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HTTP Basics

Go back to the ⬆ Table of Contents or ⬆ What's next? section.

Simply put, HTTP stands for hypertext transfer protocol and is used for transmitting data (e.g. web pages) over the Internet.

Some important information about HTTP:

  • all requests originate at the client (e.g. browser)
  • the server responds to a request
  • the requests and responses are in readable text
  • the requests are independent of each other and the server doesn’t need to track the requests

I will not describe the HTTP protocol meticulously so you have to look at this as an introduction. I will discuss only the most important things because we have some great documents which describe this protocol in a great deal of detail:

We have also some interesting books:

And finally look at this amazing series of A1-sized posters about the HTTP protocol.

Features and architecture

The HTTP protocol is a request/response protocol based on the client/server based architecture where web browsers, robots and search engines, etc. act like HTTP clients, and the Web server acts as a server. This is HTTP's message-based model. Every HTTP interaction includes a request and a response.

By its nature, HTTP is stateless. Stateless means that all requests are separate from each other. So each request from your browser must contain enough information on its own for the server to fulfill the request.

Here is a brief explanation:

  • most often the HTTP communication uses the TCP protocol

  • HTTP protocol is stateless (all requests are separate from each other)

  • each transaction of the message based model of HTTP is processed separately from the others

  • the HTTP client, i.e., a browser initiates an HTTP request and after a request is made, the client waits for the response

  • the HTTP server handles and processing requests from clients (and continues to listen and to accept other requests), after that it sends a response to the client

  • any type of data can be sent by HTTP as long as both the client and the server know how to handle the data content

  • the server and client are aware of each other only during a current request. Afterwards, both of them forget about each other

The HTTP protocol allows clients and servers to communicate. Clients send requests using an HTTP method request and servers listen for requests on a host and port. The following is a comparison:

  • client - the HTTP client sends a request to the server in the form of a request method, URI, and protocol version, followed by a MIME-like message containing request modifiers, client information, and possible body content over a TCP/IP connection

  • server - the HTTP server responds with a status line, including the message's protocol version and a success or error code, followed by a MIME-like message containing server information, entity meta information, and possible entity-body content

HTTP_steps

This infographic comes from www.ntu.edu.sg - HTTP (HyperText Transfer Protocol).

HTTP/2

HTTP/2 is a major revision of the HTTP network protocol, intended as a higher performance alternative to HTTP/1.1. It introduces several new features, while remaining semantically compatible.

I will not describe it because there are brilliant studies:

But you should remember: A client that does not support HTTP/2 will never ask the server for an HTTP/2 communication upgrade: the communication between them will be fully HTTP1/1. A client that supports HTTP/2 will ask the server (using HTTP1/1) for an HTTP/2 upgrade:

  • if the server is HTTP/2 ready, then the server will notice the client as such: the communication between them will be switched to HTTP/2
  • if the server is not HTTP/2 ready, then the server will ignore the upgrade request answering with HTTP1/1: the communication between them should stay plenty HTTP1/1

An HTTP/2 client never makes a HTTP/2 request by default. It always makes an HTTP/1.1 request with Upgrade: HTTP/2.0 header.

URI vs URL

Uniform Resource Identifier (URI) is a string of characters used to identify a name or a resource on the Internet. A URI identifies a resource either by location, or a name, or both. A URI has two specializations known as URL and URN.

I think, the best explanation is here: The Difference Between URLs, URIs, and URNs by Daniel Miessler.

