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An entity tag is an opaque validator associated with a selected HTTP representation and commonly supplied in an ETag HTTP header field. It allows requests to refer to a previously observed version or equivalence class without interpreting the representation's contents. The tag's spelling need not reveal how the server generated it.^[ref]

A strong entity tag has no weak marker; a weak entity tag has the prefix W/. Strong comparison succeeds only when both tags are non-weak and their opaque values match character by character. Weak comparison checks the opaque values while ignoring whether either tag is marked weak.^[ref]

For example, "v1" and W/"v1" match weakly but not strongly. Two occurrences of "v1" match under either comparison. Tags "AbC" and "abc" match under neither: field-name case insensitivity does not make the opaque value case-insensitive.

A useful HTTP conditional request illustrates the purpose. Suppose a client retains representation data tagged "v1", then sends GET with If-None-Match: "v1". If the selected current tag matches, this precondition fails and the server returns 304 without transferring that response content. The retained data can then be used according to the applicable caching rules. If-None-Match uses weak comparison, while If-Match uses strong comparison.^[ref]

An entity tag is therefore a protocol validator, not an instruction to lowercase or decode its opaque string. Different representations of one resource can require different validators. Compare the tag for the selected representation under the comparison rule required by the field, rather than assuming that equal resource URLs alone establish equal data.

An HTTP header field is named information carried in an HTTP message's header section. Its definition determines how its value is interpreted. Field names are case-insensitive, while values follow the rules of their particular field rather than one universal casing rule.^[ref]

For example, ETag: "AbC" and etag: "abc" use the same field name but different entity tags. The quoted opaque strings are compared character by character under the relevant tag-comparison rules. Lowercasing both names is harmless for identifying the field; lowercasing all values would erase a real distinction.^[ref]

Other fields express different kinds of information. Content-Type: text/plain; charset=utf-8 describes a media type and character encoding for content. It does not contain the content itself, and its meaning cannot be inferred by applying the entity-tag grammar to it.

In HTTP/1.1, a textual field line places a colon between the name and value; an empty line terminates the header section. This is a wire-format example, not a requirement that every HTTP version transmit identical text lines.^[ref]

Repeated field occurrences also need their own semantics. Some can be combined as ordered lists, but treating every header section as a dictionary with exactly one value per name can lose information. Read the field definition before combining lines or rearranging values.^[ref]

An HTTP request is a message asking a server to apply an HTTP request method to a target resource. It includes control information and HTTP header fields, and can include content according to the applicable method and message rules. Its meaning is distinct from the textual framing used by one protocol version.^[ref]

For an illustrative HTTP/1.1 request, the line GET /color?view=plain HTTP/1.1 selects a method and request target, while Host: example.org supplies host information. An empty line ends the header section. These pieces belong to the version's request-line and field syntax.^[ref]

Starting from https://example.org/color?view=plain#legend, the ordinary origin-form target is /color?view=plain. The query string participates in that target, while the fragment identifier does not: fragment processing is performed on the client side rather than by including the fragment in the target sent to the server.^[ref][ref]

The method cannot be inferred from a content string alone. Sending the text blue to an endpoint stipulated to replace a color and sending it to an endpoint stipulated to append a log entry request different operations. The target, method and application contract supply that distinction.

An HTTP request also differs from a returned HTTP response. A successfully transmitted request can receive an error status, and a connection can end before a final response is obtained. Reasoning about the requested operation, its eventual outcome and the transport event as separate observations makes failures easier to interpret.

An HTTP request method identifies the operation requested for an HTTP request's target. The target and method have separate roles: the same resource can be read, replaced or processed according to the chosen method and its defined semantics. Method tokens are case-sensitive.^[ref]

For example, an invented /color resource uses GET to retrieve its current color and PUT to replace that color with the supplied value. One PUT storing blue and two identical such operations leave the same intended color. A different /log resource is stipulated to append each POST value: two appends of x then produce two entries. The append behavior belongs to this example application; it is not a universal definition of every POST.

HTTP idempotence concerns repeated intended effects, not identical response messages. A repeated replacement can leave the same state while returning a different HTTP status code because the resource already exists. An HTTP safe method requests essentially read-only semantics; incidental activities such as logging do not make every safe request a request to change the resource.^[ref]

Cacheability is another property, governed by method and response rules rather than by idempotence alone. Interpret the HTTP request method, HTTP header fields and HTTP response together before deciding what a repeated or cached exchange means.^[ref]

An HTTP response is a message returned in response to an HTTP request, carrying an HTTP status code, associated HTTP header fields and any permitted content. An exchange can include informational responses before its final response.^[ref]

For an invented plain-text resource, a successful GET can return status 200, Content-Type: text/plain; charset=utf-8 and the content blue. These four letters occupy four bytes in UTF-8, so a corresponding content length is four. The HTTP representation's data, its media type and the status describing this exchange have different roles.

