HTTP (Hyper Text Transfer Protocol)

  1. HTTP is the foundation of data communication for the World Wide Web, where hypertext documents include hyperlinks to other resources that the user can easily access.
  2. HTTP/2 (2015) and HTTP/3 (2019) do not make previous versions of the protocol obsolete.
  3. HTTP functions as a request-response protocol in the client-server computing model. A web browser for example, may be the client and an application running on a computer hosting a website may be the server. The server provides resources such as HTML files and other content, or performes other functions on behalf of the client, returning a response message to the client. The response contains completion status information about the request and may also contain requested content in its message body. However both a web browser and a mobile app are user agents, as indeed is any software that accesses, consumes, or displays web content.
  4. HTTP is designed to permit intermediate network elements to improve or enable communications between clients and servers. High-traffic websites often benefit from web cache servers that deliver content on behalf of upstream servers to improve response time. Web browsers cache previously accessed web resources and reuse them, when possible, to reduce network traffic.
  5. HTTP is an application layer protocol designed within the framework of the Internet protocol suite. Its definition presumes an underlying and reliable transport layer protocol, and Transmission Control Protocol (TCP) is commonly used. However, HTTP can be adapted to use unreliable protocols such as the Universal Datagram Protocol (UDP), for example in HTTPU and Simple Service Discovery Protocol (SSDP), and in in HTTP/3 which use UDP instead of TCP.
           userinfo          host        port
       ┌───────┴───────┐ ┌────┴────────┐ ┌┴┐
http://john.doe:password@www.example.com:123/forum/questions/?tag=networking&order=newest#top
└─┬─┘ └───────────┬────────────────────────┘└─┬─────────────┘└────────┬──────────────────┘└┬─┘
scheme         authority                      path                  query             fragment



HTTP session

An HTTP session is a sequence of network request-response transactions. An HTTP client initiates a request by establishing a Transmission Control Protocol (TCP) connection to a particular port on a server (typically port 80, occasionally port 8080). An HTTP server listening on that port waits for a client's request message. Upon receiving the request, the server sends back a status line, such as "HTTP/1.1 200 OK", and a message of its own. The body of this message is typically the requested resource, although an error message or other information may also be returned.




Persistent connections

In HTTP/0.9 and 1.0, the connection is closed after a single request/response pair. In HTTP/1.1 a keep-alive-mechanism was introduced, where a connection could be reused for more than one request. Such persistent connections reduce request latency perceptibly, because the client does not need to re-negotiate the TCP 3-Way-Handshake connection after the first request has been sent. Another positive side effect is that, in general, the connection becomes faster with time due to TCP's slow-start-mechanism.

Version 1.1 of the protocol also made bandwidth optimization improvements to HTTP/1.0. For example, HTTP/1.1 introduced chunked transfer encoding to allow content on persistent connections to be streamed rather than buffered. HTTP pipelining further reduces lag time, allowing clients to send multiple requests before waiting for each response. Another addition to the protocol was byte serving, where a server transmits just the portion of a resource explicitly requested by a client.




HTTP session state

HTTP is a stateless protocol. A stateless protocol does not require the HTTP server to retain information or status about each user for the duration of multiple requests. However, some web applications implement states or server side sessions using for instance HTTP cookies or hidden variables within web forms.




HTTPS

Hypertext Transfer Protocol Secure (HTTPS) is an extension of the Hypertext Transfer Protocol (HTTP). It is used for secure communication over a computer network. In HTTPS, the communication protocol is encrypted using Transport Layer Security (TLS), or, formerly, its predecessor, Secure Sockets Layer (SSL). The protocol is therefore also often referred to as HTTP over TLS, or HTTP over SSL.

The principal motivation for HTTPS is authentication of the accessed website and protection of the privacy and integrity of the exchanged data while in transit. It protects against man-in-the-middle attacks. The bidirectional encryption of communications between a client and server protects against eavesdropping and tampering of the communication. In practice, this provides a reasonable assurance that one is communicating without interference by attackers with the website that one intended to communicate with, as opposed to an impostor.

Historically, HTTPS connections were primarily used for payment transactions on the World Wide Web, e-mail and for sensitive transactions in corporate information systems. Since 2018, HTTPS is used more often by web users than the original non-secure HTTP, primarily to protect page authenticity on all types of websites; secure accounts; and keep user communications, identity, and web browsing private.