For me, this short and clear explanation is also interesting:

URIs identify and URLs identify and locate; however, locators are also identifiers, so every URL is also a URI, but there are URIs which are not URLs.

url_urn_uri

Look at the following examples to get your mind out of confusion and take it simple:

TYPE DESCRIPTION
URL https://www.google.com/folder/page.html
URL http://example.com/resource?foo=bar#fragment
URL ftp://example.com/download.zip
URL mailto:[email protected]
URL file:///home/user/file.txt
URL /other/link.html (a relative URL)
URN www.pierobon.org/iis/review1.htm#one
URN urn:ietf:rfc:2648
URN urn:isbn:0451450523
URI http://www.pierobon.org/iis/review1.htm.html#one

The graphic below explains the URL format:

url_format

If it is still unclear to you, I would advise you to look at the following articles:

HTTP Headers

When a client requests a resource from a server it uses HTTP. This request includes a set of key-value pairs giving information like the version of the browser or what file formats it understands. These key-value pairs are called request headers.

The server answers with the requested resource but also sends response headers giving information on the resource or the server itself.

See these articles about HTTP headers:

HTTP headers allow the client and the server to pass additional information with the request or the response. An HTTP header consists of its case-insensitive name followed by a colon :, then by its value (without line breaks).

Header compression

The role of header compression:

Header compression resulted in an ~88% reduction in the size of request headers and an ~85% reduction in the size of response headers. On the lower-bandwidth DSL link, in which the upload link is only 375 Kbps, request header compression in particular, led to significant page load time improvements for certain sites (i.e. those that issued large number of resource requests). We found a reduction of 45 - 1142 ms in page load time simply due to header compression.

  • HTTP/2 supports a new dedicated header compression algorithm, called HPACK
  • HPACK is resilient to CRIME
  • HPACK uses three methods of compression: Static Dictionary, Dynamic Dictionary, Huffman Encoding

Please see also:

HTTP Methods

The HTTP protocol includes a set of methods that indicate which action to be done for a resource. The most common methods are GET and POST. But there are a few others, too:

  • GET - is used to retreive data from a server at the specified resource

  • POST - is used to create or append a resource to an existing resource

  • PUT - is used to create or update (replace) the existing resource

  • HEAD - is almost identical to GET, except without the response body

  • TRACE - is used for used for diagnostic/debugging purposes which echo's back input back to the user

  • OPTIONS - is used to describe the communication options (HTTP methods) that are available for the target resource

  • PATCH - is used to apply partial modifications to a resource

  • DELETE - is used to delete the specified resource

Request

A request consists of: (1) a command or request + (2) optional headers + (4) optional body content:

                      FIELDS OF HTTP REQUEST       PART OF RFC 2616
---------------------------------------------------------------------
  Request       = (1) : Request-line                 Section 5.1
                  (2) : *(( general-header           Section 4.5
                          | request-header           Section 5.3
                          | entity-header ) CRLF)    Section 7.1
                  (3) : CRLF
                  (4) : [ message-body ]             Section 4.3

Example of form an HTTP request to fetch /alerts/status page from the web server running on localhost:8000:

http_request

Additional information about requests:

  • route to endpoint
  • rewrite path/query
  • deny access (acls)
  • headers modification
Request line

The Request-line begins with a method, followed by the Request-URI and the protocol version, and ending with CRLF (\r\n). The elements are separated by space SP characters:

Request-Line = Method SP Request-URI SP HTTP-Version CRLF
Methods
METHOD DESCRIPTION
GET is used to retreive data from a server at the specified resource

For more information look at the HTTP/1.1 RFC for the definition of GET.

For example, say you have an API with a /api/v2/users endpoint. Making a GET request to that endpoint should return a list of all available users.

Requests with GET method does not change any data (the state of resource) and should never be used to submit new information to the server. This method is almost identical to HEAD. If GET /users returns a list of users, then HEAD /users will make the same request but won't get back the list of users.

At a basic level, these things should be validated:

  • check that a valid GET request returns a 200 status code
  • ensure that a GET request to a specific resource returns the correct data

When executing a GET request, you ask the server for one, or a set of entities. To allow the client to filter the result, it can use the so called "query string" (?) of the URL.

