Important points for Chapter 3

Application layer is the starting point and the ending point of data communication

When data is transmitted, data will be processed through various layers of the OSI model. This is known as Encapsulation. Headers are added.

When data is received, data will be processed through the same layers of the OSI model. This is known as De-encapsulation. Headers are removed.

Devices at 2 points must adopt the same communications model, different models do not work

Application layer has 3 entities - Applications, Services and Protocols
Applications - Microsoft Outlook, FileZilla, LimeWire, Bearshare
Protocols - Rules to govern the transmission of data
Services - Combination of Application and Protocols to provide services such as email, files upload and messaging.

As we know that the OSI model is a reference model while the TCP/IP model is a protocol model, the application layers at both models are related. The application, presentation and session layers at the OSI model is equal to the application layer at the TCP/IP model. The main reason is due to legacy issues as OSI model is developed after TCP/IP model and encompass more features compared to the latter model.

The various protocols for the application layer are DNS, HTTP, SMTP, POP, DHCP. All these protocols are important for your tests and exams

Most application layer protocols followed the client server model. The client sends a resource request to the server and the server will process the request and allocate the resources and send a response back to the client. Usually there is one server connected to multiple clients

For telnet, the server is running a daemon which actively listen to telnet requests from clients. When there is a request, the server will create a child process to handle the telnet session. This way, the server is able to monitor various clients at the same time.

Peer to peer networking model is different as any PC can be both client and server at the same time. Information is processed simulataneously at both ends and the transmission is full duplex.

Main function of DNS is the resolve domain names to ip addresses. When a user enter a domain name into the browser, the domain name is passed to the resolver (nslookup). The resolver will query the domain server for the correct ip address, if it is found then the ip address is passed to the PC. The PC will direct the data packets using the ip address.

HTTP is responsible for reliable transmission for web data from one end to another. It is achieved by means of POST and GET requests to the web server.

SMTP is used to send and forward mail information. MUA is your email software while MTA is used as a forwarder to forward email information to the right email server. The MDA is used to store your email on the server before it is transferred to the MUA. The MUA will retrieve email from the MDA using POP.

Telnet is used to create remote access to a particular device. The transmission is using in plaintext and is not secure. Nowadays, we used SSH (secure shell) to perform remote access.

SMB (Server Message Block) is a protocol which is used for folder and file sharing over the network. It is used mainly for Microsoft Windows. It follows a server and client model

Gnutella, on the other hand, is a protocol for folder and file sharing over the network using the peer networking model. It is used by bearshare and limewire. There is another protocol known as BitTorrent which is also peer to peer.

DHCP is used for dynamic assignment of ip addreses to clients. The four main messages in a DHCP exchange are DHCPDISCOVER, DHCPOFFER, DHCPREQUEST, DHCPACK. For a large enterprise network, having DHCP will remove a lot of manual work needed to configure ip addresses.

Need to remember the port numbers.