: bytes 500-999/1234 o All except for the first 500 bytes: bytes 500-1233/1234 o The last 500 bytes: bytes 734-1233/1234 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. For example, HTTP/1.1 206 Partial content Date: Wed, 15 Nov 1995 06:25:24 GMT Last-modified: Wed, 15 Nov 1995 04:58:08 GMT Content-Range: bytes 21010-47021/47022 Content-Length: 26012 Content-Type: image/gif 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 MIME message. The multipart MIME content-type used for this purpose is defined in this specification to be "multipart/byteranges". See appendix 19.2 for its definition. A client that cannot decode a MIME multipart/byteranges message should not ask for multiple byte-ranges in a single request. 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 invalid, the server should treat the request as if the invalid Range header field
did not exist. (Normally, this means return a 200 response containing the full entity). The reason is that the only time a client will make such an invalid request is when the entity is smaller than the entity retrieved by a prior request. 14.18 Content-Type 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. Content-Type = "Content-Type" ":" media-type Media types are defined in section 3.7. An example of the field is Content-Type: text/html; charset=ISO-8859-4 Further discussion of methods for identifying the media type of an entity is provided in section 7.2.1. 14.19 Date 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 822. The field value is an HTTP-date, as described in section 3.3.1. Date = "Date" ":" HTTP-date An example is Date: Tue, 15 Nov 1994 08:12:31 GMT If a message is received via direct connection with the user agent (in the case of requests) or the origin server (in the case of responses), then the date can be assumed to be the current date at the receiving end. However, since the date--as it is believed by the origin--is important for evaluating cached responses, origin servers MUST include a Date header field in all responses. Clients SHOULD only send a Date header field in messages that include an entity- body, as in the case of the PUT and POST requests, and even then it is optional. A received message which does not have a Date header field SHOULD be assigned one by the recipient if the message will be cached by that recipient or gatewayed via a protocol which requires a Date.
In theory, the date SHOULD 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. The format of the Date is an absolute date and time as defined by HTTP-date in section 3.3; it MUST be sent in RFC1123 -date format. 14.20 ETag The ETag entity-header field defines the entity tag for the associated entity. The headers used with entity tags are described in sections 14.20, 14.25, 14.26 and 14.43. The entity tag may be used for comparison with other entities from the same resource (see section 13.3.2). ETag = "ETag" ":" entity-tag Examples: ETag: "xyzzy" ETag: W/"xyzzy" ETag: "" 14.21 Expires The Expires entity-header field gives the date/time after which the response should be considered stale. A stale cache entry may not normally be returned by a cache (either a proxy cache or an 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). See section 13.2 for further discussion of the expiration model. 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 3.3; it MUST be in RFC1123-date format: Expires = "Expires" ":" HTTP-date
An example of its use is Expires: Thu, 01 Dec 1994 16:00:00 GMT Note: if a response includes a Cache-Control field with the max-age directive, that directive overrides the Expires field. HTTP/1.1 clients and caches MUST treat other invalid date formats, especially including the value "0", as in the past (i.e., "already expired"). To mark a response as "already expired," an origin server should use an Expires date that is equal to the Date header value. (See the rules for expiration calculations in section 13.2.4.) To mark a response as "never expires," an origin server should use an Expires date approximately one year from the time the response is sent. HTTP/1.1 servers should not send Expires dates more than one year in the future. The presence of an Expires header field with a date value of some time in the future on an response that otherwise would by default be non-cacheable indicates that the response is cachable, unless indicated otherwise by a Cache-Control header field (section 14.9). 14.22 From The From request-header field, if given, SHOULD contain an Internet e-mail address for the human user who controls the requesting user agent. The address SHOULD be machine-usable, as defined by mailbox in RFC 822 (as updated by RFC 1123 ): From = "From" ":" mailbox An example is: From: email@example.com This header field MAY be used for logging purposes and as a means for identifying the source of invalid or unwanted requests. It SHOULD NOT be used as an insecure form of access protection. 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. For example, when a request is passed through a proxy the original issuer's address SHOULD be used. Note: The client SHOULD not send the From header field without the user's approval, as it may conflict with the user's privacy interests or their site's security policy. 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. 