METHOD DESCRIPTION
POST is used to send data to the sever to modify and update a resource

Look at the HTTP/1.1 RFC for the definition of POST:

The POST method is used to request that the origin server accept the entity enclosed in the request as a new subordinate of the resource identified by the Request-URI [...] The posted entity is subordinate to that URI in the same way that a file is subordinate to a directory containing it, a news article is subordinate to a newsgroup to which it is posted, or a record is subordinate to a database.

When executing a POST request, the client is actually submitting a new document to the remote host but the server decides the location where it is stored under the user collection.

Requests with POST method change data (the state of resource) on the backend server by modifying or updating a resource.

If you do not know the actual resource location, for instance, when you add a new article, but do not have any idea where to store it, you can POST it to an URL, and let the server decide the actual URL.

Here are some tips for testing POST requests:

  • create a resource with a POST request (the server could respond with a 201 Created)
  • next, make a GET request to ensure a 200 OK status code is returned (the data was saved correctly)
  • add tests that ensure POST requests fail with incorrect or ill-formatted data

Modify and update a resource:

POST /items/<existing_item> HTTP/1.1
Host: example.com

The following is an error:

POST /items/<new_item> HTTP/1.1
Host: example.com
METHOD DESCRIPTION
PUT is used to send data to the sever to create or overwrite a resource

Look at the HTTP/1.1 RFC for the definition of PUT:

The PUT method requests that the enclosed entity be stored under the supplied Request-URI. If the Request-URI refers to an already existing resource, the enclosed entity SHOULD be considered as a modified version of the one residing on the origin server. If the Request-URI does not point to an existing resource [...] the origin server can create the resource with that URI.

The PUT specification requires that you already know the URL of the resource you wish to create or update. On create, if the client chooses the identifier for a resource a PUT request will create the new resource at the specified URL.

If you know that a resource already exists for a URL, you can make a PUT request to that URL to replace the state of that resource on the server.

Check for these things when testing PUT requests:

  • repeatedly call a PUT request always returns the same result (idempotent)
  • if an existing resource is modified, either the 200 OK or 204 No Content response codes SHOULD be sent to indicate successful completion of the request
  • PUT requests should fail if invalid data is supplied in the request - nothing should be updated

For a new resource:

PUT /items/<new_item> HTTP/1.1
Host: example.com

To overwrite an existing resource:

PUT /items/<existing_item> HTTP/1.1
Host: example.com

For me, the difference between POST and PUT is sometimes unclear. I think both can be used for creating and the big difference is that a POST request is supposed to let the server decide how to (and if at all) create a new resource.

In my opinion, there is one more good explanation:

The POST method is used to send user-generated data to the web server. For example, a POST method is used when a user comments on a forum or if they upload a profile picture. A POST method should also be used if you do not know the specific URL of where your newly created resource should reside.

The PUT method completely replaces whatever currently exists at the target URL with something else. With this method, you can create a new resource or overwrite an existing one given you know the exact Request-URI.

Look also at this amazing answer by Brian R. Bondy.

Request URI

The Request-URI is a Uniform Resource Identifier and identifies the resource upon which to apply the request. The exact resource identified by an Internet request is determined by examining both the Request-URI and the Host header field.

The most common form of Request-URI is that used to identify a resource on an origin server or gateway. For example, a client wishing to retrieve a resource directly from the origin server would create a TCP connection to port 80 of the host example.com and send the following lines:

GET /pub/index.html HTTP/1.1
Host: example.com

The absoluteURI form is required when the request is being made to a proxy:

GET http://example.com/pub/index.html HTTP/1.1

Note that the absolute path cannot be empty; if none is present in the original URI, it MUST be given as / (the server root).

The asterisk * is used when an HTTP request does not apply to a particular resource, but to the server itself, and is only allowed when the method used does not necessarily apply to a resource.

HTTP version

The last part of the request indicating the client's supported HTTP version. HTTP has four versions — HTTP/0.9, HTTP/1.0, HTTP/1.1, and HTTP/2.0. Today the versions in common use are HTTP/1.1 and HTTP/2.0.

