csg1105_Module_One.pdf

Module One CSG1105 Applied Communications CSG1105 Applied Communications Module One 1 / 28  Section 1 Introduction CSG1105 Applied...

Module One CSG1105 Applied Communications CSG1105 Applied Communications Module One 1 / 28  Section 1 Introduction CSG1105 Applied Communications Module One 2 / 28  The Chicken or the Egg? Which came first? In dealing with this material, we are often faced with this problem. One or more of the topics uses concepts from another topic we have not yet covered. It may seem that we are continually returning to topics already covered, but it may be necessary to deal with interactions Progression through the material is not necessarily linear: you may need to revisit earlier topics multiple times CSG1105 Applied Communications Module One 3 / 28  Section 2 What is a Web Browser? CSG1105 Applied Communications Module One 4 / 28  What is a Web Browser? We’re all familiar with a Web browser, be it on a PC, Tablet or phone. We use it to access information in the way of text, images, audio, video or even applications. A Web browser is a HTTP Client It communicates with a Web Server, AKA a HTTP Sever We will look at HTTP a little later CSG1105 Applied Communications Module One 5 / 28  Client/Server 1 Browsers, (Firefox, Chrome, Edge etc.) are Web clients and that Web sites host Web servers. So, what are clients and servers? Firstly, a negative definition: A server is not hardware A server is a process, running at an advertised location providing a service. A process is a running program. A single computer will have many running processes, some may be servers, some clients A client is a process that consumes the product of a service Generally, there are many clients using the service provided by a server simultaneously. Generally, client and server are on separate computers, but not always. CSG1105 Applied Communications Module One 6 / 28  Client/Server 2 CSG1105 Applied Communications Module One 7 / 28  Client/Server 3 There are thousands of types of servers We will be covering a number of commonly used servers in this unit Some examples are: ▶ Web (http) server ▶ Email (smtp) server ▶ DNS server ▶ Directory (ldap) server ▶ File (smb) server CSG1105 Applied Communications Module One 8 / 28  Section 3 Protocols CSG1105 Applied Communications Module One 9 / 28  Protocols The clients and servers for any service communicate using an accepted protocol The previous slide identifies the protocols used (http,smtp,DNS,ldap,smb) A protocol is simply an accepted method of exchanging messages between the client and server Question: Who creates the protocols? CSG1105 Applied Communications Module One 10 / 28  Standards Bodies There are Open and Proprietary standards Some standards are created by International groups, some by Industry bodies Many protocols used on the Internet are Open standards Three of the major international organizations are: ▶ The IETF - Internet Engineering Task Force ▶ The IEEE - Institute of Electrical and Electronic Engineers ▶ The ISO - International Standards Organization CSG1105 Applied Communications Module One 11 / 28  HTTP Protocol We will use some simple HTTP messages to illustrate a protocol in action If you don’t specify a particular page on a Web server, it will return a default page, often index.html The browser will establish a connection to the web server (more on this later) CSG1105 Applied Communications Module One 12 / 28  Request A request is sent: GET / HTTP/1.1 The request consists of: ▶ the Method GET ▶ the path ”/” ▶ the Protocol Version 1.1 ▶ This line will be followed by a list of headers detailing the client CSG1105 Applied Communications Module One 13 / 28  Response If the page exists, it is returned with a protocol version from the server and a status code and message. This will be followed by headers giving details of the returned content. A new line then separates the headers from the content, in this case, a HTML page HTTP/1.0 200 OK Server: SimpleHTTP/0.6 Python/3.6.9 Date: Tue, 04 Feb 2020 06:11:46 GMT Content-type: text/html Content-Length: 91 Last-Modified: Tue, 04 Feb 2020 06:08:17e CSG1105 Applied Communications Module One 14 / 28  Response for missing page If the page DOESN’T exist GET /badpage.html HTTP/1.1 Error message HTTP/1.0 404 File not found CSG1105 Applied Communications Module One 15 / 28  HTTP 2 The HTTP protocol has many more features which may be found in RFC2616 found at https://tools.ietf.org/html/rfc2616 What we have looked at is an example of a request/response style protocol Many of the protocols used on the Internet follow this mode of operation CSG1105 Applied Communications Module One 16 / 28  Section 4 Uniform Resource Locators (URLs) CSG1105 Applied Communications Module One 17 / 28  Uniform Resource Locators (URLs) The HTTP protocol allows the browser to make requests and receive responses over an existing connection Earlier, we defined a server as a process running at an advertised location The method of specifying a location is a Uniform Resource Locator (URL) The formal specification of URLs is documented in RFC 3986 available at https://tools.ietf.org/html/rfc3986 CSG1105 Applied Communications Module One 18 / 28  URL Syntax A simple URL consists of three elements ▶ The Scheme ▶ The Authority ▶ The Path Using the address from the previous slide as an example: https://tools.ietf.org/html/rfc3986 CSG1105 Applied Communications Module One 19 / 28  Scheme The scheme is the Protocol used for the connection Some examples of protocols used in URLs ▶ HTTP ▶ FTP (file transfer protocol) ▶ mailto (send to email address) ▶ file (a file on the client computer) There are many other protocols, some of which are now obsolete e.g. gopher CSG1105 Applied Communications Module One 20 / 28  Authority The Authority provides the location of the resource The syntax is userinfo@host:port For the moment, we are going to focus on the host and consider the others later The host portion of the Authority is a Fully-Qualified Domain Name (FQDN) which we will look at later CSG1105 Applied Communications Module One 21 / 28  Path The Path locates where the resource is stored on the target machine The Web server has a tree structured set of folders to contain content The path specifies where in relation to a specified root folder As a machine may implement more than one web server, this is usually a virtual location CSG1105 Applied Communications Module One 22 / 28  FQDN FQDN - Fully Qualified Domain Name An FQDN is the complete domain name computer on the Internet It consists of two parts: ▶ The host name and ▶ The domain name An example: www.curtin.edu.au CSG1105 Applied Communications Module One 23 / 28  Section 5 DNS - Domain Name System CSG1105 Applied Communications Module One 24 / 28  DNS - Domain Name System Your phone has a Contacts app that don’t have to remember them Stan 0412 345 678 Lee +64 0422 987 655 DNS is the ”Contacts app of the Internet” A world-wide distributed directory service used to index computers, services and other resources DNS is a protocol with clients and servers It is a tiered system, where each level has delegated control over a portion of the namespace One purpose is to translate human readable FQDN strings into a numerical address (IP address) CSG1105 Applied Communications Module One 25 / 28  DNS Figure 2: DNS Tree CSG1105 Applied Communications Module One 26 / 28  Tracing a DNS Query 1 Our browser sends a query to ECU’s DNS server 2 ECU’s web server queries the.edu.au root server for Curtin’s DNS server address 3 The.edu.au server returns the address of Curtin’s DNS server 4 The ECU DNS server sends a query to the Curtin DNS Server 5 The Curtin DNS server returns the address of www.curtin.edu.au 6 ECU’s DNS server returns the address of www.curtin.edu.au to the client PC 7 The client PC can use the address to contact www.curtin.edu.au CSG1105 Applied Communications Module One 27 / 28  DNS Query CSG1105 Applied Communications Module One 28 / 28