A response need not contain a representation body. A successful PUT can return 204 without response content, and a HEAD response carries no content even when the corresponding GET would retrieve data. Interpreting a response therefore requires the request method as well as the status code.^[ref]

In HTTP/1.1, HTTP/1.1 200 OK is a possible status line, followed by field lines, an empty line and any message body allowed by the framing rules. This spelling is one version's concrete syntax for the broader response abstraction.^[ref]

The received content can change between requests while the resource identifier stays the same. Conversely, the same content can accompany different exchange metadata. Inspect status, fields and content together instead of treating any one of them as the entire response or as a guarantee of an application-specific result.

An HTTP status code is a three-digit code in an HTTP response describing the outcome or progress of handling an HTTP request. Its first digit identifies a broad class: informational 1xx, successful 2xx, redirection 3xx, client-error 4xx or server-error 5xx. The particular code and request method refine the meaning.^[ref]

For an invented /color resource, a PUT can store blue and return 204 to indicate successful fulfillment without response content. A later GET can return 200 with the text blue. A HEAD response can also have status 200 while carrying no response content. Thus a successful code does not by itself determine whether there is a body to read.^[ref]

In HTTP/1.1, a status line might be written HTTP/1.1 200 OK. The numerical code supplies the standardized status; the accompanying reason phrase is optional and is not the authority for interpretation. Changing the descriptive phrase does not redefine code 200.^[ref]

An HTTP status code describes this exchange rather than proving that every application-level goal succeeded. A request can be transferred correctly and still receive an error status. Conversely, receiving 200 does not establish that an application-specific calculation inside the returned content is correct. Interpret status, HTTP header fields, content and the application's contract together.

An HTTP trailer field is a message field carried in a trailer section after the content. It can provide metadata whose final value becomes available while the content is being generated or processed. Its placement lets a sender stream content before that metadata is complete.^[ref]

For a concrete HTTP/1.1 example, define an illustrative extension field Example-Count to report the number of content octets and explicitly permit it in trailers. This is a definition for the example, not a standard field. A response can announce Transfer-Encoding: chunked and Trailer: Example-Count in its header section. Its following transfer-coded body, using visible escape notation, is 3\r\nabc\r\n0\r\nExample-Count: 3\r\n\r\n.

Here \r\n denotes a carriage-return/line-feed pair in the actual stream. The three-octet chunk supplies abc. The zero-sized chunk ends the chunks, and Example-Count: 3 then supplies the trailer metadata before the final empty line. The content is just abc; the trailer field is not appended to the representation data.^[ref]

The initial Trailer field names metadata the sender anticipates sending. A recipient can use this announcement to prepare its processing, but it does not guarantee that every announced field will actually arrive. The field value itself remains in the later trailer section.^[ref]

Placement is constrained by semantics. A sender must know that a field's definition permits trailer use; fields needed earlier for HTTP message framing, routing or content interpretation generally cannot simply be moved to the end. A recipient must not merge trailer fields into the header section unless their definitions explicitly permit and define the merge.^[ref]

Trailers can also be discarded in transit, so an application cannot assume that arbitrary metadata placed there will reach or be acted upon by every recipient. Keeping HTTP trailer fields distinct from HTTP header fields preserves the distinction between information available before content processing and information learned afterwards.^[ref]

The Hypertext Transfer Protocol is an application-level communication protocol for exchanging requests and responses about resources. In the client-server model, an HTTP request supplies an HTTP request method and target; an HTTP response supplies an HTTP status code and associated information. HTTP header fields convey metadata, and messages can carry content. These semantics are distinct from the particular wire format used by a protocol version.^[ref]

For a concrete exchange, a client can send method GET for https://example.org/manual. A successful response can carry status 200 and an HTML representation of the resource. A response with status 404 instead says that the server did not find a current representation, or is unwilling to disclose one; successful communication does not require successful retrieval.^[ref]

The request's URL identifies the target, while the response describes the outcome of this exchange. A later exchange can return different content or a different status. The protocol's stateless request semantics also do not prevent an application from maintaining state: the distinction concerns what a request must mean, not whether a user can have an ongoing session.^[ref]

A media type identifies a data format and its intended interpretation using a type/subtype name, optionally followed by parameters. It lets a recipient choose an appropriate interpretation rather than inferring everything from a file name. Media types are used in the Hypertext Transfer Protocol and other Internet protocols.^[ref]

For example, text/plain; charset=utf-8 has type text, subtype plain and a charset parameter naming UTF-8. In an HTTP header field such as Content-Type: text/plain; charset=utf-8, this describes the accompanying representation data; it is not itself that data.^[ref]

A format and a character encoding answer different questions. A media type can describe plain text or an image format, while a character encoding explains how text characters correspond to bytes. For example, the PNG format has media type image/png; it is not interpreted by treating all its bytes as a sequence of displayed text characters.

A file suffix is another convention. Two files with the same suffix need not actually contain the same format, and a response's type metadata need not be derived from the last characters of its URL. Correct interpretation requires consistent metadata and data, not merely a suggestive name.

Type/subtype tokens and parameter names have their specified case-insensitive matching rules, but parameter values follow the rules of their own definitions. Lowercasing arbitrary values is therefore not a general parsing strategy.^[ref] A useful inspection separates the name, each parameter and the actual bytes being interpreted.