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The response provided by a cache is stale (the content's age exceeds a maximum age set by a Cache-Control header or heuristically chosen lifetime). 111 Revalidation Failed The cache was unable to validate the response, due to an inability to reach the origin server.
HTTP/2 [2] and HTTP/3 instead use a binary protocol, where headers are encoded in a single HEADERS and zero or more CONTINUATION frames using HPACK [3] (HTTP/2) or QPACK (HTTP/3), which both provide efficient header compression. The request or response line from HTTP/1 has also been replaced by several pseudo-header fields, each beginning with ...
curl was first released in 1996. [9] It was originally named httpget and then became urlget before adopting the current name of curl [10] [11] The original author and lead developer is the Swedish developer Daniel Stenberg, who created curl because he wanted to automate the fetching of currency exchange rates for IRC users.
The request/response message consists of the following: Request line, such as GET /logo.gif HTTP/1.1 or Status line, such as HTTP/1.1 200 OK, Headers; An empty line; Optional HTTP message body data; The request/status line and headers must all end with <CR><LF> (that is, a carriage return followed by a line feed).
The Upgrade header field is an HTTP header field introduced in HTTP/1.1.In the exchange, the client begins by making a cleartext request, which is later upgraded to a newer HTTP protocol version or switched to a different protocol.
Normally, such header fields would be sent in the message's header; however, it may be more efficient to determine them after processing the entire message entity. In that case, it is useful to send those headers in the trailer. Header fields that regulate the use of trailers are TE (used in requests), and Trailers (used in responses).
The HTTP Location header field is returned in responses from an HTTP server under two circumstances: To ask a web browser to load a different web page (URL redirection). In this circumstance, the Location header should be sent with an HTTP status code of 3xx. It is passed as part of the response by a web server when the requested URI has:
The general format of the field is: [2] X-Forwarded-For: client, proxy1, proxy2 where the value is a comma+space separated list of IP addresses, the left-most being the original client, and each successive proxy that passed the request adding the IP address where it received the request from.