14.23 Host The Host request-header field specifies the Internet host and port number of the resource being requested, as obtained from the original URL given by the user or referring resource (generally an HTTP URL, as described in section 3.2.2). The Host field value MUST represent the network location of the origin server or gateway given by the original URL. This allows the origin server or gateway to differentiate between internally-ambiguous URLs, such as the root "/" URL of a server for multiple host names on a single IP address. Host = "Host" ":" host [ ":" port ] ; Section 3.2.2 A "host" without any trailing port information implies the default port for the service requested (e.g., "80" for an HTTP URL). For example, a request on the origin server for <http://www.w3.org/pub/WWW/> MUST include: GET /pub/WWW/ HTTP/1.1 Host: www.w3.org A client MUST include a Host header field in all HTTP/1.1 request messages on the Internet (i.e., on any message corresponding to a request for a URL which includes an Internet host address for the service being requested). If the Host field is not already present, an HTTP/1.1 proxy MUST add a Host field to the request message prior to forwarding it on the Internet. All Internet-based HTTP/1.1 servers MUST respond with a 400 status code to any HTTP/1.1 request message which lacks a Host header field. See sections 5.2 and 19.5.1 for other requirements relating to Host. 14.24 If-Modified-Since The If-Modified-Since request-header field is used with the GET method to make it conditional: if the requested variant has not been modified since the time specified in this field, an entity will not
be returned from the server; instead, a 304 (not modified) response will be returned without any message-body. If-Modified-Since = "If-Modified-Since" ":" HTTP-date An example of the field is: If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT A GET method with an If-Modified-Since header and no Range header requests that the identified entity be transferred only if it has been modified since the date given by the If-Modified-Since header. The algorithm for determining this includes the following cases: a)If the request would normally result in anything other than a 200 (OK) status, 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. b)If the variant has been modified since the If-Modified-Since date, the response is exactly the same as for a normal GET. c)If the variant has not been modified since a valid If-Modified-Since date, the server MUST 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 that the Range request-header field modifies the meaning of If-Modified-Since; see section 14.36 for full details. Note that If-Modified-Since times are interpreted by the server, whose clock may not be synchronized with the client. Note that if a client uses an arbitrary date in the If-Modified-Since header instead of a date taken from the Last-Modified header for the same request, the client should be aware of the fact that this date is interpreted in the server's understanding of time. The client should consider unsynchronized clocks and rounding problems due to the different encodings of time between the client and server. 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.
14.25 If-Match 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. 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. As a special case, the value "*" matches any current entity of the resource. If-Match = "If-Match" ":" ( "*" | 1#entity-tag ) If any of the entity tags match the entity tag of the entity that would have been returned in the response to a similar GET request (without the If-Match header) on that resource, or if "*" is given and any current entity exists for that resource, then the server MAY perform the requested method as if the If-Match header field did not exist. A server MUST use the strong comparison function (see section 3.11) to compare the entity tags in If-Match. If none of the entity tags match, or if "*" is given and no current entity 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 status, then the If-Match header 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 14.43) exists, and MUST NOT be performed if the representation does not exist.
A request intended to update a resource (e.g., a PUT) MAY include an If-Match header field to signal that the request method MUST NOT be applied if the entity corresponding to the If-Match value (a single entity tag) is no longer a representation of that resource. This allows the user to indicate that they do not wish the request to be successful if the resource has been changed without their knowledge. Examples: If-Match: "xyzzy" If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" If-Match: * 14.26 If-None-Match The If-None-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 none of those entities is current by including a list of their associated entity tags in the If-None-Match header field. 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 a resource which was not known to exist. As a special case, the value "*" matches any current entity of the resource. If-None-Match = "If-None-Match" ":" ( "*" | 1#entity-tag ) If any of the entity tags match the entity tag of the entity that would have been returned in the response to a similar GET request (without the If-None-Match header) on that resource, or if "*" is given and any current entity exists for that resource, then the server MUST NOT perform the requested method. Instead, if the request method was GET or HEAD, the server SHOULD respond with a 304 (Not Modified) response, including the cache-related entity-header fields (particularly ETag) of one of the entities that matched. For all other request methods, the server MUST respond with a status of 412 (Precondition Failed). See section 13.3.3 for rules on how to determine if two entity tags match. The weak comparison function can only be used with GET or HEAD requests. If none of the entity tags match, or if "*" is given and no current entity exists, then the server MAY perform the requested method as if the If-None-Match header field did not exist.
