Topic:Application Layer
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The application layer is the highest layer in the protocol stack, and contains many of the technologies that power the web, and yet we take for granted nearly every day. In this topic, we examine in great detail the architectures contained within this layer, and the how we use them'
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[edit] Architectures
There are 3 ways in which we can use the Application Layer
- Client-Server
- Peer to Peer
- Hybrid
[edit] Client-server
The Client Server Model operates mainly on the concept of an always-on server, to whom a client may connect to. The server normally has a static IP address, and an associated DNS record to allow hosts to look it up. Clients may come and go, and are never directly connected to each other. Applications that communicate between hosts using the client server model must send information through the server, which may be problem if the server happens to go offline.
An example of the Client Server Architecture is the web server and browser. Wikiversity is hosted a number of mediawiki servers, which are always on, allowing you and other people from across the globe to view this web page.
[edit] Peer to Peer
Also known as P2P, the peer to peer architecture does not require the use of centralised servers. Instead, hosts communicate directly with each other. Hosts may have dynamic IP addresses and may join or leave such networks at any time.
A greatly known example of P2P is the bittorrent file sharing protocol.
[edit] Hybrid
Hybrid architectures use a combination of both P2P and client server architecture.
An example of this is the skype protocol. A central server keeps the address books of all individual users, as well as the IP addresses of the users in order for other users to establish connections to. When a person wants to make a call to another person, Skype provides the IP address to the destination host and a peer to peer connection is established between the 2 users.
[edit] Common Ports used in the Application Layer
| Protocol | Port |
|---|---|
| FTP/data (File Transfer Protocol) | 20 |
| FTP/control (File Transfer Protocol) | 21 |
| SSH (Secure Shell) | 22 |
| Telnet | 23 |
| SMTP (Simple Mail Transfer Protocol) | 25 |
| DNS (Domain Name Service) | 53 |
| DHCP (Dynamic Host Control Protocol) | 68 |
| HTTP (Hypertext Transport Protocol) | 80 |
| POP3 (Post Office Protocol, ver 3) | 110 |
| NTP (Network Time Protocol) | 123 |
| IMAP (Internet Message Access Protocol) | 143 |
| SSL (Secure Socket Layer) | 443 |
| SMTP (Simple Mail Transfer Protocol/"submission") | 587 |
