Network Working Group R. Fielding Request for Comments: 2616 ...

MAY require that behavior be consistent with the directives.Pragma = "Pragma" ":" 1#pragma-directivepragma-directive = "no-cache" | extension-pragmaextension-pragma = token [ "=" ( token | quoted-string ) ]When the no-cache directive is present in a request message, anapplication SHOULD forward the request toward the origin server evenif it has a cached copy of what is being requested. This pragmadirective has the same semantics as the no-cache cache-directive (seesection 14.9) and is defined here for backward compatibility withHTTP/1.0. Clients SHOULD include both header fields when a no-cacherequest is sent to a server not known to be HTTP/1.1 compliant.Fielding, et al. Standards Track [Page 136]RFC 2616 HTTP/1.1 June 1999Pragma directives MUST be passed through by a proxy or gatewayapplication, regardless of their significance to that application,since the directives might be applicable to all recipients along therequest/response chain. It is not possible to specify a pragma for aspecific recipient; however, any pragma directive not relevant to arecipient SHOULD be ignored by that recipient.HTTP/1.1 caches SHOULD treat "Pragma: no-cache" as if the client hadsent "Cache-Control: no-cache". No new Pragma directives will bedefined in HTTP.Note: because the meaning of "Pragma: no-cache as a responseheader field is not actually specified, it does not provide areliable replacement for "Cache-Control: no-cache" in a response14.33 Proxy-AuthenticateThe Proxy-Authenticate response-header field MUST be included as partof a 407 (Proxy Authentication Required) response. The field valueconsists of a challenge that indicates the authentication scheme andparameters applicable to the proxy for this Request-URI.Proxy-Authenticate = "Proxy-Authenticate" ":" 1#challengeThe HTTP access authentication process is described in "HTTPAuthentication: Basic and Digest Access Authentication" [43]. UnlikeWWW-Authenticate, the Proxy-Authenticate header field applies only to