If the request would, without the If-None-Match header field, result in anything other than a 2xx status, then the If-None-Match header MUST be ignored. 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 14.43) exists, and SHOULD be performed if the representation does not exist. This feature may be useful in preventing races between PUT operations. Examples: If-None-Match: "xyzzy" If-None-Match: W/"xyzzy" If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz" If-None-Match: * 14.27 If-Range If a client has a partial copy of an entity in its cache, and wishes to have an up-to-date copy of the entire entity in its cache, it could use the Range request-header with a conditional GET (using either or both of If-Unmodified-Since and If-Match.) However, if the condition fails because the entity has been modified, the client would then have to make a second request to obtain the entire current entity-body. The If-Range header allows a client to "short-circuit" the second request. Informally, its meaning is `if the entity is unchanged, send me the part(s) that I am missing; otherwise, send me the entire new entity.' If-Range = "If-Range" ":" ( entity-tag | HTTP-date ) If the client has no entity tag for an entity, but does have a Last- Modified date, it may use that date in a If-Range header. (The server can distinguish between a valid HTTP-date and any form of entity-tag by examining no more than two characters.) The If-Range header should only be used together with a Range header, and must be ignored if the request does not include a Range header, or if the server does not support the sub-range operation.
If the entity tag given in the If-Range header matches the current entity tag for the entity, then the server should provide the specified sub-range of the entity using a 206 (Partial content) response. If the entity tag does not match, then the server should return the entire entity using a 200 (OK) response. 14.28 If-Unmodified-Since The If-Unmodified-Since request-header field is used with a method to make it conditional. If the requested 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 were not present. If the requested variant has been modified since the specified time, the server MUST NOT perform the requested operation, and MUST return a 412 (Precondition Failed). If-Unmodified-Since = "If-Unmodified-Since" ":" HTTP-date An example of the field is: If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT If the request normally (i.e., without the If-Unmodified-Since header) would result in anything other than a 2xx status, the If- Unmodified-Since header should be ignored. If the specified date is invalid, the header is ignored. 14.29 Last-Modified The Last-Modified entity-header field indicates the date and time at which the origin server believes the variant was last modified. Last-Modified = "Last-Modified" ":" HTTP-date An example of its use is Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT 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 may be just the file system last-modified time. For entities with dynamically included parts, it may be the most recent of the set of last-modify times for its component parts. For database gateways, it may be the last-update time stamp of the record. For virtual objects, it may 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 entity 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 entity's modification time, especially if the entity changes near the time that the response is generated. HTTP/1.1 servers SHOULD send Last-Modified whenever feasible. 14.30 Location The Location response-header field is used to redirect the recipient to a location other than the Request-URI for completion of the request or identification of a new resource. For 201 (Created) responses, the Location is that of the new resource which was created by the request. For 3xx responses, the location SHOULD indicate the server's preferred URL for automatic redirection to the resource. The field value consists of a single absolute URL. Location = "Location" ":" absoluteURI An example is Location: http://www.w3.org/pub/WWW/People.html Note: The Content-Location header field (section 14.15) differs from Location in that the Content-Location identifies the original location of the entity enclosed in the request. It is therefore possible for a response to contain header fields for both Location and Content-Location. Also see section 13.10 for cache requirements of some methods. 14.31 Max-Forwards The Max-Forwards request-header field may be used with the TRACE method (section 14.31) to limit the number of proxies or gateways that can forward the request to the next inbound server. This can be useful when the client is attempting to trace a request chain which appears to be failing or looping in mid-chain. Max-Forwards = "Max-Forwards" ":" 1*DIGIT
