CCNA Routing and Switching
<< IP and Network Fundamentals Course
>> Models, Protocols, and Standards Section
- Understanding TCP/IP and OSI Models
- Understanding Packet Flow Across the Network Part1
- Understanding Packet Flow Across the Network Part2
- Understanding Ethernet LAN Standards
- Understanding Ethernet MAC Addresses
- Understanding WAN Technologies
- Understanding Data Link Layer Encapsulation
- Understanding EoMPLS WAN Technology
- Understanding IP Routing Process Concept
- Understanding Web Browser DNS Lookup
- Understanding ARP Broadcast Message
- Understanding TCP and UDP Protocols
- Understanding TCP and UDP Multiplexing
- Understanding Application Layer Services
- Understanding Quality of Service Concept
Server’s Applications (Services), PC’s Applications, and how they complete each other? Understanding Application Layer Services will help you grasp the features and functions located at Layer 7 of either Server or PC. Technically speaking and from Layer 7 perspective, Applications at OSI Module are called services, specific Rolls of those services are meant for Servers only. Server Operating System with proper Hardware configuration, is meant to run multiple Rolls or dedicated Roll. PC Operating Systems in design was not meant to host Services the way Servers do.
Table of Contents
TCP/IP Apps & Services
Mainly, the Operating system being used distinguish the difference between Network device running as Server Roll and a Network device running as Personal Computer. Nevertheless, too many clients devices can run Services as well for testing purpose.
E.g. running Apache Server on Windows7 Machine, but is Win7 meant for Apache production? The Answer of course No. So, understanding Applications and Services Rolls Between Servers and Clients help you understand why we have 2 different rolls – Server and Client.
Understanding the Difference
An Application: Installed at the client side, usually meant to Request data from a Servers. Hence, PC’s Applications receive data from servers.
A Service: Installed at the Server side, usually meant to Receive requests from Client’s Application. Hence, Server’s Services respond to clients request with required data.
Note: Understanding the difference between Sending and Receiving procedures by Servers and Clients will make it so easy to troubleshoot IP Networks.
Clients Applications
As you know, there are too many Applications used on the clients, however, I will talk mainly about Web Browsers and the pieces used to request Web page Data. The HTTP client’s application (Internet Browser) such Firefox, Safari, and IE should have 3 main components to browse the internet:
1. Hypertext Transfer Protocol http:// – Layer 7 Service meant for client side.
2. World Wide Web (www) – Indicating that you are browsing Internet Articles.
3. Universal Resource Locator – Indicates a complete Web address or Web Link.
When combined all together, they Uniform a web address or Web Link such https://www.ccnahub.com
http:// – is the foundation of data communication for the World Wide Web. After establishing a TCP connection between the Browser and a Web Server, http file transfer starts between these 2 machines. HTTP protocol meant to transfer files between 2 network devices as article format.
HTTP uses several commands to make this happened, the browser uses GET request command to get the index.html file from the a web server, then the web server send back Get response command with a code of 200 indicating OK message. If you notice, one of the most known HTTP code is 404, meaning the file not found or the page cannot be displayed.
www – for short W3 or mostly known by “The Web”. Umbrella term that includes all web servers around the world, a system of interlined hypertext Articles accessed via the internet. The web indicates that you are browsing Articles or information as “Web Pages” such this Article.
Briefly, Web is a collection of Articles; both clients’ browsers and HTTP Servers such Apache or Nginx makes it possible to the user to request and read Articles as web pages using http protocol.
Domain Name – There are levels associated with Domain Names. Top level is .com, .net, .org, and so on. 2nd level is ccnahub.com, the name. The 3rd level, known by sub-domains, and it can be mail.ccnahub.com, ftp.ccnahub.com and so on, depends on the business needs.
URL – Known by Web Address or sometimes called Uniform Resource Locator. The browser has a Web Address field used to type a URL. URL can be used with other protocols as well and not specifically http. FTP is a file transfer protocol as well. Its URL is as follows ftp://ftp.ccnahub.com/uploads/.
