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HTTP (codes (Depending on the availability or otherwise of the Resource,…

- - - - Extended Validation SSL
        
        the organization purchasing it needs to provide documentation as proof of the legal status of the company
        
        has the green browser bar and padlock and it shows the company’s legal name in the browser’s address bar.
      - Standard SSL
        
        it shows the green padlock it does not show the company’s legal name in the browser’s address bar.
- - - - Headers
        
        In the HTTP/1.1 protocol, all header fields except Host are optional.
        
        Content-Type
        
        content negotiation
        
        media-type
        
        convention: type "/" [tree "."] subtype ["+" suffix] *[";" parameter]
        
        examples : application/zip[json//javascript/x-www-form-url-encoded/ pdf/sql], text/css[html/plain/csv] image/gif[png/jpeg], multipart/form-data
        
        types : application, audio, example, font, image, message, model, multipart, text and video
        
        a two-part identifier for file formats and format contents transmitted on the Internet; consists of a type which is further structured into a tree and a subtype ; can optionally define a suffix and parameters
        
        A resource may be available in several different representations; for example, it might be available in different languages or different media types.
        
        mechanisms defined as a part of HTTP that make it possible to serve different versions of a document (or more generally, representations of a resource) at the same URI, so that user agents can specify which version fits their capabilities the best
        
        content negotiation mechanisms
        
        server-driven (proactive)
        
        Accept-Language: de; q=1.0, en; q=0.5
        Accept: text/html; q=1.0, text/; q=0.8, image/gif; q=0.6, image/jpeg; q=0.6, image/; q=0.5, /; q=0.1
        
        is performed by algorithms on the server which choose among the possible variant representations.
        
        agent-driven (reactive)
        
        is performed by algorithms in the user-agent which choose among the possible variant representations.
        
        Browsers indicate their preferences by headers in the request
        
        The Media type of the body of the request (used with POST and PUT requests).
        
        Accept
        
        media-types that are acceptable by the request
        
        Accept-Encoding
        
        HTTP data is compressed before it is sent from the server: compliant browsers will announce what methods are supported to the server before downloading the correct format; browsers that do not support compliant compression method will download uncompressed data. The most common compression schemes include gzip and Deflate;
        
        Compression scheme negotiation
        
        web client advertises which compression schemes it supports by including a list of tokens in the HTTP request, then If the server supports one or more compression schemes, the outgoing data may be compressed by one or more methods supported by both parties. If this is the case, the server will add a Content-Encoding or Transfer-Encoding field in the HTTP response with the used schemes, separated by commas.
        
        formats
        
        deflate - algorithm
        
        exi – W3C Efficient XML Interchange
        
        gzip - the most popular, uses deflate algorithm for compression, but the data format and the checksum algorithm differ from the "deflate" content-encoding
        
        Accept-Language
        
        Accept-Language: en-US
        
        Content-Length
        
        The length of the request body in octets (8-bit bytes)
        
        Cookie
        
        if
        
        If-Unmodified-Since
        
        If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
        
        If-Match
        
        Only perform the action if the client supplied entity matches the same entity on the server. This is mainly for methods like PUT to only update a resource if it has not been modified since the user last updated it
        
        If-Modified-Since
        
        Allows a 304 Not Modified to be returned if content is unchanged. If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
        
        Transfer-Encoding
        
        Each chunk is preceded by its size in bytes. The transmission ends when a zero-length chunk is received. The chunked keyword in the Transfer-Encoding header is used to indicate chunked transfer.
        
        Proxy-Authorization
        
        Max-Forwards
        
        Limit the number of times the message can be forwarded through proxies or gateways.
        
        origin
        
        Initiates a request for cross-origin resource sharing (asks server for Access-Control-* response fields).
        
        ETag
        
        An ETag is an opaque identifier assigned by a web server to a specific version of a resource found at a URL
        
        entity tag - is part of HTTP and one of several mechanisms that HTTP provides for web cache validation , which allows a client to make conditional requests. This allows caches to be more efficient, and saves bandwidth
        
        If the resource representation at that URL ever changes, a new and different ETag is assigned.
        
