Web servers are the literal machines that power the web.
Whether secure, using the SSL protocol, or unencrypted plaintext over http, the HyperText Transport Protocol is the most common and well used protocol online.
In this section you will learn about the basic principles of the protocol, it's differing versions but not web servers themselves.
To be an effective and talented technical SEO a good understanding of the protocol is important. You do not need to know every intimate detail but by having great knowledge you will be able to see and intuitively know when things are broken and not working in the way they should.
You may not be the person who fixes these issues, but by identifying them, and speaking with authority due to your knowledge, I can assure you that technical departments and colleagues will be more receptive at listening and taking care of the "tech guy" in that marketing department.
So without further ado, let's get down to some details.
As with all the sections in this training, I will give a base level of knowledge and information. You SHOULD research these topics further.
I strongly suggest you find resources to understand the relatively new protocol of HTTP 2 and it's predecessor SPDY - HINT Wikipedia and links from there are your friend...
HTTP functions as a request–response protocol in the client–server computing model. A web browser, for example, may be the client and an application running on a computer hosting a website may be the server. The client submits an HTTP request message to the server. The server, which provides resources such as HTML files and other content, or performs other functions on behalf of the client, returns a response message to the client. The response contains completion status information about the request and may also contain requested content in its message body.
A web browser is an example of a user agent (UA). Other types of user agent include the indexing software used by search providers (web crawlers), voice browsers, mobile apps, and other software that accesses, consumes, or displays web content.
HTTP is designed to permit intermediate network elements to improve or enable communications between clients and servers. High-traffic websites often benefit from web cache servers that deliver content on behalf of upstream servers to improve response time. Web browsers cache previously accessed web resources and reuse them when possible to reduce network traffic. HTTP proxy servers at private network boundaries can facilitate communication for clients without a globally routable address, by relaying messages with external servers.
HTTP is an application layer protocol designed within the framework of the Internet protocol suite. Its definition presumes an underlying and reliable transport layer protocol and Transmission Control Protocol (TCP) is commonly used. However HTTP can be adapted to use unreliable protocols such as the User Datagram Protocol (UDP), for example in HTTPU and Simple Service Discovery Protocol (SSDP).
HTTP resources are identified and located on the network by Uniform Resource Locators (URLs), using the Uniform Resource Identifiers (URI's) schemes http and https. URIs and hyperlinks in HTML documents form inter-linked hypertext documents.
HTTP/1.1 is a revision of the original HTTP (HTTP/1.0). In HTTP/1.0 a separate connection to the same server is made for every resource request. HTTP/1.1 can reuse a connection multiple times to download images, scripts, stylesheets, etc after the page has been delivered. HTTP/1.1 communications therefore experience less latency as the establishment of TCP connections presents considerable overhead.