The Max-Forwards value is a decimal integer indicating the remaining number of times this request message may be forwarded. Each proxy or gateway recipient of a TRACE request containing a Max- Forwards header field SHOULD check and update its value prior to forwarding the request. If the received value is zero (0), the recipient SHOULD NOT forward the request; instead, it SHOULD respond as the final recipient with a 200 (OK) response containing the received request message as the response entity-body (as described in section 9.8). If the received Max-Forwards value is greater than zero, then the forwarded message SHOULD contain an updated Max- Forwards field with a value decremented by one (1). The Max-Forwards header field SHOULD be ignored for all other methods defined by this specification and for any extension methods for which it is not explicitly referred to as part of that method definition. 14.32 Pragma The Pragma general-header field is used to include implementation- specific directives that may apply to any recipient along the request/response chain. All pragma directives specify optional behavior from the viewpoint of the protocol; however, some systems MAY require that behavior be consistent with the directives. Pragma = "Pragma" ":" 1#pragma-directive pragma-directive = "no-cache" | extension-pragma extension-pragma = token [ "=" ( token | quoted-string ) ] When the no-cache directive is present in a request message, an application SHOULD forward the request toward the origin server even if it has a cached copy of what is being requested. This pragma directive has the same semantics as the no-cache cache-directive (see section 14.9) and is defined here for backwards compatibility with HTTP/1.0. Clients SHOULD include both header fields when a no-cache request is sent to a server not known to be HTTP/1.1 compliant. Pragma directives MUST be passed through by a proxy or gateway application, regardless of their significance to that application, since the directives may be applicable to all recipients along the request/response chain. It is not possible to specify a pragma for a specific recipient; however, any pragma directive not relevant to a recipient SHOULD be ignored by that recipient.
HTTP/1.1 clients SHOULD NOT send the Pragma request-header. HTTP/1.1 caches SHOULD treat "Pragma: no-cache" as if the client had sent "Cache-Control: no-cache". No new Pragma directives will be defined in HTTP. 14.33 Proxy-Authenticate The Proxy-Authenticate response-header field MUST be included as part of a 407 (Proxy Authentication Required) response. The field value consists of a challenge that indicates the authentication scheme and parameters applicable to the proxy for this Request-URI. Proxy-Authenticate = "Proxy-Authenticate" ":" challenge The HTTP access authentication process is described in section 11. Unlike WWW-Authenticate, the Proxy-Authenticate header field applies only to the current connection and SHOULD NOT be passed on to downstream clients. However, an intermediate proxy may need to obtain its own credentials by requesting them from the downstream client, which in some circumstances will appear as if the proxy is forwarding the Proxy-Authenticate header field. 14.34 Proxy-Authorization The Proxy-Authorization request-header field allows the client to identify itself (or its user) to a proxy which requires authentication. The Proxy-Authorization field value consists of credentials containing the authentication information of the user agent for the proxy and/or realm of the resource being requested. Proxy-Authorization = "Proxy-Authorization" ":" credentials The HTTP access authentication process is described in section 11. Unlike Authorization, the Proxy-Authorization header field applies only to the next outbound proxy that demanded authentication using the Proxy-Authenticate field. When multiple proxies are used in a chain, the Proxy-Authorization header field is consumed by the first outbound proxy that was expecting to receive credentials. A proxy MAY relay the credentials from the client request to the next proxy if that is the mechanism by which the proxies cooperatively authenticate a given request. 14.35 Public The Public response-header field lists the set of methods supported by the server. The purpose of this field is strictly to inform the recipient of the capabilities of the server regarding unusual methods. The methods listed may or may not be applicable to the
Request-URI; the Allow header field (section 14.7) MAY be used to indicate methods allowed for a particular URI. Public = "Public" ":" 1#method Example of use: Public: OPTIONS, MGET, MHEAD, GET, HEAD This header field applies only to the server directly connected to the client (i.e., the nearest neighbor in a chain of connections). If the response passes through a proxy, the proxy MUST either remove the Public header field or replace it with one applicable to its own capabilities. 14.36 Range 14.36.1 Byte Ranges Since all HTTP entities are represented in HTTP messages as sequences of bytes, the concept of a byte range is meaningful for any HTTP entity. (However, not all clients and servers need to support byte- range operations.) Byte range specifications in HTTP apply to the sequence of bytes in the entity-body (not necessarily the same as the message-body). A byte range operation may specify a single range of bytes, or a set of ranges within a single entity. ranges-specifier = byte-ranges-specifier byte-ranges-specifier = bytes-unit "=" byte-range-set byte-range-set = 1#( byte-range-spec | suffix-byte-range-spec ) byte-range-spec = first-byte-pos "-" [last-byte-pos] first-byte-pos = 1*DIGIT last-byte-pos = 1*DIGIT The first-byte-pos value in a byte-range-spec gives the byte-offset of the first byte in a range. The last-byte-pos value gives the byte-offset of the last byte in the range; that is, the byte positions specified are inclusive. Byte offsets start at zero.