        The method by which ETags are generated has never been specified in the HTTP specification.
        Common methods of ETag generation include using a collision-resistant hash function of the resource's content, a hash of the last modification timestamp, or even just a revision number.
        
        A strongly validating ETag match indicates that the content of the two resource representations is byte-for-byte identical and that all other entity fields (such as Content-Language) are also unchanged.
        
        A weakly validating ETag match only indicates that the two representations are semantically equivalent, meaning that for practical purposes they are interchangeable and that cached copies can be used.
        
        In typical usage, when a URL is retrieved, the web server will return the resource's current representation along with its corresponding ETag value, which is placed in an HTTP response header "ETag" field:
        ETag: "686897696a7c876b7e"
        The client may then decide to cache the representation, along with its ETag. Later, if the client wants to retrieve the same URL resource again, it will first determine whether the local cached version of the URL has expired (through the Cache-Control and the Expire headers). If the URL has not expired, it will retrieve the local cached resource. If it is determined that the URL has expired (is stale), then the client will contact the server and send its previously saved copy of the ETag along with the request in a "If-None-Match" field.[2]
        If-None-Match: "686897696a7c876b7e"
        On this subsequent request, the server may now compare the client's ETag with the ETag for the current version of the resource. If the ETag values match, meaning that the resource has not changed, then the server may send back a very short response with a HTTP 304 Not Modified status. The 304 status tells the client that its cached version is still good and that it should use that.
        However, if the ETag values do not match, meaning the resource has likely changed, then a full response including the resource's content is returned, just as if ETags were not being used. In this case the client may decide to replace its previously cached version with the newly returned representation of the resource and the new ETag.
        
        Cache-Control
        
        Used to specify directives that must be obeyed by all caching mechanisms along the request-response chain. e.g. Cache-Control: no-cache
        
        three basic mechanisms for controlling caches
        
        Freshness allows a response to be used without re-checking it on the origin server, and can be controlled by both the server and the client. For example, the Expires response header gives a date when the document becomes stale, and the Cache-Control: max-age directive tells the cache how many seconds the response is fresh for.
        
        Validation can be used to check whether a cached response is still good after it becomes stale. For example, if the response has a Last-Modified header, a cache can make a conditional request using the If-Modified-Since header to see if it has changed. The ETag (entity tag) mechanism also allows for both strong and weak validation.
        
        Invalidation is usually a side effect of another request that passes through the cache. For example, if a URL associated with a cached response subsequently gets a POST, PUT or DELETE request, the cached response will be invalidated.
        
        Expect
        
        Indicates that particular server behaviors are required by the client. Expect: 100-continue
        
        Head
        
        The domain name of the server (for virtual hosting), and the TCP port number on which the server is listening. The port number may be omitted if the port is the standard port for the service requested.Mandatory since HTTP/1.1.[13] If the request is generated directly in HTTP/2, it should not be used
        
        Authorization
      - An empty line
      - An optional message body
      - Request line
        
        e.g., GET /images/logo.png HTTP/1.1
  - - - ETag
        
        is used to determine if a cached version of the requested resource is identical to the current version of the resource on the server.
      - Content-Type specifies the Internet media type of the data conveyed by the HTTP message,
      - Content-Length indicates Internet media type length in bytes
      - Age
        
        The age the object has been in a proxy cache in seconds Age: 12
      - Allow Valid methods for a specified resource. To be used for a 405 Method not allowed Allow: GET, HEAD
      - Cache-Control
        
        Tells all caching mechanisms from server to client whether they may cache this object. It is measured in seconds Cache-Control: max-age=3600
        
        no-store is intended to instruct a browser application to make a best effort not to write it to disk, but it cannot prevent this action.
        
        max-age=0; This instructs the user agent that the content is stale and should be validated before use.
        