As mentioned, URL can consists of several parts such http:// or ftp://, if it’s for web (articles) then it might include www or might not such http://archive.ccnahub.com, or for FTP requests such ftp://us.ccnahub.com/uploads/ and so on.
HTTP as Application
At the client side, called Web Browser, the client version of http protocol such Firefox, Safari, and IE, used to browse articles from the internet which can be located at any Hosting Server.
Most of the clients such PCs and smart devices, use Hypertext Transfer Protocol (http) along with the World Wide Web (www) addressing structure to browse contents from the Web Servers.
Remember: when client’s browser request articles from the internet, the client’s Layer 4 picks up a random dynamic TCP port as source port (as stated in Understanding Multiplexing TCP and UDP Sockets), and port 80 as destination port to form TCP socket as HTTP service. Once the socket is formed, IP packets that include data (articles) start to flow.
Web Hosting Services
There are too many services out there for a Server Roll, however, I will focus mainly on those used by a Hosting Server. At the end, the concept of either HTTP, FTP, MAIL, and any service is the same among the Network TCP/IP services which is: Server side and Client side.
HTTP as a Service
Called Web Server, served by Apache or Nginx. You can call it Web Server or Web Service. The word “Hosting Server” meant as Umbrella word of multiple services or Servers such Web or Mail Servers hosted on one machine.
The program installed on the Hosting Server to serve http requests, is called HTTP service (it can be Apache or Nginx). Again, Hosting Servers might house multiple services such web, mail, FTP, DNS, and so on.
HTTP server or service uses known registered ports to serve clients requests, TCP port 80 for clear text communication, and TCP port 443 for secure communication.
Besides, there are special Web Servers as well that can be used on devices such routers, switches, printers, or even inside your fridge at home. Those web servers are not meant to service articles, rather, it’s used specifically for configuration and monitoring.
Most of these special web servers uses custom ports such :8080, :10000, :8181, and so on, depends on the design. E.g. Configuring Network printer through its Web Interface.
Domain Name Services (DNS)
Layer 3 understands only IP addresses and people don’t remember IP addresses, therefore, there must be a way to translate human names to IP addresses. DNS Servers and Name Servers are used to translate domain name such ccnahub.com to its public IP address; DNS utilizes TCP or UDP port 53 to serve DNS requests that comes from DNS clients.
When a user requests ccnahub.com through a web browser, DNS client asks the DNS server to translate ccnahub.com to its Public IP address, hence, ccnahub.com home page can be loaded.
POP3 and IMAP Services
Hosting Servers use it all the time to deliver mail to clients through Push or Sync mail technology. Post Office Protocol Version 3 (POP3) utilizes POP3 TCP port 110 using Push Technology. And Internet Mail Access Protocol (IMAP) utilizes TCP port 143 using Sync Technology.
On the Client side, Outlook or similar client programs can be configured to communicate with either POP3 or IMAP service. POP3 and IMAP services are used to deliver mail to clients with either push or sync technology.
The difference between the 2 services: POP3 push everything to the client mailbox (Outlook) and empty the mail server, where using IMAP service will sync mail box between the the server and the Client inbox, hence, keep emails on all devices would up2date. Each of POP3 and IMAP got its own advantage and disadvantage.
For secure and encrypted communication, POP3 can provide its secure service through TCP port 995, and IMAP can provide its secure service through TCP port 993 as secure socket.
SMTP/SMTPS Service
Simple Mail Transfer Protocol (SMTP) and (SMTPS) used to send mail initiated by clients. SMTP uses TCP port 25 for clear communication, and TCP port 465 (SSL), or TCP port 587 (TLS) for secure communication.
Telnet and Secure Shell Services
Telnet and SSH used to remotely manage network devices. Telnet server utilizes TCP port 23 and SSH (the secure version of Telnet) utilizes TCP port 22. Secure Shell can be used for different services as well such securing FTP, SCP uses SSH, and VPN tunnel can be used through SSH. SSH mostly known for managing Cisco Routers, Switches, or Linux Machines.