If the last-byte-pos value is present, it must be greater than or equal to the first-byte-pos in that byte-range-spec, or the byte- range-spec is invalid. The recipient of an invalid byte-range-spec must ignore it. If the last-byte-pos value is absent, or if the value is greater than or equal to the current length of the entity-body, last-byte-pos is taken to be equal to one less than the current length of the entity- body in bytes. By its choice of last-byte-pos, a client can limit the number of bytes retrieved without knowing the size of the entity. suffix-byte-range-spec = "-" suffix-length suffix-length = 1*DIGIT A suffix-byte-range-spec is used to specify the suffix of the entity-body, of a length given by the suffix-length value. (That is, this form specifies the last N bytes of an entity-body.) If the entity is shorter than the specified suffix-length, the entire entity-body is used. Examples of byte-ranges-specifier values (assuming an entity-body of length 10000): o The first 500 bytes (byte offsets 0-499, inclusive): bytes=0-499 o The second 500 bytes (byte offsets 500-999, inclusive): bytes=500-999 o The final 500 bytes (byte offsets 9500-9999, inclusive): bytes=-500 o Or bytes=9500- o The first and last bytes only (bytes 0 and 9999): bytes=0-0,-1
o Several legal but not canonical specifications of the second 500 bytes (byte offsets 500-999, inclusive): bytes=500-600,601-999 bytes=500-700,601-999 14.36.2 Range Retrieval Requests HTTP retrieval requests using conditional or unconditional GET methods may request one or more sub-ranges of the entity, instead of the entire entity, using the Range request header, which applies to the entity returned as the result of the request: Range = "Range" ":" ranges-specifier A server MAY ignore the Range header. However, HTTP/1.1 origin servers and intermediate caches SHOULD support byte ranges when possible, since Range supports efficient recovery from partially failed transfers, and supports efficient partial retrieval of large entities. If the server supports the Range header and the specified range or ranges are appropriate for the entity: o The presence of a Range header in an unconditional GET modifies what is returned if the GET is otherwise successful. In other words, the response carries a status code of 206 (Partial Content) instead of 200 (OK). o The presence of a Range header in a conditional GET (a request using one or both of If-Modified-Since and If-None-Match, or one or both of If-Unmodified-Since and If-Match) modifies what is returned if the GET is otherwise successful and the condition is true. It does not affect the 304 (Not Modified) response returned if the conditional is false. In some cases, it may be more appropriate to use the If-Range header (see section 14.27) in addition to the Range header. If a proxy that supports ranges receives a Range request, forwards the request to an inbound server, and receives an entire entity in reply, it SHOULD only return the requested range to its client. It SHOULD store the entire received response in its cache, if that is consistent with its cache allocation policies.