        no-cache - a web browser or other caching system (intermediate proxies) must not use the response to satisfy subsequent requests without first checking with the originating server (this process is called validation). This header field is part of HTTP version 1.1, and is ignored by some caches and browsers. It may be simulated by setting the Expires HTTP version 1.0 header field value to a time earlier than the response time. Notice that no-cache is not instructing the browser or proxies about whether or not to cache the content. It just tells the browser and proxies to validate the cache content with the server before using it (this is done by using If-Modified-Since, If-Unmodified-Since, If-Match, If-None-Match attributes mentioned above). Sending a no-cache value thus instructs a browser or proxy to not use the cache contents merely based on "freshness criteria" of the cache content.
      - Content-Disposition
        
        An opportunity to raise a "File Download" dialogue box for a known MIME type with binary format or suggest a filename for dynamic content. Quotes are necessary with special characters. Content-Disposition: attachment; filename="fname.ext"
      - Content-Location
        
        An alternate location for the returned data Content-Location: /index.htm
      - Content-Range
        
        Where in a full body message this partial message belongs Content-Range: bytes 21010-47021/47022
        
        byte serving
        
        One advantage of this capability is when a large media file is being requested, and that media file is properly formatted, the client may be able to request just the portions of the file known to be of interest. This is essential for serving video files; if a server lacks this feature, videos hosted on that server may not be playable until the entire file has been downloaded by the client,
        
        the process of sending only a portion of an HTTP/1.1 message from a server to a client. Byte serving begins when an HTTP server advertises its willingness to serve partial requests using the Accept-Ranges response header. A client then requests a specific part of a file from the server using the Range request header. If the range is valid, the server sends it to the client with a 206 Partial Content status code and a Content-Range header listing the range sent. If the range is invalid, the server responds with a 416 Requested Range Not Satisfiable status code.
      - Expires
        
        Gives the date/time after which the response is considered stale (in "HTTP-date" format as defined by RFC 7231) Expires: Thu, 01 Dec 1994 16:00:00 GMT
      - Last-Modified
      - Location
        
        Used in redirection, or when a new resource has been created and adequate status code should be added (2xx or 3xx)
        Example 1: Location: http://www.w3.org/pub/WWW/People.html
        Example 2: Location: /pub/WWW/People.html*
      - Pragma
        
        Implementation-specific fields that may have various effects anywhere along the request-response chain.
      - Proxy-Authenticate
        
        Request authentication to access the proxy. Proxy-Authenticate: Basic
      - Retry-After
        
        If an entity is temporarily unavailable, this instructs the client to try again later. Value could be a specified period of time (in seconds) or a HTTP-date.
        Example 1: Retry-After: 120
        Example 2: Retry-After: Fri, 07 Nov 2014 23:59:59 GMT
      - Set-Cookie
        
        An HTTP cookie Set-Cookie: UserID=JohnDoe; Max-Age=3600; Version=1
      - Strict-Transport-Security
        
        A HSTS Policy informing the HTTP client how long to cache the HTTPS only policy and whether this applies to subdomains. Strict-Transport-Security: max-age=16070400; includeSubDomains
      - Transfer-Encoding
        
        Currently defined methods are: chunked, compress, deflate, gzip, identity.
        Must not be used with HTTP/2.
      - WWW-Authenticate
        
        Indicates the authentication scheme that should be used to access the requested entity. WWW-Authenticate: Basic
- - - - request headers
        
        Access-Control-Request-Method
        
        Access-Control-Request-Headers
        
        Origin
      - response headers
        
        Access-Control-Allow-Origin
        
        Access-Control-Allow-Credentials
        
        Access-Control-Expose-Headers
        
        Access-Control-Max-Age
        
        Access-Control-Allow-Methods
        
        Access-Control-Allow-Headers
    - - If service.example.com is willing to accept the action, it may respond with the following headers:
        Access-Control-Allow-Origin: http://www.example.com
        Access-Control-Allow-Methods: PUT, DELETE
      - Server will return an error page if the server does not allow the cross-origin request
      - Access-Control-Allow-Origin: *
        
        The value of "*" is special in that it does not allow requests to supply credentials, meaning it does not allow HTTP authentication, client-side SSL certificates, or cookies to be sent in the cross-domain request