ITN Module 15: Application Layer PDF

Module 15: Application Layer Introduction to Networks v7.0 (ITN) 15.3 Web Protocols © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 2 Web and Email Protocols Hypertext Transfer Protocol and Hypertext Markup Language When a web address or Unif...

Module 15: Application Layer Introduction to Networks v7.0 (ITN) 15.3 Web Protocols © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 2 Web and Email Protocols Hypertext Transfer Protocol and Hypertext Markup Language When a web address or Uniform Resource Locator (URL) is typed into a web browser, the web browser establishes a connection to the web service. The web service is running on the server that is using the HTTP protocol. To better understand how the web browser and web server interact, examine how a web page is opened in a browser. Step 1 The browser interprets the three parts of the URL: http (the protocol or scheme) www.cisco.com (the server name) index.html (the specific filename requested) © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 3 Web and Email Protocols Hypertext Transfer Protocol and Hypertext Markup Language (Cont.) Step 2 Step 3 The browser then checks with a In response to the request, the server sends name server to convert the HTML code for this web page to the www.cisco.com into a numeric IP browser. address, which it uses to connect to the server. The client initiates an HTTP request to a server by sending a GET request to the server and asks for the index.html file. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 4 Web and Email Protocols Hypertext Transfer Protocol and Hypertext Markup Language (Cont.) Step 4 The browser deciphers the HTML code and formats the page for the browser window. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 5 Web and Email Protocols HTTP and HTTPS HTTP is a request/response protocol that specifies the message types used for that communication. The three common message types are GET, POST, and PUT: GET - This is a client request for data. A client (web browser) sends the GET message to the web server to request HTML pages. POST - This uploads data files to the web server, such as form data. PUT - This uploads resources or Note: HTTP is not a secure protocol. content to the web server, such as For secure communications sent across the internet, HTTPS should be used. an image. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 6 15.4 IP Addressing Services © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 7 IP Addressing Services Domain Name Service Domain names were created to convert the numeric IP addresses into a simple, recognizable name. Fully-qualified domain names (FQDNs), such as http://www.cisco.com, are much easier for people to remember than 198.133.219.25. The DNS protocol defines an automated service that matches resource names with the required numeric network address. It includes the format for queries, responses, and data. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 8 IP Addressing Services DNS Message Format The DNS server stores different types of resource records that are used to resolve names. These records contain the name, address, and type of record. Some of these record types are as follows: A - An end device IPv4 address NS - An authoritative name server AAAA - An end device IPv6 address (pronounced quad-A) MX - A mail exchange record When a client makes a query, the server DNS process first looks at its own records to resolve the name. If it is unable to resolve the name by using its stored records, it contacts other servers to resolve the name. After a match is found and returned to the original requesting server, the server temporarily stores the numbered address in the event that the same name is requested again. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 9 IP Addressing Services DNS Message Format (Cont.) DNS uses the same message format between servers, consisting of a question, answer, authority, and additional information for all types of client queries and server responses, error messages, and transfer of resource record information. DNS message section Description Question The question for the name server Answer Resource Records answering the question Authority Resource Records pointing toward an authority Additional Resource Records holding additional information © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 10 IP Addressing Services DNS Hierarchy DNS uses a hierarchical system to create a database to provide name resolution. Each DNS server maintains a specific database file and is only responsible for managing name- to-IP mappings for that small portion of the entire DNS structure. When a DNS server receives a request for a name translation that is not within its DNS zone, the DNS server forwards the request to another DNS server within the proper zone for translation. Examples of top-level domains:.com - a business or industry.org - a non-profit organization.au - Australia © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 11 IP Addressing Services The nslookup Command Nslookup is a computer operating system utility that allows a user to manually query the DNS servers configured on the device to resolve a given host name. This utility can also be used to troubleshoot name resolution issues and to verify the current status of the name servers. When the nslookup command is issued, the default DNS server configured for your host is displayed. The name of a host or domain can be entered at the nslookup prompt. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 12 15.5 File Sharing Services © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 13 File Sharing Services File Transfer Protocol FTP was developed to allow for data transfers between a client and a server. An FTP client is an application which runs on a computer that is being used to push and pull data from an FTP server. Step 1 - The client establishes the first connection to the server for control traffic using TCP port 21. The traffic consists of client commands and server replies. Step 2 - The client establishes the second connection to the server for the actual data transfer using TCP port 20. This connection is created every time there is data to be transferred. Step 3 - The data transfer can happen in either direction. The client can download (pull) data from the server, or the client can upload (push) data to the server. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 14