14.37 Referer The Referer[sic] request-header field allows the client to specify, for the server's benefit, the address (URI) of the resource from which the Request-URI was obtained (the "referrer", although the header field is misspelled.) The Referer request-header allows a server to generate lists of back-links to resources for interest, logging, optimized caching, etc. It also allows obsolete or mistyped links to be traced for maintenance. The Referer field MUST NOT be sent if the Request-URI was obtained from a source that does not have its own URI, such as input from the user keyboard. Referer = "Referer" ":" ( absoluteURI | relativeURI ) Example: Referer: http://www.w3.org/hypertext/DataSources/Overview.html If the field value is a partial URI, it SHOULD be interpreted relative to the Request-URI. The URI MUST NOT include a fragment. Note: Because the source of a link may be private information or may reveal an otherwise private information source, it is strongly recommended that the user be able to select whether or not the Referer field is sent. For example, a browser client could have a toggle switch for browsing openly/anonymously, which would respectively enable/disable the sending of Referer and From information. 14.38 Retry-After The Retry-After response-header field can be used with a 503 (Service Unavailable) response to indicate how long the service is expected to be unavailable to the requesting client. The value of this field can be either an HTTP-date or an integer number of seconds (in decimal) after the time of the response. Retry-After = "Retry-After" ":" ( HTTP-date | delta-seconds ) Two examples of its use are Retry-After: Fri, 31 Dec 1999 23:59:59 GMT Retry-After: 120 In the latter example, the delay is 2 minutes.
14.39 Server The Server response-header field contains information about the software used by the origin server to handle the request. The field can contain multiple product tokens (section 3.8) and comments identifying the server and any significant subproducts. The product tokens are listed in order of their significance for identifying the application. Server = "Server" ":" 1*( product | comment ) Example: Server: CERN/3.0 libwww/2.17 If the response is being forwarded through a proxy, the proxy application MUST NOT modify the Server response-header. Instead, it SHOULD include a Via field (as described in section 14.44). Note: Revealing the specific software version of the server may allow the server machine to become more vulnerable to attacks against software that is known to contain security holes. Server implementers are encouraged to make this field a configurable option. 14.40 Transfer-Encoding The Transfer-Encoding general-header field indicates what (if any) type of transformation has been applied to the message body in order to safely transfer it between the sender and the recipient. This differs from the Content-Encoding in that the transfer coding is a property of the message, not of the entity. Transfer-Encoding = "Transfer-Encoding" ":" 1#transfer- coding Transfer codings are defined in section 3.6. An example is: Transfer-Encoding: chunked Many older HTTP/1.0 applications do not understand the Transfer- Encoding header. 14.41 Upgrade 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 server
MUST use the Upgrade header field within a 101 (Switching Protocols) response to indicate which protocol(s) are being switched. Upgrade = "Upgrade" ":" 1#product For example, Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11 The Upgrade header field is intended to provide a simple mechanism for transition from HTTP/1.1 to some other, incompatible protocol. It does so by allowing the client to advertise its desire to use another protocol, such as a later version of HTTP with a higher major version number, even though the current request has been made using HTTP/1.1. This eases the difficult transition between incompatible protocols by allowing the client to initiate a request in the more commonly supported protocol while indicating to the server that it would like to use a "better" protocol if available (where "better" is determined by the server, possibly according to the nature of the method and/or resource being requested). The Upgrade header field only applies to switching application-layer protocols upon the existing transport-layer connection. Upgrade cannot be used to insist on a protocol change; its acceptance and use by the server is optional. The capabilities and nature of the application-layer communication after the protocol change is entirely dependent upon the new protocol chosen, although the first action after changing the protocol MUST be a response to the initial HTTP request containing the Upgrade header field. The Upgrade header field only applies to the immediate connection. Therefore, the upgrade keyword MUST be supplied within a Connection header field (section 14.10) whenever Upgrade is present in an HTTP/1.1 message. The Upgrade header field cannot be used to indicate a switch to a protocol on a different connection. For that purpose, it is more appropriate to use a 301, 302, 303, or 305 redirection response. This specification only defines the protocol name "HTTP" for use by the family of Hypertext Transfer Protocols, as defined by the HTTP version rules of section 3.1 and future updates to this specification. Any token can be used as a protocol name; however, it will only be useful if both the client and server associate the name with the same protocol.