HTTPS creates a secure channel over an insecure network. This ensures reasonable protection from eavesdroppers and man-in-the-middle attacks, provided that adequate cipher suites are used and that the server certificate is verified and trusted.

Because HTTPS piggybacks HTTP entirely on top of TLS, the entirety of the underlying HTTP protocol can be encrypted. This includes the request URL (which particular web page was requested), query parameters, headers, and cookies (which often contain identity information about the user). However, because host (website) addresses and port numbers are necessarily part of the underlying TCP/IP protocols, HTTPS cannot protect their disclosure. In practice this means that even on a correctly configured web server, eavesdroppers can infer the IP address and port number of the web server (sometimes even the domain name e.g. www.example.org, but not the rest of the URL) that one is communicating with, as well as the amount (data transferred) and duration (length of session) of the communication, though not the content of the communication.




TCP head-of-line blocking

Although the design of HTTP/2 effectively addresses the HTTP-transaction-level head-of-line blocking problem by allowing multiple concurrent HTTP transactions, all those transactions are multiplexed over a single TCP connection, meaning that any packet-level head-of-line blocking of the TCP stream simultaneously blocks all transactions being accessed via that connection. This head-of-line blocking in HTTP/2 is now widely regarded as a design flaw, and much of the effort behind QUIC and HTTP/3 has been devoted to reduce head-of-line blocking issues.

Message format


The request message consists of the following:

  1. Request Message

    A request line (e.g., GET /images/logo.png HTTP/1.1, which requests a resource called /images/logo.png from the server.

  2. Request header fields

    e.g. Accept-Language: en, or Host: en.wikipedia.org:8080 etc

  3. An empty line

  4. An optional message body

    For example "Hello world!"

Request methods

HTTP defines methods to indicate the desired action to be performed on the identified resource. What this resource rpresents, whether pre-existing data or data that is generated dynamically, depends on the implementation of the server.

Method names are case sensitive. This is in contrast to HTTP header field names which are case-insensitive.

HTTP Method Description
GET The GET method requests a representation of the specified resource. Requests using GET should only retrieve data and should have no other effect. (This is also true of some other HTTP methods.)
HEAD The HEAD method asks for a response identical to that of a GET request, but without the response body. This is useful for retrieving meta-information written in response headers, without having to transport the entire content.
POST The POST method requests that the server accept the entity enclosed in the request as a new subordinate of the web resource identified by the URI. The data POSTed might be, for example, an annotation for existing resources; a message for a bulletin board, newsgroup, mailing list, or comment thread; a block of data that is the result of submitting a web form to a data-handling process; or an item to add to a database.
PUT The PUT method requests that the enclosed entity be stored under the supplied URI. If the URI refers to an already existing resource, it is modified; if the URI does not point to an existing resource, then the server can create the resource with that URI.
DELETE The DELETE method deletes the specified resource.
TRACE The TRACE method echoes the received request so that a client can see what (if any) changes or additions have been made by intermediate servers.
OPTIONS The OPTIONS method returns the HTTP methods that the server supports for the specified URL. This can be used to check the functionality of a web server by requesting '*' instead of a specific resource.
CONNECT The CONNECT method converts the request connection to a transparent TCP/IP tunnel, usually to facilitate SSL-encrypted communication (HTTPS) through an unencrypted HTTP proxy.
PATCH The PATCH method applies partial modifications to a resource.

Summary table

HTTP Method Request body Response body Safe Idempotent Cacheable
GET Optional Yes Yes Yes Yes
HEAD Optional No Yes Yes Yes
POST Yes Yes No No Yes
PUT Yes Yes No Yes No
DELETE Optional Yes No Yes No
CONNECT Optional Yes No No No
OPTIONS Optional Yes Yes Yes No
TRACE No Yes Yes Yes No
PATCH Yes Yes No No No

Idempotence

From a RESTful service standpoint, for an operation (or service call) to be idempotent, clients can make that same call repeatedly while producing the same result. In other words, making multiple identical requests has the same effect as making a single request. Note that while idempotent operations produce the same result on the server (no side effects), the response itself may not be the same (e.g. a resource's state may change between requests).




Request methods

The response message consists of the following:

  1. A Status Line

    Which includes the status code and reason message (e.g., HTTP/1.1 200 OK, which indicates that the client's request succeeded.)

