Age HTTP Header - Original Server Response

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   The Age header contains the time in seconds the object has been in a proxy cache.

HTTP Upgrade Header - HTTP/2

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   The HTTP Upgrade mechanism is used to establish HTTP/2 starting from plain HTTP. The client starts an HTTP/1.1 connection and sends an Upgrade: h2c header. If the server supports HTTP/2, it replies with HTTP 101 Switching Protocol status code. The HTTP Upgrade mechanism is used only for cleartext HTTP2 (h2c). In the case of HTTP2 over TLS (h2), the ALPN TLS protocol extension is used instead.

Vary Header check for User-Agent

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   When using the Vary: User-Agent header, caching servers should consider the user agent when deciding whether to serve the page from cache

Vary Header Accept-Encoding

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   The Accept-Encoding request HTTP header advertises which content encoding, usually a compression algorithm, the client is able to understand. Using content negotiation, the server selects one of the proposals, uses it and informs the client of its choice with the Content-Encoding response header.

X-Cache HTTP Header

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   The resource was not in the website cache, and was pulled from the origin server like Varnish servers.

SPF

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   The Sender Policy Framework (SPF) is an email-authentication technique which is used to prevent spammers from sending messages on behalf of your domain. With SPF an organisation can publish authorized mail servers.

Apache

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   The Apache HTTP Server, colloquially called Apache, is free and open-source cross-platform web server software, released under the terms of Apache License 2.0. Apache is developed and maintained by an open community of developers under the auspices of the Apache Software Foundation.

Server partial requests support

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   The Accept-Ranges response HTTP header is a marker used by the server to advertise its support of partial requests. The value of this field indicates the unit that can be used to define a range.

Gzip Content-Encoding

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   The Content-Encoding entity header is used to compress the media-type. When present, its value indicates which encodings were applied to the entity-body. It lets the client know how to decode in order to obtain the media-type referenced by the Content-Type header.