The HTTP protocol defines the [@https://tools.ietf.org/html/rfc7230#section-2.1 client and server roles]: clients send requests and servers send back responses. When a client and server have established a connection, the client sends a series of requests while the server sends back at least one response for each received request in the order those requests were received.
A request or response is an [@https://tools.ietf.org/html/rfc7230#section-3 HTTP message] (referred to hereafter as "message") having two parts: a header with structured metadata and an optional variable-length body holding arbitrary data. A serialized header is one or more text lines where each line ends in a carriage return followed by linefeed (`"\r\n"`). An empty line marks the end of the header. The first line in the header is called the ['start-line]. The contents of the start line contents are different for requests and responses.
Every message contains a set of zero or more field name/value pairs, collectively called "fields". The names and values are represented using text strings with various requirements. A serialized field contains the field name, then a colon followed by a space (`": "`), and finally the field value with a trailing CRLF.
Clients send requests, which contain a [@https://tools.ietf.org/html/rfc7230#section-3.1.1 method] and [@https://tools.ietf.org/html/rfc7230#section-5.3 request-target], and [@https://tools.ietf.org/html/rfc7230#section-2.6 HTTP-version]. The method identifies the operation to be performed while the target identifies the object on the server to which the operation applies. The version is almost always 1.1, but older programs sometimes use 1.0.
This request has a method of "GET", a target of "/", and indicates HTTP version 1.1. It contains a single field called "User-Agent" whose value is "Beast". There is no message body.
Servers send responses, which contain a [@https://tools.ietf.org/html/rfc7231#section-6 status-code], [@https://tools.ietf.org/html/rfc7230#section-3.1.2 reason-phrase], and [@https://tools.ietf.org/html/rfc7230#section-2.6 HTTP-version]. The reason phrase is [@https://tools.ietf.org/html/rfc7230#section-3.1.2 obsolete]: clients SHOULD ignore the reason-phrase content. Here is a response which includes a body. The special [@https://tools.ietf.org/html/rfc7230#section-3.3.2Content-Length] field informs the remote host of the size of the body which follows.
This response has a [@https://tools.ietf.org/html/rfc7231#section-6 200 status code] meaning the operation requested completed successfully. The obsolete reason phrase is "OK". It specifies HTTP version 1.1, and contains a body 13 octets in size with the text "Hello, world!".
Messages may optionally carry a body. The size of the message body is determined by the semantics of the message and the special fields Content-Length and Transfer-Encoding. [@https://tools.ietf.org/html/rfc7230#section-3.3 rfc7230 section 3.3] provides a comprehensive description for how the body length is determined.
Certain fields appearing in messages are special. The library understands these fields when performing serialization and parsing, taking automatic action as needed when the fields are parsed in a message and also setting the fields if the caller requests it.