FTP, TFTP, and SCP Services
File Transfer Protocol (FTP) uses two TCP ports, 20 for FTP data and 21 for FTP control. FTP server requires both ports open at the firewall or Router in order to serve its clients. Important to remember, that FTP service is not secure, therefore, FTP can be served over SSL certificate as FTPS through 443 port, or SFTP which FTP utilizing SSH TCP port 22 for secure connection. In Linux, usually OpenSSH is used to setup SFTP on Linux Servers.
Trivial File Transfer Protocols (TFTP) service mostly used to transfer firmware and OS images such cisco IOS from a PC to a switch or Router, it utilizes UDP port 69. TFTP server can be installed on windows as well; you can find a lot of free versions online that runs as TFTP servers. Routers and Switches come ready with tftp client using copy command.
Secure Copy Protocol (SCP) service utilizes SSH TCP port 22 as well, and client interface is a command line interface used by UNIX and Linux machines. WinSCP client application available for windows machines, it can connect to SCP, SFTP, FTP, or FTPS service.
SNMP Service
Simple Network Management Protocol is crucial agent component to monitor and gather specs information from network devices. SNMPv3 agent added encryption and is used widely to monitor Routers, switches, Servers, and mostly any network device.
To make SNMP useful, you need to have Network Management System (NMS) installed on one of your Windows or Linux Servers. The manager (NMS) service uses UDP port 162 to receive notifications/traps from SNMP agents.
SNMP agents operate at the Application Layer of OSI model (installed or configured on Network Device such Router or Switch) with UDP port 161 to communicate with NMS service. A free known version for monitoring network devices even cross the globe is Spiceworks.com, install it on a Server Machine, configure all your network devices with SNMP agents and let spicework engine scan your network.
NMS could be a huge system such as HP OpenView, and Cisco of course own line of products to offer monitoring for routers and switches, one of them is NetFlow. Cisco NetFlow is a Cisco developed flow technology that allows bandwidth monitoring of a network. Another Advanced Cisco solution that uses SNMP is: Cisco Prime Network where it can monitor all your Network Devices.
SNMP and NMS work together to gather and view useful information about network devices; it can alert Administrators about low performance and fault devices. Free version of NMS is Spiceworks.com and OpenNMS. Finally, SNMP agent can be configured at Cisco devices with few commands.
Using both TCP and UDP Ports
Some applications may use both TCP and UDP ports. For example, the low overhead of UDP enables DNS to serve many client requests very quickly. Sometimes, however, sending the requested information may require the reliability of TCP. In this case, the well known port number 53 is used by both protocols TCP and UDP. So, DNS Port 53 can be used by TCP and UDP when ever needed.
Subject Related
By Wikipedia App Layer | HighTech App Layer | TCP/IP Guide App Layer | Tutorials Point Application protocols | R&S ICND1 and ICND2 | Introduction To Network | Practical Packet | Computer Network | Introduction to Networking | Who is Running the Internet Book | Networking Self-Teaching Guide
CCNA Routing and Switching
<< IP and Network Fundamentals Course
>> Models, Protocols, and Standards Section
- Understanding TCP/IP and OSI Models
- Understanding Packet Flow Across the Network Part1
- Understanding Packet Flow Across the Network Part2
- Understanding Ethernet LAN Standards
- Understanding Ethernet MAC Addresses
- Understanding WAN Technologies
- Understanding Data Link Layer Encapsulation
- Understanding EoMPLS WAN Technology
- Understanding IP Routing Process Concept
- Understanding Web Browser DNS Lookup
- Understanding ARP Broadcast Message
- Understanding TCP and UDP Protocols
- Understanding TCP and UDP Multiplexing
- Understanding Application Layer Services
- Understanding Quality of Service Concept
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