14.19 ETag 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 14.24, 14.26 and 14.44. The entity tag MAY be used for comparison with other entities from the same resource (see section 13.3.3). ETag = "ETag" ":" entity-tag Examples: ETag: "xyzzy" ETag: W/"xyzzy" ETag: "" 14.20 Expect The Expect request-header field is used to indicate that particular server behaviors are required by the client. Expect = "Expect" ":" 1#expectation expectation = "100-continue" | expectation-extension expectation-extension = token [ "=" ( token | quoted-string ) *expect-params ] expect-params = ";" token [ "=" ( token | quoted-string ) ] 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 417 (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 417 (Expectation Failed) status. Comparison of expectation values is case-insensitive for unquoted tokens (including the 100-continue token), and is case-sensitive for quoted-string expectation-extensions.
The Expect mechanism is hop-by-hop: that is, an HTTP/1.1 proxy MUST return a 417 (Expectation Failed) status if it receives a request with an expectation that it cannot meet. However, the Expect request-header itself is end-to-end; it MUST be forwarded if the request is forwarded. Many older HTTP/1.0 and HTTP/1.1 applications do not understand the Expect header. See section 8.2.3 for the use of the 100 (continue) status. 14.21 Expires The Expires entity-header field gives the date/time after which the response is considered stale. 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). 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 3.3.1; it MUST be in RFC 1123 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 (see section 14.9.3), 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 sends 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 sends 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 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 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. The client SHOULD NOT send the From header field without the user's approval, as it might 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 URI 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 naming authority 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/> would properly 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 . If the requested URI does not include an Internet host name for the service being requested, then the Host header field MUST be given with an empty value. An HTTP/1.1 proxy MUST ensure that any request message it forwards does contain an appropriate Host header field that identifies the service being requested by the proxy. All Internet-based HTTP/1.1 servers MUST respond with a 400 (Bad Request) status code to any HTTP/1.1 request message which lacks a Host header field. See sections 5.2 and 188.8.131.52 for other requirements relating to Host. 14.24 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. Entity tags are defined in section 3.11. 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 13.3.3) 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 or 412 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 14.44) 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: * 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. 14.25 If-Modified-Since The If-Modified-Since request-header field is used with a 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 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 14.35 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 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. 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. 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 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 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, 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 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 entities tags match. The weak comparison function can only be used with GET or HEAD requests.
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, then the If-None-Match header MUST be ignored. (See section 13.3.4 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 14.44) exists, and SHOULD be performed if the representation does not exist. This feature is intended to 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: * 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. 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 an 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 or 412 status, the If-Unmodified-Since header SHOULD be ignored. If the specified date is invalid, the header 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. 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 URI for automatic redirection to the resource. The field value consists of a single absolute URI. Location = "Location" ":" absoluteURI An example is: Location: http://www.w3.org/pub/WWW/People.html Note: The Content-Location header field (section 14.14) 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 provides a mechanism with the TRACE (section 9.8) and OPTIONS (section 9.2) methods 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 or OPTIONS request containing a Max-Forwards header field MUST check and update its value prior to forwarding the request. If the received value is zero (0), the recipient MUST NOT forward the request; instead, it MUST respond as the final recipient. If the received Max-Forwards value is greater than zero, then the forwarded message MUST contain an updated Max-Forwards field with a value decremented by one (1). The Max-Forwards header field MAY 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 might 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 backward 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 might 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 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. Note: because the meaning of "Pragma: no-cache as a response header field is not actually specified, it does not provide a reliable replacement for "Cache-Control: no-cache" in a response 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" ":" 1#challenge The HTTP access authentication process is described in "HTTP Authentication: Basic and Digest Access Authentication" . 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 might 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 "HTTP Authentication: Basic and Digest Access Authentication"  . 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 Range 14.35.1 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 syntactically invalid. The recipient of a byte-range- set that includes one or more syntactically invalid byte-range-spec values MUST ignore the header field that includes that byte-range- set. 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. If a syntactically valid byte-range-set includes at least one byte- range-spec whose first-byte-pos is less than the current length of the entity-body, or at least one suffix-byte-range-spec with a non- zero suffix-length, then the byte-range-set is satisfiable. Otherwise, the byte-range-set is unsatisfiable. If the byte-range-set is unsatisfiable, the server SHOULD return a response with a status of 416 (Requested range not satisfiable). Otherwise, the server SHOULD return a response with a status of 206 (Partial Content) containing the satisfiable ranges of the entity-body. Examples of byte-ranges-specifier values (assuming an entity-body of length 10000): - The first 500 bytes (byte offsets 0-499, inclusive): bytes=0- 499 - The second 500 bytes (byte offsets 500-999, inclusive): bytes=500-999 - The final 500 bytes (byte offsets 9500-9999, inclusive): bytes=-500 - Or bytes=9500- - The first and last bytes only (bytes 0 and 9999): bytes=0-0,-1 - 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 14.35.2 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 ought to 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: - 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). - 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 might 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. 14.36 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 relative URI, it SHOULD be interpreted relative to the Request-URI. The URI MUST NOT include a fragment. See section 15.1.3 for security considerations. 14.37 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. This field MAY also be used with any 3xx (Redirection) response to indicate the minimum time the user-agent is asked wait before issuing the redirected request. 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. 14.38 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 14.45). Note: Revealing the specific software version of the server might allow the server machine to become more vulnerable to attacks against software that is known to contain security holes. Server implementors are encouraged to make this field a configurable option.
