Outline

• Finish intro to browsers and web servers from last day
• Quiz
• Apache
• .htaccess files
• MIME
• HTTP

The World Wide Web

• For this semester, the protocol on top of TCP/IP we will be interest in is called HTTP (Hypertext Transfer Protocol).
• It is used to retrieve from servers hypertext documents (one's with links and images. i.e., HTML) over the internet.
• It was first proposed in 1989 by a group including Tim Berners-Lee at CERN.
• The World Wide Web is the collection of servers and client that use this protocol.

Web Browsers

• Web Browsers are clients used to request hypertext documents from HTTP servers.
• The first browser developed by Berners-Lee was called WWW.
• The first widely used browser was NSCA Mosaic.
• It served as the basis for the first commercial browser Netscape.
• The most common modern browsers are: Internet Explorer, Edge, Chrome, Firefox, Safari, Opera, and Brave.
• Under the hood, browsers use a layout engine. For example, Internet Explorer uses Trident, Edge uses EdgeHTML (now also based on WebKit variant), Firefox uses Gecko, and Safari, Chrome, and Opera, including on mobile platforms use some variant of WebKit such as Blink.
• The site evolt.org has executables for many old browsers.
• Today, as indicated by the wide-spread use of WebKit, most cell phones have browsers which support W3C compliant HTML and Javascript/ECMAscript.
• As a web programmer you should code to standards not any particular browser. In fact, large organizations (such as SJSU) might be required to produce only pages which conform to standards such as accessibility standards.

Web Servers

• The most common web servers as a fraction of the top million trafficked sites according to Netcraft as of January, 2021 are Apache (~25.03%), nginx (~25.03%), IIS (~6.89%) (Microsoft Internet Information Server), and Litespeed (~2.04%).
• As Apache is free, I expect you to have it downloaded and installed by the time the first homework is due.
• Apache is included in many all in one web development packages such as WAMP, MAMP, XAMPP, so if you hate installing software you might consider using one of these.

Quiz

Which of the following statements is true?

1. In this course, you can use insurance points to recover up to half of what you lost on an exam.
2. This course teaches only client side web development.
3. We will have two midterms and six homeworks this semester.

Web Server Operation

• In a typical exchange between a browser and a web server, the browser makes a TCP connection into port 80 or 443 (encrypted) of the web server and makes a GET request for a particular document.
• You can, as a human, simulate this connection by telneting/openssl'ing into port 80 or 443. For example,

  openssl s_client -crlf -quiet -connect pollett.org:443
  

• A web server has two sets of directories, the root of one of these is called the document root of the server. The web server looks in this directory to try to find the file to service the client's request.
• The root of the other directory the server uses is called the server root. Under it is stored the server and its support software.
• To initiate an exchange as above, the user typically supplies the browser with the protocol, say http; the machine, say pollett.org; and the path to the file /index.php in one string

  https://pollett.org/index.php

  called a URL.

Apache

• This is the most common web server.
• It has one main configuration file, usually named something like httpd.conf or apache2.conf, which may be self-contained or may load several other additional configuration files.
• On Linux variant it can usually be found in the /etc/apache2 folder.
• Configuration files consists of a bunch of lines in a pseudo-tagged based language. # is used for comments.
• Here is a brief list of common directives:
  • ServerName --- names the server system
  • ServerRoot --- path to the server root (used when relative path given in over the commands such as LoadModule)
  • DocumentRoot --- path to the document root
  • LoadModule --- loads a module (like PHP to be used with the server)
  • AddModule --- makes the added module available for use
  • AddType --- associate a file name extension with a MIME type.
  • Alias -- alias a URL path to a physical directory path
  • <Directory> </Directory> --- control how the contents of a particular directory can be served. .htaccess files
• Even if you don't have access to the httpd.conf file, if the webserver admin allows it, you might be able to use .htaccess files in the directories you control.
• An .htaccess file allows you to do configurations similar to what can be done in <Directory></Directory> tags.

Example of an .htaccess File to Password Protect a Directory

AuthUserFile  "/Volumes/Library/WebDocuments/library/.htpasswd"
AuthGroupFile /dev/null
AuthName library
AuthType Basic

<Limit GET POST>
require valid-user
</Limit>

MIME

• MIME stands for Multipurpose Internet Mail Extensions.
• It was originally developed as a way to send attachments containing non-text documents over the internet.
• Nowadays, it provides a way for a browser to determine the format of the documents it receives from the server.
• A MIME specification has the format:

  type/subtype.

• The most common types are text, image, video.
• Some example complete specifications are: text/plain, text/html, image/gif, image/jpeg, video/mpeg.
• In addition to the above, you sometimes see types such as application/xhtml+xml, application/xml, or experimental types which begin with an x-. For example, video/x-msvideo

HTTP

• The details of HTTP (hypertext transfer protocol) can be found in RFC 1945 (HTTP 1.0), RFC 2616 (HTTP 1.1), RFC 7549 (HTTP/2), and draft-ietf-quic-http (HTTP/3). Additional specifications for things like security, links to which can be found at the W3C HTTP Spec page.
• HTTP 1.0 and HTTP 1.1 use text based messages which can be sent over the network unencrypted.
• HTTP 2 and 3 require encryption. Although many of the messages are the same as HTTP/1.1 they are sent using a more succinct binary format.
• HTTP 2 allows persistent connections with multiple streams of data, server pushes, and special headers that can tell a browser related resources that should be pre-fetched.
• HTTP 3 is essentially HTTP 2 using the QUIC protocol for the connection rather than TCP. One reason for the switch is that lost frames in TCP could cause all streams in an HTTP 2 connection to stall.
• HTTP 3 is, as of Jan 2021, close to being rolled out in the wild on a large-scale, but not there yet.