  2. response header fields

    e.g. Content-Type: text/html etc

  3. An empty line

  4. An optional message body

    For example some html

Example server response

 HTTP/1.1 200 OK
 Date: Mon, 23 May 2005 22:38:34 GMT
 Content-Type: text/html; charset=UTF-8
 Content-Length: 138
 Last-Modified: Wed, 08 Jan 2003 23:11:55 GMT
 Server: Apache/1.3.3.7 (Unix) (Red-Hat/Linux)
 ETag: "3f80f-1b6-3e1cb03b"
 Accept-Ranges: bytes
 Connection: close

 <html>
   <head>
     <title>An Example Page</title>
   </head>
   <body>
     <p>Hello World, this is a very simple HTML document.</p>
   </body>
 </html>

Transmission Control Protocol (TCP)

The Transmission Control Protocol (TCP) is one of the main protocols of the Internet protocol suite. It originated in the initial network implementation in which it complemented the Internet Protocol (IP). Therefore, the entire suite is commonly referred to as TCP/IP. TCP provides (Of a stream of bytes between applications running on hosts communicating via an IP network):

  1. Reliable delivery
  2. Ordered delivery
  3. Error-checked delivery


Major internet applications such as the World Wide Web, email, remote administration, and file transfer rely on TCP.

Applications that do not require reliable data stream service may use the User Datagram Protocol (UDP), which provides a connectionless datagram service that emphasizes reduced latency over reliability.

TCP is still dominantly used for the web, i.e. for the HTTP protocol, and later HTTP/2, while not used by latest standard HTTP/3.

TCP is optimized for accurate delivery rather than timely delivery and can incur relatively long delays (on the order of seconds) while waiting for out-of-order messages or re-transmissions of lost messages. Therefore, it is not particularly suitable for real-time applications such as voice over IP. For such applications, protocols like the Real-time Transport Protocol (RTP) operating over the User Datagram Protocol (UDP) are usually recommended instead.

TCP is a reliable stream delivery service which guarantees that all bytes received will be identical and in the same order as those sent. Since packet transfer by many networks is not reliable, TCP achieves this using a technique known as positive acknowledgement with re-transmission. This requires the receiver to respond with an acknowledgement message as it receives the data. The sender keeps a record of each packet it sends and maintains a timer from when the packet was sent. The sender re-transmits a packet if the timer expires before receiving the acknowledgement. The timer is needed in case a packet gets lost or corrupted.

Universal Datagram Protocol (UDP)

The User Datagram Protocol (UDP) is one of the core members of the Internet protocol suite. With UDP, computer applications can send messages, in this case referred to as datagrams, to other hosts on an Internet Protocol (IP) network. Prior communications are not required in order to set up communication channels or data paths.

UDP uses a simple connectionless communication model with a minimum of protocol mechanisms. UDP provides checksums for data integrity, and port numbers for addressing different functions at the source and destination of the datagram. It has no handshaking dialogues, and thus exposes the user's program to any unreliability of the underlying network; there is no guarantee of delivery, ordering, or duplicate protection. If error-correction facilities are needed at the network interface level, an application may use Transmission Control Protocol (TCP) or Stream Control Transmission Protocol (SCTP) which are designed for this purpose.

UDP is suitable for purposes where error checking and correction are either not necessary or are performed in the application; UDP avoids the overhead of such processing in the protocol stack. Time-sensitive applications often use UDP because dropping packets is preferable to waiting for packets delayed due to retransmission, which may not be an option in a real-time system.

  1. It is transaction-oriented, suitable for simple query-response protocols such as the Domain Name System or the Network Time Protocol.
  2. It provides datagrams, suitable for modeling other protocols such as IP tunneling or remote procedure call and the Network File System.
  3. It is simple, suitable for bootstrapping or other purposes without a full protocol stack, such as the DHCP and Trivial File Transfer Protocol.
  4. It is stateless, suitable for very large numbers of clients, such as in streaming media applications such as IPTV.
  5. The lack of retransmission delays makes it suitable for real-time applications such as Voice over IP, online games, and many protocols using Real Time Streaming Protocol.
  6. Because it supports multicast, it is suitable for broadcast information such as in many kinds of service discovery and shared information such as Precision Time Protocol and Routing Information Protocol.

HTTP Response Codes

Here is a handy .net enum for these. Needless to say these codes should always be matched against an enum in code, for correctness.