14.42 User-Agent The User-Agent request-header field contains information about the user agent originating the request. This is for statistical purposes, the tracing of protocol violations, and automated recognition of user agents for the sake of tailoring responses to avoid particular user agent limitations. User agents SHOULD include this field with requests. The field can contain multiple product tokens (section 3.8) and comments identifying the agent and any subproducts which form a significant part of the user agent. By convention, the product tokens are listed in order of their significance for identifying the application. User-Agent = "User-Agent" ":" 1*( product | comment ) Example: User-Agent: CERN-LineMode/2.15 libwww/2.17b3 14.43 Vary The Vary response-header field is used by a server to signal that the response entity was selected from the available representations of the response using server-driven negotiation (section 12). Field- names listed in Vary headers are those of request-headers. The Vary field value indicates either that the given set of header fields encompass the dimensions over which the representation might vary, or that the dimensions of variance are unspecified ("*") and thus may vary over any aspect of future requests. Vary = "Vary" ":" ( "*" | 1#field-name ) An HTTP/1.1 server MUST include an appropriate Vary header field with any cachable response that is subject to server-driven negotiation. Doing so allows a cache to properly interpret future requests on that resource and informs the user agent about the presence of negotiation on that resource. A server SHOULD include an appropriate Vary header field with a non-cachable response that is subject to server-driven negotiation, since this might provide the user agent with useful information about the dimensions over which the response might vary. The set of header fields named by the Vary field value is known as the "selecting" request-headers. When the cache receives a subsequent request whose Request-URI specifies one or more cache entries including a Vary header, the cache MUST NOT use such a cache entry to construct a response to the new request unless all of the headers named in the cached Vary header
are present in the new request, and all of the stored selecting request-headers from the previous request match the corresponding headers in the new request. The selecting request-headers from two requests are defined to match if and only if the selecting request-headers in the first request can be transformed to the selecting request-headers in the second request by adding or removing linear whitespace (LWS) at places where this is allowed by the corresponding BNF, and/or combining multiple message- header fields with the same field name following the rules about message headers in section 4.2. A Vary field value of "*" signals that unspecified parameters, possibly other than the contents of request-header fields (e.g., the network address of the client), play a role in the selection of the response representation. Subsequent requests on that resource can only be properly interpreted by the origin server, and thus a cache MUST forward a (possibly conditional) request even when it has a fresh response cached for the resource. See section 13.6 for use of the Vary header by caches. A Vary field value consisting of a list of field-names signals that the representation selected for the response is based on a selection algorithm which considers ONLY the listed request-header field values in selecting the most appropriate representation. A cache MAY assume that the same selection will be made for future requests with the same values for the listed field names, for the duration of time in which the response is fresh. The field-names given are not limited to the set of standard request-header fields defined by this specification. Field names are case-insensitive. 14.44 Via The Via general-header field MUST be used by gateways and proxies to indicate the intermediate protocols and recipients between the user agent and the server on requests, and between the origin server and the client on responses. It is analogous to the "Received" field of RFC 822 and is intended to be used for tracking message forwards, avoiding request loops, and identifying the protocol capabilities of all senders along the request/response chain.
Via = "Via" ":" 1#( received-protocol received-by [ comment ] ) received-protocol = [ protocol-name "/" ] protocol-version protocol-name = token protocol-version = token received-by = ( host [ ":" port ] ) | pseudonym pseudonym = token The received-protocol indicates the protocol version of the message received by the server or client along each segment of the request/response chain. The received-protocol version is appended to the Via field value when the message is forwarded so that information about the protocol capabilities of upstream applications remains visible to all recipients. The protocol-name is optional if and only if it would be "HTTP". The received-by field is normally the host and optional port number of a recipient server or client that subsequently forwarded the message. However, if the real host is considered to be sensitive information, it MAY be replaced by a pseudonym. If the port is not given, it MAY be assumed to be the default port of the received-protocol. Multiple Via field values represent each proxy or gateway that has forwarded the message. Each recipient MUST append its information such that the end result is ordered according to the sequence of forwarding applications. Comments MAY be used in the Via header field to identify the software of the recipient proxy or gateway, analogous to the User-Agent and Server header fields. However, all comments in the Via field are optional and MAY be removed by any recipient prior to forwarding the message. For example, a request message could be sent from an HTTP/1.0 user agent to an internal proxy code-named "fred", which uses HTTP/1.1 to forward the request to a public proxy at nowhere.com, which completes the request by forwarding it to the origin server at www.ics.uci.edu. The request received by www.ics.uci.edu would then have the following Via header field: Via: 1.0 fred, 1.1 nowhere.com (Apache/1.1) Proxies and gateways used as a portal through a network firewall SHOULD NOT, by default, forward the names and ports of hosts within the firewall region. This information SHOULD only be propagated if explicitly enabled. If not enabled, the received-by host of any host behind the firewall SHOULD be replaced by an appropriate pseudonym for that host.