14.39 TE The TE request-header field indicates what extension transfer-codings it is willing to accept in the response and whether or not it is willing to accept trailer fields in a chunked transfer-coding. Its value may consist of the keyword "trailers" and/or a comma-separated list of extension transfer-coding names with optional accept parameters (as described in section 3.6). TE = "TE" ":" #( t-codings ) t-codings = "trailers" | ( transfer-extension [ accept-params ] ) The presence of the keyword "trailers" indicates that the client is willing to accept trailer fields in a chunked transfer-coding, as defined in section 3.6.1. This keyword is reserved for use with transfer-coding values even though it does not itself represent a transfer-coding. Examples of its use are: TE: deflate TE: TE: trailers, deflate;q=0.5 The TE header field only applies to the immediate connection. Therefore, the keyword MUST be supplied within a Connection header field (section 14.10) whenever TE is present in an HTTP/1.1 message. A server tests whether a transfer-coding is acceptable, according to a TE field, using these rules: 1. The "chunked" transfer-coding is always acceptable. If the keyword "trailers" is listed, the client indicates that it is willing to accept trailer fields in the chunked response on behalf of itself and any downstream clients. The implication is that, if given, the client is stating that either all downstream clients are willing to accept trailer fields in the forwarded response, or that it will attempt to buffer the response on behalf of downstream recipients. Note: HTTP/1.1 does not define any means to limit the size of a chunked response such that a client can be assured of buffering the entire response. 2. If the transfer-coding being tested is one of the transfer- codings listed in the TE field, then it is acceptable unless it is accompanied by a qvalue of 0. (As defined in section 3.9, a qvalue of 0 means "not acceptable.")
3. If multiple transfer-codings are acceptable, then the acceptable transfer-coding with the highest non-zero qvalue is preferred. The "chunked" transfer-coding always has a qvalue of 1. If the TE field-value is empty or if no TE field is present, the only transfer-coding is "chunked". A message with no transfer-coding is always acceptable. 14.40 Trailer 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. Trailer = "Trailer" ":" 1#field-name An HTTP/1.1 message SHOULD include a Trailer header field in a message using chunked transfer-coding with a non-empty trailer. Doing so allows the recipient to know which header fields to expect in the trailer. If no Trailer header field is present, the trailer SHOULD NOT include any header fields. See section 3.6.1 for restrictions on the use of trailer fields in a "chunked" transfer-coding. Message header fields listed in the Trailer header field MUST NOT include the following header fields: . Transfer-Encoding . Content-Length . Trailer 14.41 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-coding 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
If multiple encodings have been applied to an entity, the transfer- 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. Many older HTTP/1.0 applications do not understand the Transfer- Encoding header. 14.42 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. 14.43 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 14.44 Vary The Vary field value indicates the set of request-header fields that fully determines, while the response is fresh, whether a cache is permitted to use the response to reply to a subsequent request without revalidation. For uncacheable or stale responses, the Vary field value advises the user agent about the criteria that were used to select the representation. A Vary field value of "*" implies that a cache cannot determine from the request headers of a subsequent request whether this response is the appropriate representation. See section 13.6 for use of the Vary header field by caches. Vary = "Vary" ":" ( "*" | 1#field-name ) An HTTP/1.1 server SHOULD include a Vary header field with any cacheable 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 MAY include a Vary header field with a non-cacheable 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 varies at the time of the response. 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 for 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. A Vary field value of "*" signals that unspecified parameters not limited to the request-headers (e.g., the network address of the client), play a role in the selection of the response representation. The "*" value MUST NOT be generated by a proxy server; it may only be generated by an origin server. 14.45 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 represents 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. 14.46 Warning The Warning general-header field is used to carry additional information about the status or transformation of a message which might not be reflected in the message. This information is typically used to warn about a possible lack of semantic transparency from caching operations or transformations applied to the entity body of the message. Warning headers are sent with responses using: Warning = "Warning" ":" 1#warning-value warning-value = warn-code SP warn-agent SP warn-text [SP warn-date] warn-code = 3DIGIT warn-agent = ( host [ ":" port ] ) | pseudonym ; the name or pseudonym of the server adding ; the Warning header, for use in debugging warn-text = quoted-string warn-date = <"> HTTP-date <"> 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 2047 . Warning headers can in general be applied to any message, however some specific warn-codes are specific to caches and can only be applied to response messages. New Warning headers SHOULD be added after any existing Warning headers. A cache MUST NOT delete any Warning header that it received with a message. 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 ought to inform the user of as many of them as possible, in the order that they appear in the response. If it is not possible to inform the user of all of the warnings, the user agent SHOULD follow these heuristics: - Warnings that appear early in the response take priority over those appearing later in the response. - 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. Requirements for the behavior of caches with respect to Warnings are stated in section 13.1.2. This is a list of the currently-defined warn-codes, each with a recommended warn-text in English, and a description of its meaning. 110 Response is stale MUST be included whenever the returned response is stale. 111 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. 112 Disconnected operation SHOULD be included if the cache is intentionally disconnected from the rest of the network for a period of time. 113 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. 199 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, besides presenting the warning to the user.
214 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, or the entity-body of the response, unless this Warning code already appears in the response. 299 Miscellaneous persistent 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. If an implementation sends a message with one or more Warning headers whose version is HTTP/1.0 or lower, then the sender MUST include in each warning-value a warn-date that matches the date in the response. If an implementation receives a message with a warning-value that includes a warn-date, and that warn-date is different from the Date value in the response, then that warning-value MUST be deleted from the message before storing, forwarding, or using it. (This prevents bad consequences of naive caching of Warning header fields.) If all of the warning-values are deleted for this reason, the Warning header MUST be deleted as well. 14.47 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 "HTTP Authentication: Basic and Digest Access Authentication" . User agents are advised to take special care in parsing the WWW- Authenticate field value as it might contain more than one challenge, or if more than one WWW-Authenticate header field is provided, the contents of a challenge itself can contain a comma-separated list of authentication parameters.