TODO: Put some sort of quote in about documents which are linked together.
Under the hood, the web is all about the HyperText Transfer Protocol. HTTP is the means by which a client (such as a web browser, or a search engine bot) can ask a server for something and get a response which either includes whatever was requested or a message that says something was wrong with the request.
Let's take a look at an HTTP request.
GET / HTTP/1.1␍
Host: example.com␍
␍
The first line is the Request-Line. This has three parts.
The request method (GET) describes what you want to do to the resource.
The request URI (/) identifies which resource the request is for.
The protocol version (HTTP/1.1) states the version of HTTP in use.
1.1 has been around since the 90s. Version 2 is under development
but it will be a long time before we see it in production.
The line then ends with a carriage return and a line feed
(CRLF).
After the request line we have the HTTP request headers.
Both requests and responses come in two parts. A set of headers and then, sometimes, some content.
The format of a header is very simple:
- A name
- A colon (
:) - A value
There are lots of different headers in HTTP, and more from extensions, and we'll cover the more important ones throughout the book.
If this was HTTP 1.0 then we wouldn't need to send any headers, but IPv4 addresses are a precious commodity and we often have multiple websites hosted on a single computer with a single network interface, all with their own hostname.
HTTP 1.1 makes it mandatory to state which website the request is for using a header.
Host = "Host" ":" host [ ":" port ] ;
TODO: http://tools.ietf.org/html/rfc2616#section-5.2 - Read this properly. Can the host header be omitted if an absolute URI is in the header? Rephrase the bits about the Host header if that is an option.
Finally, the request header is ended by two CRLF in a row.