For organizations that have strong privacy requirements for hiding internal structures, a proxy MAY combine an ordered subsequence of Via header field entries with identical received-protocol values into a single such entry. For example, Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy could be collapsed to Via: 1.0 ricky, 1.1 mertz, 1.0 lucy Applications SHOULD NOT combine multiple entries unless they are all under the same organizational control and the hosts have already been replaced by pseudonyms. Applications MUST NOT combine entries which have different received-protocol values. 14.45 Warning The Warning response-header field is used to carry additional information about the status of a response which may not be reflected by the response status code. This information is typically, though not exclusively, used to warn about a possible lack of semantic transparency from caching operations. Warning headers are sent with responses using: Warning = "Warning" ":" 1#warning-value warning-value = warn-code SP warn-agent SP warn-text warn-code = 2DIGIT warn-agent = ( host [ ":" port ] ) | pseudonym ; the name or pseudonym of the server adding ; the Warning header, for use in debugging warn-text = quoted-string A response may carry more than one Warning header. The warn-text should be in a natural language and character set that is most likely to be intelligible to the human user receiving the response. This decision may be based on any available knowledge, such as the location of the cache or user, the Accept-Language field in a request, the Content-Language field in a response, etc. The default language is English and the default character set is ISO- 8859-1. If a character set other than ISO-8859-1 is used, it MUST be encoded in the warn-text using the method described in RFC 1522 .
Any server or cache may add Warning headers to a response. New Warning headers should be added after any existing Warning headers. A cache MUST NOT delete any Warning header that it received with a response. However, if a cache successfully validates a cache entry, it SHOULD remove any Warning headers previously attached to that entry except as specified for specific Warning codes. It MUST then add any Warning headers received in the validating response. In other words, Warning headers are those that would be attached to the most recent relevant response. When multiple Warning headers are attached to a response, the user agent SHOULD display as many of them as possible, in the order that they appear in the response. If it is not possible to display all of the warnings, the user agent should follow these heuristics: o Warnings that appear early in the response take priority over those appearing later in the response. o Warnings in the user's preferred character set take priority over warnings in other character sets but with identical warn-codes and warn-agents. Systems that generate multiple Warning headers should order them with this user agent behavior in mind. This is a list of the currently-defined warn-codes, each with a recommended warn-text in English, and a description of its meaning. 10 Response is stale MUST be included whenever the returned response is stale. A cache may add this warning to any response, but may never remove it until the response is known to be fresh. 11 Revalidation failed MUST be included if a cache returns a stale response because an attempt to revalidate the response failed, due to an inability to reach the server. A cache may add this warning to any response, but may never remove it until the response is successfully revalidated. 12 Disconnected operation SHOULD be included if the cache is intentionally disconnected from the rest of the network for a period of time. 13 Heuristic expiration MUST be included if the cache heuristically chose a freshness lifetime greater than 24 hours and the response's age is greater than 24 hours.
14 Transformation applied MUST be added by an intermediate cache or proxy if it applies any transformation changing the content-coding (as specified in the Content-Encoding header) or media-type (as specified in the Content-Type header) of the response, unless this Warning code already appears in the response. MUST NOT be deleted from a response even after revalidation. 99 Miscellaneous warning The warning text may include arbitrary information to be presented to a human user, or logged. A system receiving this warning MUST NOT take any automated action. 14.46 WWW-Authenticate The WWW-Authenticate response-header field MUST be included in 401 (Unauthorized) response messages. The field value consists of at least one challenge that indicates the authentication scheme(s) and parameters applicable to the Request-URI. WWW-Authenticate = "WWW-Authenticate" ":" 1#challenge The HTTP access authentication process is described in section 11. User agents MUST take special care in parsing the WWW-Authenticate field value if it contains more than one challenge, or if more than one WWW-Authenticate header field is provided, since the contents of a challenge may itself contain a comma-separated list of authentication parameters.