All HTTP response status codes are separated into five classes (or categories). The first digit of the status code defines the class of response. The last two digits do not have any class or categorization role. There are five classes defined by the standard:

  • 1xx Informational – the request was received, continuing process
  • 2xx Successful – the request was successfully received, understood and accepted
  • 3xx Redirection – further action needs to be taken in order to complete the request
  • 4xx Client Error – the request contains bad syntax or cannot be fulfilled
  • 5xx Server Error – the server failed to fulfill an apparently valid request



1XX Informational response

An informational response indicates that the request was received and understood. It is issued on a provisional basis while request processing continues. It alerts the client to wait for a final response. The message consists only of the status line and optional header fields, and is terminated by an empty line. As the HTTP/1.0 standard did not define any 1xx status codes, servers must not send a 1xx response to an HTTP/1.0 compliant client except under experimental conditions.

Code Description
100 Continue The server, has received the request headers and the client should proceed to send the request body (in the case of a request for which a body needs to be sent; for example, a POST request). Sending a large request body to a server after a request has been rejected for inappropriate headers would be inefficient. To have a server check the request's headers, a client must send Expect: 100-continue as a header in its initial request and receive a 100 Continue status code in response before sending the body. If the client receives an error code such as 403 (Forbidden) or 405 (Method Not Allowed) then it shouldn't send the request's body. The response 417 Expectation Failed indicates that the request should be repeated without the Expect header as it indicates that the server doesn't support expectations (this is the case, for example, of HTTP/1.0 servers).
101 Switching Protocols The requester has asked the server to switch protocols and the server has agreed to do so.
102 Processing A WebDAV request may contain many sub-requests involving file operations, requiring a long time to complete the request. This code indicates that the server has received and is processing the request, but no response is available yet. This prevents the client from timing out and assuming the request was lost.
103 Early Hints Used to return some response headers before final HTTP message.



2XX Success

This class of status codes indicates the action requested by the client was received, understood and accepted.

Code Description
200 OK Standard response for successful HTTP requests. The actual response will depend on the request method used. In a GET request, the response will contain an entity corresponding to the requested resource. In a POST request, the response will contain an entity describing or containing the result of the action.
201 Created The request has been fulfilled, resulting in the creation of a new resource.
202 Accepted The request has been accepted for processing, but the processing has not been completed. The request might or might not be eventually acted upon, and may be disallowed when processing occurs.
203 Non-Authoritative Information (since HTTP/1.1) The server is a transforming proxy (e.g. a Web accelerator) that received a 200 OK from its origin, but is returning a modified version of the origin's response.
204 No Content The server successfully processed the request and is not returning any content.
205 Reset Content The server successfully processed the request, but is not returning any content. Unlike a 204 response, this response requires that the requester reset the document view.
206 Partial Content (RFC 7233) The server is delivering only part of the resource (byte serving) due to a range header sent by the client. The range header is used by HTTP clients to enable resuming of interrupted downloads, or split a download into multiple simultaneous streams.
207 Multi-Status (WebDAV; RFC 4918) The message body that follows is by default an XML message and can contain a number of separate response codes, depending on how many sub-requests were made.
208 Already Reported (WebDAV; RFC 5842) The members of a DAV binding have already been enumerated in a preceding part of the (multistatus) response, and are not being included again.
IM Used (RFC 3229) The server has fulfilled a request for the resource, and the response is a representation of the result of one or more instance-manipulations applied to the current instance.




3XX Redirection

This class of status code indicates the client must take additional action to complete the request. Many of these status codes are used in URL redirection.

A user agent may carry out the additional action with no user interaction only if the method used in the second request is GET or HEAD. A user agent may automatically redirect a request. A user agent should detect and intervene to prevent cyclical redirects.