Determining the appropriate version of the HTTP protocol is very important because it allows you to set specific HTTP method or required headers (e.g. cache-control for HTTP/1.1).

There is a nice explanation about How does a HTTP 1.1 server respond to a HTTP 1.0 request?.

Request header fields

There are three types of HTTP message headers for requests:

  • General-header - applying to both requests and responses but with no relation to the data eventually transmitted in the body

  • Request-header - containing more information about the resource to be fetched or about the client itself

  • Entity-header - containing more information about the body of the entity, like its content length or its MIME-type

The Request-header fields allow the client to pass additional information about the request, and about the client itself, to the server.

What is an accepted maximum allowed size for HTTP headers? Read this great answer. HTTP does not define any limit. However most web servers do limit size of headers they accept. Server will return 413 Entity Too Large error if headers size exceeds that limit.

Message body

Request (message) body is the part of the HTTP request where additional content can be sent to the server.

It is optional. Most of the HTTP requests are GET requests without bodies. However, simulating requests with bodies is important to properly stress the proxy code and to test various hooks working with such requests. Most HTTP requests with bodies use POST or PUT request method.

Generate requests

How to generate a request?

  • curl

    curl -Iks -v -X GET -H "Connection: keep-alive" -H "User-Agent: X-AGENT" https://example.com

  • httpie

    http -p Hh GET https://example.com User-Agent:X-AGENT --follow

  • telnet

    telnet example.com 80
GET /index.html HTTP/1.1
Host: example.com

  • openssl

    openssl s_client -servername example.com -connect example.com:443
...
---
GET /index.html HTTP/1.1
Host: example.com

For more examples, see Testing chapter.

Response

After receiving and interpreting a request message, a server responds with an HTTP response message:

                     FIELDS OF HTTP RESPONSE       PART OF RFC 2616
---------------------------------------------------------------------
  Request       = (1) : Status-line                  Section 6.1
                  (2) : *(( general-header           Section 4.5
                          | response-header          Section 6.2
                          | entity-header ) CRLF)    Section 7.1
                  (3) : CRLF
                  (4) : [ message-body ]             Section 4.3

Example of form an HTTP response for a request to fetch the /alerts/status page from the web server running on localhost:8000:

http_request

Additional information about responses:

  • caching objects (browser/proxy)
  • headers modification
  • body modification
Status line

The Status-line consisting of the protocol version followed by a numeric status code and its associated textual phrase.

Status-Line = HTTP-Version SP Status-Code SP Reason-Phrase CRLF
HTTP version

When an HTTP/1.1 message is sent to an HTTP/1.0 recipient or a recipient whose version is unknown, the HTTP/1.1 message is constructed such that it can be interpreted as a valid HTTP/1.0 message if all of the newer features are ignored.

Status codes and reason phrase

For more information please see:

Be sure to take a look at this:

http_decision_diagram

This amazing diagram comes from for-GET/http-decision-diagram repository.

Response header fields

There are three types of HTTP message headers for responses:

  • General-header - applying to both requests and responses but with no relation to the data eventually transmitted in the body

  • Response-header - these header fields give information about the server and about further access to the resource identified by the Request-URI

  • Entity-header - containing more information about the body of the entity, like its content length or its MIME-type

The response-header fields allow the server to pass additional information about the response.

Message body

Contains the resource data that was requested by the client.

HTTP client

HTTP client is a client that is able to send a request to and get a response from the server in HTTP format. Clients also originates a connection, passes any necessary authentication tokens and delivers the request for a specific piece of data.

The clients are anything that send requests to the back-end.

Back-End web architecture

I recommend to read these great repositories:

The back-end, or the "server side", is all of the technology required to process the incoming request and generate and send the response to the client. This typically includes three major parts:

  • server - this is the computer that receives requests (backend/origin server)
  • app - this is the application running on the server that listens for requests, retrieves information from the database, and sends a response
  • databases - are used to organize and persist data