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LibrariesinsidecoreHTTP

Updated Jul 2025

HTTP Client Terminology

  • Server

    The HTTP server is the central server to which HTTP clients can connect. The HTTP server receives HTTP requests and sends only HTTP responses. An HTTP response is sent only when an HTTP request is received and interpreted.

  • Client

    The HTTP client connects to an HTTP server. The HTTP client sends only HTTP requests to the server and receives HTTP responses.

  • Request

    The type of HTTP message that the client sends to the server. It includes a Request-Line, a variable number of header lines, and an optional request body.

    Example HTTP request:

    1GET /site-image.jpg HTTP/1.1\r\n
    2

    3Host: FreeRTOS.org\r\n
    4

    5Connection: keep-alive\r\n`  
    6

    7\r\n
    8

    9Optional request body.`

  • Response

    The type of HTTP message that the server sends back to the client in reply to an HTTP request. It includes a Status-Line, a variable number of header lines, and an optional response body.

    Example HTTP response:

    1HTTP/1.1 403 Not found\r\n
    2

    3Accept-Ranges: bytes\r\n
    4

    5Content-Length: 1024\r\n
    6

    7Connection: close\r\n
    8

    9\r\n
    10

    11<html lang="en-US"> . . .

  • Request-Line

    The first line in a request message. It includes the request method token, the request path, and the protocol version. This line ends with a carriage return and line feed. 

    Example:

    1GET /path_to_item.txt?optional_query=search HTTP/1.1\r\n

  • Status-Line

    The first line in a response message. It includes the protocol version, the response status code, and a response status phrase. This line ends with a carriage return and line feed.

    Example:

    1HTTP/1.1 200 OK\r\n

  • Method

    The HTTP request method token is an indication of what should be done with the path in the HTTP request. Currently the IoT HTTPS Client Library supports methods: GET, HEAD, PUT, POSTexternal_link (Sections 9.3 - 9.6).  These tokens are the first characters in the Request-Line. 

  • Path

    The path in the Request-Line is a Uniform Resource Locator Identifier (URLI) that identifies the resource to apply the request to. It is contained in the Request-Line of the request.

  • Status Code

    The response status code is a three digit integer noting the result of the attempt to understand and satisfy the request. The phrase following the status code gives human readable reasons for the status code. For example a request to GET a page from an invalid address path results in the status code 404 and the phrase "not found" being returned. The response status is contained in the Status-Line. Status codes are well defined in the HTTP/1.1 standard please see RFC2616 Section 10external_link.

  • Header Line

    In both the response and request messages there are header lines after the Status-Line or the Request-Line. Each header line consists of a header field and header value separated by a colon and space. Each header line ends with a carriage return and line feed. The last header line in an HTTP message is followed by an extra carriage return and line feed.

    Example header lines:

    1Host: FreeRTOS.org\r\n
    2

    3Content-Length: 512\r\n

  • Header Field

    The header field is the name of the header in a header line. The header field precedes a colon ":" in the header line.

    In the example header line below "Content-Length" is the header field.

    1Content-Length: 256\r\n

  • Header Value

    The header value follows the colon and space in the header line.

    In the example header line below "256" is the header value.

    1Content-Length: 256\r\n

  • Persistent Connection

    In a persistent HTTP connection, the server does not close the connection from the client after sending its response to the client's request message. The connection persisted when the client sends the "Connection: keep-alive" header line to the server in each request. Typically web servers will close a connection after 60 seconds if there are not requests received. The client will needs to send "Connection: keep-alive" before the server closes the connection. Servers are not required to honor a "Connection: keep-alive" request.

  • Non-Persistent Connection

    In a non-persistent HTTP connection, the server closes the connection from the client after sending a response to the client's request message. The client can request a non-persistent connection with a "Connection: close" header in the request.