Code Description
300 Multiple Choices Indicates multiple options for the resource from which the client may choose (via agent-driven content negotiation). For example, this code could be used to present multiple video format options, to list files with different filename extensions, or to suggest word-sense disambiguation.
301 Moved Permanently This and all future requests should be directed to the given URI.
302 Found (Previously "Moved temporarily") Tells the client to look at (browse to) another URL. 302 has been superseded by 303 and 307. This is an example of industry practice contradicting the standard. The HTTP/1.0 specification (RFC 1945) required the client to perform a temporary redirect (the original describing phrase was "Moved Temporarily"), but popular browsers implemented 302 with the functionality of a 303 See Other. Therefore, HTTP/1.1 added status codes 303 and 307 to distinguish between the two behaviours. However, some Web applications and frameworks use the 302 status code as if it were the 303.
303 See Other (since HTTP/1.1) The response to the request can be found under another URI using the GET method. When received in response to a POST (or PUT/DELETE), the client should presume that the server has received the data and should issue a new GET request to the given URI.
304 Not Modified (RFC 7232) Indicates that the resource has not been modified since the version specified by the request headers If-Modified-Since or If-None-Match. In such case, there is no need to retransmit the resource since the client still has a previously-downloaded copy.
305 Use Proxy (since HTTP/1.1) The requested resource is available only through a proxy, the address for which is provided in the response. For security reasons, many HTTP clients (such as Mozilla Firefox and Internet Explorer) do not obey this status code.
306 Switch Proxy No longer used. Originally meant "Subsequent requests should use the specified proxy."
307 Temporary Redirect (since HTTP/1.1) In this case, the request should be repeated with another URI; however, future requests should still use the original URI. In contrast to how 302 was historically implemented, the request method is not allowed to be changed when reissuing the original request. For example, a POST request should be repeated using another POST request.
308 Permanent Redirect (RFC 7538) The request and all future requests should be repeated using another URI. 307 and 308 parallel the behaviors of 302 and 301, but do not allow the HTTP method to change. So, for example, submitting a form to a permanently redirected resource may continue smoothly.




4XX Client errors

This class of status code is intended for situations in which the error seems to have been caused by the client. Except when responding to a HEAD request, the server should include an entity containing an explanation ofthe error situation, and whether it is a temporary or permanent condition. These status codes are applicable to any request method. User agents should display any included entity to the user.

Code Description
400 Bad Request The server cannot or will not process the request due to an apparent client error (e.g., malformed request syntax, size too large, invalid request message framing, or deceptive request routing).
401 Unauthorized (RFC 7235) Similar to 403 Forbidden, but specifically for use when authentication is required and has failed or has not yet been provided. The response must include a WWW-Authenticate header field containing a challenge applicable to the requested resource. See Basic access authentication and Digest access authentication. 401 semantically means "unauthorised",[34] the user does not have valid authentication credentials for the target resource. Note: Some sites incorrectly issue HTTP 401 when an IP address is banned from the website (usually the website domain) and that specific address is refused permission to access a website.
402 Payment Required Reserved for future use. The original intention was that this code might be used as part of some form of digital cash or micropayment scheme, as proposed, for example, by GNU Taler,[35] but that has not yet happened, and this code is not usually used. Google Developers API uses this status if a particular developer has exceeded the daily limit on requests. Sipgate uses this code if an account does not have sufficient funds to start a call. Shopify uses this code when the store has not paid their fees and is temporarily disabled.
403 Forbidden The request contained valid data and was understood by the server, but the server is refusing action. This may be due to the user not having the necessary permissions for a resource or needing an account of some sort, or attempting a prohibited action (e.g. creating a duplicate record where only one is allowed). This code is also typically used if the request provided authentication via the WWW-Authenticate header field, but the server did not accept that authentication. The request should not be repeated.
404 Not Found The requested resource could not be found but may be available in the future. Subsequent requests by the client are permissible.
405 Method Not Allowed A request method is not supported for the requested resource; for example, a GET request on a form that requires data to be presented via POST, or a PUT request on a read-only resource.
406 Not Acceptable The requested resource is capable of generating only content not acceptable according to the Accept headers sent in the request.
407 Proxy Authentication Required (RFC 7235) The client must first authenticate itself with the proxy.
408 Request Timeout The server timed out waiting for the request. According to HTTP specifications: "The client did not produce a request within the time that the server was prepared to wait. The client MAY repeat the request without modifications at any later time."
409 Conflict Indicates that the request could not be processed because of conflict in the current state of the resource, such as an edit conflict between multiple simultaneous updates.
410 Gone Indicates that the resource requested is no longer available and will not be available again. This should be used when a resource has been intentionally removed and the resource should be purged. Upon receiving a 410 status code, the client should not request the resource in the future. Clients such as search engines should remove the resource from their indices.[42] Most use cases do not require clients and search engines to purge the resource, and a "404 Not Found" may be used instead.
411 Length Required The request did not specify the length of its content, which is required by the requested resource.
412 Precondition Failed (RFC 7232) The server does not meet one of the preconditions that the requester put on the request header fields.
413 Payload Too Large (RFC 7231) The request is larger than the server is willing or able to process. Previously called "Request Entity Too Large".
414 URI Too Long (RFC 7231) The URI provided was too long for the server to process. Often the result of too much data being encoded as a query-string of a GET request, in which case it should be converted to a POST request.[47] Called "Request-URI Too Long" previously.
415 Unsupported Media Type (RFC 7231) The request entity has a media type which the server or resource does not support. For example, the client uploads an image as image/svg+xml, but the server requires that images use a different format.
416 Range Not Satisfiable (RFC 7233) The client has asked for a portion of the file (byte serving), but the server cannot supply that portion. For example, if the client asked for a part of the file that lies beyond the end of the file. Called "Requested Range Not Satisfiable" previously.
417 Expectation Failed The server cannot meet the requirements of the Expect request-header field.
418 I'm a teapot (RFC 2324, RFC 7168) This code was defined in 1998 as one of the traditional IETF April Fools' jokes, in RFC 2324, Hyper Text Coffee Pot Control Protocol, and is not expected to be implemented by actual HTTP servers. The RFC specifies this code should be returned by teapots requested to brew coffee.[53] This HTTP status is used as an Easter egg in some websites, including Google.com.
421 Misdirected Request (RFC 7540) The request was directed at a server that is not able to produce a response[56] (for example because of connection reuse).
422 Unprocessable Entity (WebDAV; RFC 4918) The request was well-formed but was unable to be followed due to semantic errors.
423 Locked (WebDAV; RFC 4918) The resource that is being accessed is locked.
424 Failed Dependency (WebDAV; RFC 4918) The request failed because it depended on another request and that request failed (e.g., a PROPPATCH).
425 Too Early (RFC 8470) Indicates that the server is unwilling to risk processing a request that might be replayed.
426 Upgrade Required The client should switch to a different protocol such as TLS/1.0, given in the Upgrade header field.
428 Precondition Required (RFC 6585) The origin server requires the request to be conditional. Intended to prevent the 'lost update' problem, where a client GETs a resource's state, modifies it, and PUTs it back to the server, when meanwhile a third party has modified the state on the server, leading to a conflict.
429 Too Many Requests (RFC 6585) The user has sent too many requests in a given amount of time. Intended for use with rate-limiting schemes.
431 Request Header Fields Too Large (RFC 6585) The server is unwilling to process the request because either an individual header field, or all the header fields collectively, are too large.
451 Unavailable For Legal Reasons (RFC 7725) A server operator has received a legal demand to deny access to a resource or to a set of resources that includes the requested resource.[60] The code 451 was chosen as a reference to the novel Fahrenheit 451 (see the Acknowledgements in the RFC).




5XX Server errors

The server failed to fulfill a request.

Response status codes beginning with the digit "5" indicate cases in which the server is aware that it has encountered an error or is otherwise incapable of performing the request. Except when responding to a HEAD request, the server should include an entity containing an explanation of the error situation, and indicate whether it is a temporary or permanent condition. Likewise, user agents should display any included entity to the user. These response codes are applicable to any request method.

Code Description
500 Internal Server Error A generic error message, given when an unexpected condition was encountered and no more specific message is suitable.
501 Not Implemented The server either does not recognize the request method, or it lacks the ability to fulfil the request. Usually this implies future availability (e.g., a new feature of a web-service API).
502 Bad Gateway The server was acting as a gateway or proxy and received an invalid response from the upstream server.
503 Service Unavailable The server cannot handle the request (because it is overloaded or down for maintenance). Generally, this is a temporary state.
504 Gateway Timeout The server was acting as a gateway or proxy and did not receive a timely response from the upstream server.
505 HTTP Version Not Supported The server does not support the HTTP protocol version used in the request.
506 Variant Also Negotiates (RFC 2295) Transparent content negotiation for the request results in a circular reference.
507 Insufficient Storage (WebDAV; RFC 4918) The server is unable to store the representation needed to complete the request.
508 Loop Detected (WebDAV; RFC 5842) The server detected an infinite loop while processing the request (sent instead of 208 Already Reported).
510 Not Extended (RFC 2774) Further extensions to the request are required for the server to fulfil it.
511 Network Authentication Required (RFC 6585) The client needs to authenticate to gain network access. Intended for use by intercepting proxies used to control access to the network (e.g., "captive portals" used to require agreement to Terms of Service before granting full Internet access via a Wi-Fi hotspot).